This page is very old and much work has been done on the active optics system, including a major upgrade in 2014.  The value of the information here is uncertain.
 

Active optics System

Short instructions for normal use

 J.Baldwin, 15 October 1998

 

1.0 PRIMARY MIRROR CORRECTION FROM LOOKUP TABLE.

Corrections should normally be ON for all foci (prime, f/8).
The present table causes the primary mirror to be bent to correct for a systematic astigmatism effect which is due to problems with the primary mirror support system.

2.0 F/14 COMA CORRECTION FROM LOOKUP TABLE.

Should normally be OFF whenever f/14 is in use.

3.0 IMAGE ANALYZER.

3.1 Before start of night,

• If the IMAN software is running, stop it by typing quit<return>
• Reboot the IMAN PC. It is best to turn its power off and on. If there is an error message, reboot it again.
• Once the IMAN PC is rebooted with no error messages, start the NFS link by typing: gonfs<return>. Answer "i" (ignore) to the error message about Authentication Failure.
• Go to the cass cage and check that the power switch for the iman camera's electronics box (which is mounted on the side of the offset guider module) is set to "ON (NO TEC)".This setting disables the ThermoElectric Cooler, which is what is wanted on all but the warmest of nights. The camera power switch should be left in this setting at all times.. The power can then be switched on and off remotely from the console room.
   

3.2 Just before using IMAN

• Turn the camera ON using the TCP menu commands
  • ACTIVE CONTROL
  • IMAGE ANALYZER
  • POWER ON CAMERA
• Only after the camera is ON, start the IMAN software in the IMAN PC by typing
  • iman<return>
  • Software should come up in IDLE mode. If not, type s<return>
  • On startup, the gain is automatically set to 2 and the offset set to the corresponding value of 231. These are the nominal values which should be used for IMAN measurements. (Modified slightly 18Dec00 r.c.and b.g.)

 

3.3 Measuring a star.

• Move rotator mirror to position 1or 2
• Move guide probe to optical axis
• Select FLAT MIRROR OUT and then PELLICLE IN (and vice versa if you are going to use the flat mirror). Now 10% of the starlight will go to the guider TV, and 90% to IMAN. The flat mirror in the guider optics will be left out of the beam for all remaining steps.
• Select SMALL APERTURE from the IMAGE ANALYZER menu. A 13 arcsec dia. aperture will be put into the focal plane ahead of the IMAN optics, but will not affect the field of view of the guider camera.
• Put the Shec Guider cursor at the following position: x=257, y=106 if you are using the pellicle, and x=200, y=153 if you are using the flat mirror (LAST UPDATED: 3 Feb 2001). Move the telescope to center a 11 mag star (10.5 to 11.5 should work) on the guide probe cursor. Guide the telescope either with the autoguider to keep the star at this position.
• Ensure that the star image on the guider is in focus. Adjust the guide probe focus if it is not.
• If you are measuring the TWEAK correction in a new part of the sky, use the RESET option in the TWEAK ADJUST command (from the PRIMARY MIRROR CONTROL menu).
• Select STAR SEQUENCE (enter * on the keyboard). This will cause a series of three 30 second exposures to be taken and analyzed. A blue window will open on the Telescope Operator's Sun and the results will start to appear.
• Use the CTRL-F2 keys to get rid of the blue windows which show IMAN results and messages.
• Select OBSERVE POSITION. This will turn off camera power, move the pellicle out of the way, and move the flat mirror to the GDR position. The guider is now ready for normal use.
• Move the guide probe out of the way of the instrument.
• On the IMAN PC's keyboard, type quit<return> to stop the IMAN software.
• To start another measurement later in the night, return to step 3.02.

 

4.0 TWEAKING.

• At the end of the STAR SEQUENCE, a red box will appear on the TCP screen giving you the options of tilting the secondary, bending the primary, or quitting.
• If you elect to tilt the secondary or bend the primary, you will then be asked whether you want to accept the recommendations made by the IMAN program. These are the "Y" and "N" entries in the Tweak? line in the IMAN output. Normally, you should not change anything unless a change is recommended ("Y").
• If at some later time you wish to tilt the secondary to correct for the coma, measured here, use either LAST LOG ENTRY or OLD LOG ENTRY from the TILT SECONDARY or PRIMARY MIRROR CONTROL menus, as appropriate.
• If you apply a tweak to the primary mirror, you must then make sure that the ENABLE option in the TWEAK ADJUST command has been selected (the TCS Status Display will show a flashing "TWEAK ON" message).
• If the tweak correction was larger than 2 microns, it is worth taking another IMAN measurement and repeating the tweak if necessary.

 

5.0 ERROR RECOVERY

• Some common IMAN error messages:
  • ERROR -- STAR IS TOO BRIGHT. Too few spots have been found and more than 1000 pixels (average of about 5 per spot) have signal levels of 4095 (CCD saturation). Find a fainter star.
  • ERROR -- STAR TOO FAINT. Too few spots have been found and less than 500 pixels are more than 150 ADU above the background. Find a brighter star.
  • ERROR -- BACKGROUND TOO BRIGHT? Too few spots have been found and the average ADU/pixel is more than half the saturation value. Find a darker sky.
  • ERROR -- FOUND TOO FEW SPOTS. Too few spots have been found and none of the 3 previous errors have been detected. take another star sequence and watch the IMAN image display monitor as the 30 second exposures are read out. Is the star way off center? Does the image turn to noise half-way through the picture?
  • ...and see "IMAN Image Analyzer" Sect. 4.7 for further exciting possible disasters.
• Image displayed on IMAN monitor is full of garbage. A noise spike has gotten into the data stream. The reduction program probably will skip over the bad image and not include it in the average values for the aberrations. If so, and if you get two good images out of a star sequence of three images, that probably is good enough. If not, take another star sequence.
• Images continue to be full of garbage. Stop the program on the Iman PC. Turn camera power off, then back on. Restart the IMAN PC starting at step 3.01 above.
• During star sequence, the message in the blue box on the TCP screen freezes. You should stop and restart the TCP program... it has lost communication with the Sun. The message in the blue box initially should read "Starting file deletion, waiting for first image", but should then quite quickly change to say "image size = 0", then larger sizes as the first 30 second exposure starts to be transfered, etc. Then the box should show the reduction results as they are built up. To restart the TCP program:
  • go to the highest-level TCP menu and select "Exit TCP". The window with the TCP menu should disappear.
  • Then open the console window and type "tcp4m <return>". The window with the menu should reopen.
  • Now try running IMAN again.
  • Aperture wheel or flat mirror do not complete their motions. Try the "iman stop" command in the TCP command window, then try again to move the device. You must turn the camera power back on after using "iman stop".
  • Timeout messages while moving a device. Probably a fatal hardware failure. It's time to call for help.

