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Last updated
20 December 2001 11:59:05

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Straylight Evaluation : Point Source


TITLE : Straylight Evaluation : Point Source

CATEGORY : Verification


The main goal is to determine the sources of straylight, the morphology of the scattered light on the HRC and WFC chips, and their relative contribution to the background levels. Saturated and unsaturated RAS/HOMS PSFs will be scanned across and along the chips' edges and in the gap separating the two WFC chips.


Execute once.


Flight build detectors HRC#1 (gain=2, offset=3) and WFC#4 (gain=1, offset=3).


The RAS/HOMS fiber spots for the HRC and WFC have been aligned correctly and their intensities adjusted uniformly. The focus and tip-tilt SMS procedures have been successfully executed and the corrector mechanisms are set at their nominal positions.

Original Mask Edges

The straylight paths in ACS have been extensively studied in past campaigns. In preparation for the Ball campaign in June 2000, G. Hartig summarized some of the observed effects in SER OAT-066 :

1. Scatter from corner of housing (HRC)

Light reflected from the CCD surface up into the D amp corner of the HRC housing cover scatters back onto the CCD. Examples are shown below in Fig 1 (ID 1229-1230) and Figs 2-3 (ID 1233-1236) acquired on 12 Nov 1998 (see gc1998316.log : HRC#2 0.1 sec exposures through F625W). The diffuse scatter can be seen over the entire field, shadowed at the edges by the CCD mask when a bright, saturated PSF is placed at the field centre or near the A-C and B-D edges. A black mirror finish replaced the previous gold finish to supress this effect. The effectiveness of the new coating will be evaluated by obtaining RAS/HOMS images of a point source, and estimating the ratio of the background scatter level to the flux in the (saturated) image. ND filters can be used to determine the approximate point source intensity. Images will be obtained at a variety of field positions.

2. Scatter from edge of mask knife edge (HRC)

Light was found to scatter, due to grazing incidence reflection, off of the edge of the HRC CCD mask, especially on the top (C-D) and right (B-D) edges. Examples are shown in Figs 4-7 (ID 1052-1071) below, acquired on 6 Nov 1998 (see gc1998310.log : HRC#2 0.1 and 10 sec exposures through F625W).A RAS/HOMS point source (A10 fiber) was scanned across each edge. The effectiveness of the new mask design, which inverts the knife edge on the top and right edges, and is sharper and black-coated, will be investigated in a similar manner for both the HRC and WFC.

3. Scatter from back of mask knife edge

Another scatter path involves reflection off the top edge of the CCD onto the back edge of the original mask knife edge, which was angled in such a way as to reflect the light back onto the CCD. For example, see Fig. 8 for HRC images 1231-1232 ( gc1998316.log). The problem was also seen in the WFC on the bottom and left edges of the field (opposite to the HRC, due to the differences in the CCD orientations) as streaks along those edges in flat-field illumination, as seen in Fig. 9 for WFC#1 ID 882-884 (17 Sep 1998), gc1998260.log). This has been corrected by angling the offending knife edges in the opposite direction and coating them black. The effectiveness of the new design will be investigated in similar fashion on both HRC and WFC.

Corrected Mask Edges : BATC Jun 2000

The most recent straylight evaluation was performed during the RAS/HOMS campaign in June 2000 (HRC#1 : gc2000155b.log, WFC#3 : gc2000157b.log) :


In Fig. 10, shadowing by the edge of the CCD mask on the HRC is still evident at the top of the field (C-D edge) when a saturated PSF is placed at the chip centre (compare with Fig. 1), even with the black coating. Its intensity is significantly reduced in comparison to the previous gold finish. In Figure 11, the A10 fiber spot was scanned vertically down from the top of the chip along column X=536 and at rows Y=955,935,914,892,873,853,832,812. The dragon's breath is most prominent between rows 832 and 935. In Figure 12, a saturated PSF was placed near each amplifier corner. The dragon's breath is evident at the top of the chip (amps C and D). The amp B image shows a remarkable doughnut-shaped feature in the upper-right quadrant as well as an apparent double image of the Fastie finger. In Figure 13, the A10 PSF is scanned vertically upward over the HRC C-D mask edge along column X=536 and at rows Y=1056,1061,1064,1067,1073. The dragon's breath is most prominent in the intermediate positions.


In Fig. 14 (ID 14269), a WFC image is displayed with saturated fiber spots. All nine fiber spots possess a doughnut-shaped ghost that stretches radially away from the central fiber A1. Some consist of two overlapping doughnuts, such as the A3, A4, A7, and A8 ghosts. The horizontal and vertical "spider" crosshairs from the pupil mask are also visible in some doughnuts. The ghosts are spatially offset from their respective fiber spots except for spot A1 whose ghost lies on top of it. Additionnally, two fainter and smaller "linked" doughnuts are seen between A1 and A7 (bottom-left quadrant). These dougnuts are probably caused by multiple reflections between the dewar window and the filter above it. The interference fringes in the doughnuts could be due to internal reflections between the substrates of the filter. The radial elongation and offset of the doughnuts may result from a non-normal incidence of the rays away from the field centre.

The fiber spots were shifted downward to find evidence of the dragon's breath along the A-B edge. The sequence of images is shown in Fig. 15. A very slight dragon's breath from knife edge scattering is detected for the N5 fiber spot (lower-right amp B corner) in Fig. 16.


ACS is in the normal RAS/HOMS configuration with the default HeNe laser wavelength of 6328 Ang. The ND filters are available and ready to be inserted in the light path.


IDL and/or IRAF routines to view the images and evaluate the contribution of straylight.

COMMAND MODE : Real time.


The A10 fiber spot will be scanned across the HRC field and its edges and near the four amplifier corners. PSF scans along and across the Fastie finger will also be performed. Similarly, the nine fiber spots will be translated across the WFC chip and its edges and the central fiber A1 across the central gap. The PSFs will be saturated and masked with ND filters to evaluate their intensities and the background levels. All images will be obtained at the standard RAS/HOMS wavelength of 6328 Ang through the F625W filter.


Approximately 40 WFC and 40 HRC frames are necessary for a successful completion of this calibration item.


The minimum time to acquire all the images is ~7 hours. But realistically, this number should be roughly doubled to account for overhead to translate the fibers, test exposures, contingencies, etc..., for a total of ~14 hours.


The location, morphology, and intensity of the straylight patterns will be analyzed with IDL and/or IRAF routines. These parameters will be correlated to the geometry of the detectors and their housing.