Subject: Re: ARC 3.5m User's committee meeting, Jan 13, 2003

From: Russet McMillan

Submitted: Tue, 21 Jan 2003 23:32:46 -0700 (MST)

Message number: 651 (previous: 650, next: 652 up: Index)

Michael's minutes included these questions:

>  We all agreed that further progress on image quality will require
>one or more dedicated people to investigate the problem(s).  In the
>meantime, it would be very useful to have a summary of:
>   -All known and possible problems affecting image quality
>	(wind-shake?  Primary mirror support?  Not enough tension 
>	in the secondary truss? Catastrophic slippage giving rise to
>	double images? Focus instability? High-speed vibrations caused
>	by the drives?)
>   -What is currently known (and not known) about their contribution
>        to the error budget. 
>   -What we need to do to get more information about these things. 

>  Without this in hand, it is difficult to make decisions of the
>relative importance of these different items. 

This is actually a message I composed a few weeks ago, just before the
AAS meeting.  It was originally written as an attempt to get my own
thoughts in order following the various Shack-Hartmann tests last
quarter.  I considered sending it out to the user community, but there
was something of a deluge of messages at that time.  However, I think
it addresses most of the questions from the minutes, including those
for which we don't yet have firm answers.


I'm trying to sum up what we currently know about various things that
have an effect on image quality, so that we can figure out which is
the tallest pole in the tent and try to identify areas where it's
possible to get the most improvement for the least cost.


What we know about collimation:

1) Coma is probably the worst aberration we suffer.  Astigmatism is
   close behind, but harder to fix.  Coma is caused by tilting or
   translation of the secondary and primary mirrors relative to one
   another; either mirror could be moving and cause the same effect,
   and therefore it's sometimes hard to identify the reason for a
   change in coma.

2) The secondary mirror tilt required to correct the coma changes by
   significant amounts on a timescale of a few days, and also shows
   large changes as the telescope cools over the course of an evening.

3) The secondary encoders indicate no correlation between the
   SH-measured coma and the secondary mirror tilt relative to the
   mirror frame.  This is the single strongest result we've gotten
   from the Shack-Hartmann monitoring program of Autumn 2002.

4) The primary LVDTs do show some variation, with a slight tendency
   toward a bi-valued distribution.  However, the motion is less than
   a micron on each actuator, and shows no clear correlation with the
   SH results.  The worst PMSS oscillations only cause the star to
   wobble on the SH video by a few tenths of an arcsecond.  I believe
   (but I have not confirmed) this indicates that the primary can't be
   tilting far enough to produce a significant change in coma.  When
   the secondary is tilted by the smallest amount significant for
   collimation, the star motion is on the order of 10 arcsec.

5) My information on mirror translation as opposed to mirror tilt is a
   little less solid since both the primary and the secondary only
   offer translation measurements in one direction.  However, the
   information that is available matches the two previous conclusions:
   the secondary translation relative to its frame does not correlate
   with Shack-Hartmann collimation measurements, and the translation
   seen on the primary is probably too small to produce the observed
   changes in collimation.

6) The strain gauges on the secondary support vanes do show occasional
   jumps in resistance which might indicate that the entire frame of
   the secondary is being moved around.  Because only twice-daily
   monitoring of the strain gauges was possible, we have not firmly
   linked any of these jumps to a change in collimation.  Nor have we
   been able to find a clear correlation between the strain gauge
   resistance (or the collimation) and weather conditions, although
   there might be a weak link between resistance changes and humidity.
   We need more data with better time-sampling in order to confirm any
   such relationships; all we know for now is that it's plausible that
   the support vanes could be causing the changes in collimation.

   In the three observed cases where the resistance changed on the
   strain gauges, all four accessible strain gauges (out of the total
   of eight) changed at the same time in the same direction by
   similar amounts.  We therefore hope that it is possible to judge
   the behavior of all eight support vanes by monitoring only one
   strain gauge.


