How to use zz.cl for reducing Hartmann screen data

Revision of J. Baldwin's notes from 6 July98

All the updated programs are stored in /ua76/boccas/4m/to_run_shap/

SETUP:

•  LOAD IRAF TASKS:
  • images
  • proto
  • lists
  • digiphot
  • apphot
• GET THE FOLLOWING EXECUTABLES AND FILES into the directory you are using:
  • shap
  • shapav9
  • newshap
  • map4
  • setup.cl
  • zz.cl
  • modcheck.cl
  • ann_zer.ann
  • ela_ann.dat

plus a .coo file appropriate for the telescope/f-ratio you are using:
• • 36 INCH: 36inch.coo
  • 4m pf and f/8: model.coo
  • 4m f/14: f14.coo
  • 4m f/30: f30.coo

• THEN:
  • task setup=setup.cl
  • task modcheck=modcheck.cl
  • task zz=zz.cl
  • task $shap=$shap9

• PLUS A COUPLE OF HANDY ALIASES
  • alias te 'textedit \!* &'
  • alias lpflip 'enscript -r -l -fCourier9 -Pnpt0x2 \!*'

• SET TELESCOPE AND CCD PARAMETERS in setup:
    epar setup TASK = setup image = obj134 Image: coofile = f14.coo coordinate file: (pixsize= 0.048) CCD pixel size (mm) (apfill = 1.0733) Hart screen fill factor, 4m=1.0733, 36in=1.0881 (mirdia = 3947.7) Mir dia (mm), 4m=3947.7, 36in=914 (fratio = 14.45) 4m=14.45,7.8,2.904; 36in=13.5 (cursor = ) (coof2 = f14.coo) (mode = al)

• TAKE A FIRST TEST FRAME as early as possible in the night.

• RUN setup. Mark a pair of spots in the top and bottom rows of the Hartmann pattern... their separation is used by Shap so you cannot just pick any spots anywhere in the pattern. Note screwy feature where you must mark 1st spot with cursor + any key, then get cursor back to IRAF window and hit RETURN in order to get prompt for 1st spot number. Setup will produce files shap.par, modshap.coo, and zz.input setup obj134 f14.coo

• Use textedit or any other editor to remove bad spots from modshap.coo

• Run modcheck to be sure you have everything set up correctly.
    modcheck obj134

• NOW EPAR A FEW VITAL PARAMETERS FOR phot:
    centerp: cbox should be set to pacing between points maxshift same fitskypars: annulus tune to dot size dannulus tune to spacing between dots photpars: aperture set to same value as annulus

• CHECK SHAP CALIBRATIONS, ANGLE ZERO POINTS, ETC:
  • determine directions on sky using offset command.
  • take shap data while forcing in in 4 mu astig at PA 0, PA 90.

• MAKE SKY MAPS WITH CORRECTIONS OFF.

• NOW RUN zz ON EACH IMAGE. 3 exposures 30 sec each.
  • For each new object or telescope configuration: 'newshap' (enter title. If mapping sky, enter ha, dec in hh.h dd.d format; eg. degrees and fraction of degree, rather than deg.min)
  • For each image: zz objxxx
  • as needed: shapav9, tail shap.work, tail shap.sumry

•  where shapav9 averages together all data between successive applications of newshap and writes detailed log file in shap.sumry, averages in shap.av

• TO PLOT RESULTS OF SKY MAP: use texteditor to copy relevant sections of shap.av into temporary file. map4

 Maxime Boccas, 4Feb01

INSTRUCTIONS TO BUILD LOOKUP TABLES FOR THE 4M ACTIVE OPTICS

Revision of J.Baldwin's notes from 17Dec98 

All the updated programs are stored in /ua76/boccas/4m/make_lookup/

STEP 0:

