Observing Strategies

What's the best way to start a science exposure with DEIMOS?

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.

Is there a good way to check the seeing?

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.

Should I check my slitmask alignment between exposures?

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.

Is it okay to move the various DEIMOS stages during readout?

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.

What is the minimum allowed slit width on a DEIMOS mask?

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).

What is the maximum allowed tilt for a slit on a DEIMOS 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°.

How can I know if my mask design is correct?

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)

DSIMULATOR generates MONGO plot files that I would like to view, but I don't have MONGO. Is there another way to plot these files?

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.

I received a message indicating a problem with my mask, and giving me the BSLIT value. How can I figure out which object is in that slit?

Elementary, my dear slitmask observer! Proceed as follows:

  1. Note the BSLIT value indicated in the error message.
  2. Log in to the slitmask database
  3. Click on the button Show Me My Mask Inventory
  4. In the mask list, locate the mask of interest and in the rightmost column click the Detail button to get the details page.
  5. In the BluSlits table, search the bSlitId column for the BSLIT number you are seeking.
  6. In the dSlitId column, note the corresponding DSLIT value.
  7. In the SlitObjMap table, search the dSlitId column for the DSLIT value.
  8. Note the OBJECT value in the ObjectId column.
  9. Click the hyperlink under ObjectId to go to the corresponding entry in the Objects table.
  10. The entry in the Objects table indicates the name and characteristics of the object in the slit.
Please note that multiple objects could be assigned to each slit.

The alignment boxes on my DEIMOS slitmasks got changed into slits by DSIMULATOR -- what happened?

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.


Which slider should I put my grating into?

Slider 3 is better behaved than Slider 4 in terms of flexure. Therefore, we recommend to install your higher priority grating in Slider 3.

Is ghosting a significant issue with any DEIMOS gratings?

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.

Is the DEIMOS resolution expected to be constant across the field of view?

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.


Can I use LRIS filters in DEIMOS?

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.

If I want to bring my own DEIMOS filter, what are the physical constraints on its size?

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.

Can I use the V and BAL12 filters for my run?

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.


I tried opening my DEIMOS image with ds9 but it only displayed a small part of it. Why?

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 > Open
which will display only the first image extension, you'll need to use
	File > Open Other > Open Mosaic IRAF
to view the entire image.

Is there a handy routine that will read my full DEIMOS image into IDL or IRAF?

For IDL, please see the program 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.

I tried to bin my spectra, but no image was taken. Why?

Likely the deiccd keyword "autopane" was set to an incompatible value (FullFrame, MaskArea, AlignArea) and not Manual.

  1. Set autopane to manual via the dashboard or modify -s deiccd autopane=manual from a deimosserver xterm.
  2. Re-set your binning and window mode
  3. Take your images
For complete info on autopane, please see the Autopane Webpage.

Alternatively, you can use the script:

that can be used to automatically configure the binning.