Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.stsci.edu/hst/wfc3/documents/ISRs/WFC3-2008-15.pdf Äàòà èçìåíåíèÿ: Thu Jul 17 00:43:09 2008 Äàòà èíäåêñèðîâàíèÿ: Sat Sep 6 20:50:24 2008 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: guide 8.0

ST-ECF Instrument Science Report WFC3-2008-15

The TV2 ground calibrations of the WFC3 NIR grisms
H. Kuntschner, H. Bushouse, J. R. Walsh, M. KÝmmel July 08, 2008

ABS

TRACT

Based on thermal vacuum tests (TV2; June - August 2007), the performance of the WFC3 near-IR G102 and G141 grisms has been assessed. The locations of the different orders relative to exposures taken through a direct imaging filter are determined, trace and wavelength solutions are derived, and the absolute throughput of the different orders is quantified. The trace and wavelength solutions were found to be linear functions varying smoothly as function of field of view. Aperture corrections are given for both grims as function of wavelength. Furthermore, we describe flat-field cubes that provide pixel-to-pixel information as function of wavelength to an accuracy of about 1%. New detectors in the UV and IR channels are being tested in TV3 (March ­ April 2008), therefore updates for the sensitivity calibrations are to be expected while geometric properties are predicted to remain stable.

1.

Introduction

The Wide Field Camera 3 (WFC3) is fitted with three grisms for slitless spectroscopy. In the UVIS channel there is one grism, G280, for the near-UV to visible range (200 - 400nm). The NIR channel has two grisms (G102 and G141) for the shorter (800 - 1150nm) and longer NIR wavelengths (1100-1700nm).
The Sp ace Telescope European Coordinating Facility. All Rights Reserved.


ST-ECF Instrument Science Report WFC3-2008-15 The fundamental design parameters of a grism are the deflection of the incident beam by the grism (defined by the prism angle), the dispersion in the various orders and the energy in each order (defined by the groove frequency and profile). In order to extract slitless spectra from grism images it is necessary to know these parameters and their variation with position in the field. With a good parameterization, then extraction software, such as the aXe tasks developed at the ST-ECF (http://www.stecf.org/software/slitless_software/), can be applied to extract multiple slitless spectra from sky images in a semi-automatic fashion. A further use of the parameterization is to provide a simulation package which can predict two-dimensional dispersed images of a given sky region if an input catalogue is provided. The ST-ECF has developed such a package (aXeSIM; KÝmmel, Kuntschner, Walsh 2007), which was available to the Cycle 17 proposal deadline. During the Thermal Vacuum testing of WFC3 in June - August 2007 (TV2), specific calibrations for the grisms were included. The tests were designed to allow determination of the spectral trace, the dispersion solution and the absolute instrument throughput by using a combination of monochromator and white light source measurements. Additionally, the detector flat-field was determined with 10nm wide bandpasses in steps of 20nm. This ISR describes the implementation of the tests and the analysis of the results. Tables provide the details of the field position dependent spectral trace and the dispersion solutions for the two IRchannel grisms; measurements of the total instrument throughput and information on the wavelength dependent flat-field are also provided. This instrument science report (ISR) is a follow up on the report by Larsen, Bushouse, & Walsh (WFC3-2005-07) from the TV1 calibrations.

2.

Calibration set-up for TV2

The grism test procedures were executed during June - August 2007, with the WFC3 in a flight-like thermal-vacuum environment. The WFC3 external optical stimulus system, CASTLE, was used to provide the necessary source targets for the tests. For these tests, the WFC3 IR detector (IR1 "FPA129") was at an operating temperature of -123°C and the IR Filter Select Mechanism (FSM) was at a temperature of -42°C. The IR16S30 series and IR16S40 series test procedures use sets of point-source exposures, all of which were obtained with the CASTLE quartz-tungsten-halogen (QTH) lamp, and a 10 micron pinhole to provide an unresolved target, which is used to determine the dispersion solution and spectral trace for each grism. The IR14S13 and IR14S14 test procedures use an extended source (200 micron pinhole) to provide high S/N photometric measurements of the absolute instrument throughput with the grisms in place. 2


ST-ECF Instrument Science Report WFC3-2008-15

3.