• Do a sky map with Iman or Shap at the standard positions all over the sky (see "Calibration Positions for 4MAP Lookup Tables [8]"). There are 2 plotting programs, one for Iman data and one for Shap data, that will produce graphs of the sky in (HA,Dec) coordinates showing the aberration vectors (the center of the graph is the zenith):
• In order to plot the sky map data taken with Iman (which measures aberration amplitude in microns), use the program 'listmap' with the input file 'list.in' (which is a copy, that you can edit, of 'iman.log.av'). See the format of 'iman.log.av' in paragraph 4.5 of the Iman manual [9]). You will simply be asked for the aberration desired. Output can be displayed by selecting '/Xwin' or copied into a poscript file by selecting 'name.ps/ps'. 'Listav' is another useful program that will calculate the average aberrations in your sky map (especially useful when there is a constant aberration over the sky).
• In order to plot the sky map data taken with Shap (which measures aberration amplitude in nanometers), use the program 'map4' with the input file 'shap.in' (which is a copy, that you can edit, of 'shap.av'). You will be asked to specify the columns containing HA, Dec, aberration value, aberration pa. In 'shap.av', HA is column 1, Dec is 2, Defocus is 3, Spherical is 4, Coma is 5, Coma pa is 6, Astig is 7, Astig pa is 8, Tref is 9, Tref pa is 10, Quad is 11, Quad pa is 12. Output can be displayed by selecting '/Xwin' or copied into a poscript file by selecting 'name.ps/ps'.

STEP 1:

•  Use the fortran program 'xyiman' or 'xyshap' (depending on whether the data are in iman or shap format) to convert (r,theta) results for aberrations into (x,y) vector components. Either program writes its output into the files 'xgrid.out' and 'ygrid.out'.
• For xyiman, the input is read from file 'xyiman.in', which is in the format written by the program 'imanav' (see paragraph 4.1 of the Iman manual [10]) into the file 'iman.log.av'.
• The normal way to run 'xyiman' is:
  • cp iman.log.av xyiman.in
  • textedit xyiman.in (cut out any unwanted runs)
  • xyiman [aberration]
  • aberration = number from list below
    • + 1 = defocus
    • + 2 = decenter
    • + 3 = coma
    • + 4 = spherical
    • + 5 = astigmatism
    • + 6 = trefoil
    • + 7 = quadrafoil
• For xyshap, the input is read from file 'xyshap.in' (a copy of 'shap.av'), and you will be asked to specify the columns containing HA, Dec, aberration value, aberration pa. In 'shap.av', HA is column 1, Dec is 2, Defocus is 3, Spherical is 4, Coma is 5, Coma pa is 6, Astig is 7, Astig pa is 8, Tref is 9, Tref pa is 10, Quad is 11, Quad pa is 12. HA and Dec must be in decimal format (this is done when you use 'newshap', see shapzz.help [1]).

STEP 2:

• Then use the IRAF routine ctio.stsdas.toolbox.imgtools.xyztable to fit to x components, and provide output x components interpolated to the standard "Calibration Positions for 4MAP Lookup Tables [8]". The desired output HA and Dec are in the file outpos.in.
• lpar xyztable

STEP 3:

•   Use xyztable again for ygrid.out --> yinterp.out

STEP 4:

• At this point, there are a pair of files xinterp.out,yinterp.out which list the standard position HA & Dec, and then either the x or y component of the coma vector at that position.
• Now run the fortran program 'tabmake'. It reads in the files xinterp.out and yinterp.out, assuming that THE FIRST DATA VALUE IS FOR THE ZENITH, and writes an output file in the correct format for the lookup table.
• If you are making a coma lookup table, it needs to be referenced to zero coma at the zenith. The program tabmake does this if you answer 'Y' or 'y' to the question "Is this a coma lookup table?". tabmake then converts the output (r,theta) values into the units wanted by the TCP keyboard entry command for tilting the f14 secondary.
• If you answer that it is not a coma lookup table, then tabmake produces an astigmatism (or trefoil, or quadrafoil) lookup table, for which the value at zenith is NOT subtracted off.
• Tabmake writes 12 lines of output in the correct format for the lookup table (amplitude must be in nm for astigmatism, trefoil and quadrafoil lookup tables, and in degrees for a coma lookup table). BUT, you must now use a texteditor to copy the first line of output to also be a final (13th) line. At the same time you can add comment lines at the start, with a * as their first character.
• Here is a sample coma lookup table called f14jul98.cof:
  • f14tbl.cof
  • skymap for f14 tilt. Made 9 Jul '98 using data taken 12 Mar '98 and 8 Jul '98
  • J.Baldwin, M.Boccas
  • zenith collimation value obtained 9 July was:
  • secondary tilt .0650, PA 147 for primary tilt NW,S,NE=-19.2,-5.0,-13.5
  • the first 5 columns are amplitudes and the last 5 are angles
  • each line corresponds to a zenith distance, in steps of 15deg

