Summary
When you start a DEIMOS science exposure, it's critical that the
Flexure Compensation System (FCS) be in
Tracking mode
when the exposure starts. If the exposure begins before FCS has
settled then the image will move on the detector during the
first few minutes of the exposure, resulting in image blur.
Instead of simply clicking the
Expose button on the
DEIMOS dashboard to begin an exposure, we suggest that you enter
the following command into a
polo xterm window:
wffcs ; goi
This will cause the system to wait until FCS is tracking, then
begin the exposure. You can run this command even while
reconfiguring DEIMOS, since the FCS will not be tracking until
the instrument is done being reconfigured. Thus, you can run
this command as soon as your slitmask alignment is complete and
you are sending the grating back from the zeroth order imaging
position to the science wavelength.
Although the guider software does not measure a true FWHM
parameter, we have a handy utility for measuring the seeing off
of slitmask alignment images. From within IRAF, simply type the
command
get_seeing; this script will fit Gaussian
profiles to the stars in your alignment boxes and compute and
print a median seeing value.
We recommend that you verify the alignment of your slitmask by
running the
align_check
script regularly during your exposure sequence. Open a
polo xterm window and initiate the
align_check command while a spectral exposure is in
progress. The script will wait for the exposure to complete,
then reconfigure the grating to the imaging position. It will
then acquire a single alignment image and return the grating to
the science wavelength so that FCS can re-establish lock. All
you need to do is run
do_xbox in IRAF (do this after
the spectral image has read out) and analyze the resulting
direct image. Send any required moves and then press the
<Enter> in the xterm running
align_check to initiate the next exposure.
One possible reason is that you changed gratings; the mapping
from position on the slitmask to position on the CCD image
depends on the grating; thus, when you change gratings you need
to re-run the
tune_qmodel program to allow
xbox to locate your alignment boxes.
However, incorrect observing strategy can also require you to
recalibrate xbox. Although it is tempting to
increase your observational efficiency by reconfiguring DEIMOS
during a telescope slew, this strategy can be
counterproductive. Since the DEIMOS FCS cannot track when the
instrument is being reconfigured, you prevent the FCS from
doing its job right when the most flexure is occurring! The
typical result is that when you get onto your new target and
acquire a direct image for slitmask alignment, you find that
the xbox software is unable to locate your
alignment boxes due to the large uncorrected flexure that
occured during the slew. If you then run the
tune_qmodel task to update the mapping from mask to
CCD coordinates, you will fix the problem...but only
temporarily. The next time you go to align a mask, the FCS
will now have updated the flexure corrections and hence your
updated coordinate mapping will be incorrect again, forcing
you to run tune_qmodel yet again!
The preferred method for dealing with this is to allow DEIMOS
to remain in its spectroscopic configuration during the
telescope slew. You can complete the coarse alignment phase
of the slitmask alignment (through running gmov) while the FCS is making
corrections. Once the FCS status changes to
Tracking, click the GO ZERO button to
switch to imaging mode and acquire your exposures for running
xbox. This should ensure that:
- xbox can find your alignment boxes without
needing to re-run tune_qmodel, and
- the FCS will rapidly return to Tracking mode
once you complete the slitmask alignment process and return
to spectroscopic mode.
Most DEIMOS stages can be moved during CCD readout without
compromising the CCD readout noise. However, the dewar focus
mechanism and the dewar filter wheel mechanism will inject a few
electrons of fixed pattern noise if they are moved while the CCD
is reading out. This is due to the fact that the motors for
those mechanisms are in extremely close proximity to the DEIMOS
CCD electronics boxes and associated cabling.
If
xbox fails to find certain boxes on a mask,
inspect the alignment image on
ds9 to see whether
some regions of the image are badly out of focus. If so, try
the following steps to fix the mask:
- Re-insert the mask by clicking the Retract
Mask button on the Slitmask Panel and re-selecting
the mask, then take another image and check the slits.
- If this fails, have the OA rotate the instrument to a
drive angle of +60, then retract and re-insert the mask and
take an image to check the slits. Try as many times as
your patience allows.
- If this fails, switch to another mask.
By default, the slitmask droplists on the LRIS/DEIMOS run
configuration form list all of the slitmasks which have "use
dates" that occur during your run. If you wish to use a mask
which has a use date outside of this range, simply do the
following:
- Select the first item in the slitmask droplist, labeled
EDIT.
- After you have clicked EDIT, use the
Backspace/Delete keys to remove the word EDIT.
- Type in either the barcode number or name for your slitmask.
Any newly submitted mask which has a "guiname" that matches any
existing mask which is intended to be used in the future is
assigned a new guiname. This is required in order to prevent
confusion about the conflicting masknames. When a mask is
renamed, the slitmask owner should receive an email about the
change.
Unforunately, no. Once the slitmask information has been
ingested, it will be transferred to the milling site and the
slitmask should be milled. Since the database computer in Santa
Cruz has no way of knowing whether the mask data have been
transferred to the slitmask mill on the Mauna Kea summit, it is
not safe to change the slitmask name once the mask has been
ingested. If we did change the name in the database, we could
end up having the name recorded in the database disagree with
the name milled into the physical mask. There is no way to
change the metal, and the inscription on the mask is used as a
cross-check by the technicians who are loading the masks. We
wish to prevent such confusion.
The answer depends on which grating is to be used. The DEEP2
pipeline seems to work better with certain grating/slider
combinations than others:
- 600ZD is preferred in slider 3.
- 830G is preferred in slider 4.
- 900ZD works well in either slider 3 or slider 4.
- 1200G is preferred in slider 3.
Unfortunately, no. The LRIS filters (241×241mm) are much larger
than the DEIMOS filters (165×165 mm) and so the former would not
fit in the DEIMOS holders. Also, the beam in DEIMOS is quite
fast (f/1.29) and thus the existing LRIS narrowband interference
filters would probably give signficant passband variations
across the FOV.
DEIMOS filters must measure (165×165 mm) in size to fit into the
filter holders and be 6.0±0.1 mm thick in order to allow the
camera to focus.
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Last modified: Wed Oct 8 23:49:48 HST 2008