	Contents:
	1.0	Basic Principles
		1.1	Active Optics
		1.2	The Wavefront
		1.3	Wavefront Errors
		1.4	Correcting the Errors
	2.0	Correcting the Errors
		2.1	Primary Mirror
		2.2	Secondary Mirror
		2.3	Image Analyzer
	3.0	IMAN output
	4.0	TCS Menu Items
	5.0	Instructions for Normal Use, Active Optics (Cross Reference)
	6.0	More about tweaking
		6.1	Tweak corrections are additive
		6.2	The TWEAK ADJUST command
		6.3	Ways to enter TWEAK values
	7.0	Be careful! here's the trap with the tweak correction!
	8.0	And the IMAN power is another trap!
	9.0	Solving Problems (problems? what poroblems?)

This page is very old and much work has been done on the active optics system, including a major upgrade in 2014.  The value of the information here is uncertain.

J.Baldwin, 14 November 1995

Last revised: M. Boccas, 3 February 2001

1.0 BASIC PRINCIPLES

1.1 Active Optics

Active Optics systems are now in use on a number of telescopes (ESO NTT, WIYN) and are planned for all future large telescopes. They correct the shape and alignment of the telescope optics on a slow time scale (once every few minutes). This greatly simplifies getting the telescope properly tuned up to start with, and then allows the optics to be continually adjusted in order to compensate for flexure, etc. as the telescope moves around the sky. There is generally a lookup table which automatically changes the Active Optics corrections as a function of telescope position, and often also an image analyzer which uses fairly bright stars to make measurements during the night for additional fine tuning. Gemini will use an image analyzer in this mode almost all of the time.

The system on the 4m Blanco Telescope operates primarily from lookup tables which contain pre-calibrated correction values which can be interpolated to the present telescope position. There also is a provision for occasionally using an image analyzer on a bright star to fine tune (or "tweak") the corrections before observations for which high angular resolution is of special importance, but this will take enough telescope time (10 min?) that it is not expected to be the normal mode of operation.

Active Optics is not Adaptive Optics. Adaptive Optics refers to high speed corrections for seeing effects in real time. Sorry, all we offer is the boring low-speed stuff. (The f/14 secondary, now under development, offers high speed tip-tilt corrections to the image position to compensate for image motion arising from dome and atmospheric seeing and wind-shake of the telescope. This will be our first implementation of ADAPTIVE optics.)

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1.2 The Wavefront

The telescope intercepts light waves coming from distant objects and brings them to a focus. A wavefront is a locus of adjacent points where the electromagnetic wave has the same phase. Except for seeing, the incoming wavefronts, before striking the primary mirror, would be perfectly flat planes perpendicular to the direction to the object being observed. After bouncing off the mirror(s), when approaching the foci, the perfect wavefronts would be spherical in order to arrive at focus in phase.

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1.3 Wavefront Errors

However, there is seeing, and the telescope is not perfect, so the actual wavefronts are distorted. The distortions can be described as the amplitude A of the displacement of the wavefront, along the direction of travel, from where it should be in the perfect case. It is convenient to use a circular coordinate system oriented perpendicular to the direction of travel. Any point can be specified by radial coordinate r and angular coordinate phi.
The amplitude of the wavefront displacements, A, at that arbitrary point can then be described as a superposition of a series of terms of different radial and angular shapes; this is analogous to describing a complex sound as a sum of simple musical tones, or frequency spectrum.

The typical way to describe the wavefront errors is to use Zernike Polynomials. These are rather complicated functions, usually depending on more than one power of r, which have the nice property (among others) of being mathematically independent of each other (orthogonal). We don't do that. Instead, we follow the example of the ESO NTT (that's where we stole our software from), and describe the wavefront as:

A = c(1,1) * r * cos(phi) + c(2,2) * r² * cos(2 phi) + ...

... + c(n,m) * rⁿ * cos (m * phi)

 

summed over all possible values of the integers n and m. c(n,m) is a coefficient giving the amplitude of each term.

The individual terms in this series are called the "Quasi-Zernike Polynomials". The terms are not precisely orthogonal to each other, but under the real conditions in the real telescope, they are close enough.

The Active Optics System includes an image analyzer (IMAN) which measures the shape of the wavefront and then calculates a set of a few low-order quasi-Zernike functions which accurately represent the shape of the wavefront.

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1.4 Correcting the Errors

There are only a limited number of alignment or bending adjustments which we can make to the telescope's mirrors. Conveniently, each of these potential errors can be related to a different Quasi-Zernike mode. These are all low-spatial-frequency modes, with small values of m and n. The higher frequency modes are caused mostly by seeing and by small-scale polishing errors on the mirror surfaces; the active optics system cannot correct these because the mirror is too stiff.

Table 1 shows the low-order errors that we can measure with the image analyzer and how they are removed (cured) using the Active Optics System.

 

Table 1: Wavefront errors

Aberration	Quasi-Zernike	Cure	Comments
Defocus	r2cos(0*pi)	Refocus	Easily confused with spherical
Spherical	r4cos(0*pi)	Bend Primary	Or move focal plane, change primary-secondary spacing
Decenter	r1cos(1*pi)	Repoint	Easily confused with telescope. astigmatism
Coma	r3cos(1*pi)	Translate or tilt secondary
Astigmatism	r2cos(2*pi)	Bend primary	Easiest way to bend mirror
Trefoil	r3cos(2*pi)	Bend Primary	Usually print-through from hard points
Quadrafoil	r4cos(4*pi)	Bend primary	Not expected to be significant

 

2.0 OPERATIONAL CONCEPTS

The Active Optics System has three main components: the 4m Active Primary system (4MAP), the Secondary Mirror Alignment System, and the Image Analyzer (IMAN).

2.1 Primary Mirror

The 4M Active Primary System (4MAP) is able to bend the mirror in modes which will correct for spherical aberration, astigmatism, trefoil and quadrafoil. For each aberration (but spherical aberration) and each focus, there is a lookup table containing corrections as a function of telescope position.

