Failed Blue Shutter Observing

Background

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.
- 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
- 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.

	Failed Blue Shutter Observing
LRIS Home Page NEWS Config form Slitmask database Index Keck Instrument Ready? Archive (KOA) Observer Home Page Tel Schedule Instruments Home Keck Home Observing Info ToO Policies Weather MK Forecast CFHT SkyProbe MK Seeing CFHT Cloudcam Summit Cams Keck Cloud Cams Planning Tools Starlist Tool Finding Charts IRSA Finder Charts Observing Planning Tool Offset/PA Tool Contact Us Contact Info Post-Observing Comment Form WWW Keck Web Site	Background 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.
Last modified: 03/21/2017 06:00 Send questions or comments to:LRIS Support