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
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.)
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.
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.
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 |
The Active Optics System has three main components: the 4m Active Primary system (4MAP), the Secondary Mirror Alignment System, and the Image Analyzer (IMAN).
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.
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.
The Image Analyzer (IMAN) consists of four components:
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.
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.
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.
| 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.
This accompanying document gives current instructions for:
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.
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".
up arrow
down arrow
PgUp
PgDn
CTRL-Home
CTRL-End
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.
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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Check the list of error messages provided on the IMAN page.
This message usually indicates a failure in the NFS link between IMANPC and IMANSUN.
See Section 3.4 of the Iman Image Analyzer WWW page or manual.
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.
Links
[1] http://www.ctio.noao.edu/noao/content/short-instructions-normal-use