[[REVISION 3Feb01:]] These tables are text files which are stored in /ut02/4map/ and are called: 4mapXY.cof, where X is the aberration (2 is astigmatism, 3 is trefoil and 4 is quadrafoil) and Y is the focus (pf, f8 or f14). Thus there are 9 '.cof' files overall. In addition, there is a file called zero.cof that is a null table (ie. filled with 0) which can be used to replace whatever 4mapXY.cof to cancel/zero the corrections whenever one doesn't want to use the lookup table (note that the telescope operator is instructed to ALWAYS use the lookup tables by selecting 'Corr ON focusxx' in the TCS menu). At a specific focus, the fact that you activate the M1 corrections means that 3 lookup tables -one for astigmatism, one for trefoil and one for quadrafoil- are under use, their values being added vectorially. Usually, only the astigmatism table actually contains numbers, the trefoil and quadrafoil tables beeing filled with 0 (this is because the telescope doesn't suffer from significant trefoil or quadrafoil aberrations). When the 4MAP PC boots, it first reads these 9 files in /ut02/4map/ in order to update its default files to the latest versions. This modification (putting the 4MAP PC on the network) was made in order to allow updating remotely the lookup tables, instead of having to physically seat in front of the 4MAP PC on the mountain as in the old days. Therefore, in order to make effective a newly-entered lookup table, one has to bring the telescope to zenith, turn off the air to M1, exit the 4MAP program and start it again (the 4MAP booting message will tell actually that it updated its .cof file). Correction values are automatically interpolated from this table (which contains 49 standard positions in the sky) to whatever is the current telescope position. [[end REVISION]]

Optionally, we can also apply an additional small constant correction for each aberration. We call it the "tweak" correction. The lookup-table and tweak values are added together vectorally. The contents of the lookup table are only rarely changed (as an engineering-time activity), while the tweak values can be remeasured (using IMAN) each time the telescope is moved to a new part of the sky, if the astronomer wants to take the time. If the astronomer prefers to take the default image quality, using only the lookup tables, the tweak correction can be disabled.

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2.2 Secondary mirror

The f/8 and f/14 secondary mirrors each have their own computer-controlled collimation system which tilts the mirror around a point near its vertex. This system permits the removal of coma, and is part of the Active Optics package. In addition, there is a system for manually translating the mirror sideways, intended as a rare daytime adjustment, to handle cases when the tilt adjustment does not have enough range.

Tests show that the collimation does not change significantly as the telescope moves around the sky during the night, but that it does occasionally change (for unknown reasons) over a period of weeks or months. The standard operating procedure therefore is to use the image analyzer on a regular once-per-week basis to check the collimation (and adjust it if necessary), but otherwise to leave it unchanged during routine operation. Any time the collimation value is changed, the new value should be entered in the Active Optics logbook and also written on the white board.

[[REVISION 3Feb01:]] A coma lookup table is now implemented to take into account loss of optimum collimation (due to flexures) when the telescopes moves around the sky. That table is called Xtbl.cof (where X is the focus, either f14 or f8) and is stored in /ut20/tcp4m/tcp/. The coma lookup table is similar to the 4MAP lookup tables of the primary mirror, except that it acts only by producing a tilt adjustment (a 'tweak') of the secondary mirror on top of the nominal tilt values determined by the collimation procedure using IMAN (stored in Last Log Entry). Once you select that option, an 'ON' label will show up next to the focus number in the central window of the TCP blue status window. The label will say 'OFF' if the coma lookup table is not active. For the time being, it should normally always be OFF. [[end REVISION]]

In addition, observers have the option of using the image analyzer to measure the collimation error at any time during their run, and then tilting the secondary to remove that error. This is the equivalent of making a tweak correction to the primary mirror, except that the new correction should be valid all over the sky. If the telescope is recollimated in this way, the new collimation value should be entered in the Active Optics logbook and also written on the white board, and should become the new default value until the next routine check is made.

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2.3 Image Analyzer

The Image Analyzer (IMAN) consists of four components:

• the IMAN OPTICS, which are part of the cass guider.
• the IMAN CAMERA SYSTEM, which includes the CCD, the camera head and the IMANPC in the computer room.
• the IMAN REDUCTION SYSTEM, which runs on the IMANSUN (currently CTIOt2).
• the IMAN CONTROL SYSTEM, which is part of the TCS. This accepts commands from the telescope operator and then translates them into other commands which are issued to the optics, camera and reduction systems in the correct sequence.

IMAN is always available at f/8 and f/14. It can be used by the night assistant at any time. It writes its results into a log file. With easy-to-use TCS commands, the night assistant can take results from this log file and use them as input for changing the collimation or the tweak values. There are options to take either the results from the most recent IMAN measurement, or to search through the log file and select some earlier result, or to type in values at the terminal. The IMAN program also makes a recommendation about which tweak values need to be changed and which do not. When tweak values are taken from the log file, the TCS program allows you to either follow these recommendations (the default) or override any of them.

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3.0 IMAN OUTPUT

This will be found in /ut22/iman/iman.log The results from a typical measurement will look like:

***************************************************************************

UT 00:44 08/27/95        HA -01:14; DEC -31:23            f/8             ROT 90.0
SECONDARY			PRIMARY
	coma3		spher	astig		triang		quad		d80
	um	d	um	um	d	um	d	um	d	arcsec
1	0.22	80	-1.57	0.64	440	0.02	367	0.17	12	0.47
1	0.28	73	-160	0.64	452	0.03	273	0.20	6	0.49
1	0.33	-71	-193	0.64	471	0.08	292	0.18	9	0.50
Average	0.09	36	-1.70	0.63	94	0.04	-64	0.18	9
Sigma	0.15		0.17	0.02		0.03		0.01
d80	0.01		0.19	0.21		0.01		0.08
Tweak?	N		N	Y		N		N

 

						d80 (arcsec)			TEL.FOCUS=172301 GDR: x=0.045 y=-0.04
npts			defoc	decen		init	coma	full
1	1	218	1.34	21.17	219	0.57	0.56	0.47
2	2	218	1.24	24.00	216	0.56	0.56	0.49
3	3	217	1.71	24.57	214	0.59	0.59	0.50

The output first shows results for the three independent 30 sec measurements. Magnitudes of the aberrations are given in microns (um), and the position angles in degrees (d). The rightmost column shows the residual 80% encircled-energy diameter that the image would have after correcting for all of the fitted aberrations (this residual includes the effects of slowly changing dome seeing components, but most of the effects of atmospheric seeing have been averaged out).

The next line gives the vector average for each aberration. After that is a line giving the standard deviation (1 sigma) of the magnitude of each aberration, and then a line giving the 80% encircled image diameter (in arcsec) which would be expected from each average value.

The line labelled "Tweak?" gives a recommendation about whether or not a correction should be made for each aberration: yes (Y) ==> make a correction; no (N) ==> do not change anything. A tweak adjustment is generally recommended for aberrations producing d80 values in excess of 0.1 arcsec, unless there is large scatter in the individual measurements. However, the spherical aberration measurements tend to show huge scatter, and we currently do not recommend making a tweak adjustment for that under any circumstances.