    0.0000	0.0170	0.0360	0.0568	0.0793	0	191	190	190	190
    0.0000	0.0122	0.0253	0.0373	0.0455	0	181	182	183	185
    0.0000	0.0056	0.0121	0.0167	0.0158	0	155	163	169	177
    0.0000	0.0053	0.0074	0.0085	0.0084	0	90	90	102	119
    0.0000	0.0117	0.0177	0.0172	0.0129	0	90	90	90	90
    0.0000	0.0162	0.0291	0.0356	0.0263	0	90	90	90	90
    0.0000	0.0156	0.0293	0.0412	0.0511	0	90	90	90	90
    0.0000	0.0098	0.0141	0.0165	0.0457	0	356	343	288	240
    0.0000	0.0042	0.0117	0.0271	0.0495	0	287	247	229	221
    0.0000	0.0095	0.0237	0.0412	0.0604	0	220	217	216	217
    0.0000	0.0154	0.0339	0.0538	0.0743	0	206	206	208	213
    0.0000	0.0183	0.0389	0.0616	0.0857	0	198	198	199	202
    0.0000	0.0170	0.0360	0.0568	0.0793	0	191	190	190	190

STEP 5:

• Tabmake also writes a file called 'tabmake.out' (with 4 columns: HA, Dec, aberration amplitude, aberration pa), which can be used as the input file to the program 'map4'. You should run map4, and check that this map looks like a smoothed version of the input data.

STEP 6:

• Finally, once you have your lookup table all made and edited, run the fortran program 'maplookup' on it. This will produce a sky map in the same format as is produced by map4.
• If it is an astigmatism (or trefoil, or quadrafoil) lookup table, the map of the lookup table should look like the sky map of aberrations which you are trying to correct, with the vectors pointing in the SAME directions in the two maps.
• For a coma lookup table, you should see that the vector value at the zenith has been subtracted at all points on the sky.

Maxime Boccas, 4Feb01

Collimation Procedure for the 1.5-m Telescope

The collimation of this telescope is difficult mostly because the secondary mirror cells don't have position gauges (allowing control of what you are doing) and the f/13.5 cell is not kinematic at all but rather a complex puzzle where center and tilt are sometimes coupled on a single screw... So follow this guideline to avoid a nightmare. For your reference, the best images (FWHM) obtained for typical R-band 30sec exposures are: 0.60" at F/13.5 on-axis (classical Cassegrain) and 0.85" at F/7.5 over the FOV. (RC Cassegrain)

• By design and fabrication, and as our starting hypothesis, the primary mirror cell is such that it is centered with the instrument rotator. The primary mirror (M1) is centered in its cell through 4 radial adjustable pads inside the chimney (check exact procedure).

• Set the instrument rotator in the nominal position (30deg), install the 3 dial gauges in the back of the M1 cell (make sure you see the indicators move as they touch the mirror and you tighten them). The dial gauges fit in open holes of the cell at the same angle as the 3 collimation screws: East, Northwest and Southwest. The dial gauges, which have 3 readings each (tenths, hundredths and thousandths of an inch) are about 680mm off-axis thus a 1/1000" (smallest division) corresponds to a 7.7 arcsec tilt of M1. The collimation screws have 20 threads/inch so 1 turn moves the mirror up and down by 1.27mm. Located at an off-axis distance of 500mm, one turn of the screw thus corresponds to a 0.14deg tilt of M1.

• Install the K&E base plate on the rotator with the 3 short conic-head screws at 120deg (they provide an autocentering of the baseplate). Install the K&E in its support. Secure it with the 3 tightening screws on the left side of the support (the left one is used to open up the tube if the K&E is hard to remove). Through the window in the GAM, install the K&E target on the extremity of the optical tube. Make sure the target itself is not loose and slide at maximum on the tube. Orientate the target marks with the cardinal directions. Make sure the dials positionning the reticle of the K&E are set at 0. Never touch them later and check regularly that they are at 0.

• Install a light at the GAM window so that it shines on the target AND upward to the corrector (in the chimney), and another lamp at the top of the chimney shining upward to the secondary mirror.

F/7.5 : start with this focus, it is easier!

