WFC3 Thermal Vacuum Testing: IR Grism Focus and Tilt Anomalies
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The WFC3 thermal-vacuum testing performed in the Fall of 2004 has revealed anomalies concerning the two grisms installed in the IR channel of the instrument. First, the grisms do not produce in-focus images, the cause of which has been traced to a 90 degree rotation error in the way the grisms were mounted in the filter wheel. Second, both grisms show a residual tilt of their dispersion axes relative to the detector axes of ~8.3 degrees, which is far larger than what would be expected from random uncertainties in their mounting.Keywords:
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The files that provide WFC3 support in the SYNPHOT package represent a compilation of data from various sources, including industry, government labs, and STScI staff. The purpose of this report is simply to record the sources of data I used in creating these files. The FITS files summarize this information in the headers, but that information is detailed and slightly expanded here, to provide a permanent record and to compile all of this information in one easily accessible document. Background The characterization of the Wide Field Camera 3 (WFC3) performance relies upon measurements from a variety of sources, including industry, government labs, and STScI staff. I utilized these measurements to create the FITS files that provide WFC3 support in the IRAF SYNPHOT package. The SYNPHOT package combines the throughputs for the various components to give the end-to-end throughput for the instrument in a particular observing mode, which can then be used to estimate expected count rates resulting from a given source. The SYNPHOT package can provide count rates from astronomical sources, Zodiacal background, geocoronal emission, Earthshine, and (in the case of WFC3 and NICMOS) thermal background. To the extent possible, the data files associated with WFC3 support in SYNPHOT reflect actual measurements on each component. Where tests during WFC3 ambient and thermal vacuum (TV) campaigns have demonstrated inaccuracies in those data, I have corrected the data. Discrepancies in the end-to-end throughput that could not be attributed to any single component have been incorporated into a distinct “correction” file instead of folded into arbitrary instrument components. Although the source of the data is summarized in the FITS headers, I summarize and expand upon this information here, to provide a permanent and easily accessible record. Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration UVIS Channel Optics The components in this section are implicit to any SYNPHOT obsmode using the UVIS channel of WFC3. They are not specified by a keyword when using SYNPHOT. filename component description source of data hst_ota_007_syn.fits HST OTA throughput 12/1988 memo by Chris Burrows, extrapolated into the UV and IR. wfc3_uvis_mir1_001_syn.fits Reflectivity of UVIS mirror 1 Data provided by J.Sullivan at Ball Aerospace, 10/2003. wfc3_uvis_mir2_001_syn.fits Reflectivity of UVIS mirror 2 same wfc3_uvis_owin_001_syn.fits Transmission of UVIS outer window same wfc3_uvis_iwin_001_syn.fits Transmission of UVIS inner window same wfc3_pom_001_syn.fits Reflectivity of pickoff mirror Data provided by J.Sullivan at Ball Aerospace, 10/2003. Produced by merging POM measurements done in the UVIS and IR. UVIS Channel Filters All data were retrieved from the NASA/GSFC WFC3 filter archive, merging high-resolution in-band data with low-resolution out-of-band data (Brown 2006, WFC3 ISR 2006-03). Multiple versions were created of each filter, each with a distinct serial number, with the best installed as the flight filter and the second best designated as a spare filter. The measurements of the filter transmission were performed by engineers at either GSFC or JPL on the date specified in the table below. The filter name in the table below is also the keyword a user would specify in the SYNPHOT obsmode. Note that the UVIS grism is included in the “Miscellaneous” section. filename filter serial # lab date wfc3_uvis_f200lp_syn.fits F200LP 312 GSFC 07/2006 wfc3_uvis_f218w_syn.fits F218W 312 GSFC 07/2005 Instrument Science Report WFC3 2007-06
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In March of 2001, a routine analysis of FR533N VISFLAT images revealed an anomaly: the filter wheel appeared to be offset by about 0.42 in some images. This offset angle is close to 0.5 degrees, which corresponds to one “filter step,” (i.e., one increment in movement of the filter wheel by the filter electronics). These offsets are not a recent phenomena, and have been found in data as far back as mid-1994. This document describes a more exhaustive analysis of the filter wheel rotation anomaly: flatfield images for all available filters in the archive were inspected to determine if they exhibited similar behavior. Only filters that projected unique features (such as pinholes) onto the image were useful because these features were needed to detect the offsets. In addition to the previouslyreported problem with FR533N, filter wheel rotation offsets have also now been found in these additional filter configurations: F375N, FR418N, FR533N18, FR533N33, FR533P15, FR680N, FR680N33, FQCH4N, FQCH4N15, and FQCH4P15. The offset angle was found to be 0.42 +/0.06 . The occurrence rate of the offset error ranges from 0% to 40% for these filters (based on samples of 20 images or larger). It is also apparent that some filters (e.g. F160BW) show no rotation errors. In general, the errors will have little impact on GO science though photometric errors can reach a few percent in worst-case scenarios. There is also some hint that the problem may be getting worse with time. Currently, the cause of the anomaly is not completely understood. Cycle 10 and cycle 11 calibration programs will continue monitoring these filter wheel offsets. ° °
Anomaly (physics)
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We describe many of the less-common artifacts and anomalies which appear in WFPC2 images. Examples are given with brief descriptions of their cause, correction, and prevention.
