Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/hst/HST_overview/documents/calworkshop/workshop2005/papers/mack.pdf Дата изменения: Wed Apr 5 00:02:25 2006 Дата индексирования: Sun Dec 23 23:00:20 2007 Кодировка: Поисковые слова: 47 tuc

The 2005 HST Calibration Workshop Space Telescope Science Institute, 2005 A. M. Koekemoer, P. Goudfrooij, and L. L. Dressel, eds.

ACS Flat Field Update and New SBC L-flats
J. Mack, R. C. Bohlin, R. L. Gilliland, and R. P. van der Marel Space Telescope Science Institute, Baltimore, MD 21218 Abstract. We present an overview of the methods used to create the flat fields which are currently in use by the ACS calibration pip eline. L-flats for the SBC detector have recently b een computed using dithered star cluster observations. Large corrections to the existing flats are required at the 10-20 p ercent level, dep ending on wavelength. A comparison with sky flats obtained via observations of the bright Earth indicate that the current pip eline flats are accurate at the 1-2 p ercent level.

1.

Introduction

The flat field reference files currently in use by the ACS calibration pip eline were derived via different methods, dep ending on the detector and the filter used. These LP-flats are a combination of two files, a P-flat which accounts for the pixel-to-pixel variations in sensitivity, and an L-flat which models the low-frequency variations in sensitivity over the detector field of view. The CCD P-flats were created in the lab oratory using an external illumination source, while the P-flats for the SBC were taken inflight using the internal lamps. The L-flats were derived from dithered stellar observations of 47 Tuc for the CCDs and NGC 6681 for the SBC MAMA. The two files were then multiplied to make the LP-flats used for calibration. This pap er consists of several topics. Section 2 describ es the techniques used to derive the LP-flats and their uncertainties for each detector/filter combination. Section 3 discusses the new SBC L-flats, which have b een recently b een computed and delivered to the calibration pip eline. Section 4 describ es using Earth flats as an indep endent means of validating the stellar L-flat technique and gives an estimate of the exp ected errors in the existing flats. Finally, Section 5 describ es known artifacts which may b e present in calibrated images and which may b e misinterpreted as flat fielding defects. 2. Flat fielding Overview

At visible wavelengths (greater than 4000 °), the CCD pip eline flats were created from A a combination of P-flats taken in the lab oratory prior to launch (Bohlin et al. 2001) and L-flats derived inflight from dithered observations of 47 Tuc (Mack et al. 2002; van der Marel 2003). From the stellar photometry, local variations in the detector resp onse of ±3% to ±9% were found, where the required correction to the lab flats increases with wavelength. These corrections were derived from observations in 6 broad-band WFC filters and 8 broadband HRC filters, and are lab eled `47 Tuc' in Table 1. The accuracy of the LP-flats derived with this technique is exp ected to b e b etter than 1%. Assuming a simple linear dep endence on wavelength, the L-flat correction for the remaining broad, medium, and narrow-band filters was computed. Using the pivot wavelength of each filter for the interp olation, these L-flats were determined from the weighted average L-flat of the two filters nearest in wavelength. These filters are lab eled `Interp' in Table 1 and are exp ected to b e accurate at the 2 - 3% level. More than 3 years of followup 67 c Copyright 2005 Space Telescop e Science Institute. All rights reserved.


68

Mack et al.

observations of the same field have b een obtained and will b e used to improve the accuracy of the interp olated flats and to quantify any temp oral changes in the L-flats while in orbit. These data will also provide a means of investigating any loss in absolute sensitivity as the detectors age. For the HRC UV filters, 47 Tuc does not provide enough signal to adequately determine the L-flats, so observations of the bright Earth were used to create the pip eline LP-flats (Bohlin & Mack 2003). While the Earth is a p oor flat field source at optical wavelengths due ° to structure in the cloud cover, the Earth is a uniform source of diffuse light b elow 4000 A due to the high optical depth ab ove the cloud layer. Unfortunately, red leaks in F220W and F250W are so large that the out-of-band light dominates, and the lab oratory flats are sup erior to the observed Earth flats. Hence, large variations in the low-frequency resp onse may still exist for these filters. Dithered observations of NGC 6681 have recently b een obtained with the HRC UV-filters will b e used to quantify the L-flat corrections required for these two filters. The HRC F330W and F344N pip eline flats, on the other hand, are defined entirely by the Earth flat data. With 20-30 observations each, over the course of 3 years, these flats have very high signal-to-noise and show rep eatability to much b etter than the required 1% accuracy.

