Straylight Evaluation : Field

PROCEDURE :  -

TITLE : Straylight Evaluation : Field

CATEGORY : Verification

PURPOSE :

The main goal is to determine the sources of external straylight, particularly from sources outside the nominal field of view, with the aid of inverse field masks. The morphology of the scattered light on the HRC and WFC chips, and their relative contribution to the background levels will be determined.

FREQUENCY :

Execute once.

DETECTOR :

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

PREREQUISITES/BACKGROUND :

In preparation for the Ball campaign in June 2000, G. Hartig summarized some of the expected effects in SER OAT-066 for the baffle efficacy and the proposed observations. This was the fourth item on his list :

4. Baffle efficacy

Straylight from other reflections, particularly from sources outside of the nominal field of view, can be assessed with the aid of an inverse field mask installed on the RAS source plate. The nominal FOV (and a small border around it) is dark and the surrounding field is bright. This is produced with a transmissive diffuser (e.g. opal glass) illuminated from behind with fiber lights, and a pair of masks to cover any pinholes and assure a very dark field. Care must be taken to minimize illumination of the back (RAS-facing) side of the mask by scatter within the RAS enclosure. The RAS source plate may require adjustment to center the mask. Masks with 0.5, 1.0 and 2.0 mm borders around the nominal FOV are available. Differences between images taken with the various masks may be useful for identifying the source of any scatter that is detected.

Examples of straylight patterns obtained through various filters (and CLEAR) and with 0.5 and 1.0 mm inverse field masks are shown in Figs 1 and 2 below (see (gc1998261.log). Straylight is generally detected along all four edges of both detectors. Additionally, a diagonal streak is seen in the WFC data in the amp C corner (upper-left).

• Figure 1 : HRC#2 ID 887,890,891,892,893,894,895,896,899 for various filter and field mask combinations.
• Figure 2 : WFC#1 ID 885,886,888,889,897,898,900 for various filter and field mask combinations.

HARDWARE REQUIREMENTS :

ACS is in the normal RAS/HOMS configuration with the default HeNe laser wavelength of 6328 Ang. The inverse field masks and a transmissive diffuser are available and ready to be inserted at the RAS source plate location. The intensity of the fiber illumination is adjusted appropriately.

SOFTWARE REQUIREMENTS :

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

COMMAND MODE : Real time.

PROGRAM/EXPOSURES :

The inverse field masks will be successively inserted in the light path and exposures of varying exposure times will be acquired, followed by exposures with no field masks, for comparison. All images will be obtained at the standard RAS/HOMS wavelength of 6328 Ang through the F625W filter.

TOTAL EXPOSURES:

Approximately 10 WFC and 10 HRC frames are necessary for a successful completion of this calibration item.

TOTAL TIME :

The minimum time to acquire all the images is ~2 hours. But realistically, possible overhead increases the total time to ~8 hours.

ANALYSIS :

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 ACS.