Internal Flat Field Monitoring II. Stability of the Lamps, Flat Fields, and Gain Ratios

Transcription

1 Instrument Science Report WFPC Internal Flat Field Monitoring II. Stability of the Lamps, Flat Fields, and Gain Ratios C. O Dea, M. Mutchler, and M. Wiggs April 27, 1999 ABSTRACT We have compared the on-orbit behavior of the VISFLAT and INTFLAT lamps and the flat fields that they produce over the period from early 1994 to early We find the following. The VISFLAT lamps continue to degrade in intensity at the expected rate. The loss is less in the blue due to the Welch-Allyn Bulb becoming hotter. One (or more) INTFLAT lamps have increased output slightly (the net change is ~1.3% in 4 years). On timescales of ~6-35 s, the output of the Carley bulbs is relatively constant. The relative gain ratios of the electronics are stable in WF2 and WF4, though there is a steady 0.3% drift in the WF3 gain ratio, and a jump in PC gain ratio at SM2 of 0.7%. The pixel-to-pixel response of the chips is very stable. There are variations in the flat fields (donuts and blemishes) not due to the chips but due to dust on optical surfaces either moving or appearing. There are changes in the large-scale structure of the INTFLAT fields with time probably related to varying lamp brightness. The effects of contamination (location of worms ) are stable with time. Overall, the results from the VISFLATs and INTFLATs are in good agreement. If the VISFLAT lamps become unusable, the INTFLAT lamps would provide useful diagnostics of all properties except possibly for the stability of the flat fields on large scales which is affected by the variations in the Carley bulbs. 1. Introduction WFPC2 contains two internal optical calibration lamp systems (the VISFLAT and INTFLAT lamps) which are used to perform diagnostics on the instrument performance, especially flat field stability. The VISFLATs are a new feature of WFPC2 and are described by Majorana and Smith (1993); while the INTFLATs are the same design as in WFPC1. The VISFLAT system consists of lenses and diffusers together with two types of incandescent tungsten bulbs, (1) two Carley bulbs with a color temperature of 2060 K 1

2 which generates the green to red part of the spectrum and, (2) a Welch Allyn bulb (with a backup currently not being used) with a color temperature of 2550 K which generates the blue part of the spectrum. However, the integrated spectrum of the VISFLAT lamps is still very red (Figure 1 of Stiavelli and Baggett, 1996). The VISFLAT lamps are located where the UV light pipe was in WFPC1, and a flip mirror is used to place the light in the WFPC2 optical path. The INTFLAT system consists of four Carley bulbs located at four edges of the shutter opening on the CCD side of the shutter. With the shutter closed, the lamps are turned on and the light reflects off the backside of the shutter and down the optical path. The on-orbit properties of the VISFLATs and INTFLATs based on data taken in the first two years after installation was reported by Stiavelli and Baggett (1996). At that time it was discovered that the VISFLAT lamps were dimming with use. The use of VISFLATs was sharply cut back and use of INTFLATs was increased. Currently, VISFLAT sweeps are done twice a year while at least some INTFLAT observations are done about twice a month. In this ISR we report a study of the on-orbit properties of the VISFLATs and INTFLATs over a four year period from Spring of 1994 to Spring of We provide more detail on the INTFLATs because of their current dominant role in the WFPC2 calibration program. 2. Long Term Stability of the Lamps INTFLAT Lamps We consider the INTFLAT lamps first since these are a single type of bulb, the Carleys. We extracted the MEANC300 value from the header which gives the average value of the counts in the central 300x300 pixel region of each chip. (The resulting counts are slightly dependent on which shutter is used (A or B blade). Here we use only the blade A data.) We obtained counts from data taken during the period of March 1994 to February 1998 for exposures taken in both gain 7 and 15. The counts in the various chips for the 10 s Gain 7 and 20s Gain 15 images are plotted in Figure 1. The count rate from the lamps initially increased by about 1.1% due to the change in WFPC2 CCD operating temperature to -88 C on April 23, The results of least squares fits to the slow rise between May 1994 to Feb 1997 are given in Table 1. The average slope is about 0.44% per year. After Feb 1997 the increase in the counts levels off. Figure 2 shows the residuals from the fit for WF4 in Gain 15. There is a hint that superimposed on the gradual rise is a slight wiggle in the counts with an amplitude of about 0.4% and a period of about 290 days. These variations can be attributed to the lamps rather than the chips since (1) similar changes in the throughput have not been seen in the photometric monitoring data, and (2) the chip gains (especially WF2 and WF4) are relatively stable. Caveats to both of these are discussed below. 2

