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 ; goiThis 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.
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.
NOTE: see also the Slitmask Database FAQ page.
The minimum slit width is 0.63 arcsec for a slit which is untilted (i.e., the position angle of the slit matches the position angle of the mask).
The answer depends on your slit width, according to the formula
pa_max = acos(0.63/w)*pi/180.where pa_max is the maximum PA of the slit (relative to the mask), and w is the slit width in arcsec. For a slit width of 1.0 arcsec, the slit may be tilted by up to 50° relative to the mask position angle. For a slit width of 0.7 arcsec, the maximum tilt is 26°.
The final check to make sure that all your slits and alignment boxes will be correctly milled is to check the blueprints in the slitmask database, once the slitmask has been submitted (see sanity-check your slitmasks)
Yes, we have written an IRAF utility called plotmask which converts these files into a format compatible with the STSDAS IGI graphics program and plots them. Please feel free to download and use the utility.
Elementary, my dear slitmask observer! Proceed as follows:
We have seen this happen in several instances in which mask designers used the t (toggle) function in DSIMULATOR to change program objects (defined as targets with priority codes greater than zero) into guide stars. The t key should only by used to switch stars (i.e., objects with priority codes of -1 or -2) between alignment and guide star.
To fix this, you can simply re-generate your slitmasks by using the text files generated by DSIMULATOR as input. The text file will correctly list the priority codes for the guide stars as -2, and so when you use this as input to DSIMULATOR it will generate appropriate alignment boxes for these targets.
Slider 3 is better behaved than Slider 4 in terms of flexure. Therefore, we recommend to install your higher priority grating in Slider 3.
Yes. Due to the large acceptance angle of the DEIMOS camera, all gratings may produce zeroth-order ghosts at some grating tilts. The 830G and 900ZD gratings are particularly prone to this at typical grating angles. These gratings are certainly usable with slitmasks, but observers using these gratings should be prepared for ghosts in their data. Please read the notes on ghosting with the 900ZD grating and heed the advice on how to acquire calibrations with the 830G and 900ZD gratings.
No. Variations of order 10% are observed from the top to the bottom of the detector, probably as a result of changes in anamorphic factor with slit position.
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. Please refer to DEIMOS drawing 5303 for dimensions and tolerances.
The original DEIMOS V-band and BAL12 filters were damaged in 2004 and had significant scratches. The BAL12 filter was refurbished at UCO/Lick and returned to service in 2006. A new V-band filter was installed and commissioned in December 2015. Therefore, it is possible to use both the BAL12 and V-band filters.
Images from the DEIMOS mosaic camera are stored in a format called multi-HDU FITS (as opposed to the “simple FITS” format employed on earlier-generation Keck instruments such as the original HIRES, LRIS, ESI, etc.) and must be opened in a different manner in order to read the entire image into ds9. Instead of simply using using the option
File > Openwhich will display only the first image extension, you'll need to use
File > Open Other > Open Mosaic IRAFto view the entire image.
For IDL, please see the program deimos_read_file.pro distributed as part of the DEEP2 DEIMOS data reduction package. In IRAF, see the MSCRED package. The Keck IRAF package includes a task called deimos.dmosdisplay which uses mscred.mscdisplay to display the full DEIMOS image on the IRAF image display client.
Likely the deiccd keyword "autopane" was set to an incompatible value (FullFrame, MaskArea, AlignArea) and not Manual.
Alternatively, you can use the script:
set_detector_configthat can be used to automatically configure the binning.