Calibration measurements

For each IR grism calibration measurements were carried out to allow determination of the spectral trace, the dispersion solution and the absolute instrument throughput as well as a wavelength dependent flat-field. A summary of all NIR grism calibration procedures is given in Table 1. Absolute throughput is determined for a central position only, while trace and dispersion calibrations are performed in a 5-point pattern (center and the four quadrants) to allow for a 2-dim solution covering the full field-of-view. Figure 1 shows graphically the distribution of the individual pointings. Since the 1st order spectrum is located to the right of the direct image position (see also Figures 2 and 3), the point source positions are not centered on the detector but shifted towards smaller x-axis values.

Figure 1: Distribution of the point source positions for the G102 trace and wavelength calibrations. The setup for G141 is identical. Quadrants as noted in Table 1 are counted such that Quadrant 1 is in the upper left corner and then numbers increase anti-clockwise.

3


ST-ECF Instrument Science Report WFC3-2008-15 Table 1. Summary of NIR grism calibrations Test procedure IR14S11 IR14S12 IR14S13 IR14S14 IR16S30 IR16S30 IR16S31 IR16S32 IR16S33 IR16S34 IR16S40 IR16S40 IR16S41 IR16S42 IR16S43 IR16S44 Date 200720072007200720072007200720072007200720072007200720072007200707 07 07 07 07 08 08 08 08 08 07 08 08 08 08 08 15 16 27 27 06 20 20 20 20 20 06 20 20 20 20 20 G G G G G G G G G G G G G G G G G rism 102 141 102 141 102 102 102 102 102 102 141 141 141 141 141 141 Purpose Flat-field Flat-field Absolute throughput Absolute throughput Dispersion (center) Dispersion (center) - repeat Dispersion (quadrant 1) Dispersion (quadrant 2) Dispersion (quadrant 3) Dispersion (quadrant 4) Dispersion (center) Dispersion (center) - repeat Dispersion (quadrant 1) Dispersion (quadrant 2) Dispersion (quadrant 3) Dispersion (quadrant 4)

The absolute throughput measurements were carried out with a calibrated monochromator source covering for the G102 grism the range 780nm - 1180nm in steps of 20nm; and for the G141 grism the range 1040nm ­ 1700nm in steps of 20nm. After the calibrations were carried out, corrections to the effective gain and neutral density filters became known (September 2007). All results presented in this report are using a gain value of 2.4 and the correction factors for the stimulus flux (OSFLUX keyword) were applied. The trace and dispersion calibrations were carried out starting with a direct image and grism pairs using a white light stimulus. monochromator steps covering for the G102 grism the range 760nm steps of 20nm; and for the G141 grism the range 1000nm ­ 1700nm 50nm, were performed. In all cases, the monochromator bandwidth was set of three After that 1180nm in in steps of 10 nm.

Additionally, the detector flat-field was determined with 10nm wide bandpasses in steps of 20nm, covering for the G102 grism the range 780nm - 1180nm; and for the G141 grism the range 1060nm ­ 1700nm.

4.

Analysis

In this section we describe the analysis of the calibration data yielding calibrations for the trace, wavelength and absolute throughput.

4


ST-ECF Instrument Science Report WFC3-2008-15

4.1.