3 Table 1. Slope of INTFLAT Lamp Variations (May 94 to Feb 97)) Chip Gain Slope% per year PC WF WF WF PC WF WF WF

4 Figure 1: Counts in the INTFLAT Lamps. The line is a fit of the data between May 94 to Feb Modified Julian Day Modified Julian Day 4

5 Figure 2: Residuals to linear fit to counts in INTFLAT lamp in WF4 Gain 15. Note slight periodic signal with timescale of 290 days. INTFLATS WF4 Residual to Fit 20 F555W, Meanc300, Blade A, Gain Modified Julian Day 5

6 VISFLATs We analyzed all VISFLAT exposures taken since April 1994 (when the WFPC2 CCDs were cooled to -88C) to monitor the continuing degradation of the calibration lamps. We determined count rates (using the MEANC300 data) with a normalization to take into account the different exposure times used, since the lamp output is not constant on scales of 20 s (see below). We extracted only the 2 or 3 most common exposure times which were enough to show the trends in order to minimize the amount of normalizing required. Monthly totals of lamp cycles and on-time were obtained from Sito Balleza. We fit a curve to this data, so that we could create a reasonable estimate of the cumulative lamp cycle and on-time for any VISFLAT based on the modified Julian date of the observations. Plots of relative intensity in B (F439W), V (F555W), and I (F814W) versus date, lamp cycles, and lamp on-time are presented in Figures 3-7. The decline in lamp intensity with cumulative lamp on-time continues as expected, consistent with the results of Stiavelli (1997). After about 270 hours of lamp on-time the intensity seen through F555W has dropped to about 89%. The fact that the INTFLAT lamps which are Carley bulbs show no such decline suggests that the decline in the VISFLATs is due to the Welch-Allyn bulb. We also note that the lamp degradation is somewhat slower at the shorter wavelengths. The lamp intensity through the F439W filter has dropped only to about 93%. This is consistent with the Welch-Allen bulb getting hotter as it dims so that the decline in the blue is less than the decline in the green (as suggested by Stiavelli and Baggett, 1996). 6

7 Figure 3: Intensity in VISFLAT lamp through F439W vs. cumulative on-time. 7

8 Figure 4: VISFLATs lamp intensity through F555W filter vs. cumulative lamp cycles. 8

9 Figure 5: VISFLATs lamp intensity through F555W vs. MJD. 9

10 Figure 6: VISFLATs intensity through F555W vs. cumulative on-time. 10

11 Figure 7: VISFLATs lamp intensity through F814W vs. cumulative on-time. 11

12 3. Short Term Variability of the INTFLAT Lamps. The counts from MEANC300 (the mean value of the inner 300x300 pixels in the uncalibrated images) in WF2, gain 7 are shown as a function of MJD in Fig 8a (top left). The corresponding requested exposure times for the data are shown in Fig 8b (top right). (Note that the values of 9 and 15 s are illegal and the actual exposure times in the data were rounded down to 8 and 14 s, respectively). Fig 8c (bottom left) shows that the count rates (MEANC300 counts divided by requested exposure time) are roughly constant with time at the different exposure times, which is consistent with the very slow long term trends shown in Figure 1. This indicates that the output of the lamp is repeatable and roughly constant on time scales of a few to a few tens of seconds. We used the sweep in exposure time obtained on Oct 17, 1996 (ID=6907) to examine the short time-scale behavior of the lamps in more detail. Since the MEANC300 values are determined from the uncalibrated data, we first subtracted the average values of the biases for each chip and gain (from Table 2 of ISR WFPC ). The first point at 6 s is determined by dividing the bias corrected MEANC300 value by 6 sec; thus this is the average count rate in the first 6 s of lamp exposure. The second point at 8 s is determined by taking the difference in the counts at 8 and 6 s and dividing by the difference in the exposure times (2s), thus this is the average countrate between 6 and 8 sec. Each subsequent point represents the average count rate during the preceeding time interval. We include the counts from the gain 15 images (multiplied by 2) which extend to larger exposure times. The results are shown in Fig 8d (bottom right). Except for slightly low values at the 6 s point, the lamp output appears to be relatively contstant (to ~3%) over the entire time interval sampled (6 to 35 s). Thus, it may be possible to use the INTFLATs to test for CCD linearity as well as stability of the instrumental gains. 12