Finally, additional information about each measurement is grouped at the bottom left of the output. "npts" is the number of spots used in the fit; "defoc" is the fitted defocus term (in microns); "decen" gives the fitted decentering term (in microns and degrees). The entries under "d80" are 80% encircled energy diameters at three different levels of correction: "init" is for no corrections; "coma" is with coma removed; "full" is with all fitted aberrations removed.

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4.0 TCS MENU ITEMS

Commands are invoked by typing the first letter of the command, except for the STAR SEQUENCE and CAL SEQUENCE commands which are invoked with * and /, respectively.
 

ACTIVE CONTROL

IMAGE ANALYZER
CALIBRATION POSITION
LARGE APERTURE
SMALL APERTURE
OBSERVE POSITION	(to power off the camera)
POWER ON CAMERA
*STAR SEQUENCE
MORE STARS
/ CAL SEQUENCE
ABORT STAR SEQUENCE
FLAT MIRROR (IN or OUT)	(IN for GUIDER; OUT for IMAN)
!! PELLICLE (IN or OUT)	(IN for IMAN; OUT for GUIDER)
IMAN COMMAND TO PC
TILT SECONDARY
INIT TILT
RELATIVE TILT	(tilt to new value)
ABSOLUTE TILT	(tilt to new value)
LAST LOG ENTRY	(tilt to last value in IMAN log file)
OLD LOG ENTRY	(select any value from IMAN log file)
DISPLAY TILT
!!  ON/OFF AUTO TILT	(activate or not the Coma lookup table)
!!  PERFORM AUTO TILT	(adjust the tilt to the value of the Coma lookup table for the current position)
!!  SET TO REFERENCE TILT	(adjust to tilt stored in Last Log Entry)
PRIMARY MIRROR CONTROL
GO
HALT
RESET ERRORS
CORRECTIONS ON/OFF	(whether or not to use the Lookup Tables for each focus)
TWEAK ADJUST ON/OFF	(options are ENABLE, DISABLE, RESET)
SHOW TWEAK	(display entries for lookup table & tweak)
MIRROR ADJUST
LAST LOG ENTRY	(set tweak to last value in IMAN log file)
OLD LOG ENTRY	(set tweak to any value from IMAN log file)
KEYBOARD ENTRY	(set tweak to values entered from keyboard)

 

!! shows the REVISED TEXT (3Feb01).

When the LAST LOG ENTRY or OLD LOG ENTRY commands are used from the PRIMARY MIRROR CONTROL menu, the user is asked:

USE DEFAULTS ?

If Y, the changes TO THE PRIMARY MIRROR FIGURE recommended by the IMAN program will be made. This command cannot change the Secondary Mirror's tilt.

If N, then the user is asked:

SPHER :
ASTIG :
TREFOIL :
QUAD :
Answer Y to cause the corrections to be applied.

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5.0 SEE "Instructions for Normal Use" document [9]

This accompanying document gives current instructions for:

• Primary Mirror Correction from Lookup Table.
• Image Analyzer.
• Tweaking.

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6.0 MORE ABOUT TWEAKING

6.1 Tweak corrections are additive

The idea of the tweak correction is that if the adjustment of the optics is not quite right, you should use IMAN to measure the error and then change the adjustment by the required amount. Therefore, you want to add that change to whatever was the previous setting.

For the secondary mirror tilt, tweaking consists of applying a RELATIVE TILT correction (see Section 6.3), which is always a differential tilt correction from the mirror's present position.

In the case of the primary mirror, if the previous tweak values are not reset to zero (see below) at the time a new tweak command is sent out, the new tweak values get added (vectorially) to the old tweak values. If the lookup table is "ON", the total tweak corrections get added to the lookup table corrections. Normal use is to leave the Lookup Table "ON" (if the f/8 focus is being used; otherwise leave it OFF), but to reset the tweak values to zero (Section 6.2) before making an IMAN measurement to determine the tweak values in a new part of the sky.

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6.2 The TWEAK ADJUST command (Primary Mirror Control Menu)

This command is used to enable/disable/reset the tweak corrections. When the corrections are "enabled", the TCS Status Screen shows a flashing "TWEAK ON" message and whatever values are in the tweak table are applied to the primary mirror. When the tweak is "disabled", the values in the tweak table are left unchanged, but no tweak correction is applied to the primary mirror shape and the TCS status screen says "TWEAK OFF". When "reset" is selected, the values in the tweak table are set to zero, the tweak correction is disabled, and the TCS status screen says "TWEAK OFF".

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6.3 Ways to enter TWEAK Values

• LAST LOG ENTRY, in both the Primary Mirror Control and Tilt Secondary menus. Applies last entry from the iman.log file. For the secondary mirror, a RELATIVE TILT command will be issued automatically. For the primary mirror, you will first be asked "ACCEPT DEFAULT VALUES (Y/N)?" If you answer "Y", the Y/N recommendations produced by the IMAN analysis program will be followed. If you answer "N", a table will appear showing the recommendation for each aberration and giving you the chance to override it.
   
• OLD LOG ENTRY, in both the Primary Mirror Control and Tilt Secondary menus. Produces a window which lets you scroll through the iman.log file and pick out the entry of your choice. To scroll, use the following commands:

up arrow
down arrow
PgUp
PgDn
CTRL-Home
CTRL-End

• Usually you will want to start with CTRL-End, and then scroll backwards to find the entry you want. To select an entry, place the cursor anywhere on the line which starts with the word "Average", and then press Return. The software will then proceed as for the LAST LOG ENTRY command. To abort without selecting a log entry, press ESC.
• KEYBOARD ENTRY, in the Primary Mirror Control menu. This lets you enter your own values for the correction for each aberration. In fact, a blue box pops up and you get to edit the current tweak values. Use Enter and the up, down arrow keys to move between fields. To accept the entered values press Enter when the cursor is in the lower right field (quadrapole angle). The TWEAK ADJUST must be Enabled (On) before the tweak corrections will be applied; if TWEAK is OFF the adjustments will be stored in the table but not be sent to the mirror until ENABLE is selected.
   
• RELATIVE TILT, in the Tilt Secondary Menu. Tilts the mirror relative to its present position. Always use this command when entering coma values measured by IMAN.
   
• DON'T USE the command "absolute tilt", which applies an amplitude and direction of tilt measured relative to an arbitrary zero point. This command should only be used to restore the secondary tilt to a previous value which has been read from the output of the Display Tilt command; it is typically used to restore the default mirror tilt after a TCS crash or an operator error.

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7.0 BE CAREFUL! HERE'S THE TRAP WITH THE PRIMARY MIRROR TWEAK CORRECTION!