Trace calibration

The ST-ECF aXe package for reduction of slitless spectroscopy data treats the spectral traces and wavelength solutions defined with respect to the position of the source in the direct image. The centroids of the continuum source images in the direct imaging exposures (Xref, Yref) were determined with the IRAF task imexam. These positions were assumed not to change through the duration of the remaining measurements. The continuum lamp spectra (see also Figure 2 and 3) were traced as a function of X=X-Xref in the detector X-direction by measuring the centroids of 5-7 pixel wide bins, using custom-written IDL programs. We found the traces of all orders (excluding the zeroth order) to be well fit by straight lines with standard deviations of 0.07 pixels. The trace definitions are of the form (YYref)=DYDX_0 + DYDX_1*X, where DYDX_0 and DYDX_1 are field dependent and given in the usual format used by the ST-ECF aXe reduction package, e.g., DYDX_1 = a0 +a1*Xref + a2*Yref + ...(see also aXe manual for more details). The coefficients of our field dependent fits are given in Table 2 and 3 for the G102 and G141 grisms, respectively. Given the sparse sampling of the field-of-view (see Figure 1) only linear terms are considered in the field dependent fits. With an accuracy of better than one pixel the traces of the orders are collinear. We also derive trace solutions for the 0th order, however, these are only approximate, and hence meaningful errors estimates could not be derived (see Tables 2 and 3).

Figure 2: Sum of a direct image (F105W) and a G102 exposure of the continuum source. A circle marks the location of the source in the direct image, and the grism +1st and +2nd orders can be seen extending towards the right. The grism 0th order is also visible as a point source like feature to the left of the direct image. The full extent of the detector (1014 pixels) is shown in x-direction. Several "strips" of bad pixels are visible just above the second order. 5


ST-ECF Instrument Science Report WFC3-2008-15

Figure 3: Sum of a direct image (F139W) and a G141 exposure of the continuum source. A circle marks the location of the source in the direct image, and the grism +1st, +2nd and +3rd orders can be seen extending towards the right. The grism 0th order is again visible as a point source like feature to the left of the direct image. Table 2: Field dependent trace descriptions for G102. Errors are given in brackets. Term +1st order DYDX_A_0 DYDX_A_1 0th order DYDX_B_0 DYDX_B_1 XOFF_B +2nd order DYDX_C_0 DYDX_C_1 a0 -6.2287E-01 (8.9206E-02) 9.3587E-03 (8.4666E-04) -3.6549E+00 0.0 -2.5918E+02 5.4198E(4.6494E8.0690E(7.4165E01 02) 03 05) a1(X) 9.7066E(1.8662E1.6237E(1.7712E05 04) 07 06) a2(Y) -1.4220E-03 (1.4483E-04) 3.6163E-06 (1.3746E-06) -1.1890E-03 1.5011E-02 -3.4318E-03 (1.1598E-04) 5.3395E-06 (1.8500E-07)

8.4494E-04 -1.7154E-03 -6.5235E-04 (2.7063E-04) -1.0805E-06 (4.3170E-07)

6


ST-ECF Instrument Science Report WFC3-2008-15 Table 3: Field dependent trace descriptions for G141. Errors are given in brackets. Term +1st order DYDX_A_0 DYDX_A_1 0th order DYDX_B_0 DYDX_B_1 XOFF +2nd order DYDX_C_0 DYDX_C_1 +3rd order DYDX_D_0 DYDX_D_1 a0 1.9770E+00 (7.0509E-02) 4.9871E-03 (9.3977E-04) 6.5863E-01 0.0 -1.9224E+02 2.2577E+00 (5.8889E-02) 3.6282E-03 (1.1997E-03) 2.1320E+00 (7.3501E-02) 5.9995E-03 (2.1223E-04) a1(X) 4.7900E(1.4747E8.7047E(1.9655E05 04) 07 06) a2(Y) -1.8890E-03 (1.1442E-04) 4.3156E-06 (1.5250E-06) -1.6251E-03 1.1109E-02 -3.0160E-03 (9.5561E-05) 6.9699E-06 (1.9468E-06) -2.7934E-03 (1.8324E-04) 2.4976E-06 (5.2910E-07)

1.5840E-04 -2.3144E-03 1.6404E(1.2317E2.6386E(2.5092E04 04) 07 06)

2.9334E-04 (4.2827E-04) -5.6133E-07 (1.2366E-06)

4.2.

