Lick Infrared Camera User's Guide

Reduction Technqiues

A Suggested Reduction Sequence

The usual goal of the reduction process is the removal of dark current, cosmetic defects, pixel-to-pixel response variations, and sky contribution from individual images. Targets may be photometrically calibrated. Finally, individual images are combined. Most reduction strategies suggested by various `experts' for the treatment of near-IR array data follow the same general theme as the one presented below.
  1. Dark Subtraction A master dark frame is created by averaging a number of dark frames of a given exposure time. Dark frames may be scaled with caution, remembering not to scale the bias if it is present. The master dark frame is subtracted from all source and calibration frames with the same exposure time.
  2. Bias Subtraction If baseline subtraction was disabled (default condition for LIRC-II), dark frames will have included the ADC bias, and it will be automatically removed in step one, above. Separate bias subtraction may be done, using a master bias frame made by averaging a series of `zero' second darks. If baseline subtraction was enabled, the bias will have been removed from the raw frames at the time they were made.
  3. Removing Cosmetic Defects Hot or dead pixels should now be removed from both sky and source frames. If allowed to remain, spurious counts in bad pixels may affect the flat-field renormalization. The customary procedure is to interpolate over bad pixels. A .i.bad pixel map; can be generated from a single suitable sky frame at the beginning of the data reduction.
  4. Flat-fielding Flat field frames, be they twilight or night-sky frames, are averaged, using a median filtering or sigma clipping algorithm to remove any stars. It is assumed here that individual images, or sets of images, were offset from one another, so that the same star falls on different pixels in different frames. Each source frame is divided by the flat-field, and renormalized by multiplying by the mean of the flat-field frame.
  5. Sky Subtraction Master sky frames are made for each observation by averaging and filtering its associated set of sky frames. The master frame is then subtracted from each source frame. Bear in mind that in many cases, source frames, taken at slightly different positions, can serve as one another's sky frames.
  6. Coaddition The individual, reduced source frames are now combined, registering them as necessary, to create a final source frame with the best possible signal to noise. The use of a clipping algorithm here will eliminate cosmic ray events.
  7. Flux Calibration For absolute photometric work, the standard-star frames are reduced exactly like source frames. The data are then calibrated using the known standard star magnitudes and converting them to observed counts as in visual photometry.

Quick Looks

The foregoing reduction recipes are not suited for quick looks at new images while at the telescope. Simply subtracting a raw sky frame from a raw image frame of the same exposure time provides an adequate first look. Where the signal-to-noise in individual images is low, it may be necessary to coadd and average a series of sky-subtracted, co-aligned frames.

Lirctop provides a sky-subtraction procedure under its `quick-reductions' option.

Registering Images

A single image in its final form is usually the sum of a number of reduced source frames. Source frames are likely to be offset from one another due to small errors in telescope tracking, deliberate offsets, or mosaicing to cover regions of sky larger than the detector format. Images must be properly registered when being coadded or fitted into a mosaic.

One or more bright point sources, common to adjacent frames, serve well as registration marks. Frames without such reference points must be registered on the basis of telescope coordinates. It is therefore critical to have accurate records of the telescope's position for each offset. Note that the FITS header includes the telescope position at the time of the observation, so long as `telco access' is enabled in the data-taker. While absolute positions recorded in this way are only as good as the pointing calibration of the telescope, relative positions within a limited field are quite accurate.