The lookup-table and tweak corrections can be individually toggled ON and OFF using the LOOKUP TABLE and TWEAK commands in the PRIMARY MIRROR menu. After a tweak correction is enabled, it's up to the astronomer or night assistant to decide when to (and remember to) turn it off. The telescope status screen tells whether LOOKUP TABLE and TWEAK are ON or OFF. "OFF" can mean that the tweak has either been disabled or reset to zero; use the SHOW TWEAK command if you need to know which.

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8.0 AND THE IMAN POWER IS ANOTHER TRAP!

The CCD camera head incorporates a Peltier electrical cooler of the same type as are used with the CCDTV. This is located *inside* the offset guider module, and generates a considerable amount of heat which can escape up the telescope's chimney, directly in the light path. The cooler is not always enabled, but when it is, leaving the IMAN power on for a long time is likely to generate bad seeing. The power is therefore remotely controlled, and should only be turned on for brief bursts when IMAN is actually in use. Use the menu command POWER ON CAMERA to turn it on; use OBSERVE POSITION to turn it off.

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9.0 SOLVING PROBLEMS

9.1 Error message in blue IMAN data reduction box on TCS screen:

Check the list of error messages provided on the IMAN page.

9.2 TCS screen shows "sequence error from IMAN":

This message usually indicates a failure in the NFS link between IMANPC and IMANSUN.

• Go to the IMANPC (it's monitor is at the far right end of the computer room, against the wall. It's keyboard is just to the left of the monitor.)
• Look for the Green Light... there is a green LED on the front of the IMAN PC (far right end of the computer room), labelled "CCD Readout in Progress". When the camera is waiting for a new command, this light should normally be blinking every second or so.
• The IMAN program should be running. The screen should be mostly blue, but with a red box at the top with the words "IMAN STATUS". If this program is not running, follow the IMANPC restart procedure.
• If the IMAN program is running, but there is an error message indicating a problem with Drive E, then follow the IMANPC restart procedure.

9.3 IMANPC restart procedure

See Section 3.4 of the Iman Image Analyzer WWW page or manual.

9.4 SEQUENCE ABORT message on TCP screen

Appears in a separate small blue box if a star sequence is aborted using the ABORT STAR SEQUENCE command in the IMAN menu. Use CTRL-F2 to clear the blue box from the screen.

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	1.	Menu Commands
	2.	Command Mode commands
	3.	Units, etc

This page is very old and much work has been done on the active optics system, including a major upgrade in 2014.  The value of the information here is uncertain.

J.Baldwin, G. Schumacher, 14 November 1995

The f/7.8 secondary mirror is controlled by a CTIO "Smart Motor Controller". The mirror can be both focused and tilted by the operation of three computer-controlled jack screws which are spaced 120 degrees apart on the back of the mirror cell. Each screw is driven by by its own servo motor, which includes an incremental encoder. In addition, a Futaba linear encoder is mounted next to each of the jack screws, and gives an independent reading of the position of the jack screw to an accuracy of nominally 1 micron.

 

1.0 Menu Commands.

Control commands are normally issued from the "Tilt Secondary" and "Focus Secondary" menus on the TCS screen. The commands are:

TILT SECONDARY

INIT TILT
RELATIVE TILT (tilt to new value)
ABSOLUTE TILT (tilt to new value)
LAST LOG ENTRY (tilt to last value in IMAN log file)
OLD LOG ENTRY (select any value from IMAN log file)
DISPLAY TILT

FOCUS SECONDARY

INIT FOCUS (reset zero points of encoders, then return to present focus position)
MOVE TO VALUE
STEP FOCUS

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2.0 Command Mode commands.

Commands for the mirror can also be typed into the TCS using the Command mode:

sec encoder

Return readings of Futaba encoders A, B1 and B2, and differences, in the order A B1 B2 (B1-A) (B2-A). Units are microns of motion at the secondary mirror. All other commands dealing with focus motions use units of microns of movement of the focal plane.

sec tilt [tilt amplitude] [tilt azimuth]

Tilt secondary to specified absolute position.

sec focus [value]

Change focus by specified DIFFERENCE from present focus.

sec afocus [value]

Change focus to specified absolute value.

sec display

Show present values of tilt and azimuth.

sec fast [value]

Set fast focus speed, in arbitrary units. Default = 120.

sec slow [value]

Set slow focus speed, in arbitrary units. Default = 50.

sec init

Moves mirror to fiducial position and rezeros encoders. Does NOT restore previous focus value (unlike the menu command).

sec reset

Zeros out all registers and hardware; leaves smart motor controller ready to receive commands.

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3.0 Units, etc.

The three jack screws and their accomapnying Futaba encoders are labelled A, B1 and B2. A is on the South side of the secondary mirror when it is in its observing position; B1 is on the NW side and B2 is on the NE side.

Focus motions are achieved by driving all three jack screws by the same amount. Focus units are microns of travel of the focal plane, = 9.56819 times the motion at the secondary mirror (but Beware!, the command "sec encoder" returns the values at the secondary mirror). Positive focus changes move the focal plane upwards.

Tilt is produced by driving the three jack screws by differing amounts, so as to tilt the mirror about a point located 4.68 inches behind its vertex. This center of tilt was chosen because it is in the plane defined by the three rollers which provide the lateral support beween the inner mirror cell (which moves) and the outer mirror cel (which doesn't move).

Tilt units are microns of wavefront error for coma3 at the edge of the pupil (the units returned by IMAN), and the azimuth of the error. The mirror will then tilt so as to remove that amount of coma. This is to maintain consistency between values pulled from the IMAN log and values entered manually. The conversion to the actual angular tilt of the mirror is:

.0133 degrees of tilt = 1 micron of coma3.

The tilt azimuth is defined as 0 deg azimuth to the west, and then increasing as you go around to the north. Including the fact that the mirror is moved so as to remove the entered coma value, a tilt request containing a positive coma amplitude will cause the side of the secondary mirror in the specified PA to move closer to the primary mirror while at the same time the opposite side moves away from the primary mirror. Therefore, relative to the tilt=0 fiducial position:

	sec tilt 1 0	Lowers W side, raises E side through a 0.0133 degree tilt.
	sec tilt 1 90	Lowers N side, raises S side.

The above tilt changes cause the absolute tilt (read out using the DISPLAY TILT menu command) to change by the requested amplitude but with a position angle which is the requested PA - 180 degrees. This will be added vectorially to the previous absolute tilt. For example, a relative tilt of:

sec tilt 1 90

 

will produce an absolute tilt of

	1.0 micron PA 270	if the starting point was abs. tilt = 0 0.
	1.4 micron PA 225	if the starting point was abs. tilt = 1 180

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	Contents
	1.0	Primary Mirror control Program description
	2.0	Command description
	3.0	Program Databases
	4.0	Program Maintanance
	5.0	Position Angle conventions
	6.0	Amplitude conventions

This page is very old and much work has been done on the active optics system, including a major upgrade in 2014.  The value of the information here is uncertain.

