Image Gallery
This page contains 'typical' images along with some examples of 'odd'
images with a brief explanation of what is causing the problem and some
advice on how to fix it.
Pupil images, and hence the FPIA (Focal Plane Image Analysis) code,
may be affected by the conditions of the telescope and mirror (is
there a temperature gradient over the mirror or telescope structure?;
are all actuators functioning?) as well as seeing. If seeing is greater than
1", it is probably not useful to spend too much time trying to focus and
collimate with FPIA; the collimation lookup table should do well enough.
The following images show typical pupil images for which FPIA converged
(Zernike coefficients less than ~400 nm) in 1-2 iterations and those for
which it did not and manual intervention was needed.
| Pupil images on which FPIA works |
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Pupil image when well-collimated. Rotator angle = 6
degrees. Note the small scale variations in illumination which are a sign
of good seeing, but the overall large-scale uniformity, a sign that the
system is reasonably well-collimated. This was taken before starting a
10-pt dither sequence, the first image of which had stars with ~0.7" FWHM.
Obtained 12-Dec-06 UT**
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Another pupil image when well-collimated. This was
taken at the end of the 10-pt dither, ~20min after the image above and at
the same RA/DEC. The last image of the dither sequence had stars of ~0.75"
FWHM at B. Rotator angle = 17 degrees. Obtained 12-Dec-06 UT**
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This pupil image was taken on
21-Nov-06 UT in 0.5 arcsec seeing. Note the uniformity of the pupil and
the crispness of the swing arm shadow. Rotator angle = -193 degrees. There
is a bit of residual thermal gradient leaving the image with slightly too
much positive spherical aberration. |
| Pupil images for which manual intervention is needed |
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The mirror temperature was warmer than ambient
which caused a radial temperature gradient over the primary after opening up.
This manifests itself as a bright ring around the outer and inner
perimeters of the pupil, similar to a higher order spherical aberration
which FPIA does not correct. Manual intervention is needed. Ask the
operator to add ~1000 nm of Z11 (spherical) and try FPIA on the pupil
image taken after that. Obtained 04-Oct-06. Rotator angle = -245 deg |
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Another manifestation of the radial
temperature gradient. In these pupils the bright ring around the outer
perimeter of the pupil is gone, there is no discernable central hole, and
instead there is a steep fall-off in the illumination with increasing
distance from the center. Obtained 04-Oct-06. Rotator angle = 13 deg
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This pupil image from 12-Dec-06
shows again the strong fall-off in illumination from the center to outer
perimeter of the pupil. On this night, the dome had been shut for several
days and the mirror was 10 deg C above the ambient temperature, the limit
of operation. The inner ring is more visible than in the previous example.
Adding ~500-1000 nm of Z11 brought the pupil into the regime where FPIA
could converge
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Uneven faceplate temperature distribution
This blobby pupil image was obtained on a night when mirror
ventilation was not working well. Later it was discovered that a hose had
been disconnected and the air was not distributed through the mirror cell.
If you see an blobby pupil image, ask the telescope operator to plot the
faceplate temperature distribution. ** The shutter was stuck open this
night, but these pupils are bright enough that the readout trails do not
significantly affect the pupil illumination and structure and the images
are fine for illustrative purposes.
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| Coma and poor seeing |
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This pupil image
was obtained at the start of a night when these was some difference
between the ambient temperature (-7.5C) and mirror (-5C). The bright
crescent near the bottom is indicative of coma and was corrected manually
using the chart ByEyeAOa? .pdf in ByEyeAO. The pupil image is so blurry,
because of poor seeing, that the spider doesn't show up and the rotator
angle was read from the header. The bright crescent is on the same side as
the spider opening angle, which according to the cartoon, indicates that
negative Z7 should be added. Z7 = -1000 nm was given.
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Less coma and poor seeing
This pupil image was taken immediately after giving Z7 =
-1000 nm. It still shows coma, but FPIA was able to take over and after
2-3 more iterations converge.
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A comatic pupil image from the beginning of the night on 8-Jan-2007. |
| Other pupil images |
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A 32 second pupil
image taken near zenith with rotator disabled. It is easy to overlook the
fact that the Telescope box in the OB execution page should be checked to
enable rotator tracking, as well as the telescope functions (slewing and
telescope open-loop tracking). The pupil image uses a subset of rows on
chip 2 near the rotator center. Note the pupil shapes show rotational
trailing. |
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A 16 second LBC-Blue pupil
image taken when there was ice or condensation on the dewar window.
If this is seen, you can visually inspect the window by
opening the shutter with a long OB or using the manual switch on the hub and
looking into the LBC from the L1 lens. In this case there was about 1.5
inches of ice at the center of the dewar window.
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| Science Images |
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Light Dagger Scattered light pattern produced by glowing optical
fibers. Fibers were re-routed in January 2007 to suppress the stray light
and this is no longer a problem.
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U flat The pattern is from the CCD manufacture and is removed in
flat fielding. It shows up more in U than in B or V. The rings on the left
center and bottom right are out of focus stars. There is vignetting around
the edges for all filters. |
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Shutter Stuck Open The image shows trails in the positive
y direction caused by the shutter remaining open during readout. The
problem first occurred in December 2006, and was cured in January 2007 by
replacing a damaged ball bearing in the shutter drive.
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Horizontal banding on Red Array Some low background images
(e.g. biases and standard stars), show horizontal banding across all 4 chips.
The amplitude peak-to-valley is about 10-15 ADU (see plot below) and is about
10% of the background. |
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For chip 2 of the above image, sum of 2048 columns plotted against row (cnts vs pixel) |