Wavelength solutions

The trace definitions derived in Section 4.1 were inserted into a configuration file for the spectral extraction software (aXe) and each of the monochromator spectra were then extracted using the standard aXe task AXECORE. The software automatically extracts the spectral orders as separate "beams", defined with respect to the location of the object in the direct image. The FITS files produced by aXe were then analyzed by custom built IDL scripts where the (X-Xref) location of the peak of each monochromator spot was measured by fitting a Gaussian. The wavelength solutions were found to be well approximated by linear fits to wavelength versus pixel offset. For G102 the standard deviation around the linear fit was typically 2 å for the 1st order (dispersion ~25 å/pixel) and 3-4 å for the second order (dispersion ~12 å/pixel). For G141 the standard deviation around the linear fit was typically 2 å for the 1st order (dispersion ~47 å/pixel) and 1-3 å for 7


ST-ECF Instrument Science Report WFC3-2008-15 the second order (dispersion ~23 å/pixel). Within the uncertainties, the wavelength solutions behave as expected, with the +1st and -1st orders having similar dispersions, and the +2nd and +3rd orders having dispersions of 1/2 and 1/3 of that of the 1st order. The 0th order spectra also show a small dispersion. We do not characterize the 3rd order for G102 since it is mostly outside the detector area for the available pointings. Similar to the trace solutions, the dispersion solutions also show variations across the field of view that can be approximated by linear fits. For example, for the G102 grism (1st order), the dispersion varies from 24.0 to 25.8 å/pixel across the field of view mostly as function of y-axis position, while for the G141 grism it varies from 45.5 to 48.0 å/pixel. The coefficients of our field dependent fits are given in Table 4 and 5 for the G102 and G141 grisms, respectively.

Table 4: Field dependent dispersion descriptions for G102. Errors are given in brackets. Term +1st order DLDP_A_0 DLDP_A_1 0th order DLDP_B_0 DLDP_B_1 +2nd order DLDP_C_0 DLDP_C_1 a0 6.3571E+03 (7.1589E-01) 2.3924E+01 (1.2344E-02) 1. (1. 1. (1. 0436E 8076E 3707E 3176E +04 +02) +03 +02) a1(X) 5.3489E-02 (1.6472E-03) -1.3224E-04 (2.8403E-05) -1.3314E+00 (8.7153E-01) -5.5924E-01 (6.3526E-01) 8.8933E-02 (9.4755E-03) -2.0007E-04 (7.1378E-06) a2(Y) 3.9486E(1.2695E1.4786E(2.1891E5.2446E(6.0628E5.3204E(4.4192E03 03) 03 05) 01 01) 01 01)

3.2105E+03 (1.0574E+00) 1.2058E+01 (7.9651E-04)

-1.3463E-02 (3.5520E-03) 7.7748E-04 (2.6757E-06)

8


ST-ECF Instrument Science Report WFC3-2008-15

Table 5: Field dependent dispersion descriptions for G141. Errors are given in brackets. Term +1st order DLDP_A_0 DLDP_A_1 0th order DLDP_B_0 DLDP_B_1 +2nd order DLDP_C_0 DLDP_C_1 +3rd order DLDP_D_0 DLDP_D_1 a0 8.9124E+03 (3.0080E+00) 4.5312E+01 (3.0552E-02) 1. (3. 2. (1. 3220E 4769E 3240E 6974E +04 +01) +03 +02) a1(X) 1.0482E-01 (6.9200E-03) -2.0978E-04 (7.0284E-05) -7.5031E-01 (1.6754E-01) 1.6366E-01 (5.2257E-02) -4.1939E-04 (1.2159E-04) -3.0712E-04 (3.1585E-02) -4.2190E-05 (3.1516E-05) a2(Y) 9.1073E(5.3318E2.8211E(5.4153E03 03) 03 05)

-1.7850E-01 (1.1657E-01) -6.8672E-02 (1.9562E-02) 1.6253E-03 (4.5518E-05) 2.7126E(1.1824E8.9823E(1.1798E02 02) 04 05)

4.5015E+03 (5.8268E+00) 2.2829E+01 (1.3558E-02) 3.0548E+03 (3.5218E+00) 1.5183E+01 (3.5142E-03)

4.3.

