AUTHORS : A.R. Martel and G. Hartig
The primary goal is to measure and compare any changes in the focus and alignment of the flight build detectors HRC#1 and WFC#4 in the pre- and post-acoustics campaigns at GSFC. The optimal focus is determined at five field points in the FOV of the two detectors with RAS/Cal mounted on RAMP and the results between the two campaigns are compared.
The pre-acoustics campaign was conducted on Mar 15-20, 2001 and the post-acoustics session on Mar 30-Apr 2, 2001. The first day of the pre-acoustics campaign was dedicated to troubleshooting the software and hardware. Generally, one day was dedicated to measuring the focus of the five field points of one detector. On Mar 20, the pre-acoustics measurements of WFC were repeated to assess the stability and quality of the optimal focii after "bumping" RAMP+RAS/Cal and realigning it with ACS. On Mar 19, the daily session was concluded with an additional measurement of the central field point of WFC (W1). Similarly, on Mar 20 and 30, the H1 point was measured at the end of the five WFC measurements.
The acoustics session itself was conducted on the afternoon of Mar 23 in Acoustics Test Facility (ATF) in Building 10 of GSFC. The procedure is described in detail in the "ACS Acoustics Test Procedure" (Shah, K.C. et al., June 2000). Essentially, ACS is integrated in the ASIPE under N2 purge and piezoelectric accelerometers monitor any mechanical vibrations of the instrument. The instrument is subjected to a composite spectrum of the sound pressure levels in the orbiter's cargo bay at 142 dB for one minute. The results of the test are based on post-acoustic visual inspection, electrical functional tests, and optical verification of ACS.
INSTRUMENT CONFIGURATION :
ACS is configured in the TBF on an optical table inside the SSDIF High Bay Cleanroom of Building 29 at GSFC. The vortex chillers are turned on at ~7:30 AM daily and the detectors are allowed to cool down to approximately -78 C (WFC) and -83 C (HRC) in the following hour. RAS/Cal is mounted atop RAMP in front of the entrance aperture of ACS. A 4 um pinhole in RAS/Cal is fed with a HeNe laser beam (6328 Ang) coupled to a fiber. RAMP permits a computer-controlled translation of RAS/Cal's beam to different positions in the FOV of the two detectors. An assortment of ND filters can be placed in the light path to control the peak intensity. Both ACS and RAS/Cal are covered with sheets of lumalloy to block out external light sources, in particular the cleanroom's ceiling lights which are left on throughout the test. The cleanroom operators for this campaign were Kevin Redman and Vicky Roberts. They moved RAS/Cal to the different field positions, adjusted the flux with the ND filters, and performed metrology measurements for each field point.
In Fig. 1, we show a sequence of images showing ACS when it was returned to SSDIF after the acoustics test and moved from the ASIPE to the handling dolly (Mar 26). Similarly, in Fig. 2, ACS was moved from the dolly to the TBF and placed on the optical table (Mar 27). The instrument is partially masked by the HFMS. These images were grabbed from the SSDIF WebCam.
A detector and field point are specified to the cleanroom operators and RAS/Cal is moved to that position. Test exposures are then acquired to verify the location and peak flux level of the PSF. For the test exposures, the gain, bias offset, amps, and exposure times should be set to the same values as the SMSs (HRC#1: gain=1, offset=0, amp B read-out, 0.1 sec; WFC#4 : gain=1, offset=3, 0.5 sec). Typically, we aimed for counts of ~40000-55000 DN/pix in the PSF's peak to avoid digital saturation (65536 DN/pix). Since the focus sweeps were executed with SMSs at gains of 1 e-/DN (see below) for both detectors, there was no risk of reaching the full well for either chip (~80000 e-/pix for WFC and ~120000 e-/pix for HRC).
Since the HeNe chief ray is angled at the entrance aperture, the PSFs usually suffer from a strong coma/asymmetry. The amount of correction is measured with the IDL routine 'qfit' on the test exposures. This procedure gives the number of steps, as well as their direction, to add to the inner and outer correctors to remove the coma. If the correction is large, the PSF location (X and Y) on the chip may move considerably. 'qfit' also returns estimates of the optimal focus. The resultant corrected PSF will likely have a larger peak intensity so possible saturation should be checked again.
After the coma has been corrected and an initial focus determined, a focus sweep is executed with the SMSs. The duration of the HRC SMS JRHH01A is ~33 min and each of the WFC SMSs lasts ~44 min. Each SMS sweeps through 7 different focii bracketing the initial estimate by stepping the IM1 (WFC) and M1 (HRC) mechanisms longitudinally. Two images are acquired at each focus. These are treated independently, that is no cosmic ray rejection is done. At the end, the focus is returned to its starting value and two final images are taken. Each SMS therefore generates 16 paired images, with images 7-8 and 15-16 at the focus of the initial estimate. In Figs 3 and 4, the RAMP field points are shown for both detectors and their coordinates are listed in the tables of the RAS/Cal description (GSFC : The CDS and RAS/Cal). In this campaign, only the central and four corner field points were imaged.
