Background

It is highly desirable to obtain standard star spectra on every available clear night with DEIMOS, to track the degradation of the silver coatings on the collimator mirror and the tent mirror. To ensure uniformity and intercomparability, the following procedures should be followed.

The observations are taken in slitless mode with no guiding. Slitless mode is feasible for a roughly 20 minute period starting about 25 minutes after sunset and extending until 5 minutes past 12 degree twilight. Similar rules with opposite sign apply at sunrise.

No arcline calibrations are needed.

Observing Procedure (Automated)

An automated script can be used to acquire the standard star spectra for certain common spectrograph configurations (see shaded items in the configuration table below). Follow these steps:

1. Insert the desired grating.
2. Select the appropriate standard star for your current observing time and slew to the object. This will generally be written on the whiteboard by your Support Astronomer. Note: coordinates for all of the DEIMOS throughput standards are in the file /kroot/starlists/0000_Throughput_standards.list.
3. Have the OA set the DEIMOS rotator to the appropriate SKYPA as shown in the configuration table below.
4. Have the OA set the pointing origin to REF, which will place the star in the center of the pickoff mirror. (Note: the script will move the star to the location where the slit would be, so it will disappear from the guider once the script stars running).
5. Acquire data by typing the following command in a polo or vm-deimos xterm window:

    observe_flux_standard

Built into the script are the appropriate settings for wavelength, filter, and integration time for each grating. The script will:

• verify that the telescope is pointed to a known DEIMOS flux standard star by checking the DCS keyword TARGNAME
• check the current grating and build appropriate exposure list
• save the current DEIMOS configuration
• verify correct pointing origin and SKYPA for this star
• configure DEIMOS for slitless spectroscopy
• acquire a series of 3 exposures using various combinations of filter and central wavelength (note: if appropriate order-blocking filters are not loaded then the script may skip one or more of the three exposures)
• restore the original DEIMOS configuration

The running time will be about 10 minutes for the 1200-line grating, slightly less for others due to shorter exposure times.

Observing Procedure (Manual)

1. Select a suitable star from the list below. Please try to keep the airmass below 1.5 if possible.
2. Tell the OA to move the telescope to the star coordinates. The stars are all in the standard star list available to the OA (/kroot/starlists/0000_Throughput_standards.list), so you need specify only the star name. However, you do need to specify the SKYPA. This rotation is needed to keep the spectrum of the standard star (taken in slitless mode) from overlapping that of other stars in the field. Tell the OA to place the star at the REF pointing origin (middle of the pickoff mirror in the lower third of the guider image.)
3. A table of potential gratings, tilts, and filter combinations is given below. These have all been observed before, so any one of them is admissable. However, there are certain combinations that have been heavily used and have become "standard." These are indicated by * and ** in the table below. Use these if the current spectrograph setup permits.
4. While the telescope is moving into position, choose the desired grating, grating tilt, and filter on the GUI and move them into position in DEIMOS. Set the exposure time corresponding to the star from the table below and select full-frame spectral readout for the CCD.
5. When DEIMOS is at the proper SKYPA and the star is visible in the guider, identify the field using the standard star finder charts (click on the star name in the list below to view the chart). Note that the field will in general be rotated from the charts, which always have North up.
6. Now ask the OA to move the star to the SLIT pointing origin. This places the star in the slitmask opening at the Y location of the longslit and at X location such that the spectrum should appear at position X ~ 5100px on the CCD mosaic. No guiding is necessary.
7. Take the first exposure using the indicated exposure time. When it has read out, verify that the spectrum is visible at the proper X location in the image. Using the "region/project" option in DS9, plot a horizontal cut across the spectrum at its brightest part (near the CCD gap) and eyeball the brightness of the sky background and the peak brightness of the spectrum above background. The sum of the total should not exceed 60,000 counts, and the peak brightness of the star should be around 5-10,000 counts per pixel.
8. Adjust the exposure time to meet these requirements and retake the exposure if necessary.
9. If time permits, take more exposures using other combinations of tilts and filters with the same grating. If yet more time is available, change gratings and follow the same routine. If one is doing more than one setup, it is helpful to write out a script in advance.

Preferred DEIMOS Standard Stars

Most frequently used standards are starred. If a standard in this list is not available, then choose one from the usual Keck standard star list. Any star is better than none.

Keck-formatted starlist of the above table

Standard Spectrograph Configurations

Starred values are preferred. Those shaded in green can be completed using the automated script.

See Also

Observation log