salt image quality: may 2006 1 salt image quality status report web-based documentation of the...

SALT Image Quality: May 2006

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SALT Image

Quality Status Report

Web-based documentation of the problem, updated on an ongoing basis, at:http://www.salt.ac.za/partners-login/ Partners Pages Data Quality

The Image Quality StoryUsername (salt) and password (tlas) needed

SALT AstronomersHitesh GajjarLuis BalonaJames O’Connor+ unswerving support from Herman, David, Phil + SAAO staff (e.g. mech workshop)+ Arek

SAAO, Cape Town

Darragh O’Donoghue SALT Operations: Optical Support

&The SALT Operations Team:

SALT Science Working Group: May 2006


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The problem can be summarized as follows:

• It is not possible to get optimal focus for all parts of the field of view at the same time.

• There is a focus gradient across the field: as telescope focus is changed to optimize one quadrant, the opposite quadrant is in worst focus and vice versa. The effect is not nearly so strong in the other two quadrants.

• This is not simply a misalignment of the detectors with the optical focal plane. The out-of-focus images show significant optical aberrations.

• The aberrations seem to be mainly focus and astigmatism but it’s v. likely higher order aberrations are present, as doubled star images, cashew-nut shaped star images and occasionally even more exotic images are seen.


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There are 3 major optical sub-systems in SALT:

• The primary mirror (91 segs, spherical)

• The spherical aberration corrector (SAC)

• The 2 instruments: SALTICAM (imager); RSS (formerly PFIS) which has an imaging mode

Both instruments are in a payload which rotates tocorrect for field rotation. This capability is a powerful

tool for diagnosis


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Slides 4-10 show IQ in images of the globular cluster 47 Tuc. Expanded scale views are also included. The instruments, the acquisition camera SALTICAM and the imaging spectrometer RSS, are mounted in a rotating payload. Slides 4-7 show images from SALTICAM with the payload at one rotation angle; slides 8-10 show images with the payload rotated around the telescope optical axis by 90 degrees. These slides (along with every other time this kind of test procedure was executed) show that:


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47 Tuc: S200511240007.fits: November 2005


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47 Tuc: S200511240007.fits: top right


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47 Tuc: S200511240007.fits: top left


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47 Tuc: S200511240007.fits: bottom right


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47 Tuc: S200511240009.fits


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47 Tuc: S200511240009.fits: top right


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47 Tuc: S200511240009.fits: top left


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Another pair of images: “typical” at the edge of the field; “best” at the centre of the field and in good seeing.


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Astigmatic images 1 arcsec images


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Slide 12 shows a “quasi-simultaneous” image taken with the two instruments, SALTICAM and RSS, in the rotating payload. An image was taken with SALTICAM and then immediately the fold flat feeding light to SALTICAM was removed, and “straight through” light from the SAC reached RSS.

This slide (along with many others obtained with the same kind of procedure) show that:

Both instruments see the same poor image quality, in this case doubled star images.


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Simultaneous bad images in SALTICAM (right) and RSS (left)


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This leads To Two Alibis ForThe Instruments:

• The same poor images are seen in both instruments in quasisimultaneous data.

• All the aberrations “go around with the rho stage” “follow the stars” the aberrations are ‘fixed’ in the reference frame of the telescope structure

These are two pretty unshakeable alibis


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This leaves the other two optical sub-systems:

• The Primary Mirror

• The Spherical Aberration Corrector (SAC)


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The Primary Mirror

My number 1 suspect for a long time because:

• Image quality variability (Primary is re- aligned every night)

• Doubled images (Primary is aligned in 2 halves)

• Images are astigmatic; astigmatism is easy to generate in the Primary, difficult in the SAC


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We felt that the doubling of star images has been a significant feature of the image quality problem which needs explanation.

The SALT tracker does not allow all the primary mirror to be seen by the Shack-Hartmann alignment camera in the tower alongside the telescope. So the primary mirror is always stacked in two separate halves.

An experiment was therefore conducted in which half the primary mirror was tilted away, unaligned, and the other half was used to take images. The expectation was that there would be no sign of doubled images. To our surprise this expectation was NOT fulfilled: see the next slide…


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The Primary Mirror

It was then a big surprise when this and other similar images were obtained:

Ignore:These areFrom the discardedhalf of thePrimary

Doublestar imagesfrom ½ thePrimary !


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The Primary Mirror

So I analysed the Shack-Hartmann residuals after aligning the primary and found no problems large enough to explain the observed poor image quality

(details can be supplied if you would like to see them).


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The Primary Mirror

Lastly, the Primary is the entrance pupil of the optical system. Problems in the primary, except for differential vignetting (which is not sufficient to be the explanation), will affect all field angles equally.

HOWEVER, the image quality problem we see is differential image quality over the field with the main aberration being a difference in focus (so it is not possible to achieve good focus across the field at the same time), but also in the places where the focus is poor, the images may be astigmatic (elongated), or more exotic (doubled etc).


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The Primary Mirror

Just to confirm the points made in the last slide, I carried out Zemax simulations where I first induced 3rd order coma and then astigmatism in the primary and inspected the spot diagrams for the centre and edges of the field of view, confirming that all parts of the field of view are affected similarly, and that all parts of the field of view have the same best focus.

These two properties are in contradiction to what is seen with SALT.


