F814W Flat:

This FLAT is an improved version of the Investigation Definition Team (IDT) F606W flat, e391433bu, where a correction has been applied to spatial scales between ~7 pixels and 800 pixels. The resulting flat is effectively the F606W preflight thermal-vaccuum flat, with an improved on-orbit illumination correction applied to scales >7 pixels. On the very largest scales, the chip-to-chip normalization of the IDT flat, e391433bu, has been preserved.

The improved illumination correction was derived using F502N EARTHFLAT's, and the previous F502N IDT flat, e3809359u, as follows:

The following F502N EARTHFLAT's were used. These are all 1.2 sec. exposures taken on shutter "A", and are corrected for shutter shading.

Dataset         Dateobs.  Days from nearest decon.

u2a31403t       94 Apr 28    +4
u2a31504t       94 May 4     +11
u2a31c03t       94 May 9     +16
u2a31g03t       94 May 16    -7
u2a31j03t       94 May 21    -2
u2a31n03t       94 May 28    +5
u2a31r03t       94 Jun 4     -9
u2a31w04t       94 Jun 9     -4
u2a31z04t       94 Jun 17    +4
u2a32503t       94 Jun 25    +12
u2a32b04t       94 Jun 30    -10
u2a32j04t       94 Jul 16    +5
u2a30r03t       94 Jul 31    +3
u2a32r03t       94 Jul 31    +3
These were combined using the STSDAS/IRAF task XSTREAKFLAT. The PC pixels of the resulting streakflat were then scaled by the ratio of the WF/PC pixel areas (4.7835) to bring the PC up to the intensity of the WF CCD's.

This streakflat was then normalised and inverted by running it through the STSDAS/IRAF task NORMCLIP, with the maximum threshold set to 10. Bad columns and pixels were then removed by interpolation using the WFIXUP task.

This image was then divided by the IDT F502N flatfield, e3809359u, to form an improved illumination correction image.

Single pixels in the correction image which differed from surrounding pixels by more than ~2% were reset to the local mean. This was an effort to minimize the impact of bad pixels on surrounding data. (These discrepant pixels were also marked in the data quality file for the output flat.) The result was then smoothed by convolving it with sigma=3 pixel Gaussian function over most of the image; near the pyramid vertices smoothing was done in only one dimension (parallel to the vertice).

Each CCD was then divided by the average pixel value in it's central 400x400 region, (PC=0.8918, WF2=0.8821, WF3= 0.8670, WF4=0.8646), so as to set the mean correction to unity over the center region of each CCD.

The final step was to multiply this illumination correction image into the current F814W reference flat, e391434fu. Regions of the output flat exceeding 2.0 were also marked in the data quality file as being suspect (regions in pyramid shadows).

The RMS (over central 600x600 pixels) and maximum corrections applied to the IDT flat are as follows:

Group		RMS Correction		Max.  Correction
1                   0.2%                      2%
2                   0.2%                      1%
3                   0.3%                      4%
4                   0.4%                      2%
In general, the largest corrections occur near the CCD edges. Other features are an apparent shift in the position of the bright diagonal X on all CCDs. This must be caused by a change in the alignment of the camera optics.

The corrections were checked by dividing both the old and new flat by a skyflat (smoothed with sigma=30 pixel Gaussian) provided by the Medium Deep Survey group. The illumination corrections appear to give some improvement in the agreement between the flat and skyflat, especially on WF3:

Group		Range (peak-to-peak) of difference from skyflat.
                      Old Flat              New Flat

1                        3.8%                  3.6%
2                        1.8%                  1.2%
3                        7%                    2.0%
4                        1.6%                  1.8%
This work performed by John Biretta and Michael S. Wiggs, 14 Dec. 1995.

Copyright © 1997 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.