HRC-I Degap Lookup from Capella Data


In an earlier memo, HRC-I Degap Lookup Table, I described a method of deriving an improved set of degapping corrections using on-orbit observations of near-on-axis, point sources. The calibration program for cycle 7 included a set of twenty observations of Capella, performed at varying SIM translation offsets, to provide an initial portion of the data that can be used to derive updated degapping corrections. This memo describes the steps used in deriving the candidate CALDB update. The candidate is available here: hrci_degaplookup_candidate.fits.

Reduction Steps

The degapping corrections were derived from the combined data from the twenty Capella observations. For each of the individual observations the source events were extracted from the event list using an 8-pixel-radius circle centered on the source, where the center was determined by iteratively-clipped centroiding starting from a 20-pixel-radius centered on Capella's RA and Dec. Using the extracted source events and aspect solution from each observation and the fits to the location of the nominal telescope aim-point in the CHIP coordinate system as the SIM is translated (from the memo HRC-I Rotation Angle), the modeled detector position of Capella in HRC-I U and V for each source event time was calculated. Modeled position uncertainties were calculated using the uncertainties from the aspect solution and from the fits of the aim-point CHIP position as a function of SIM translation.

For each axis the events were binned according to coarse position (CRSU or CRSV), amplifier scale (AMP_SF), and modeled source position and the mean raw event position (RAWX or RAWY) for the modeled source position and uncertainty calculated as well as the mean deviation and its uncertainty. Figure 1 shows the result for one the U-axis coarse positions. The upper panel shows the deviation of the modeled position from the modeled position while the lower panel shows the number of events that were used in the determination at each modeled position. Data from the different amplifier scales are plotted in different colors (AMP_SF = 1 - blue, 2 - black, 3 - red).

Deviations for CRSU = 29
Figure 1 Upper: Mean of the differences between the modeled source location on the detector along the U-axis, derived from the aspect solution, and the reported RAWX value. Only events that were reported as centered on U-axis tap number 29 are included. Different colors are used to distinguish the values calculated from data with different amplifier scale factors (AMP_SF = 1 - blue, 2 - black, 3 - red). Vertical dashed lines indicate the nominal pixel range covered by the coarse position. The solid black curve is the difference implied by the degapping corrections in the CALDB file hrciD1999-07-22gaplookupN0001.fits.
Lower: Number of events at the modeled source location used in determining the mean differences plotted above. Data set colors are the same as in the upper panel.

The links below are to plots similar to figure 1 but for other coarse positions on either the U or V axis.

CRSU: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
CRSV: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30
31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The mean deviations found above provide the data on which the degap corrections are based. The degapping correction values are indexed by the event RAW position along the axis. The correction value for a given RAW position is selected from the data on the coarse position that nominally generates the RAW position (CRS = RAW/256; e.g. a RAW range of 7424-7679 is CRS 29). If Capella data is not available for the range of RAW positions (a whole coarse position range or just part of one) the current CALDB lookup value is used.

On a coarse position where Capella data is available, for each of the amplifier scales the modeled position bins that are within the nominal coarse position range and that had events are selected. These selected modeled positions and the deviations of the modeled position from the mean RAW position are used to calculate the corresponding RAW positions. The weighted mean of the deviations that have corresponding RAW positions at a given RAW position are interpolated between the minimum and maximum corresponding RAW positions. Typically, when the modeled position covered the end of the nominal coarse position range the minimum/maximum of the RAW positions do not. The depapping corrections in these gap regions were set to put the events at the nominal coarse position boundary. An example of the resulting degapping corrections is shown in figure 2.

Updated degapping
		corrections for CRSU = 29
Figure 2:: Updated degapping corrections in the CRSU = 29 region derived from the Capella observations. Color indicates the amplifier scale setting for the corrections with AMP_SF = 1 - blue, 2 - black, 3 - red. The vertical dashed lines are the nominal boundaries of the coarse position region.

The resulting corrections for each coarse position and amplifier scale were visually examined. For the CRSU range 4—11 and the CRSV range 5—12 there are too few events in the AMP_SF = 3 setting for reasonable degapping corrections to be determined using data from that scale. Rather than continuing to use the current CALDB correction on this one scale setting, the degapping correction in the update is the average of the AMP_SF 1 and 2 values.


I have used this candidate degapping correction to reprocess a few observations; the resulting level 2 events files can be obtained at the following links:

Figure 3 shows a comparison of the encircled counts fraction of the ObsID 1385 (AR Lac) data between the using the degapping corrections currently in the CALDB (3.2.1) and the updated corrections generated here. Using the updated degapping corrections results in a tighter point-spread function.

encircled counts fraction
Figure 3: Fractional encircled counts as a function of radius for the on-axis source AR Lac (ObsID 1385). The black curve is the result using the degapping corrections in CALDB 3.2.1; the red curve is the result using the candidate degap generated from the Capella observations.

Last modified: Wed Apr 19 10:14:42 EDT 2006

Dr. Michael Juda
Harvard-Smithsonian Center for Astrophysics
60 Garden Street, Mail Stop 70
Cambridge, MA 02138, USA
Ph.: (617) 495-7062
Fax: (617) 495-7356