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The Primary Mirror: Through Focus: Coma & Astigmatism

Through Focus

Field

Coma Astigmatism

Key Points: No variation with position in the science field No focus change with position in the science field


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We conclude that the Primary Mirror is notthe cause of SALT’s image quality problems:

Doubled images when using either half of the Primary

Testing of the Primary at CCAS shows no evidence for problems

Primary is pupil optic: all field angles affected the same


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Unfortunately, this leaves the most complex optical subsystem in the telescope: the SAC

M2M3

M5

M4


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The SAC

Initially I thought it couldn’t be the SAC as, if you misalign it or any of its 4 mirrors, this generates large amounts of coma. Here is anexample of a misalignment of the entire SAC :


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The SAC

… but… control of tip/tilt provided by the Tracker is very accurate (1-2 arcsec) and enables all coma to be “tuned out”. If there’s coma, the auto-collimator offsets can be, and are, adjusted until it is minimized. This leaves SAC astigmatism:

Through FocusSpot Diagram


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The SAC

In fact, only SAC mirrors M4 and M5 can deliver focus and astigmatism varying over the field of view: M2 and M3 behave like the Primary: M3 is a pupil mirror and M2 nearly so. On M4/5, different field angles see different parts of the mirror:

“Footprint” Of Light From Opposite Sides Of Science Field On M1/M4

SAC M4Primary DifferentialVignetting

Mostly non-overlapping


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So Let’s Summarize The Logic So Far:

• It’s not the instruments:

Aberrations go round with the rotating stage Same poor images seen in SALTICAM and RSS

• It’s not the Primary Mirror:

Doubled images seen using only ½ the mirror CCAS testing shows no evidence for problems Primary mirror affects all field angles ~the same

• This leaves the SAC. And not only that, it must be M4/5

Only M4/M5 give differential field effects Best current ray tracing guess gives very plausible ‘star images’ qualitatively, but not yet quantitatively.


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How can we further diagnose what the problem is?


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Hartmann Testing At Prime Focus:

Install Hartmann mask at exit pupil of the telescope (moving baffle location)

Put telescope 3 mm out of focus (use SALTICAM with internal focus at –1 mm to compensate for “test setup” aberrations

Obtain star images over full science field

Analyse w.r.t. Zemax “nominal telescope” ray trace


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Guesstimated schedule for fixing:

Diagnosis complete by end of Sep

Completion of fix by end of Qtr I, 2007

The End

(if you have had enough)


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90- hole Hartmann Mask


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Example of one of the Hartmann images


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Data Analysis:

Centroid the spots using a CCD stellar photo- metry program.

Compare spot positions with those of “ideal” telescope. Compare spot positions with those from telescope with (i) M4 decentred in x/y; (ii) Tracker tip/tilted; (iii) M4/M5 tip/tilted; (iv) M4 coma/astigmatism; (v) M1 coma/astigmatism. (I made no attempt to decompose into Zernikes as what do we do then?).

Least squares fit linear combination of the above models.


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Preliminary Results Of Data Analysis:

So far, none of the models, or their linear combinations, provide a satisfactory fit to the Hartmann data, although only a small number of the 3 nights worth of images has so far been analysed. The best solution so far is the model in which M4 is decentred but although it does provide some fit to the Hartmann data, it does not provide a fit that can be regarded as satisfactory.


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The central question is how do we get the system fixed given the risks (schedule, technical, costetc.)?

Schedule:

• Sending the SAC to SAGEM will bring the telescope down for weeks to months. And what if the problem disappears on taking the SAC off the telescope because it is something to do with mounting it on SALT.

• Adjusting on the telescope is optimal (but probably impossible, or too risky at best).

• What seems best to me is the compromise: fix at SALT if possible


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Technical Risk:

• Damage to optics, opto-mechanics or coatings during removal/re-installation of SAC. Mitigate this through contracting Willem Esterhuyse and Leon Nel (who installed it), to remove and re-install

• Damage (as above) during transportation to Cape Town or France. Avoid if possible

• Lack of experience/equipment of local personnel with complex aspherical optics. Contract SAGEM to adjust at Sutherland if at all possible

• Incorrect adjustment. Very serious if done in France; not so damaging if done at SALT (but of course much better to get it right first time!)


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Speculations: I How Did This Situation Arise?

• It is my belief, since the exoneration of the Primary Mirror, that misalignment of some combination of M4/M5, either as individual mirrors, or as a pair, is the likely cause of the IQ problem.

• The amount of misalignment required is HUGE: ~0.5 mm of decentre in M4, or a similar amount in M5, or an appropriate tip/tilt. How on earth can such a large misalignment have arisen?

• Misalignment of M4/M5 (or any of the other mirrors) can arise at assembly at SAGEM, during shipping, during installation, or during operation.


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Speculations: II How Did This Situation Arise?

• M2, M4 and M5 were individually fabricated and tested interferometrically; M4 and M5 were assembled interferometrically.

• This “rules out” simple misalignment of M4 with respect to M5.

• The M4/M5 pair were positioned on M2 by mechanical metrology. Decentre of this pair is unlikely and would have minimal impact anyway. Tip/tilt of 0.4 degree (!) would give an error more or less of the right size. However, Zemax simulations of this possibility shows no doubling of images or cashew-shaped star images.


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Speculations: III How Did This Situation Arise?

• Shipping: container very sophisticated; no shock sensors were triggered on arrival in CT.

• Very violent treatment needed during installation/ operation to cause a 0.5 mm decentre or 0.4 degree tip/tilt. Personally I think this is very unlikely.

So far, there is no satisfactory detailed explanation of the problem.


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Summary

4 months of IQ investigation completed. IQ problems characterized and documented on internet

Science instruments and Primary Mirror found not to be responsible for IQ problems

All evidence currently point to SAC mirrors M4/M5

Hartmann testing at prime focus data obtained and analysis in progress

salt image quality: may 2006 1 salt image quality status report web-based documentation of the...

Documents

right slide

left slide

salt operations team

quasisimultaneous image

salt science working

partners pages data

outoffocus images

pair of images