Geometrical Field Distortions

PROCEDURE :  -

TITLE : HRC and WFC Geometrical Field Distortions

CATEGORY : Calibration.

PURPOSE :

The goal is to map the geometric distortions of the two flight build detectors with the OTA-like illumination of RAS/HOMS with a Ronchi checkerboard ruling. Also, the plate scale differences between orthogonal axes will be measured.

FREQUENCY :

Execute once with the flight build detectors. Repeat if any changes are made to the instrument that would affect the FOV or location/alignment of the chips.

DETECTOR :

Flight build detectors HRC#1 (amp C) and WFC#4 (amps ABCD).

PREREQUISITES/BACKGROUND :

This calibration item was performed on June 19, 2000 at BATC with the non-flight build detectors HRC#1 and WFC#3 (see HRC#1 and WFC#3 Field Distortions). Four HRC and three WFC images were acquired through the F606W (HRC) and F625W (WFC) filters at the HeNe wavelength of RAS/HOMS (6328 Ang). The Ronchi ruling was displaced by one grid pattern between each exposure. The exposure times were 5.0 sec for HRC and 0.5 sec for WFC.

HARDWARE REQUIREMENTS :

The Ronchi ruling and a piece of Mylar transmissive diffuser, taped to one side of the ruling and facing the illumination, will be placed at the location of the RAS focal plane. An operator is suited in the clean room to adjust the micrometer stage. The illumination will be provided by the HeNe lasers (6328 Ang) through the F606N narrow-band filter for both the HRC and WFC channels to minimize chromatic aberration that limited the contrast in the June test with the broader filters (F606W and F625W).

SOFTWARE REQUIREMENTS :

IDL to view the images and for data analysis.

COMMAND MODE : Real time.

PROGRAM/EXPOSURES :

The test will be repeated in a nearly identical fashion as in the June 2000 campaign. After a few trial images to estimate the correct exposure times (equivalent to ~30000 DN), focus, and contrast, the images will be acquired in five sets, each consisting of one HRC and one WFC image. The Ronchi ruling will be offset by one grid pattern between each set with the micrometer stage and a new set will then be acquired at the new position. Bias frames will be acquired before and after the full sequence.

The test will be repeated a second time for at least one of the detectors with the Ronchi ruling rotated by 90 degrees from the original orientation to verify that there is no plate scale differences in the mask (i.e. the mask consists of true squares, and not of rectangles that are nearly squares).

VARIANTS :

If time allows, observe a full sequence of both HRC and WFC frames with the mask rotated by 90 degrees.

TOTAL EXPOSURES:

A minimum of 5 HRC frames and 5 WFC full frames will be acquired. Additional data will be taken with the mask rotated by 90 degrees, if time permits.

TOTAL TIME :

The execution time is dominated by overhead to configure the instrument, adjust the focus, contrast, and exposure times, move the stage holding the mask, and read-out time. In June 2000, the entire test took 3.1 hours, including ~2 hours for verification of the focus and contrast. The data taking itself took 740 sec = 12.3 minutes for one HRC+WFC set followed by a translation of the ruling. For one full sequence of five sets, the exposure time is then ~1 hour. If the test is repeated with a rotated mask, then the minimum total time doubles to ~2 hours. Including a focus and contrast check, the total execution time becomes ~4 hours.

ANALYSIS :

The analysis will be performed by D. Lindler with the IDL procedures used in reducing the data from the June 2000 BATC campaign.