SBC FLATS: PRISM P-FLATS and IMAGING L-FLATS

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1 Instrument Science Report ACS SBC FLATS: PRISM P-FLATS and IMAGING L-FLATS R. C. Bohlin & J. Mack December 2006 ABSTRACT The internal deuterium lamp was used to illuminate the SBC detector through the PR110L and PR130L prisms for 12.2 hours each to produce a total of ~12,000 counts/pixel. This illumination does not simulate the OTA optics and, therefore, is not suitable for the production of a low frequency L-flat. However, the pixel-to-pixel P-flat is an improvement over the laboratory SBC P-flat currently used in the ACS pipeline for the two dispersing modes. In addition, short exposure internal lamp flats were obtained in the standard imaging filters. These flats have sufficient signal to define the low frequency L-flat field for five filters relative to the high signal F125LP flat, assuming that the relative lamp illumination does not vary with wavelength. These five ratio L-flats are smoother than the ratios of the current pipeline L-flats; but there is evidence for variation of the internal lamp illumination with wavelength. Thus, the current SBC L-flats may have some errors of a few percent due to local inappropriate lumpiness; but the alternative flats defined by the internal illumination may also have errors. 1. Introduction Because uniform flat field illumination in the Å region is available neither in the lab nor in space, the low-frequency L-flat field correction for the SBC filters on ACS is problematic. However, pipeline processing does not require an L-flat correction for prism data. Instead, Larsen (2006) derives sensitivities that depend on location on the detector. The internal deuterium lamp does produce a smoothly varying illumination 1

2 pattern that can be used to define a P-flat. Section 2 of this ISR addresses the creation of a new P-flat for the SBC prism modes, while section 3 discusses the relative L-flat correction for five imaging filters. 2. Prism P-flat The prism P-flat differs from the imaging filters, because the illumination has a different angle of incidence Bohlin, Hartig, and Meurer (1999, BHM). Because the P-flat for the prisms has probably changed by an amount that is comparable to the change found by Bohlin & Mack (2005) from ground to space for the imaging filters, an additional set of internal lamp exposures are analyzed to define a P-flat appropriate for on-orbit use for the prism modes. Figure 1 - Relative count rate vs. total time of illumination of the SBC detector by the internal deuterium flat field lamp. The 32 points after ~21 hours are 16 PR130L and 16 PR110L points, which are normalized to the earlier F125LP data. 2

3 The internal lamp exposure times are all 2750s for the 16 PR110L and 16 PR130L images that were obtained in program over the time period of 2005Oct4 to 2005Nov5. The total exposure time is 24.4 hours, which resulted in ~12,000 total counts/pixel near the center of the SBC MAMA detector. Figure 1 shows the degradation of the lamp output vs. exposure time. Based on pre-launch testing, the declining lamp output is probably caused by polymerization of contaminants on the lamp window. In order to verify the claim by BHM that the SBC flats are independent of wavelength, the 16 individual exposures for each prism are co-added and fit following the procedures summarized in Bohlin et al. (1999) The IDL routine flatall along with its main subroutines flats and fitflat are used to produce the P-flat by fitting the overall vignetting pattern with low order splines. This internal vignetting of the lamp illumination is about 10% from image center to the edges, while the P-flat is equal to the original co-added image divided by the fitted pattern. The data quality flags in the third extension of the flat field file are all zero, except for values of 512 around the edges, in the un-illuminated region beyond column 850, in rows 599:604 with dead MAMA diodes, and for 295 pixels listed in the bad pixel table lch1502jj_bpx.fits. The new PR110L and new PR130L P-flats are typically identical to % of excess residual scatter over the differences expected from the Poisson counting statistics. With an expected Poisson uncertainty per pixel of 0.9% in the combined 32 prism exposures, the intrinsic differences between PR110L and PR130L are small enough so that the best P-flat is made from the co-addition of all 32 internal prism exposures. The ratio of this new prism flat to the original lab P-flat is illustrated in Figure 2. In the dashed box of Figure 2, the expected Poisson scatter of 1.13% is comprised of 0.7% from the original lab flat and 0.9% from the new in-flight P-flat. The error in using the original flat is 1.08% from the combined (in quadrature) of its 0.70% Poisson statistics and the 0.82% systematic error. Assuming 0.3% systematic error caused by averaging the PR110L and PR130L images and combining with the 0.9% statistical uncertainty, the new P-flat uncertainty of 0.95% per pixel is slightly better than the 1.08% error expected in the old P-flat. The intrinsic differences at the lower right in Figure 2 increase from 0.82% to over 1% due to a slight shift in the Moire interference pattern between the micro-channel plate and the anode array of the SBC MAMA detector. Thus, the advantage of the new prism P-flat is greater in the lower right corner of the SBC. 3