• The F/7.5 cell has 2 basic adjusments: radial (3 points at West, Northeast and Southeast) for centering, and longitudinal (3 points at East, Northwest and Southwest) for tilt. Two of the centering screws push directly on pads in contact with the secondary mirror (M2) and the third one (West) pushes on the mirror via a strong spring and has a locking nut. In thee back of the mirror, there are 3 hard points (screws with square head) that one needs to rise by a few mm when starting to tilt in order to open us space for M2 to go up, and tighten up when the collimation is done (use a 5 to 7/1000" shim to leave a tiny gap between these hard points and the glass for thermal expansion). The central hole in the mirror is fitted with a sleeve and a bolt which pulls up on the mirror to a system of counterweight supposed to hold the mirror figure against gravity. The 3 counterweights bars should go through the center of their support (if they don't, screw/unscrew the bottom nut holding the sleeve until they are centered). This has to be done at the end once the mirror tilt is set.

• Looking through the K&E, focus on the sleeve and the bolt/nut on the surface of M2. There is a tiny dot in the center of the bolt: this is the reference that needs to be centered with the reticle. The bolt diameter is 1/4" to give you the scale (you can also measure any decenter with the reticle dials but DON'T forget to reset them to 0 after measuring).

• The next step is very important: MAKE SURE that the K&E is perfectly perpendicular to its base plate (the centering has good locking screws but check it with a caliper, measuring the gap between the centration bolt blocks and the disk holding the K&E). Sight the bolt and make a zoomed drawing (direction and distance) of where the reticle is compared to the dot. Now, taking extreme care at not bumping the K&E in its baseplate (which has some play unfortunately -to be checked-), unscrew the 3 conic head bolts and rotate by 180 the baseplate onto the rotator (beware: you can not use the rotator to rotate because the bearing is not perfect and the rotation doesn't happen in a plane and thus your alignment will never converge!). Tighten up again and sight. Draw the new relative position. If the position of the reticle with respect to the dot has not changed before and after the 180deg rotation, it means the K&E is perpendicular to its support, so you can proceed. If it wasn't the case, determine the center of rotation (half way between the 2 reticle positions) and use the 3 tilt screws (with nut) of the K&E mount to set the reticle on that virtual rotation center. This is difficult because you need to maintain the wished position while you tighten the tilt screws. Once you have passed that tricky exercise, sight the bolt again, draw the respective positions of the dot and the reticle and rotate the baseplate of the K&E as described before. The new sights should show no difference. If this is not true yet, iterate the previous process.

• The K&E is now perpendicular to the rotator and thus defines the mechanical axis of the telescope onto which the optical axis of M1 and M2 must be brought. Use the radial screws in the M2 cell to center the mirror. Next, defocus slightly the bolt and you shall see the white K&E target imaged by M2: by tilting the mirror, center the image of the target with the reticle. The optical axis of the F/7.5 M2 coincides now with the mechanical axis.

• Drive the telescope to 45deg zenith distance in the 4 cardinal directions and sight each time at M2 with the K&E to detect any loss of alignment (sign that the mirror might not be held properly in its cell, i.e. not tight enough). Next flip to F/13.5.

F/13.5: the tough one!!

• The F/13.5 cell also has 2 'basic' adjusments:
  • Centering : 4 radial push-pull screws at the cardinal directions, each of them has a locking nut (but the East one), which need to be loosened first with a 25/32 tube key, and tightened at the end. These screws (use a 1/4" allen key) are accessible through holes in the wall of the Secondary Cage but you need to run the focus to the correct value to have these holes open up and be able to put the tools through them. These 4 screws are push-pulling on a belt attached to the mirror. Normally, you will be able to push one screw when the other 3 are loose (principle of the push-pull system). The problem of these centering screws is that they also introduce 'random' tilt and astigmatism if too tight!!
  • tilt : 3 outside screws driving pinions and teethed sectors at West, Northeast and Southeast. They require a special key that is a tube with 2 pins at one extremity and a central long allen key. First, locate the allen key in the lock screw of the pinion, loosen it, and rotate the tube until the pins enters the pinion. You can then rotate the tube and see the pinion rotates. The full range of each pinion corresponds to a tilt of about 5 arcmin (the star goes Northeast if you rotate the Northeast pinion to see the teethed sector go to the left). Tighten the lock screw at the end. Finally, there is also, inside the Secondary Cage (enters the tube standing up on the chimney), one push-pull screw rising up and down the entire cell mechanism from the south side thus allowing tilt in a quasi North-South direction (in theory this is in addition to the West pinion, also tilting the mirror in North-South). Use a special 1/8 long allen key for that adjustment.