Absolute throughput calibrations

For each of the calibrated monochromator settings (see also Section 3) and all visible orders, the detected counts were measured with the IRAF task imexam. The aperture radius was adjusted with 3 iterations generally resulting in radii of about 17 pixel. The ratio of detected flux (using a gain of 2.4 e/DN) versus the incoming flux as recorded in the image headers (keyword OSFLUX, given as flux per second) gives then the instrument efficiency. Figures 4 and 5 present the efficiency curves for WFC3 with the G102 and G141 grisms, respectively. The plots show the efficiency of WFC3 as measured in TV2 but do not include any contributions from the OTA throughput. For G102, the peak efficiency of ~31% is reached between 1020 and 1120nm in the +1st order. Efficiency above 10% is achieved over a broad wavelength range from 820 to 1140nm. Due to the excellent design of the grism, the throughput in the +2nd 9


ST-ECF Instrument Science Report WFC3-2008-15 and zeroth order is much lower with a maximum of 4.4 and 1.4%, respectively. For G141, the peak efficiency of ~41% is reached between 1440 and 1640nm in the +1st order. Efficiency above 10% is achieved over a broad wavelength range from 1100 to 1680nm, thus giving continuous wavelength coverage from 820 to 1680nm with the two grisms together. Due to the excellent design of the G141 grism, the throughput in the +2nd, 3rd and zeroth order is much lower with a maximum of 6, 0.5 and 1.4%, respectively. Tables 6 and 7 present the efficiency measurements in tabular form for the G102 and G141 grisms, respectively.

Figure 4: Instrument efficiency of various orders for the WFC3 G102 grism as function of wavelength.

10


ST-ECF Instrument Science Report WFC3-2008-15

Figure 5: Instrument efficiency of various orders for the WFC3 G141 grism as function of wavelength. The wavelength coverage is interrupted for 0th and 2nd orders on account of insufficient signal.

11


ST-ECF Instrument Science Report WFC3-2008-15

Table 6: Instrument efficiency for WFC3 G102
Waveleng th ( nm ) 7800 8000 8200 8400 8600 8800 9000 9200 9400 9600 9800 10000 10200 10400 10600 10800 11000 11200 11400 11600 11800 0th ord er 0.0040 0.0106 0.0131 0.0139 0.0130 0.0136 0.0087 0.0109 0.0098 0.0073 0.0045 0.0036 0.0057 0.0020 0.0023 0.0018 0.0010 0.0021 0.0017 0.0006 1st ord er 0.0301 0.0886 0.1232 0.1676 0.1835 0.2323 0.2469 0.2670 0.2747 0.2908 0.3043 0.2969 0.3107 0.3089 0.3021 0.3068 0.3193 0.3136 0.2656 0.0427 0.0007 2nd order 0.0169 0.0379 0.0414 0.0441 0.0404 0.0322 0.0323 0.0234 0.0215 0.0187 0.0142 0.0135 0.0114 0.0075 0.0054 0.0046 0.0036 0.0025 -

