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.
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 F606W reference flat, e391433bu.
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:
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:
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.
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.
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.