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5 Figure 2 - Ratio of the on-orbit SBC P-flat to the laboratory P-flat for the prisms. The name of the lab flat, psbcex99marsm20prism, is the sum 20 images with external illumination obtained in March of 1999 and is also known as jref$m5t12222j_pfl in the ACS pipeline processing system. The grey scale calibration is indicated on the reference bar at the top right. The line of text "P-flat rms(%)= " indicates one-sigma values in the 101x101 pixel standard region at 452:552,462:562 (dashed box) for the total pixel-to-pixel scatter, the Poisson counting statistic, and the intrinsic rms variation of the ratio. The horizontal grey strip is the region of bad anodes, where the flats are set to unity. The vertical feature near column 578 is caused by the slightly different illumination in the region of the repeller wire. The uniform grey region beyond column 850 at right hand side is outside the field-of-view of the prisms. 3. Imaging L-flats Even though the P-flat is independent of wavelength, the L-flat is not. Mack, et al. (2005, MGVB) used observations of the NGC6681 star cluster to determine SBC L-flats on a lightly smoothed 16x16 grid. The S/N of the hot blue HB stars in NGC6681 is the highest at the shorter wavelengths. The formal uncertainties in the L-flat are the lowest for F125LP, which also has the most observations. An alternative method for defining the remaining five SBC imaging L-flats uses short exposure internal deuterium lamp flats. These flats have sufficient signal to define the low frequency L-flat field for five filters relative to the high signal stellar F125LP L-flat, assuming that the relative lamp illumination does not vary with wavelength. These five ratios of internal lamp images are smoother than the corresponding ratios of the current MGVB pipeline L-flats. For example in the worst case with the lowest S/N for F165LP, the ratio F165LP/F125LP is shown in Figure 3. If the deuterium lamp illumination of the detector-filter combination is the same for all six filters, the internal flat field lamp can be used to find differences in the L-flat between filters. Single exposures of 2500s obtained in program in each of F115LP, F122M, F140LP, F150LP, and F165LP are sufficient to define the delta L-flats relative to the F125LP L-flat from MGVB. The ratio of the MGVB stellar L-flats in panel c) of Figure 3 is lumpier than the corresponding ratio of the L-flat structure in the internal lamp images illustrated in panel d) by up to 7% in a hot spot (white) in the lower left quadrant of panel c). MGVB estimate a systematic uncertainty of ~4% for F165LP in their table 3. Alternative L-flats, L', for the five filters in Table 1 are formed by multiplying the F125LP stellar L-flat by the appropriate internal lamp ratio, eg. multiplying panel b) by the F165LP/F125LP ratio of panel d) from the internal lamp. 5

6 Figure 3 - Top panels: L-flats from MGVB a) F165LP and b) F125LP scaled to unity in the central 101x101 pixel box. Lower panels: c) Ratio of panel a) to panel b) for the stellar L-flats from MGVB. d) Corresponding ratio of L-flat response to the internal deuterium lamp for F165LP/F125LP. 6

7 Table 1 summarizes the large-scale and small-scale differences between the five L-flats derived from the deuterium lamp data and the MGVB pipeline flats. The two "64 Bin" columns are the large-scale minimum and maximum values of the MGVB/internal L/L' ratio averaged over 8x8=64 boxes of 128x128 pixels per box, while the last two columns are the local, small-scale minimum and maximum of the L-flat ratios within the central 512x512 pixels of the 1024x1024 pixel SBC image format. The total range of 64-bin differences exceeds 10% only for F165LP. Both the min and max of the range of these large-scale differences are usually at an edge of image, where any change in the internal lamp illumination pattern with wavelength would contribute the most. The min and max of the local, smaller-scale lumpiness in the central 512x512 pixel region minimize any effect of changing illumination with wavelength and demonstrate agreement between L and L' to 4%, except for F165LP. TABLE 1. Ratio of Stellar to Internal-lamp L-flats Large scale Smaller scale Filter 64 Bin Central 512 px min max min max F115LP F122M F140LP F150LP F165LP Complete reduction, including time dependent corrections, is done for all the NGC6681 F165LP data; and stellar photometry is derived using the MGVB pipeline L-flats. This standard photometry, L, can be corrected to L' photometry for the alternative internal lamp L'-flat. If the lamp L'-flat is superior to the stellar L-flat, then the rms scatter in the dithered observations of NGC6681 should be lower in the L' photometry. The biggest differences between L and L' photometry should be for stars lying nearest to the hotspot, where L/L' reaches a maximum of 1.07 in panel c) of Figure 3. There are seven stars where at least one observation lies in box centered on the hotspot at (360,320) with x-size of 64 pixels and y-size of 74 pixels. Each of these seven stars has 33 observations, so that the seven rms measures are robust. Five stars show an improved rms scatter with the L' lamp flat, where the biggest improvement is from to mag. However, the rms scatter for two of the seven stars increases; and most of the outliers for the L' lamp photometry tend to lie near the top of the image. Thus, the L'-flat may be good locally; but there may be wavelength dependent errors that slowly vary across the full field. 7

8 To test for local improvements, the only photometry considered is still for the same seven stars; but every point is restricted to lie within a box of 200x200 pixels, centered on the hotspot. For this restricted case, the number of repeat observations is 7-8 for three stars and for four stars; and the rms scatter is always better for the L' photometry by as much as a factor of two. In summary, the relative L'-flats from the deuterium internal lamp suggest that the flats derived from the stellar observations by MGVB are somewhat too lumpy; and the final L-flats should be restricted to have stiffer fits to the stellar data. Unfortunately, the relative L'-flats from the internal lamp data may have significant low order errors across the full field that could be attributed to a gradient in the relative lamp illumination that varies with wavelength. Ongoing SBC observations of NGC6681 will be combined with existing observations; and revised L-flats will be made that should have ratios similar to the internal lamp ratios, eg. Figure 3d. REFERENCES Bohlin, R., Hartig, G., Lindler, D., Meurer, G., & Cox, C. 1999, Instrument Science Report, ACS 99-01, (Baltimore:STScI) Bohlin, R., Hartig, G., & Meurer, G. 1999, Instrument Science Report, ACS 99-02, (Baltimore:STScI), BHM Bohlin, R. C., & Mack, J. 2005, Instrument Science Report, ACS , (Baltimore:STScI) Larsen, S. 2006, Instrument Science Report, ACS 06-02, (Baltimore:STScI) Mack, J., Gilliland, R., van der Marel, R., & Bohlin, R. 2005, Instrument Science Report, ACS , (Baltimore:STScI), MGVB 8