12


ST-ECF Instrument Science Report WFC3-2008-15

Table 7: Instrument efficiency for WFC3 G141
Waveleng th (nm) 10400 10600 10800 11000 11200 11400 11600 11800 12000 12200 12400 12600 12800 13000 13200 13400 13600 13800 14000 14200 14400 14600 14800 15000 15200 15400 15600 15800 16000 16200 16400 16600 16800 17000 0th ord er 0.0005 0.0016 0.0064 0.0118 0.0138 0.0136 0.0114 0.0103 0.0098 0.0082 0.0069 0.0061 0.0047 0.0044 0.0020 0.0021 0.0013 0.0014 0.0020 0.0022 0.0045 0.0061 0.0082 0.0106 0.0111 0.0050 0.0111 0.0022 1st ord er 0.0044 0.0191 0.0846 0.1724 0.2343 0.2386 0.2516 0.2697 0.2957 0.3097 0.3291 0.3266 0.3297 0.3571 0.3603 0.3665 0.3691 0.3669 0.3760 0.3956 0.4051 0.4019 0.4168 0.4127 0.4067 0.4094 0.4011 0.4147 0.4167 0.4148 0.4119 0.3899 0.1711 0.0533 2nd order 0.0025 0.0091 0.0322 0.0519 0.0604 0.0519 0.0477 0.0420 0.0376 0.0286 0.0228 0.0208 0.0157 0.0145 0.0116 0.0086 0.0066 0.0048 0.0032 0.0015 0.0026 0.0035 0.0010 0.0009 0.0006 0.0001 3rd ord er 0.0002 0.0008 0.0027 0.0039 0.0055 0.0046 0.0043 0.0031 0.0042 0.0036 0.0029 -

13


ST-ECF Instrument Science Report WFC3-2008-15

4.4.

Aperture corrections

Aperture corrections were derived for both near-IR grims by using the aXe software to extract a rectified 2-dim image around a continuum exposure. The resulting image (called "stamp" image) is wavelength calibrated and image distortions as recorded in our trace calibrations are removed. From this image covering a large region around the continuum exposure various sub-apertures (here used in the sense of a diameter) are extracted with the help of IDL scripts and compared to the flux one determines in the largest aperture. This procedure is carried out as function of wavelength in six bins for each grism. The resulting aperture correction values are given in Tables 8 and 9 for the G102 and G141 grisms, respectively. For both grisms about 95% of the flux is concentrated in an aperture (=diameter) of about 15 pixels or 1.95 arcsec.

Table 8: Aperture corrections for WFC3 G102
Apert ure [pixel] 1 3 5 7 9 11 13 15 29 59 101 201 8850 å 0.432 0.791 0.859 0.889 0.913 0.932 0.943 0.952 0.974 0.991 0.997 1.000 9350 å 0.364 0.787 0.868 0.901 0.924 0.939 0.950 0.957 0.977 0.990 0.996 1.000 9850 å 0.376 0.786 0.872 0.902 0.925 0.941 0.951 0.959 0.979 0.991 0.996 1.000 10350 å 0.450 0.800 0.871 0.901 0.925 0.940 0.950 0.959 0.979 0.991 0.996 1.000 10850 å 0.438 0.800 0.876 0.906 0.928 0.944 0.955 0.964 0.984 0.996 0.999 1.000 11350 å 0.385 0.791 0.883 0.914 0.937 0.953 0.963 0.970 0.987 0.995 0.998 1.000

14


ST-ECF Instrument Science Report WFC3-2008-15

Table 9: Aperture corrections for WFC3 G141
Apert ure [pixels] 1 3 5 7 9 11 13 15 29 59 101 201 11300 å 0.394 0.772 0.867 0.897 0.920 0.936 0.947 0.955 0.978 0.990 0.996 1.000 12300 å 0.388 0.760 0.850 0.878 0.904 0.926 0.939 0.948 0.976 0.990 0.997 1.000 13300 å 0.347 0.723 0.835 0.871 0.900 0.920 0.935 0.945 0.977 0.991 0.997 1.000 14300 å 0.313 0.698 0.834 0.874 0.897 0.916 0.933 0.944 0.977 0.991 0.997 1.000 15300 å 0.296 0.698 0.838 0.876 0.899 0.918 0.933 0.945 0.980 0.993 0.998 1.000 16300 å 0.295 0.698 0.837 0.880 0.903 0.921 0.936 0.949 0.981 0.993 0.997 1.000

4.5.