• Once again, sight M2 through the K&E. You will see the large central hole in the glass (about 60mm in diameter). Center the hole with the K&E field of view (the 2 circles are quite close so this is easy and accurate typically within 0.5mm) or use the special aluminium target in the center of M2 (and center the tiny dot with the reticle). Next, defocus very slightly the glass and you will see the image of the corrector (down in the chimney just above the K&E) and especially its perimeter as a thin bright line just outside (very close) the hole of M2. Make that circle concentric with the hole in M2. The optical axis of the F/13.5 M2 coincides now with the mechanical axis. In principle, you ought to center first and then attempt tilting with the 4 dedicated tilt screws (but if you don't have enough range, use the tilt introduced by some centering screws and iterate...)

• Now comes the funny part (hey,hey the game is not over!): tighten the mirror without loosing the adjustments... Don't forget all the locking nuts (6) and make sure that the 4 radial screws for centering are all pushing hard against the belt (otherwise the mirror will move when you flip). With a bit of patience and practice, all that goes actually quite well. Always check any screw adjustment you are doing with someone looking in real time through the K&E (or do it alone but be ready for about 100 trips back and forth between the K&E and the mirror). Finally, check that there is no loss of alignment versus telescope position. Eventually, do a flip to check F/7.5, and then back to F/13.5.

• LAST STEP: bring the optical axis of M1 onto the mechanical axis (therefore also the optical axis of the M2s). In theory, you can use anyone of both foci to do that final adjusment (but whatever focus you use, it is highly rercommended to check the other focus later). It has to be done at night-time with direct imaging (look at through-focus sequences and 'zero the coma' on axis) or with the Hartmann screen. Use the tilt dial gauges of the M1 cell to monitor the collimation iterations.

IN PRACTICE, start with F/13.5. Tweak the tilt of M1 to get proper collimation at F/13.5. Then flip to F/7.5 and if the collimation is not adequate, use ONLY the adjustment screws (center and tilt) of M2 to get it right. DO NOT modify the tilt of M1 anymore obviously. The reason of that strategy is that you can't re-tweak the F/13.5 M2 at night-time while you can with the F/7.5 M2. I repeat: DO NOT in any case use the adjustment screws of F/13.5 M2 at night time because you will mess up everything.

Notes about gravity effects and mirror flips:

• After doing the K&E alignment, move the telescope 45deg off the zenith in one cardinal direction. Even when the mirror is tightly held in the cell, you will probably notice about a 1mm 'fall' of the mirror, probably due to a sag of the secondary cage. I find about the same fall amplitude at both focii and in all cardinal directions. Curiously the tilt seems to remain unchanged. This 1mm decenter is obviously a collimation limitation (at ZD45deg). This sag must somehow be related to the M2 focus encoder drift by about 30um when moving from zenith to ZD45deg (the drift was more like 100um before some maintenance was done to the focus drive mechanism in July 00). Actually, when the mirror is held tight in the cell so that it doesn't move on its own, adding to the cage sag, it is frequent to see trefoil and/or astigmatism, so that you need to loosen a bit the pressure, taking the risk of allowing decenter.

• Reading the axial dial gauges of M1 when going off-zenith shows that M1 is stable in its cell, within a 1-2/1000" at ZD45deg (7-14 arcsec tilt of M1).

• Doing repeated flips from F/7.5 to F/13.5 can alter the collimation (try it by sighting with the K&E), decentering the mirrors by up to 1mm, which is another collimation limitation. In order to minimize that effect, the flip has to be done by making sure both conical pins of the top ring are tightened slowly and equally.
•  

Some useful notes for the Hartmann screen method:

• Use the model newha1.coo (116 holes) and Baldwin's shapzz.help

• Reduce the data on ctio60 (Solaris) with the following version of ximtool (type: /usr/local/combin/ximtool -gui /usr/local/combin/ximtool-alt.gui)
• Use binning 2x2 (faster), one amplifier so you access the axis properly

• Use star mag 9 (clear sky), defocus the telescope by about 8000 units (don't put spacers, the focus mechanism axis is sufficiently colinear with the mechanical axis)

• Angle convention of Shap: 0deg is South, 90deg is West....(this is with the normal Tek2K orientation, i.e. arcon box facing North). With that orientation in mind, lower M1 on the side indicated by the coma angle in order to decrease the coma (for example, coma angle is 270deg=East, lower=unscrew the East collimation of M1).

