LRIS Observing Procedures


Draft
There are some observing procedures that we (the DEEP group) use to reduce the effects of fringing and to obtain calibrations needed for the prototype reduction pipeline.

Calibration Mask

In order to solve for the alignment of various optical elements in LRIS, so that the pipeline software can run easily, it is necessary to use the calibration mask "cal_grid.eng" with as many different gratings, tilts, and orders as practical. Below is a typical data set:

For each grating, use appropriate internal arc lamp(s). One exposure is sufficient; slight saturation is not a problem:

Do this for at least 2 gratings, preferably 3.

Also get image with the mirror in imaging mode (zeroth order).

Dome Flats

When dome flats are available (needs OA) get the following images:

Wavelength Matching for Best Fringe Division

The LRIS CCD finges at a level of +/- 5% or so in the red. The fringe pattern is imprinted on the pixels, and moves through a full phase in about 50 Angstroms, meaning that the wavelength scale of dome flats must agree very closely (ie, << 50 A) with the science images in order to remove fringing to a high precision. Unfortunately, LRIS flexes in both the spatial and wavelength directions, resulting in wavelength shifts (bad phase match) and spatial shifts (different slit illumination) as the telescope is moved around the sky. The following procedure can be used to reduce these uncertainties.

The general procedure is this: from each science image (or set of images, using the same mask), use the night sky lines to figure out where a given arc line will fall as seen through a particular slit when the grating is tilted at the same angle. (Example for 600/7500 grating, using night sky lines at 7572 A and 7316 A to predict location of 7635 Ar I.) Then, at the end of the night, return to the nominal setup used for the science observations -- that means similar elevation (EL), physical rotator angle (ROTPPOSN) and grating tilt (GRANGLE) (these are all found in the image headers). Using the arc lamp, note where the chosen arc line in the chosen slit falls. If it doesn't land in the right place, tweak the grating angle (GRANGLE) to move it to the right spot. When it is close enough (within about 1 px or so), you have recaptured the same configuration as well as you can. (HINT: if you know how to window the CCD, reading out onlyi, say, 100 rows encompassing the chosen slit really speeds up this process!)

PROBLEMS: The encoders on the gratings seem to be fairly coarse, so small GRANGLE moves will be ignored. You may find it best to set GRANGLE to a very different value and then reset to the updated value. This can be a very time-consuming process.

How much to change GRANGLE? I have a couple of numbers in my head, namely, 0.00192 degrees/A for the 600/7500 grating, and around 0.004 deg/A for the 1200/7500 grating. I should calculate these in terms of pixels sometime ...

(oops, got to go for now ...)

Standard Disclaimer

These procedures were developed by Drew Phillips and others at Lick Observatory for the private use of the DEEP team at UCSC. They are not official Keck procedures, so please don't bother Keck folk with questions/complaints relating to the above.
Andrew C. Phillips / Lick Observatory
Last modified: 18 sep 2001
phillips@ucolick.org