What we know about the other effects of mirror tilts:

1) Tilts of the tertiary mirror cause pointing changes without
   collimation changes.  We have evidence from the echelle that large
   changes in tertiary tilt (several hundred arcseconds) happen on a
   few-day timescale, although it's usually consistent during a single
   night and showed excellent repeatability during recent pointing
   models.  Pointing appears to be more consistent at NA2, and when it
   does have problems, those are often attributable to motors rather
   than mirrors.  I have no strong evidence that tertiary tilts at NA2
   have a significant effect on pointing.  Tertiary tilt
   inconsistencies are worst at TR4, showing egregious non-repeatable
   behavior within a single night, especially at low altitudes.
   Pointing accuracy at this port may never been better than 10-15
   arcseconds. 

   In November, I implemented tertiary sag coefficients at all three
   instrument ports.  I have limited evidence from pointing models
   that the sag behavior is consistent over a timescale of weeks or
   months for NA1 and NA2, and therefore I hope the sag coefficients
   will improve the accuracy of NA1 pointing relative to NA2.  I
   haven't been able to confirm that result; adjustments to the
   tertiary tilt constant are still needed every week or two to
   improve echelle pointing.

   The echelle "tilt" changes tend to be largest in the Y-tilt
   direction, which might indicate that the problem is actually in the
   repeatability of the tertiary rotation rather than tilt, although
   we have not seen any such problems in multiple engineering tests.
   It's also possible there could be other sources of pointing error
   which can be fixed by changes in tertiary tilt, but are not caused
   by irregularities of tertiary tilt or rotation; however, I do think
   the tertiary is the most likely culprit.

2) PMSS oscillations frequently cause degradations of image quality.
   These oscillations are clearly visible on the Shack-Hartmann video
   screen, at levels of a few tenths of an arcsecond.  In science,
   most of the time the effects are quite subtle, making images only
   slightly out-of-round or masquerading as poor seeing.  However,
   given the evidence of the dataq and the confirmation from the SH
   video that PMSS oscillations truly do translate into image motion,
   I believe this is having a significant impact.

   The oscillations tend to happen in three kinds of situations.  All
   of these are worse when the PMSS is poorly tuned, but even under
   the best circumstances we usually have some of this going on.  The
   first situation is high altitude, where oscillations usually start
   showing up somewhere in the region 80-83 degrees.  The second
   situation is windshake; unavoidable in itself, but sometimes it
   sets off oscillations which are much too slow in damping out.  The
   third, least common situation is oscillations at low altitudes,
   often arising after a long slew in azimuth.  These tend to produce
   the most obvious effect on science, although again it sometimes
   masquerades as seeing because observers often attribute it to high
   airmass.


What we know about focus:

1) The current adjustments for focus as a function of temperature are
   better than nothing, but not adequate by themselves.  I believe we
   spend a considerable fraction of our observing time away from best
   focus, mostly because of temperature changes.  My experience is
   that keeping the telescope in focus requires checks at least every
   hour and sometimes every quarter-hour, especially on nights when
   the temperature is changing atypically or the seeing is
   exceptionally good.

2) My sense for the scientific instruments is that changes in focus
   can usually but not always be predicted by changes in the
   difference between the air temperature and the structure
   temperature.  When the air temperature warms suddenly, the focus
   goes up until the telescope has time to re-adjust.  During the
   evening when the telescope is not yet in equilibrium, it usually
   starts out warmer than the air with a negative focus value, then
   the value moves higher as the telescope cools.  If the telex fans
   are turned off in the middle of the night, the focus tends to move
   upward. 

   The above statements are based entirely on intuition and anecdote.
   I spent 5 hours one engineering night doing focus after focus to
   quantify this relationship, and couldn't prove anything (the
   temperature was unusually stable that night, and so was the focus).
   Nor have I been able to plot this correlation using focus data from
   our logs; all I got when I tried (in 1999? 2000?) was a
   scattergram.  Perhaps I should try checking that again with more
   recent focus data.