• Equivalence between wavefront error (1 micron) measured with Shap and the resulting image blur (EE80 in arcsec) for each aberrations: spherical 0.29", coma 0.37", astigmatism 0.88", trefoil 1.04", quadrafoil 1.12".

• Check out the focus thermal sensitivity on this graph [11]

Maxime Boccas, 9sept00, last updated 19june01

Collimation Procedure for the 4.0-m Telescope at Prime Focus

There are 3 adjustments possible:

• 1. Tilting M1: this is done by moving in or out the 3 defining hard points of the mirror cell. The screws are located in the roof of the Cassegrain cage and are equipped with dial indicators. The screws are actionned with a special tool. The scale of the dials is usually set so that it reads 0 when air is OFF. The dials are located on a radius of 54 inches while the hard points are on a radius of 68 inches. Therefore a 1/1000" turn of one screw, while the other 2 are fixed, will create a tilt of 2.34 arcsec of the mirror.

• 2. Centering the PF pedestal (ie. Prime Focus Corrector + Mosaic): there is a centering mechanism on both the northern and southern sides, that works in a push-pull mode. A ±5mm range is available for centering. Dial indicators are installed and precise centering can be done. Holding screws (22 of them) must be loosen first.

• 3. Tilting the PF pedestal: there are 10 push-pull screws all around the pedestal but no indicators are provided. Holding screws must be loosen first.

Only option 1 is readily doable any time at the telescope. Options 2 and 3 require more delicate adjustment inside the PF cage (see CH2150.540-C030 for details) as it has to be done at zenith with someone standing in the cage.

A 'pure' focal plane tilt (revealed by a defocus slope across the field) can be corrected by tilting MOSAIC at its interface with the PFC with push-pull screws. Beware at the installation of Mosaic: these captured screws must normally be loose to avoid a non-intentional focal plane tilt (it happened once)!

The PFC is designed to deliver a 48 arcmin non-vignetted field with FWHM under 0.5arcsec at the edge of the field (see Tom Ingerson's paper [12]). The PFC optics include a field corrector for coma (paraboloidal M1) and spherical aberration (Ritchey-Chretien M1) and an Atmospheric Dispersion Corrector (2 pairs of rotating cemented prisms).

In terms of alignment requirements, the PFC optics have little optical power (1.076) thus are relatively insensitive to tilt, and behave like the M2 of a Cassegrain telescope where lateral centering is important to avoid misalignment coma. If decentering coma is present, the PFC optics must be translated laterally. In practice, for small corrections, we do tilt M1 instead. Zeroing the coma on-axis is not a sufficient condition. It is also important to check the field astigmatism off-axis (see simulations below). On-axis astigmatism doesn't reflect a misalignment problem but rather a problem with the cell of the primary mirror ("mirror pinched") and must be corrected via the active optics lookup table. If spherical aberration and/or field curvature are present, the longitudinal position of the PFC is not optimum and must be corrected by adjusting the back focal distance as Mosaic does not focus independently of the PFC (this was done once in Nov99 after the installation of MosaicII).

How to correct coma by tilting M1?

This can be done quickly by looking at a through-focus sequence for an on-axis star (9x100um steps is ideal, usually coma shows up nicely 200um away from best focus for a reasonnably bright star). Determine the cardinal direction of the coma (from the coma head to the tail): this side of M1 must be raised. The cardinal directions of the Mosaic field in the image display are: east is up, north is right. Alternatively, you can look for the minimum coma point in the entire field and consequently lower that part of the mirror. Iterate the adjusting tilts until the on-axis through-focus sequence is round and uniform. When wavefront errors are available from the Hartmann screen data, 1/1000 inches tilt is equivalent to an OPD of 230nm at the edge of the pupil, thus would correct about 230nm of coma measured on-axis. This is about the smallest correction that one can do with confidence and repeatability. Always keep a record of the tilt values of M1 (ie. values of the 3 hard points -south, northeast, northwest- dial gauges with air ON).

