90prime Flat Images in May/June 2004

C. W. Engelbracht; last updated 2004-7-8

Summary

                  3  4
chip layout:
                  1  2

• The edges (~5% of the array width/height) and corners (~20% measured corner-to-corner) of the field of view (FOV) are vignetted - the fully illuminated area measures about 85% of the FOV by area, and the signal drop to the edge of the field is about 40%.
• It is difficult to remove sources from dark-sky flats without at least 8-10 sky images (depending on wavelength) to combine.
• Twilight flats make a good substitute for dark-sky flats, although there is some variation in the illumination pattern at the corners of the field, possibly due to scattered light in the twilight flats.
• The twilight flat variation in the corners of the field is worst on chips 2 and 3.
• The dark-sky flats changed slightly between May and June due to elevated dark current on chip 3 and variability in the dark current on chip 4.
• The dark-current variation discussed here can bias the flats if not corrected for. This effect was not seen in the May data, but is visible in the June data.

Data

These data were obtained during the May and June 2004 runs with 90prime. Twilight flat observations were generally obtained in sets of 5, with a 1-arcminute dither between observations. Dark sky flats were assembled from any nighttime data which did not contain a science target and consist of 4 to 20 images, depending on the chip and filter used. The images were overscan-, zero-, and dark-subtracted, then combined using the iterative technique described by Morrison et al. (1994, AJ, 108, 1191). The data are presented in Table 1, which also contains a ratio of the twilight and night-sky flats (except for the May U band data, where no twilight flat was taken).

Table 1
Date	Filter	Twilight Flat	Dark Sky Flat	(Dark Sky) / Twilight	#images in dark sky flats
		display range 0.7 - 1.3	display range 0.7 - 1.3	display range 0.9 - 1.1	chip 1/2/3/4
May 2004	U				10/15/17/9
May 2004	B				8/10/12/4
May 2004	V				4/6/8/4
May 2004	R				4/6/8/4
May 2004	I				4/6/8/4
June 2004	U				10/20/20/10
June 2004	B				8/16/16/8

Variation With Time

The flat fields changed between the May and June runs, mostly in the vignetted corners of the FOV. The ratios of May and June dark-sky flats (May / June) are plotted in Table 2. The display range in each case is 0.9 to 1.1.

Table 2
Date	Filter	Ratio
May/June	U
May/June	B