CHAPTER 10 FLATFIELDING Flat fielding is accomplished with option AR operating on the general equation: OUTPUT=(RAWFRAME-bias-DARKFRAME)*normalization/FLATFIELD. The "bias" is simply a number representing an average intensity for a subset of pixels (say in a 50 by 50 area) taken with no light entering the system and taken immediately after the exposure on the object of interest (RAWFRAME file). The DARKFRAME file, which contains the spatial variations about the zero level, can be obtained by averaging several frames taken with no light entering the system. Each of these frames must have its own "bias" subtracted from it before the average is computed. DARKFRAMES of long exposure may have too many cosmic ray defects or may be impractical to obtain. It is then appropriate to average many short exposure dark frames and add on an estimated dark current value. The FLATFIELD file is obtained by exposing on a spatially uniform source of light. Exposures on the inside of telescope domes generally are only uniform to a part in 100 and are not recommmended. Exposures on the twilight sky should be long enough (at least 10 seconds) to avoid shutter vignetting effects. Exposures on the night sky and also on the twilight sky will contain star images which must be removed. A median filter is not recommended for this operation since it does not remove the faint outer wings of stars which overlap on more than half the exposures for a given pixel. The most accurate way to remove stars is by modeling the light distribution with options PR or EA and subtracting the result with option FG. Note that the working flat field file should be temporarily flat fielded by another approximate flat field for the modeling operation to work. Sometimes, after modeling and subtracting, the centers of bright stars will have unacceptable residuals. The option OF can then be used to perform a two-dimensional linear interpolation on the areas affected. The areas can be selected with the Grinnell cursor box; either the entire area within the box can be written over or just the values which you have set to zero (with options EP, EC, or ER). F2 The box filters can clip out points > a specified multiple of the RMS and replace with a median or mean. BK Fits a two-dimensional surface to the map For IPCS spectra vignetting: RS will scale rows with the averages stored from VP and FL. To preserve (roughly) counts when dividing by a flat field: NO will normalize the flat fields to an average of 1.0 For flat fields which are out of register (but not warped): XY will shift by fractions of pixels in x and/or y. RO will rotate thru any angle. Page 2 And for bad edges on the fields: CU will trim them ME will merge the trimmed pieces.