In Fig. 5, a mosaic of the five field points on the HRC has been assembled. Even though the instrument was covered, the exposures still suffer from contaminating straylight from external sources. The circular ring surrounding the PSF comes from light reflected off the pinhole holder assembly of RAS/Cal. In Fig. 6, a mosaic of the five 400x400 WFC subarrays is shown.
After the execution of an SMS for a particular field point, the optimal focus is determined with the IDL 'cmfscan' procedure. An example of its output is shown in Fig. 7. The cleanroom operators then perform metrology measurements with theodolytes before moving to the next field point. These measurements are necessary to reproduce the exact location of the field points in future campaigns, such as the post-acoustics session. As a precursor to this later session, and to test the reproducibility of the positions and focus, the WFC measurements of Mar 19 were repeated on the following day, but after moving RAMP+RAS/Cal by 2 mm in X and Y and 1 mm in Z and then re-aligning it with ACS.
In Table 1, we list the focus measurements of the WFC field points for Mar 19 and Mar 20 and in Fig. 8, we display their absolute and relative values. Due to the move and re-alignment of RAMP+RAS/Cal on Mar 20, the absolute focus should not be identical for the two days, but the relative focus between the five field points should remain constant. This is exactly what is observed in Figs 8(a) and (b). In (a), the absolute focii between the two datasets have a nearly constant offset, in particular for W1, W7, and W9 (~30 steps). In (b), the relative focii follow the same trend for both sets. The additional measurement of W1 on Mar 19 provides an estimate of the errors expected in the focus. From (a) and (b), respectively, uncertainties of ~15 steps or ~0.7% are representative for each focus measurement. Therefore, the focii between the two days are entirely consistent and lie within these uncertainties (assuming the constant offset is removed in Fig. 8(a)).
The mean absolute focus for Mar 19 is 2045 and for Mar 20, 2013. The total observed range in focii is ~50 steps or 62.5 microns (1.25 um/focus step) between W1 and W5 which can be directly compared to the focal depth. Roughly, the focal depth is the distance from the detector at which point the focal beam has expanded beyond the size of one pixel, neglecting any lens aberrations. At f/25 and 15 micron/pixel, the WFC's focal depth is ~375 microns so the range of focii over the WFC field is within ~16% of the focal depth.
Table 1 : Focus of the WFC Field Points
19 Mar 01
20 Mar 01
In Table 2, the focus of the five HRC field points for Mar 16 are tabulated and in Fig. 9, their absolute and relative values are shown. The Mar 20 measurement of H1 can't be formally compared with the value of Mar 16 because of the move and re-alignment of RAMP+RAS/Cal on Mar 20, and so is not shown in the figure. Even so, we see from Table 2, that the H1 focus is essentially identical (2063 vs 2064) for both days. There is less variation (2%) in the relative focii of the HRC field points than in those of the WFC's (4%).
The mean absolute focus for Mar 16 is 2079. The total observed range in focii is ~36 steps or 304 microns (8.45 um/focus step) between H1 and H9 which can be directly compared to the focal depth, as we did above for WFC. At f/72 and 21 micron/pixel, the HRC's focal depth is ~1510 microns so the range of focii over the HRC field is within ~20% of the focal depth.
Table 2 : Focus of the HRC Field Points
16 Mar 01
|H1 (Mar 20)||27641-27656||2064|
Here, we consider the amount of motion or jitter a PSF suffers through the execution of an SMS, and determine its cause, if observed. For each sequence of 16 images, the centroid of the PSFs was measured with the 'phot' task in IRAF. The offsets in X and Y with respect to the first image of each sequence are shown in Figs 10-14 for WFC and 15-19 for HRC.
Considerable motion is observed for both detectors. All the WFC field points show a positive drift of up to 4 pixels in X and -2 pixels in Y. The same general behavior is observed for all five field points i.e. the offsets increase for every change in focus. Similarly, for the HRC field points H8, H5, H9, and H6, drifts of up to +2 pixels in X and +1 pixel in Y are observed and all follow the same behavior. The offsets of H1 appear much more erratic and severe : -2 pixels in X and -4 pixels in Y. Why H1 behaves differently from the other four field points is unknown. We also see that within a 16-image sequence of a field point, the two images of a pair acquired at the same focus show slight variations in their offsets. Ideally, none would be observed.