4MAP

G.Schumacher, 14 December 1995

FOR MORE INFO: Hard-copy manual "4M Active Primary Mirror Support System Operating Manual", by G.Perez et al.
Also: Calibration Positions for 4MAP Lookup Tables [10].

1.0 PRIMARY MIRROR CONTROL PROGRAM DESCRIPTION

Controlling the primary mirror consists of applying a calculated pressure to the support pads. There are 33 pads distributed uniformly in two concentric rings: one called the outer ring, having 21 pads and the other called the inner ring, having 12 pads. In the outer ring there are also three hard points, separated at 120 degrees each, where the mirror sits when there is no pressure applied.

Associated to each pad, there is a pressure controller named MAMAC CONTROLLER, that outputs a pressure proportional to a voltage applied to it. The output pressure in turn is sensed and converted back to a voltage that is read by an analog to digital converter. Therefore, in order to control each MAMAC there is a DAC and an ADC device connected to it.

A control cycle consists then on calculating the pressure to apply to each pad, convert that pressure to a voltage, instruct the DAC to generate that voltage, and read back the voltage proportional to the output pressure sensed by the ADC. In between control cycles, the program constantly monitors each device and takes several actions on error conditions.

The control program is designed to operate in one of two modes. The first mode is called EMULATION MODE, and consists of emulating the behaviour of the old "passive" mechanical controllers. That behaviour is based on applying an equal pressure to every pad in each ring, proportional to the cosine of the zenith distance of the telescope. This pressure is called the Nominal Pressure and can be defined independently for the outer and inner ring. The second mode is called ACTIVE MODE and consists of adding different pressures to the basic Nominal ones, calculated based on known aberrations, parameterized in terms of tables of coefficients for each term of the distortion model. Switching between modes is done with the ACTIVE coefficients command.

The control program runs in one of five states: The START state, the HALT state, the ERROR state, the ADJUST state and the CHECK state.

• The START state is entered by the GO command. The program checks that the previous state is HALT or otherwise rejects the command. Upon entering the START state, the DAC's and ADC's are tested by reading the status from each device. Then, the program checks the state of the air switch and of the zenith switch. If all conditions are met, the state is changed to ADJUST and the emulation mode is selected.
• The HALT state is the initial state of the program, and is also entered by the HALT command or the RESET command after an error condition. The HALT command causes the pressure to be dropped by sending a zero voltage to all the MAMACS.
• The ERROR state is entered by one or more of the following conditions:
•  air switch off
  • lift off switches activation
  • DGH modules failures
  • MAMAC modules failures
  • TCS communications broken

An error condition causes the pressure to be dropped abruptly by activating the safety valves. A zero voltage is also written to the DAC's. In order to activate the control again, it is necessary to change to the HALT state by issuing the RESET command, followed by the GO command.

The ADJUST state is entered from the START state or from the CHECK state by an ADJ command. In this state, the pressures are calculated and then converted to voltages that are applied to the MAMACS. Before applying the new voltages a check is made to lower first all pressures that will be lower than the present ones and then raising all pressures that will be higher than the present ones. This is to avoid an intermediate situation in that the mirror might be lifted due to the total sum of pressures might be larger than the mirror weight.

The CHECK state is entered after a successfull adjust process. In this state the program continuously monitors the condition of the DGH modules and the MAMAC modules. In particular, the pressure is read back and checked against the requested one. If a pressure changes, no attempt is made to correct it, but the ERROR state is entered if the change is greater than a certain limit (presently 2 psi). The TCS link is also monitored. If no TCS command is received after 1 second has elapsed from the last one, it is assumed that the RS485 link or the TCS is broken and the pressure gets dropped by openning the safety valves.

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2.0 COMMAND DESCRIPTIONS

The user interacts with the program by giving commands in a Command Window at the PC terminal. Several of the commands could also be issued from the TCP user interface. In that case, all commands should be preceded by the "box id". The id for the 4M Active Primary control PC is "4map".

act

This command turns on or off the calculation of active corrections for a given mirror position. By default, the program starts in the off state. Format:

act [on / off]

adj

This command causes the program to calculate a new set of pressures and apply them to the MAMAC controllers. This command takes as its arguments the present telescope hour angle (hours) and declination (degrees).
Format:

adj hour_angle declination
adj -1.23 -47.35

IT IS ILLEGAL AND DANGEROUS TO GIVE ADJUST COMMANDS WITH ERRONEOUS POSITION INFORMATION SINCE THE CONTROLLER WILL APPLY THE WRONG PRESSURES TO THE MIRROR.

c0, c2, c3, c4

Specify new values for the individula active corrections. c0 specifies spherical, c2 astigmatism, c3 trefoil and c4 quadrafoil.
These commands set the total value of the corresponding correction.
Format:

c0 amplitude(nm)
c0 2000
c2 amplitude(nm) PA(deg)
c2 1000 45
c3, c4 have same format as c2.

c0twk,c2twk,c3twk,c4twk

Specify values which will be added vectorially to the existing amplitude and PA of the corresponding correction. "twk" refers to the "tweak" command in the TCP. Format same as c0,c2,c3,c4.

din

This command reads the digital input port of a DGH module. The result is returned as an hex number.
Format:

din module_address
din Q

dout

This command writes to the digital output port of a DGH module. The value should be given as an hex number.
Format:

dout module_address value
dout x 2

go

This command activates the control cycle. The telescope must be at zenith with air on and the program must be in the HALT state. A test is made of all modules, and if satisfactory, the mirror gets supported with the proper pressures.

halt

This command halts the control cycle by sending a zero voltage to all the MAMACS. In this state, all periodical TCS communications ceases.

help

Lists help info on screen.

i

This command defines the DGH addresses for the inner ring pads. See the Nomenclature Diagram for the numbering scheme.
Format:

i pad_number DAC_address ADC_address
i 8 2 Y

o

This command defines the DGH addresses for the outer ring pads. See the Nomenclature Diagram for the numbering scheme.
Format:

o pad_number DAC_address ADC_address
o 8 J q

pin

This command changes the default Nominal Pressure for the inner ring. The pressure is given in units of psi.
Format:

pin [pressure]
pin 9.0

pout

This command changes the default Nominal Pressure for the outer ring. The pressure is given in units of psi.
Format:

pout [pressure]
pout 8.5

pp

This command calculates and prints the pressures on the screen. The command arguments are an hour angle and a declination. This command doesn't interfere with the normal calculations done with the adj command, so it's useful for debugging the active corrections.

pp hour_angle declination

reset

This command resets an error condition and place the program in the HALT state. This command must be given prior to go, after an error condition.status This command returns a textual description of the program status. Possible responses include:

OK CORRECTIONS ON/OFF

This message indicates that the system is active and no errors are present.