13 Figure 8: Short Term Variability of INTFLAT lamps. (a) 3000 (b) Modified Julian Day (c) (d) 300 PC1 WF2 WF3 WF Modified Julian Day exposure time (s) 4. Long Term Stability of the Gain Ratios INTFLATs The gain ratios as a function of Modified Julian Date are plotted in Figure 9 for the four chips. These are obtained by dividing the count rates (determined using the MEANC300 statistic) in the 10s gain 7 and 20s gain 15 images. Average values for the period after the April 1994 cool-down are given in Table 2. The lamp intensity appears to be constant to ~3% accuracy on the relevant timescales (Section 3), which sets the limit on our determination of the gain ratio accuracy. The gain ratios in WF2 and WF4 are very stable. The average values have a dispersion of The gain ratio in WF3 has declined gradually by about 0.3% over a 4 year period. The gain ratio in PC1 has apparently jumped by 0.7% at the time of SM2 (starting MJD 50492). This appears to be mainly due to a decline in the gain for the gain 15 channel (see Figure 1). 13

14 Figure 9: Plot of ratio of counts/s in gain 7/gain 15 INTFLAT images Modified Julian Day Modified Julian Day Table 2. Mean Value of Ratio of Gain 7/15. Chip Mean of gain ratio rms npts PC WF WF WF

15 VISFLATs Because of both short term and long term variability in the INTFLAT lamps it is important to confirm the suggestion that the gain ratios are varying with time. Although nearly all of the VISFLAT data is taken in gain 7, there are occasional observations in the linear ramp filter FR533N in both gains which are used to check the A-to-D conversion. We extracted the MEANC300 values and exposure times from the headers of the data and determined the ratios of the count rates in each gain (Figure 10). The slopes from a least squares fit to the VISFLAT and INTFLAT data are given in Table 3. There is less data for the VISFLATS, especially over the last couple of years, and there is more scatter (possibly due to the spread in time between measurements in different gains), yet the same trends can be seen as in the INTFLAT data. Thus, the VISFLAT data confirm the small variations in gain ratio seen in the INTFLAT data. Figure 10: Plot of ratio of count rates in gain 7/15 for the VISFLAT FR533N Modified Julian Day Modified Julian Day 15

16 Table 3. Slopes from a least squares fit to the gain ratios in the INTFLAT and VISFLAT data. Chip INTFLAT slope %/yr error VISFLAT slope %/yr error PC WF WF WF Photometric Monitoring We noted above that there is a jump in the gain ratio in the PC at the time of SM2 which appears to be mainly due to a drop in sensitivity of gain 15. We examined the photometric monitoring data (taken in gain 15) from the standard star GW+70D5824 which is available on the wfpc2 web pages (Gonzaga et al., 1998). The normalized data are plotted in Figure 11. The mean values before and after SM2 are / and / which give a drop of 0.6% which is formally significant at the 2.9 sigma level and which is consistent with the change in the PC gain ratios seen at SM2. Thus the photometric monitoring data confirms the change in the PC1 gain 15 channel which occurred at the time of SM2. What causes the gain changes? Changes in the electronics in each camera head would be the same for both gains and would divide out, producing no change in gain ratios. Changes in the parts of the electronics which are common to each gain would cause similar changes in all chips. Yet we see qualitatively different gain variations in the four chips. Thus the variation must occur in a piece of the electronics which is unique to each gain/chip combination. Analysis by Sito Balleza suggests that the gain changes may be occurring in the Double Sampling Processors (DSP). One possible mechanism is slight changes in the resistors in the amplifier in the DSP. Changes in resistance may occur with aging and/or with cosmic ray (CR) damage. The small changes seen (<1% over 4 years) are consistent with what is expected for the behavior of the electronics. 16