   Currently, the autofocus algorithms are based only on the structure
   temperature; the air temperature is not accessible to that
   algorithm, so it would have to be re-written to test the
   helpfulness of my intuition.

3) The Shack-Hartmann's focus appears to behave differently than the
   other instruments.  Notably, it tends to start high in the evening
   when the telescope is warm and move downward as the telescope
   cools; this occurred with the instrument mounted at both TR2 and
   TR4.  I cannot think of any explanation for this.  The focus should
   be dominated by telescope rather than instrument effects.  The
   echelle's behavior has generally been similar to the other
   instruments, as far as I could tell.  

   Since spherical aberration is supposed to indicate the ideal
   position of the focal plane, I might be able to tell if this
   phenomenon is caused by some internal temperature-related expansion
   within the instrument.  However, focal plane motions need to be
   quite large (millimeters) in order to make significant focus
   changes, and I don't see how temperature effects internal to the
   instrument could be making such a difference.

4) The focus vs. altitude algorithm appears fairly good; I have never
   had any strong evidence, either from science observations or from
   engineering, that it was imperfect.  However, I would like to check
   it more closely.  With the software tools currently available, I
   cannot compare the requested focus sag versus the secondary encoder
   positions; but that by itself wouldn't be a sufficient check, since
   sag in the truss and vanes might also affect focus.  Therefore a
   direct empirical check is needed, on a night with good seeing.  I
   would prefer to use spicam than the Shack-Hartmann, partly because
   of the strange behavior mentioned above, and also because spicam
   can measure focus at lower altitudes.


What we know about motors:

1) For the most part, it seems that the motors either work adequately
   for tracking, or they don't work at all.  The very short period
   between the appearance of the first image-quality problems and the
   complete breakdown of both azimuth motors in November 2002 is a
   case in point.

2) When the motors do produce image quality problems, it's often in
   conjunction with the PMSS, as mentioned above.  Long azimuth slews
   at low altitude sometimes set up mirror oscillations.  The PMSS
   behavior at high altitude tends to be worse in the north and south
   (where azimuth is tracking more rapidly than usual) and not so bad
   at similar altitudes in the east and west (where the tracking
   direction is primarily in altitude).

3) Tracking irregularities in the enclosure motors can be felt fairly
   easily by a person standing in the dome during observations.
   Theoretically, these should not affect the telescope because it's
   decoupled from the dome; however, we know the decoupling isn't
   perfect because we've occasionally seen footsteps in the dome
   produce twitches in an untuned PMSS.  Didn't Karen once report
   seeing a little motion on the SH video when someone was walking
   around in the dome?  So it would be good to think of a way to check
   whether the slight lurching of the enclosure is feeding over into
   our images.

4) The rotator is the most erratic of the three axes, although its
   behavior is worse on slews than on tracking.  By itself, this
   is usually not enough to affect image quality even on long
   exposures, since the science instruments are on the optical axis.
   However, secondary problems arise when the guider, which is placed
   further from the optical axis, corrects rotator eccentricities by
   making adjustments in azimuth and altitude.  The drift produced in
   this way is generally a few tenths of an arcsecond in a quarter
   hour, somewhat smaller than tracking inaccuracies without the
   guider running at all.  But it would be nice if it could be better,
   either through software (which would require having two guide stars
   in the field at the same time -- not always possible) or through
   repair of the rotator.

5) Bad magnesensor readings usually cause egregious pointing and
   tracking errors which are immediately noticed.  However, we had
   some instances lately where the first bad reading didn't ruin the
   pointing right away, and it was only when the second or third error
   showed up that the problem became obvious.  I have to wonder if
   there could have been small tracking errors after the first bad
   measurement which were not glaringly obvious, but might have
   affected image quality.  This is a fairly rare situation, however. 



Our image quality is reasonable now, but I believe if we can address
some of the problems above we would be able to reduce the number of
occasions of bad "seeing," since some of those are in fact focus or
image motion problems.  We would also make it possible to take better
advantage of the best genuine seeing conditions that might present
themselves. 



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