The LRIS blue shutter is prone to getting stuck, preventing observers
from acquiring images in the normal way. Fixing the stuck shutter is
a major repair requiring removal of the entire LRIS blue dewar; hence,
this repair will probably not be made until the conclusion of the
current LRIS run.
Fortunately, if the blue shutter fails during a run it is possible to
operate the blue side in a shutter-less mode and still obtain usable
data as long as certain procedures are followed. This document
describes the required hardware modifications and observing procedures
to carry out observing in shutter-less mode.
Hardware Changes
In the event that the blue side shutter is stuck, the following
hardware changes must be made to operate the LRIS trapdoor as a
shutter for the blue side.
Force the shutter open. This will have to be done by a summit
instrument technician if the shutter is stuck closed.
Have the summit instrument technician put the shutter into
"manual" mode via the toggle switch on the blue-side shutter
controller on the instrument.
Where to find the switch ...
put the rotator at 0 deg
Open the Blue side electronics enclosure, on the back of the
instrument. The enclosure is on the left when looking at the back of
the instrument.
There is a blue box two two buttons: one red for closing and
a black one for open. There is also a switch to change from manual and
automatic motion. Put the switch to manual and press the open button
to open the shutter.
A green LED should indicate that the shutter is open. A red
LED indicates that the shutter is closed.
If necessary, increase the speed of the trapdoor to open and
close the trapdoor in under 0.5 seconds by adjusting the air pressure
supplied to the solenoids on the trapdoor. The speed is not critical.
Observing strategies
Below are observing strategies to assist observers in acquiring
data. There are many work arounds for a failed shutter and the
observers will have to choose which observing procedures to
follow. Below are some recommendations.
MIRA - Acquire MIRA images as if everything is normal. The
success of MIRA relies on centroiding the individual
segments. Although the images are "streaked" in the readout
direction, the segment point sources are bright enough to
centroid, and the focus results are valid.
SLITMASK ALIGNMENT Acquire slitmask alignment images as if
everything is normal. Like MIRA, slitmask alignment is
robust to streaked images. Use the blue XPOSE gui to acquire
images. Do not use the scripts described below.
SCIENCE EXPOSURES
There are several strategies for acquiring science
exposures. Observers must decide what is right for their
science and targets. What works for one target or
science program may not work for another. Sometimes
observers will employ all three methods during the night.
Observe normally Science is robust to streaking in the
readout direction. This may be useful when all an
observer cares about is where emission lines are
located. Advantages: Continuous guiding. Disadvantages:
images streaked in readout direction.
Zero Second Dark After every science exposure, acquire a
zero second dark. The dark will contain the
contamination incurred during readout and may be
subtracted from science images. Advantages: Possibly to
remove streaking by subtracting dark. Continuous
guiding. Disadvantages: Additional noise, may not
completely remove streaking.
Trapdoor as shutter There is a script to use the
trapdoor as a shutter (see below). Advantages: Removes
streaking in the data. Disadvantages: Non-continuous
guiding, less efficient.
For guiding, let the OA set up on the target and
record the position of the guide star. The OA should
be able to put the guide star back to the position
when the trapdoor re-opens. This has been very
successful in the past.
Exposure times are set in the XPOSE GUIs the Blue
side exposure time must be set to 7 seconds longer
than the red when exposing using both red and blue
sides simultaneously.
Slitmask alignment: use the SAT as normal. The small effect of the shutter
failing to close shouldn't affect the alignment result.
Calibrations: Close the
trapdoor, acquire all the red side data first. then use the
scripts (see below) to acquire the Blue side cals.
Useful Scripts
The following scripts may be used to acquire the arc lamp data, flats
and science data using the trapdoor as a shutter.
cal_trap_hg_blue
Acquires a single Hg arc lamp exposure using only the blue side. The
exposure length is hard coded, and could be a variable to acomodate
different slit widths. The script does the following:
Turns on the Hg lamp for 15 seconds to allow it to warm up.
Turns off all of the lamps.
Sets the exposure time to 60 sec.
Erases the CCD.
Turns on the Hg lamp for 7 seconds.
Resets the exposure time to 7 sec to initiate readout.
Turns off the lamp.
Reads out the array.
cal_trap_cd_zn_blue
Acquires a single Cd+Zn arc lamp exposure using only the blue
side. The exposure length is hard-coded (but the script could be
modified to make this variable to accommodate different slit widths
and grisms). Prior to running the script the observers must allow
the lamps to warm up for 5 min. The script does the following:
Turns off all of the lamps.
Sets the exposure time to 60 sec.
Erases the CCD.
Turns on the Cd and Zn lamps for 5 seconds.
Resets the exposure time to 5 sec to initiate readout.
Turns off the lamp.
Reads out the array.
Turns on the Cd and Zn lamp for 5 seconds. [??]
cal_trap_flat_blue exptime
Acquires a single Halogen lamp flat using only the blue side. The
variable exptime is a required variable defining
the exposure time [in seconds] for the internal Halogen flat field
lamp. The script does the following:
Turns off all of the lamps.
Sets the exposure time to exptime+13 sec.
Erases the CCD.
Turns on the flat lamp for exptime seconds.
Resets the exposure time to exptime sec to initiate readout.
Turns off the lamp.
Reads out the array.
goitrapb [-both]
Acquires images on the blue side, operating the trapdoor as a shutter
for the blue side detector. The optional keyword "-both" will acquire
a single image on the red and blue sides simultaneously. The script
will check to ensure that the red side exposure time is 3 seconds less
than the exposure time on the blue side. The script will do the
following:
If -both is specified, starts an exposure on the red
side and pauses the exposure after the erase.
Closes the trapdoor.
Erases the blue side CCD.
Opens the trapdoor to start the blue side exposure.
Note: Guiding should resume if needed.
If -both is set, resumes the red side exposure. The red
side exposure will complete before the blue side finishes.
When the blue exposure finishes, closes the trapdoor.
Reads out both arrays.
Opens the trapdoor when the blue side finishes readout.
To focus the LRIS BLUE side CCD
The procedure is similar to the standard focusing procedure, except
that the acquisition of the blue-side images required running a
special script. Follow these steps to acquire the images:
Set the blue side exposure time to 3 seconds.
Set the blue filter to G
Set the grism to clear.
Turn on the spec dome flat field lamps.
Insert the focus_holes mask.
In a lrisserver xterm enter:
xfocuslooptrapb 7 -2500 70
Note that the -2500 should
be changed to a default focus value if appropriate.
Once the images have been acquired, you can analyze them as usual
using the XFOCUS blue IDL widget accessible from the
desktop menu.