ERROR 5: HALT

This message indicates that the system is in the HALT state. To activate it, a go command must be given.

ERROR 5: MAMAC 12 BAD 2.351 0.000

This message indicates that when checking the MAMAC voltage (number 12 in this case), a difference of more than 0.5 volts was detected. This might be due to a bad DAC, a bad ADC or a bad MAMAC. To determine the offender, a specific test should be run for each module associated with that MAMAC unit (see the Nomenclature Diagram).

ERROR 5: DGH 2 NO RESPONSE

This message indicates that the specific module (2 in this case) is not responding to commands issued to it.

test

This command orders the execution of test for the DGH modules or MAMAC units. The tests are run on all modules or units. If you want to test a specific module, use the vin or vout commands. The responses are similar to the ones described under the status command.
Format:

test dgh/mamac

vin

This command reads the voltage of one DGH module or all ADC modules. The argument is the module address. If the address is '*' then read all ADC modules.
Format:

vin module_address (or *)

vout

This command outputs a voltage to one or all DAC modules. Be aware that this is an active command so if air is on, a pressure will be applied to the pads. Use with care and only if you know what you are doing. As a rule of thumb, the relation of voltage to pressure is close to 1 to 4 (i.e. 1 volt 4 psi).
Format:

vout module_address (or *) voltage
vout * 0.0

x

This command defines the DGH addresses for modules not related with the pads. This are the ones that act on the solenoid valves or receive information on the various switches.
Format:

x module_number module_address

zero

This command sets all voltages to zero. This is equivalent to vout * 0.0.

?

Lists help info on screen.

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3.0 PROGRAM DATABASES

The program uses two databases for its proper functioning: a Parameters Database, called "4map.par" and a Coefficients Database, called "4map.cof". These are currently located on ctiot0, on the /ut02 disk, in the 4map directory.(8Jul04).

The Parameters Database is a collection of commands that defines the starting values for the program. Any valid command could be placed in this file, that gets executed at startup. In particular, the DGH addresses are to be found here so, if a module is changed the new address should be modified accordingly. A '*' character at the beggining of the line indicates a comment; therefore, the file is self documented.

The Coefficients Database contains the parameter values for the different aberrations, mapped around the sky. The map is made in terms of zenith distance and azimuth positions. Each line contains the values for a certain azimuth. Normally, there are 10 values per line, corresponding to zenith distances of 0°, 15°, 30°, 45° and 60°. The first 5 values corresponds to the amplitude parameter and the next 5 values corresponds to the angle parameter. The azimuth values span the range of 0° to 360°, in steps of 30°. Again, an '*' character at the beginning of the line indicates a comment. The hour angles and declinations at which the IMAN measurements for this table should be made are listed in "Calibration Positions for 4MAP Lookup Tables" [10].

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4.0 PROGRAM MAINTENANCE

All the source code resides in directory \AP\SOURCE on drive C: of the control PC. The program is entirely written in C and the MICROSOFT C/C++ compiler rev 8.0 is used to produce the object modules.

The process of making a new executable is automated by using the NMAKE utility. There is a MAKEFILE that declares all the files and libraries needed. The procedure then consist of editting the necessary files and then typing the command 'NMAKE'.

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5.0 POSITION ANGLE CONVENTIONS

The active force patterns are generated by using the Mamac controllers to increase or decrease the air pressure in specific air bags, as compared to the nominal air pressure required to support the mirror at a given telescope position. A positive correction to the air pressure moves the corresponding part of the mirror upwards, while a negative correction lowers the corresponding part of the mirror.
4MAP can correct abberations with the azimuthal position cos(m*phi - phi0), for the following values of m:

m	abberation
0	spherical
2	astigmatism
3	triangular (trefoil)
4	quadrafoil

     

The corresponding cos(m*phi-phi0) force patterns are then superimposed on the mirror. The sinusoidal force pattern is repeated m times going around the mirror. An amplitude and a position angle must be specified in order to generate this pattern.
The position angle convention for phi0, when a positive amplitude is requested, is:

• For phi0 = 0, the peak additional (upwards) force is always at the pad on the NORTH side of the mirror.
• For other values of phi0, the peak is moved through an angle phi0/m, which increases from north through west for positive values of phi0.
• For m=0 (spherical abberation), positive amplitudes cause an increased upwards force on the inner ring of supports, and a decreased force on theouter ring

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6.0 AMPLITUDE CONVENTIONS

When TWEAK commands are entered through the TCP menu system, the requested amplitudes are accepted in the units measured by IMAN, and are then scaled by calibration factors before being passed on to 4MAP. The current calibration factors are:

Abberation	m	Calibration factor
Spherical	0	0.00288
Astigmatism	2	0.00101
Trefoil	3	0.00117
Qaudrafoil	4	0.00123

where the values entered through the menu commands are DIVIDED by the calibration factor before being passed on to 4MAP.

However, when force patterns requests are entered directly into the 4MAP PC using the commands c0,c2,c3 or c4, the amplitudes must be specified without the scale factors. The units then correspond to the deflections predicted by a simplified analysis of the mirror performed by Lothar Noethe at ESO.

The calibration factors also convert the micron units used by IMAN into the nanometer units used by 4MAP, and would be 0.001 if the calculations by Noethe had been perfect. So Lothar's analysis came really close on every abberation except spherical.

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This page is very old and much work has been done on the active optics system, including a major upgrade in 2014.  The value of the information here is uncertain.

German Schumacher
5 December 1995

The following table lists the hour angles and declinations at which IMAN measurements should be taken in order to calibrate the lookup tables for 4MAP (the 4M Active Primary mirror support system). The HA and Dec entries are chosen to give an equally spaced grid in azimuth and zenith distance; azimuth varies as you move vertically through the table and zenith distance as you move horizontally.

