COS Data Handbook Version 2.0

In this section, we discuss sources of error that are associated with major steps in the COS calibration pipeline (calcos). Note that these steps themselves were already described in Chapter 3 and will not be repeated here; this section will only describe specific issues related to the error budget of the resulting data which were not described before.

Dark counts arise from a combination of detector effects and external sources. Calcos will remove the effects of detector background (which includes dark, scattered light, etc.) in the BACKCORR module. This is done after the X1DCORR converts the detector image to a 1D extracted spectrum. Here, we discuss the instrumental contribution, since it can be the limiting factor in the error budget for very faint sources.

The FUV detector dark rates measured on the ground were very low, of order 0.4 counts cm-2 s-1. Typical dark rates on-orbit away from the South Atlantic Anomaly (SAA) during the first year on orbit for both segments are about three times higher,
1.8 × 10-6 counts pixel-1 s-1 (or 1.25 counts cm-2 s-1). This is equivalent to 2.6×10-4 counts s-1 per resolution element in a spectrum with the default extraction height. These rates have remained stable since SM4.

When HST passes though the SAA, observations stop and the detector high voltage is lowered in order to prevent damage to the detector. Elevated dark rates of up to 30 times higher than normal (Figure 4.1) have been observed during exposures made when skimming the edge of the SAA. To minimize the observing time with higher background, the SAA model was shifted 6 degrees to the west in May 2010.

The spatial distribution of background counts on Segment A is quite uniform, independent of pulse height thresholding or proximity to the SAA (Figure 4.2). For segment B, however, there are a number of bright spots in the region where the spectra fall when all pulse heights are included. These features disappear when the appropriate pulse height thresholding (used by default in the calcos pipeline for TIME-TAG data) is applied, as shown in Figure 4.3.

There is an additional complication to FUV dark correction. As the FUV detectors have been exposed to more light, the portion of the detectors where the spectrum falls has become less sensitive. This sensitivity loss even affects the dark count rate. As a result, the background, which is estimated from rarely illuminated regions on either side of the science spectrum, tends to over estimate the dark rate at the location of the spectrum and, therefore, over correct the spectrum. This effect is small, and only affects very faint objects. Nevertheless, one should be aware of it. The team is currently studying a more robust background subtraction, which accounts for this effect.

Dark rate for FUV Segment B with no pulse height thresholding (top), and with the default thresholding used by calcos (bottom). Using the appropriate thresholding minimizes the effects of the extra features near the middle of the segment.

The STIS MAMA flat fields are dominated by a fixed pattern that is a combination of several effects including “beating” between the micro-channel plates and the anode pixel array and variations in the charge cloud structure at the anode. Similar effects are present in the COS MAMA. Intrinsic pixel-to-pixel variations measured on the ground for the COS NUV-MAMA are 5.2% rms. Analysis of the COS NUV flat-field taken during SMOV by Ake et al. (COS ISR 2010-03) found that it aligned to within one pixel of the flat field created during ground testing. Consequently, all SMOV and ground data were combined to produce a single flat field reference file for pipeline processing.

The reference file does not correct vignetting, which affects X pixels with values between 0 and 200. The vignetting can eliminate as much as 20% of the flux from X = 0 to 100, and then slowly decrease to 0 between X = 100 and 200. Since the amount of vignetting depends on the angle of illumination, and because the OSM positions do not repeat, simple corrections were inadequate. Due to the low current usage of the NUV channel, a more complex solution has not been pursued.

Studies of the on-orbit S/N achievable indicate that the Poisson limit can be reached for S/N < 70 and that a S/N > 150 can be achieved by combining high S/N exposures obtained at different FPPOS settings over most of the detector. However, the variable vignetting can introduce large, spatially coherent errors over the first 200 pixels of each stripe of the NUV spectra.

The FUV XDL detector has considerable fixed pattern noise. These include dead spots and a honeycomb pattern due to the manufacturing process used to produce the MCP and shadows from the repeller grid wires. A full, two dimensional flat field obtained during internal ground tests did not produce the signal-to-noise needed for a useful flat, and it has been deemed too costly in terms of exposure time and impact on detector lifetime to fully characterize the COS flat field using on-orbit observations.

Nevertheless, some progress has been made. The grid wire shadows, which are the largest single source of fixed pattern noise, are now corrected by the flat fields for the G130M and G160M. However, we currently do not employ a similar grid wire flat for the G140L. Instead, calcos flags the grid wire shadows and removes them in the _x1dsum files. So if G140L data are obtained at more than one FPPOS setting, a complete spectrum will result which has reduced S/N at the grid wire positions.

Note that even with the correction (for the G130M and G160M) or elimination (for the G140L) of the grid wire shadows, other large amplitude (up to 10%) fixed pattern features remain in the spectra. At present, the best approach to mitigate these is to combine observations obtained at different FPPOS settings. A complete description of the G130M and G160M grid wire flats, and estimates for the achievable S/N for these gratings from normally processed data, are given in COS ISR 2011-03.

As described in Chapter 1, the COS XDL FUV detectors experience a loss of sensitivity called gain sag. The COS FUV detectors already experienced localized gain sag in the regions of the detector that are exposed to the bright Ly α airglow line when the G130M is used. These are most serious on the FUVB side near pixels 7150 and 9100. Figure 4.4 shows the effect of changing the lower PHA cutoff from 4 to 2 on the feature centered near pixels 7150 and 9100. With a PHA cutoff of 4, the adjoining pixels are not affected, but the region of gain sag is depressed by nearly 50%. In contrast, with a PHA cutoff of 2, the gain sag regions are depressed by 10%, approximately. As the gain sag deteriorated, changing the PHA threshold no longer worked, and the high voltage had to be raised.

	Finally, in July 23, 2012 the COS FUV spectra was shifted to a new lifetime position. For a complete description consult Appendix A:COS New Spectra Position.

The XDL centroiding electronics are sensitive to thermal effects. The TEMPCORR module of calcos measures the location of the stim pulses in order to determine the shift and stretch of the detector format and correct for any changes; TEMPCORR applies a linear correction based on the position of these stims. The accuracy of this correction will influence the ability to properly register the flat field corrections and may influence the final error budget. As of this time, no comprehensive study of how well this registration is performing has been carried out, but spot checks indicate that it is working as expected.