17 Figure 11: Plot of gain 15 PC1 photometric monitoring data. Note the decrease in count rate of 0.6% at the time of SM Photometric Monitoring F555W PC1 Gain Modified Julian Day 5. Spatial Structure in the INTFLATs We selected a set of 9 INTFLATs in F555W from June 1994 and combined them using the STSDAS task crrej. We multiplied the image by the pipeline flat field (g640925nu.r4h) and normalized the image to unity. Figure 12 shows the normalized flatfielded INTFLAT. We see large scale features, i.e., diagonal bands or stripes or gradients running across the chips with amplitudes of 30-50%. In WF4 we also find two large, diffuse, donut-like features with amplitude of ~2%. The large scale features are probably due to the nonuniform illumination of the chips by the four Carley bulbs, scattered light, or obscurations. There is probably also a contribution from obscurations upstream from the INTFLAT lamps/shutter which appear in the pipeline flats and not the INTFLATs. The large donuts in WF4 are not seen in data taken with the VISFLATs or in the regular flat fields. Thus, they are not present in the light path taken by the science data. They are probably due to bright spots on the shutter. 17

18 Figure 12: Greyscale of F555W INTFLAT from June 1994 flatfielded and normalized. mos555jun94ff2-555jun94ff2[1/4] z1=0.60 z2=1.20 ztrans=linear Con=1.03 Brt=0.44 cmap=grayscale ncolors= NOAO/IRAF odea@hawkwind.stsci.edu Wed May 20 16:26:

19 6. Stability of the Flat Fields We extracted sets of VISFLATS at several epochs between 1994 and The images were combined using crrej to remove cosmic rays. We divided the April 94 by the March 98 images to look for long term changes in the flat field response. By and large the images are very smooth showing that the pixel-to-pixel response has been very stable (Figure 13). Mean values for the ratios of the images are given in Table 4. The typical standard deviation is 0.02 DN. There are several notable differences in the 94 and 98 images. There are three donuts which are seen in WF2. These features have an amplitude of a few percent. We analyzed VISFLATs at intermediate epochs and discovered that the features appeared between observations which were taken in August 96 and early Feb 97 (just before SM2). The donuts have so far remained stable in VISFLATs taken since Feb 97. These features are probably due to dust specks on an out-of-focus optical surface in the WFPC2 light path. Krist has suggested that these are on the field flattener. In WF3 and WF4 there are several positive-negative features with an amplitude of between a few and 10 percent. These are due to features which are present at all times but which appear to be shifting position slightly with time. These are due to dust on the pyramid mirror which is nearly in focus. The features are seen in image ratios involving data between 94 to 96 and so have been shifting location during that time. Since 96 the features have been relatively stable. We performed a similar analysis of the flat field stability using the INTFLATs. The statistics of the 94/98 ratio images are given in Table 4 and an image is presented in Figure 14. We find that the pixel-to-pixel stability is very good. We see the same low level features (donuts and positive-negative features) as noted in the VISFLATs. In addition we also see large-scale features at the ~1% level, e.g., diagonal bands running across the chips, and a large donut in WF4, which are present in INTFLATs during all epochs. These features must vary with time and may be due to the gradual change in the INTFLAT lamps with time (Figure 1). If the 4 bulbs vary by different amounts, the illumination of the chips may change with time, resulting in time varying structure in the INTFLATs. Some of these features may also be due to changes in the optical alignment. 19

20 Table 4. Statistics of inner 200x200 pixels of ratios of VISFLATs and INTFLATs (1994/ 1998). Lamp Filter Chip Mean Stddev VISFLAT F439W PC VISFLAT F439W WF VISFLAT F439W WF VISFLAT F439W WF INTFLAT F555W PC INTFLAT F555W WF INTFLAT F555W WF INTFLAT F555W WF

21 Figure 13: Ratio of VISFLATs in F439W (April 1994 divided by March 1998). Note the very smooth ratio indicating that the VISFLATs are very stable. A few dust features can be seen in WF2, 3, and 4 where dust has appeared or moved on optical surfaces between the two epochs. 21