SIMULATIONS:

The following simulations are made at 650nm at zenith (ADC neutral). Only coma and astigmatism patterns do change; the trefoil pattern is not modified by misalignments. Reference field aberration maps for an aligned telescope are to be found in the PF imaging section of the main optics web page.

• See on this coma [13] field aberration map the effect of a 1mm decenter of the PFC+Mosaic (with respect to M1). The field astigmatism [14] pattern changes very slightly.
• A 1mm decenter of the PFC+Mosaic (with respect to M1) corrected by the appropriate tilt of M1 (25 arcsec) returns to the ideal case of 0 coma on-axis but the astigmatism pattern [15] worsens a bit.
• Sensitivity of the 3 types of misalignments: 1660nm (=0.23" in d80) of on-axis coma is produced by 1mm decenter of the PFC, or 25arcsec tilt of M1 or 0.35deg tilt of the PFC.
• When on-axis coma is present, if one really wants to know whether it comes from a tilt of M1 or a decenter of the PFC, it is possible to look at the differences between the field astigmatism patterns. For a same amount of coma, the case of a M1 tilt does generate more astigmatism and with a particular pattern [16], where the vector axis always points away from the axis thus rotates around the field (beware, when you examine images, this pattern becomes apparent only for a minimum amount of tilt).
• Besides generating misalignement coma, a large tilt of the PFC, 0.2deg in this case, will produce very particular astigmatism patterns: radial inward if the tilt is in the Y direction [17] (ie. north-south, which is the prism axis in neutral position), and circular if the tilt is in the X direction [18]. A subsequent appropriate tilt of M1 to zero the coma on-axis would almost not decrease the astigmatism pattern.
• Conclusion:

How to quickly correct astigmatism with the active optics of M1?

If astigmatism is present on-axis in a through-focus sequence, it is possible to correct for it with a TWEAK of the primary mirror (read first the TWEAK section of the 4m active optics on-line manual!). This is a 'quick-and-dirty' method but it can improve the images when the lookup table is not working well. On-axis astigmatism is caused by a non ideal support of M1 in the cell: that problem typically varies with telescope position, thus a TWEAK is valid only for a specific telescope position ±5deg (ie. about ±20min of exposure time). So if you follow the same object, you should actualize the TWEAK about every 40min.

Run the IRAF task mscexam on each out-of-focus image of a single star and look at the ellipticity angle (called 'pa') with the ',' option for example. To make sure that you are in presence of astigmatism, pa should flip by 90deg across best focus and be fairly stable on each side of the best focus. If the pa exhibits large variation and/or does not change roughly by 90deg across focus, don't even try that method!

Following is a list of measured pa for the IN-FOCUS image (called 'pa in') and the corresponding angle to enter in the tweak table of the tcp (called 'pa tweak'):

• if pa in > 0, pa tweak = 360 -2*(pa in)
• if pa in < 0, pa tweak = -2*(pa in)
• Hint for double-checking: a 'pa in' of 90deg is equivalent to a vertical astigmatic image in the mscexam xgterm window (not in the main image display), while a 'pa in' of 0deg is equivalent to an horizontal astigmatic image.

Examples:

• pa in 90 -> pa tweak 180
• pa in 67.5 -> pa tweak 225
• pa in 45 -> pa tweak 270
• pa in 22.5 -> pa tweak 315
• pa in 0 -> pa tweak 0
• pa in -45 -> pa tweak 90
• pa in -67.5 -> pa tweak 135

What amplitude shall you enter in the tcp tweak table? This is even more empirical: in reasonnable seeing of 0.8", a 1um astigmatism will be visible at 100um off the best focus, but hardly visible at 50um... Once again, although this technique works, it is experimental and must be used with care until a better lookup table can be built. This method can actually also be used to update the astigmatism lookup table using the seeing images recorded every night (ie. not waiting for a dedicated engineering night).

Note: SCALE OF THE WAVEFRONT ERRORS (optical path difference at the edge of the pupil): at the 4m, 0.1" image degradation in d80 is produced by 0.70u of coma, 0.30um of astigmatism, 0.26um of trefoil, 0.24um of quadrafoil.

Maxime Boccas, 28March01