Table 1: CCD L-flat Summary
Filter F2 F2 F3 F3 F4 F4 F5 F5 F5 F6 F6 F6 F6 F7 F8 F8 F8 2 5 3 4 3 7 0 5 5 0 2 5 6 7 1 9 5 0 0 0 4 5 5 2 5 0 6 5 8 0 5 4 2 0 W W W N W W N W M W W N N W W N LP WFC Pivot Wave n/a n/a n/a n/a 4317 4744 5023 5360 5581 5918 6311 6584 6599 7693 8060 8915 9055 L-flat Method n/a n/a n/a n/a 47 Tuc Interp Interp 47 Tuc Interp 47 Tuc Interp Interp Interp 47 Tuc 47 Tuc Interp 47 Tuc WFC Total Correction HRC Pivot Wave 2255 2716 3363 3434 4311 4776 5023 5356 5580 5888 6296 6584 6599 7665 8115 8916 9145 L-flat Method None None Earth Earth 47 Tuc 47 Tuc Interp 47 Tuc Interp 47 Tuc 47 Tuc Interp Interp 47 Tuc 47 Tuc Interp 47 Tuc HRC Total Correction

16% 10% 14%

7% 7% 6% 7% 7% 8% 9% 12%

13% 15% 18%

The SBC MAMA flat fields were derived inflight, using internal observations of the deuterium lamp (Bohlin & Mack 2005). This illumination does not simulate the OTA optics and, therefore, does not accurately model the low frequency variations in sensitivity. Thus, the internal flats are useful only for correcting the pixel-to-pixel detector resp onse. As done for the CCDs, the SBC L-flats were derived using dithered starfield observations. Instead of 47 Tuc, the UV-bright globular cluster NGC 6681, which is rich in blue horizontal branch stars, was selected. While the data were taken just after launch, they were not analyzed until recently, due to the infrequent usage of the SBC detector. Due to the failure of STIS in August 2004, the SBC is now the only HST instrument with FUV capabilities, and the ACS team has shifted the SBC calibrations to high priority.


ACS Flat Field Up date and New SBC L-flats

69

N

Figure 1: NGC 6681 mosaic from the combined, drizzled SBC exp osures. Note that this mosaic is significantly larger than the normal SBC FOV (shown in red at the center of the figure) due to the combination of dithered observations. The `rhombus-shap ed' field of view is a result of correcting for the SBC geometric distortion. Stars used to derive the improved flat fields are circled in red. The compass at top-right indicates the direction of north, where the length of the vector is 5 . The coordinates at the field center are RA=18:43:12.75, Dec=-32:17:32.76. 3. SBC L-flats

The uniformity of the SBC detector resp onse has b een assessed using multiple dithered observations of the globular cluster NGC 6681 (Mack et al. 2005). The drizzled mosaic of this field is shown in Figure 1. By placing the same stars over different p ortions of the detector and measuring relative changes in brightness, low-frequency variations in the detector resp onse (L-flats) have b een derived and used to create improved SBC flat fields for the FUV imaging filters. Follow-up observations of the same field have b een used to investigate any temp oral dep endence in the detector sensitivity. The matrix-solution program develop ed for computing the CCD L-flats (van der Marel 2003) has b een adapted to handle the SBC data. To summarize, the observed magnitude of a star at a given p osition is assumed to b e the sum of the true magnitude plus a correction term, the L-flat, that dep ends on the p osition on the detector. When a set of dithered observations is available for a given star field, the determination of b oth the L-flat and the true instrumental magnitude of each star can b e written as an over determined matrix equation. This equation has a unique minimum 2 -solution that can b e efficiently obtained through singular-value-decomp osition techniques.