We consider five possible causes for the motion of the PSFs :
1. changes in the temperature of ACS : The instrument is powered up every morning and thus takes a few hours to reach thermal equilibrium. This slow thermal cycling may introduce slight, random shifts in the image. But the observed drifts are fairly smooth and of the same magnitude and direction through each SMS, suggesting that this effect is not very important.
2. stability of RAMP+RAS/Cal : The mechanical stability of RAMP+RAS/Cal has not been verified. It can easily be measured by taking a series of images of one field point at a fixed focus over several minutes. As for 1., this effect would probably introduce offsets in random directions, contrary to the observations, suggesting that the source of the offsets is internal to ACS rather than external.
3. change in the light distribution at each focus : A change in focus redistributes the light in the PSF's peak and slightly disturbs the centroid. This effect is expected to be small, of the order of ~1 pixel or less. If this effect were to dominate, then the X-Y offsets of images 7-8 and 15-16 should be the same since they were acquired at the same focus (the initial guess), but this is not the case for WFC - the offsets of the latter pair are considerably larger for all field points, so another effect must be contributing. On the other hand, for HRC, the X and Y offsets of pair 15-16 fall in roughly the same range as that of pair 7-8, so different light distributions at each focus position may account for some of the observed behavior of the HRC offsets.
4. heating of the IM1 and M1 motor mechanisms between each focus adjustment : The movement of the IM1 and M1 mirrors heats their motor mechanisms followed by cooling during the data acquisition and read-out. This heating-cooling cycle induces slight changes in the mirror position along the axis of motion (V1) and hence in the location of the PSF on the detector. The ACS specifications state that the motor temperature increases by 17 Celcius for every 1000 steps and a wait time of 7 minutes is advised after an 1100 step move to allow the motor to cool. We can verify if the focus sweep SMSs satisfy these criteria. For the WFC SMSs (JGCW21A-I), the focus is stepped by 474 and 3 minutes is allowed between the first image of each pair. Similarly, for the HRC (JRHH01A), the focus steps are 237 and the time between the first image of the pairs is 2 min 15 sec. So, the focus sweep SMSs are roughly within specifications. This effect most likely accounts for the small wiggle seen in the X-Y offsets of pairs of images observed at the same focus. For each focus position, the motor mechanism goes through a heat-cool cycle, slightly changing the location of the mirror along the V1 axis, thus resulting in small variations in the location of the PSF.
5. IM1/M1 motor drive : The spherical IM1/M1 mirror moves along the V1 axis and nominally, the chief ray hits the mirror at angles of incidence and reflection of ~1 degree from this axis. As the mirror is translated along V1, the chief ray will fall at a slightly different location on IM1/M1, and hence on the IM2/M2 mirror, resulting in an offset at the detector location. Since the focus sweeps for any field point of a given detector span the same range, the magnitude of the drift should be similar for each point, as is observed. The direction of the drift should also be similar for all field points. This is probably the dominant effect causing the observed offsets in the centroid of the PSFs through the execution of the SMSs.
We conclude that the PSF of an HRC and WFC field point exhibits considerable drift through the execution of its focus sweep SMS. The large-scale, smooth drifts are dominated by motion of the mirror mechanism along the V1 axis, atop which are superimposed small wiggles between images of a given pair taken at the same focus, most likely resulting from thermal heating and cooling of the IM1 and M1 motor mechanisms.
After the acoutics session on Mar 23, ACS was moved back to SSDIF and configured with RAMP+RAS/Cal as the illumination source (see Figs 1 and 2). As in the pre-acoustics campaign, focus sweeps were performed at five field points in the HRC and WFC FOVs. The measurements are listed in Tables 3 and 4 and plotted in Figs 20 and 21. The mean absolute focus of the five WFC field points on Mar 30 is 2031 and for the HRC field points of Apr 2, 2085. The additional measurement of H1 on Mar 30, 2089, with RAMP+RAS/Cal at the same location as on Apr 2, indicates that the errors on the H1 measurements are of the order of 25 steps or ~1.2%.
Table 3 : Focus of the WFC Field Points
30 Mar 01
Table 4 : Focus of the HRC Field Points
2 Apr 01
|H1 (Mar 30)||27876-27891||2089|
The acoustics will introduce mechanical flexure or displacements in the instrument resulting in a change in the alignment the HRC and WFC detectors which can potentially be detected by measuring their optimal focus over their FOVs. For the pre- and post-acoustics data, we want to compare :
CEI SPECIFICATIONS :
Within measurement errors, no changes are observed in the absolute and relative focii of the WFC and HRC detectors over their FOVs between the pre- and post-acoustics campaigns. The acoustics test did not introduce measurable flexure or displacements within the instrument and between the detectors.
"ACS Acoustics Test Procedure" (Shah, K.C. et al., June 2000)