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The WFC3/IR channel will combine sensitivity with a wide field of view, such that all long exposures will contain some saturated stars, even in sparse fields near the Galactic poles. Because such saturations in IR detectors can result in persistence (particularly when a pixel is over-saturated by factors of 10 to 100), I quantify here the frequency of 1x, 10x, and 100x saturations, as a function of position on the sky. I use the 2MASS and GSC2 catalogs for this analysis, although each has its limitations for this purpose. Away from the Galactic plane, the greater depth of the GSC2 is required to quantify the frequency of moderate saturations (1-10x). If, however, one is only interested in extreme saturations (>100x), the 2MASS catalog is more appropriate, because it is sufficiently deep for such sources and it avoids the large systematic errors (by factors of 200) that can arise due to an incorrect assumption of spectral type and extinction when extrapolating from the GSC2 optical bands into the IR. Both catalogs suffer from serious incompleteness in the Galactic plane, especially toward the Galactic center, but for such fields the 2MASS catalog would still be preferred over the GSC2 when checking for saturating objects.
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During WFC3 thermal-vacuum testing in September and October 2004, a subset of the UVIS20 test procedure, “UVIS Flat Fields”, was executed twice, both times under thermal-vacuum conditions. The results of this procedure show that high quality UVIS broadband flat fields can be easily constructed and that the CCD non-uniformities are correctable to at least <1% and in some cases to <0.5%.
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ABSTRACT The filter set of the WFC3 IR channel has until now included a filter with significant transmission in the optical, the F093W. The bandpass provided by that filter will extend well below 800 nm when the current IR flight detector (FPA64) is replaced by one with its substrate removed. The F093W filter was intended to allow alignment of the IR channel using an optical laser, but it has also been considered as a potential filter for science. Because the IR channel can now be aligned with an infrared laser, we revisit in this report the utility of the F093W filter for scientific observations. The science that can be performed with the F093W could in general be pursued with the F098M filter on the IR channel or one of the red filters on the UVIS channel, such as the F600LP. Thus, because the primary impetus for the F093W is gone, we are removing it and restoring the F140W filter, which was itself replaced by the F110W in 2002. The F140W filter provides an excellent complement to the F105W filter, straddles a gap in ground-based coverage (due to strong atmospheric absorption between J and H) and extant WFC3 filter coverage, and provides a useful direct image for the G141 grism.
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Electronic crosstalk produces a signal in one or more amplifier quadrants in response to a stimulus in another quadrant. In the WFC3 IR camera, the crosstalk arises during the readout and appears as a negative mirror image of the source. Positioned symmetrically opposite the source about the dividing line between each of the coupled readout amplifier quadrants, the crosstalk appears at a lower level than the surrounding background, at the level of ~1e-06 that of the source signal. The level is low enough that it should not be an issue for most programs; dithering can help mitigate the effect. This ISR characterizes the position, intensity and shape of the crosstalk effect as seen during TV3 ground testing. Introduction Crosstalk (CT) in the WFC3 IR channel is a type of electrical interference that occurs during the chip readout: the detected signal experiences variations due to signals present elsewhere in the system. In this case, the act of reading a signal from a source in one quadrant changes the signal levels detected from another quadrant. As a result, a relatively diffuse region of inverse signal is produced in response to a bright target in another quadrant. As shown in Figure 1, the WFC3 IR crosstalk is faint relative to other known effects such as persistence.
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A new anomaly has been discovered in the WF4 channel of WFPC2 which is characterized by sporadic images having low or zero bias levels. Early beginnings of the problem can be traced near the time of Service Mission SM3B in March 2002 where images begin to appear with progressively lower and lower bias levels. By mid-2004 some images appear with very low bias levels – far below the normal 311 DN, and in February 2005 the first images with zero bias level are seen. As of this writing all images are significantly below normal bias levels, and nearly 30% have zero bias level. While the calibration pipeline will correct for low bias levels, close examination of these images reveals faint horizontal streaks (<1 DN RMS) and low photometry (up to 70% low for faint targets at very low bias and gain 15); these effects are correctable and preliminary corrections are given in the Appendices. Images with zero bias, however, suffer much more serious problems -these are blank except for brightest pixels and the data are not recoverable. Both A-to-D gains 7 and 15 show similar anomalies, though the onset is slower at gain 15 due to its greater digitization range. Extrapolation of current trends suggests gain 7 will become completely unusable in mid2006 while gain 15 will produce some useful data until late-2006. Fortunately, the other WFPC2 CCDs appear unaffected by the anomaly, and show no evidence for early onset of similar problems. Examination of bias levels during periods with frequent WFPC2 images shows low and zero bias episodes every 4 to 6 hours. This periodicity is driven by cycling of the WFPC2 Replacement Heater, with the bias anomalies occurring at the temperature peaks. We suggest that lowering the Replacement Heater temperature set points by a few degrees C might effectively eliminate the WF4 anomaly. Copyright© 2005 The Association of Universities for Research in Astronomy, Inc. All Rights Reserved.
Digitization
Anomaly (physics)
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