AUTHORS : A.R. Martel, G. Hartig, J. McCann.
LOCATION AND DATE : GSFC (SSDIF) : Sep 15, 2000
Determine the correct focus position of the M1 mirror at a single field position for SBC and HRC Build 1 using RAS/Cal and a D2 lamp. The relative focus between the HRC#1 and SBC will serve as a baseline once the HRC flight build detector is integrated in the instrument.
ACS is located in the SSDIF at GSFC and integrated in the TBF. The illumination is provided by a Hamamatsu D2 lamp (with a strong Hg 1849 Ang line which dominates in the SBC) and a 1" MgF2 lens feeding a 4 um pinhole in RAS/Cal. All HRC images are 1.0 sec each, full-frame, and read-out with Amp B with a gain of 1 and offset of 0 through the F435W filter. The SBC images are 200 sec each, full-frame, and through the F150LP filter. The HRC sweep images have entries 19435 to 19450 and the SBC images, entries 19452 to 19459.
First, a correction for coma in the HRC images is performed, followed by the focus sweep. The initial focus is set at the optimal position determined with RAS/HOMS during the Ball campaign in June 2000. All focus positions are reached along a positive direction and the first position is overshot to remove any hysteresis. The encircled energy within a diameter of 0.15 arcsecond is measured and plotted for each focus position. The best focus maximizes the encircled energy within this reference diameter. The SBC sweep is centered on the best HRC focus determined previously. Two IDL procedures automatically analyze the focus sweeps : 'qfit' and 'cmfscan'.
Typical RAS/Cal PSFs for HRC and SBC are shown in Figs 1-2 below. For the HRC frame, the background level is essentially identical to a bias or short dark frame (~3330 DN) and for the SBC, it is null. The HRC and SBC focus sweeps are shown in Figs 3-4. The optimal HRC focus is at a position of 2600 and the SBC at a position of 2517.
CEI SPECIFICATIONS :
The optimal HRC focus is at a position of 2600 and the SBC at a position of 2517.
Below, George Hartig and Mark Erickson are preparing the instrument for the confocality test. These images were taken with the SSDIF Live Web Cam.