70

Mack et al.

F115LP

F125LP

F140LP

F150LP

F165LP

-0.15

-0.10

-0.05

0.00

+0.05

+0.10

+0.15

+0.20

Figure 2: SBC L-flat corrections, in magnitudes, where the contours corresp ond to 0.03 steps in magnitude. Positive magnitudes (black) indicate that the photometry obtained using the existing P-flats was too faint, and negative magnitudes (white) indicate that the photometry was too bright.

The required low-frequency corrections to the inflight lamp flats are summarized in Table 2, and the smoothed L-flat images are shown in Figure 2, where the corrections range from ±8 to ±20 p ercent, dep ending on wavelength. Insufficient data were available to derive a solution for F122M, so the L-flat is simply a copy of the F115LP L-flat which is closest in wavelength. Errors of several p ercent may still exist for the F122M flat field. In addition to the low-frequency spatial variations in sensitivity, a time-dep endent comp onent has also b een found, suggesting a decline in the absolute UV sensitivity of 2 to 4 p ercent p er year for the first 1.6 years in orbit. This is consistent with the situation for the STIS FUV-MAMA, where a loss of 1 to 3 p ercent p er year was measured for the first 5 years in orbit (Stys, Bohlin, & Goudfrooij 2004). Since that time, the SBC sensitivity app ears to have stabilized. Additional followup observations are ongoing. The new SBC flat fields have b een delivered for use in the calibration pip eline, and the resulting photometric accuracy is now ±1% for F115LP, F125LP, and F140LP and ±2% for F150LP and F165LP, after correcting for b oth the new L-flats and the timedep endent sensitivity. Further development to ACS pip eline software is required b efore the new sensitivity corrections can b e applied within the HST on-the-fly-reprocessing (OTFR) pip eline.


ACS Flat Field Up date and New SBC L-flats Table 2: SBC L-flat Summary
Filter F1 F1 F1 F1 F1 1 2 4 5 6 5 5 0 0 5 L L L L L P P P P P SBC Pivot Wave 1406 1438 1527 1611 1758 L-flat Method NGC 668 NGC 668 NGC 668 NGC 668 NGC 668 SBC Total Correction 16% 17% 23% 21% 40%

71

1 1 1 1 1

4.

HRC Earth Flats

In order to verify the accuracy of the pip eline flat fields, several hundred observations of the bright Earth during occultation were obtained using the full set of HRC standard filters (Bohlin et al. 2005). While most of these images show streaks or other non-uniform illumination, a significant numb er are defect-free and can b e used for comparison with the existing pip eline flats. Unfortunately, the WFC Earth flats suffer from a shutter light leak, and the Earth limb is too bright for SBC observations, so the HRC is the b est detector for comparison. In Table 3, the total numb er of streak-free flats is given in column 2 and the total range of deviation (minimum to maximum) b etween the pip eline LP-flat and the new average Earth flat is given in column 3. In general, the flat fields are confirmed to a precision of 1%, validating the stellar L-flat technique. While the `interp olated' L-flats were exp ected to have larger uncertainties, they do not app ear to b e significantly worse than the L-flats derived from direct observations. One exception is the F550M filter which shows a total deviation of more than 2%. Other exceptions are the four longest wavelength HRC filters which show large systematic differences with the pip eline flats. These differences app ear to b e caused by stray light originating from the detector surface, where most of the long wavelength photons are reflected and then scattered back from nearby focal plane structures. Any filter transmitting at these long wavelengths will see the extra pattern from this light, though the strength of the additional stray light will b e prop ortional to the total flux of the source. Thus, for large diffuse ob jects that fully illuminate the detector, these Earth flats are more appropriate for calibration than the existing pip eline flats, which are appropriate for p oint sources. Table 3: HRC Earthflat Observations
Filter F330W F344N F435W F475W F502N F555W F550M F606W F625W F658N F660N F7 F8 F8 F8 7 1 9 5 5 4 2 0 W W N LP Nobs 20 34 10 6 6 4 4 3 6 6 3 2 3 6 4 Total Deviation 0.4±0.7% 0.1±0.9% 0.9±0.9% 0.7±0.8% 1.3±0.4% 1.8±1.6% 2.4±1.3% 1.1±1.1% 1.4±0.8% 1.2±0.7% 0.9±1.0% 2 4 6 8 .6 .0 .5 .4 ± ± ± ± 0 1 1 3 .5 .9 .5 .0 % % % % L-flat Method Earth Earth 47 Tuc 47 Tuc Interp 47 Tuc Interp 47 Tuc 47 Tuc Interp Interp 47 Tuc 47 Tuc Interp 47 Tuc