Start and end at zenith. In between, do the following:

		AZIM/ZD	15	30	45	60
	HA (h m)	0	0 00	0 00	0 00	0 00
	DEC (d m)		-15 09	-00 09	14 50	29 50
WEST		30	0 31	0 58	1 24	1 52
			-16 57	03 29	10 01	23 24
		60	0 55	1 45	2 31	3 16
			-21 55	-12 39	-02 50	07 04
		90	1 08	2 14	3 16	4 13
			-29 01	-25 47	-20 48	-14 33
		120	1 04	2 19	3 38	4 56
			-36 40	-40 38	-41 22	-38 43
		150	0 40	1 40	3 17	5 30
			-42 46	-54 03	-62 13	-64 06
		180	0 00	0 00	0 00	0 00
			-45 09	-60 09	-75 09	-89 50
EAST		210	-0 40	-1 40	-3 17	-5 30
			-42 46	-54 03	-62 13	-64 06
		240	-1 04	-2 19	-3 38	-4 56
			-36 40	-40 38	-41 22	-38 43
		270	-1 08	-2 14	-3 16	-4 13
			-29 02	-25 47	-20 48	-14 33
		300	-0 55	-1 45	-2 31	-3 16
			-21 55	-12 39	-02 50	07 04
		330	-0 31	-0 58	-1 24	-1 52
			-16 57	-03 29	10 01	23 24

 

For wide-field use, especially with Hydra, a new corrector has been installed at the R/C focus of the Blanco Telescope. It is referred to as the "RCADC" (Ritchey-Chrètien Atmospheric Dispersion Compensator) corrector. It is located in the telescope chimney. Click here [11] to see just where it is.

Hydra MUST be used with this corrector. Any optical R/C instrument can use the RCADC if desired, but there is not much justification. The R/C and echelle spectrographs do not need its wide field though they sometimes might benefit from the ADC function. The RCADC can only be installed by Observer Support personnel via a motorized system operated from the Cass. cage.

The RCADC has six elements in four groups. This Optical Diagram [12] shows its configuration. It contains two meniscus "corrector" elements of fused silica at the front and back surfaces of the assembly. They provide images with D80 less than .3 arcsec over the entire 42 arcminute Hydra field. The corrector also makes the image "telecentric", which means that the pupil is located at the center of curvature of the field so that the optical axis of the images is perpendicular to the focal surface over the entire field. This minimizes light lost due to focal ratio degradation (FRD) in the fibers.

Between the two corrector elements, there are two cemented doublet prisms of silica and a light flint glass (LLF6). All surfaces of these prisms are plane, inclined appropriately so that the light passes through with zero deviation at an intermediate wavelength (4200A). Each prism provides a small amount of dispersion and rotates under control of the TCS though an angle of 360 degrees. When the two elements are oriented 180 degrees apart, their dispersions cancel so that the prisms have essentially no effect on the images. Orienting them at different angles can provide an artifical dispersion in any direction desired, which can compensate for atmospheric dispersion up to the limiting power of the prisms which in the case of the RCADC is at Air Mass 2.4 (65 degrees zenith angle).

The RCADC is coated with sol-gel over MgF2 on all eight surfaces. Sol-gel over MgF₂ has very low reflectivity over a broad wavelength range. Although it has not been directly measured, the overall transmission of the corrector is believed to be above 95% at all wavelengths from 4000-10000A. Transmission falls in the UV due to the LLF6 elements in the ADC prism. Throughput of the corrector is roughly 85% at 3500A, 60% at 3340A and 20% at 3200A.

If dispersion correction is not desired or has been disabled for some reason, the ADC elements MUST be set in the neutral position. This is easily done via the TCS. Zero and 180 degrees is the standard setting but any orientation of the prisms 180o apart is equivalent. Normally, the ADC is left on and dispersion correction is automatic.

Observers sometimes ask when ADC should be used. The safest answer is "always". If there is significant dispersion in the field, correcting for it will improve the efficiency of the observation. It will never make it worse. The only reason not to use the ADC function is to avoid any possible effect on the pointing accuracy or if the control system is malfunctioning. Optical analysis indicates that rotating the elements does not significantly alter the field model, though for lack of time this has not been explicitly verified.

The expected effect of atmospheric refraction on the observing efficiency can be estimated from the following diagrams.

1. Differential refraction at 2km altitude [13]
2. Flux captured by Hydra fibers as a function of seeing and centering [14]
3. Image movement during exposure caused by refraction [15]

The first diagram quantifies the effect of refraction while the second lists fiber efficiency using the standard (Wolff) model of the profile of images degraded by seeing. Using the two diagrams it is relatively easy to estimate the effect of refraction on system efficiency.

For example, when the seeing is 1.0 arcsec, 85% of the incident light will enter a perfectly centered Hydra fiber. If the image is decentered by 0.5 arcsec, the efficiency falls to 73%. A decenter of 1.0 arcsec decreases the efficiency to 39%. Thus, if refraction decenters a star by 0.5 arcsec, in 1" seeing, overall system efficiency will decrease by approximately 14% (.85-.73/.85). Correspondingly at this seeing the efficiency will decrease by 54% if there is a 1 arcsec centering error.)

One can study the table as a function of seeing and estimate how much effect seeing might have on overall efficiency in a particular observing situation. If (say) the 10-15% efficiency degradation produced by an .5 arcsec offset is deemed acceptable, then an overall dispersion of 1 arcsec could be tolerated. Diagram 1 then tells us that someone observing from 3500-5000A could observe to an air mass of 1.3 without using the corrector. Observations from 4000-6000A could be done to an air mass of 1.45 while observations from 6000-9500A could be made at any air mass up to 2.40.

Important! Note that these tables can be used to determine the optimum central wavelength for positioning the fibers. If the corrector is not used, centering the guide star(s) on the wrong wavelength will offset the entire field. Either a filter must be used in the FOPS guide camera or FOPS stars of appropriate color must be used. Of course if the ADC function is enabled, no filter need be installed in the guide camera and the spectral type of the FOPS stars will have no significant effect on the positioning accuracy of the system.

Yet another consequence of refraction is to cause an apparent relative movement of points in the image as the field moves across the sky. This effect is quantified in the third diagram above.

Here, the Hydra field is shown with the locations of star images at 9 points in the field at -70 degrees during ten hours as the telescope tracks from 5 hours east to 5 hours west of the meridian. As can be seen, the image appears to rotate about a point approximately on the edge of the field with an amplitude of about .5 arcsec per hour of telescope motion at the other side of the field and the images drift with respect to the overall rotaion.

This effect is relatively small though it can be significant under some circumstances. Differential motion is the reason that Hydra asks for the approximate time of the middle of the exposure before positioning the fibers. In some situations it is desirable to reposition the Hydra fibers between exposures and to select the location of the guide star(s) in the field with care.

Deciding on what, if anything to do about this effect is up to the observer. One can use the information given in the first diagram to make an estimate as to the relative size of refraction effects at different zenith angles. The information in the third diagram can be used to make an educated guess as to how the images will drift during and between exposures. Since the effect is small, this is all that is ever necessary.

29 May 2000
by T. Ingerson