22 Figure 14: Ratio of INTFLATs in F555W (1994 divided by 1998). Similar dust features are seen as in the VISFLAT ratio. We also see larger scale variations likely caused by the variation in the INTFLAT lamps. mosintv94div98n.hhh - MOSINTV94DIV98N[1/1] z1=0.90 z2=1.10 ztrans=linear Con=3.13 Brt=0.33 cmap=grayscale ncolors= NOAO/IRAF odea@hawkwind.stsci.edu Wed May 20 15:24:

23 7. Effects of Contamination We plot the ratio pre-decon/post-decon of counts in the inner sections of VIS- FLATs and INTFLATs in Fig 15. Compare with Fig 8 of ISR which shows the results for the 95 VISFLATs. Our results for the INTFLATs are in good agreement with those for the VISFLATs, with the possible exception that the INTFLATs do not show the slight increase in efficiency in the blue post-decon which is present in the VISFLATs. The overall effect in F439W is about 1% in WF2 and WF3 and 0.5% in WF4 and 0.3% in PC1 in the F439W band. The result that PC1 shows less contamination is consistent with the results of photometric monitoring (Whitmore, Heyer, & Baggett ISR 96-04; Baggett & Gonzaga ISR 98-03). The VISFLATs also show a smaller effect than the photometric monitoring, suggesting that the dominant the effect of the contamination is scattering. Note that the INTFLATs F336W data is likely to be strongly affected by the red leak in this filter, since the lamp spectrum is extremely red. An image of the pre/post DECON VISFLATs in the F439W filter are shown in Figure 16. There are chip-to-chip differences in the appearance of the contamination features. WF2 is largely covered by worms while PC1 shows worms in about 1/3 of the image. The contamination features in WF3 and WF4 are more diffuse and are not organized into worms. Similar features are seen in the pre/post DECON images for the INTFLATs (Figure 17). The agreement between the VISFLAT and INTFLAT results (obtained at different times) suggests that the location of the contamination features (worms) is stable. 23

24 Figure 15: Ratios of counts before and after DECON. (top) Inner 400x400 pixels of F439W, F555W, F675W, and F814W VISFLATs for gain 7 for the 6 March 1998 DECON. Note that the red leak in F336W reduces the effects of contamination. We plot the mean of the ratios of the images. For each filter, the chips are separated in wavelength to improve clarity of the plot. (bottom) INTFLATs inner 300x300 pixels of F336W, F439W, F555W, F675W, and F814W for gain 7 and shutter A averaged over several DECONs in We plot the ratio of the mean values of the images VISFLATS Decon 6 March INTFLATS 1997 Decons Filter Wavelength (Angstroms) 24

25 Figure 16: The ratio of VISFLATs in F439W taken before and after the March DECON. The before and after images are the result of combining two separate images with crrej. 25

26 Figure 17: The ratio of INTFLATs in F439W (before/after) spanning a series of DECONs in mosintbdecon.hhh - MOSINTBDECON[1/1] z1=0.90 z2=1.10 ztrans=linear Con=2.98 Brt=0.45 cmap=grayscale ncolors= NOAO/IRAF odea@hawkwind.stsci.edu Thu May 28 09:25:

27 Acknowledgments We are grateful to Sylvia Baggett, John Biretta, Sito Balleza, Stefano Casertano, John Krist, and Brad Whitmore for helpful discussions, to John Biretta for comments on the ISR, and to Inge Heyer for help with the formatting. We thank Sylvia for providing her master table of calibration observations. References Baggett, S. & Gonzaga, S. 1998, WFPC2 ISR Gonzaga, S., Ritchie, C., Baggett, S., Whitmore, B., & Mutchler, M. 1998, Standard Star Monitoring Memo Wfpc2_memos/wfpc2_stdstar_phot3.html Majorana, L. N., & Smith, S. T., 1993, SPIE Proc. 1938, 163 Stiavelli, M. & Baggett, S. 1996, WFPC2 ISR Stiavelli, M., 1997, WFPC2 TIR