72

Mack et al.

Figure 3: WFC bias offset (left) and scattered Earth light (right). 5. 5.1. Artifacts affecting the `flatness' of calibrated observations Bias Offsets

Calibrated WFC science images processed with CALACS often exhibit a residual offset in their absolute levels at the b oundary of the A-B and C-D amplifiers. In Figure 3 (left), a calibrated WFC F814W image, binned 8x8, shows this quadrant-dep endent residual bias offset b etween amplifiers A and B. This effect can b e attributed to uncertainties in the bias level subtraction due to random variations in the difference b etween the leading physical overscan and the bias level in the active area (Sirianni et al. 2003), where the amplitude of this residual offset is typically less than 4 DN. The total background level in each science image is therefore comp osed of the actual sky background, modulated by the spatiallyvarying quantum efficiency of the detector, plus a bias offset that is constant over the image quadrant/amplifier. Failure to remove this additional offset prior to flat fielding will result in calibrated images which app ear to have a slight flat fielding error. This effect will b e more apparent in narrow-band images which have a lower contribution from the sky background. 5.2. Scattered Earth Light

In an effort to minimize the background light in HST images, observations are nominally scheduled only when the telescop e is more than 20 degrees from the bright Earth limb. The total background contributed by scattered Earth light dep ends on the angle b etween the HST p ointing and the nearest Earth limb, and whether the limb was sunlit (bright) or dark. This additional background light is not constant, but app ears as a gradient in brightness across the field of view. This gradient may b e misinterpreted as p oor flat fielding, while in fact it is a true representation of the sky background. In Figure 3 (right), an extreme example of scattered light is shown for an Earth limb angle of 14 degrees. This WFC F606W image is binned 8x8 and shows a gradient of 15% across the detector. Such scattered light is b est removed by fitting the sky background with a two-dimensional surface, rather than assuming a constant sky value.


ACS Flat Field Up date and New SBC L-flats References

73

Bohlin, R. C., Hartig, G., & Martel, A. 2001, Instrument Science Report ACS 2001-11 (Baltimore: STScI), available through http://www.stsci.edu/hst/acs Bohlin, R. C. & Mack, J. 2003, Instrument Science Report ACS 2003-02 (Baltimore: STScI) Bohlin, R. C. & Mack, J. 2005, Instrument Science Report ACS 2005-04 (Baltimore: STScI) Bohlin, R. C., Mack, J., Hartig, G., & Sirianni, M. 2005, Instrument Science Report ACS 2005-12 (Baltimore: STScI) Mack, J., Bohlin, R. C., Gilliland, R. L., van der Marel, R. P., Blakeslee, J. P., & de Marchi, G. 2002, Instrument Science Report ACS 2002-08 (Baltimore: STScI) Mack, J., Gilliland, R. L., van der Marel, R. P., & Bohlin, R. C. 2005, Instrument Science Report ACS 2005-13 (Baltimore: STScI) Sirianni, M., Martel, A. R., Jee, M.J., van Orsow, D. & Sparks, W. B. 2003, in Proc. 2002 HST Calibration Workshop, ed. S. Arribas, A. Koekemoer, & B. Whitmore (Baltimore: STScI), p. 82 Stys, D. J., Bohlin, R. C., & Goudfrooij, P., 2004, Instrument Science Report STIS 2004-04 (Baltimore: STScI), available through http://www.stsci.edu/hst/stis van der Marel, R. P. 2003, Instrument Science Report ACS 2003-10 (Baltimore: STScI)