Flat-field determinations

In slitless spectroscopy each pixel can receive any wavelength in the range of the grism, so it is necessary to build a flat-field cube in order to assign a flat-field correction for all pixels and wavelengths. The aXe reduction software employs a flat-field cube in the form of a polynomial fit to the wavelength variation of the flat-field. The input data for constructing a flat-field cube are thus a set of flatfields at specified wavelengths. Using CASTLE and the monochromator the G102 and G141 grisms were illuminated with light over the relevant wavelength range of each grism: for G102 8000 to 11800å in steps of 200å; for G141 10600 to 17000å, also in steps of 200å. The monochromator slit was set to produce light of wavelength width 100å. Figure 6 shows the G102 image at 10000 å. There is a modest (~20%) illumination pattern increasing from left to right and bottom to top, which was common to all the exposures for both grisms. This was removed by fitting a surface using the IRAF task imsurfit. It was found that the illumination is rather flat to the right of column 300 so better fits could be obtained by splitting the image at row 300 and separately performing the surface fits for these two sections, then combining the results. A spline was used for the fitting function with order 5 in X and order 12 in Y for both grisms. Figure 7 shows the resulting flat-field for G102 at 10000å. Only the G102 flat is shown here; the G141 flat-field images are very similar in appearance. The sets of flat-fields were fitted pixel-by-pixel with wavelength to 15


ST-ECF Instrument Science Report WFC3-2008-15 produce the flat-field cubes. A 3rd order polynomial was found to be adequate and the residual images on the fit showed mean values of 1.2 and 0.8% for the G102 and G141 grisms, respectively. As demonstration of the wavelength dependence of the flat-fields, Figure 8 shows examples for a few selected pixels. Two pixels lying at the mean flat-field response of 1.0 are show in blue and red, as representative of the majority of the pixels. The green points show the wavelength dependence for a 'high' pixel and the magenta points for a 'low' pixel. The open circles show the points generated from the polynomial fit. One of the reasons for using monochromatic light with width 100å was to investigate any possible fringing with wavelength of the detector. Comparing images at different wavelengths, no evidence for fringing to a level of about 1% was seen. This is also apparent from the plots on Figure 8 where the flat-field behaviour is rather flat with wavelength, even for the high and low response pixels.

Figure 6: Example of the monochromatic image at 10000å taken with the G102 grism. The mean level is 8500 ADU and the greyscale range is mean 3600 - 10150 (black - white).

16


ST-ECF Instrument Science Report WFC3-2008-15

Figure 7: The resulting normalised flat-field image for the G102 grism at 10000å, derived from the image shown in Figure 6. The greyscale is -/+ 33% about the mean.

17


ST-ECF Instrument Science Report WFC3-2008-15

Figure 8: The selected pixels. plot shows the coordinates of (magenta) and which is stored

variation of the flat-field as a function of wavelength for a few Each point refers to the mean value over nine pixels. The upper behaviour for the G102 grism and the lower plot for G141. The the pixels are indicated. The open circles (for the low pixel high pixel (green) values) are derived from the polynomial fit, in the flat-field cube.

18


ST-ECF Instrument Science Report WFC3-2008-15

5.

Conclusions

This ISR presented the WFC3 IR grism calibrations carried out during thermal vacuum 2 (TV2). The grisms show linear dispersion relations and trace definitions, which vary across the field of view and can be well approximated by linear fits. The average dispersion is ~25 å/pixel and ~47 å/pixel for the G102 and G141 grisms, respectively. The new IR grisms show excellent design characteristics with more than 90% of the power in the first order. Peak efficiency of WFC3 with the G102 and G141 grisms reach ~31% and ~41%, respectively. The resulting configuration and sensitivity files were used in a simulation package (aXeSIM) for HST/WFC3 grism observations and are available from our Web pages (http://www.stecf.org). Aperture corrections are given for both grims as function of wavelength. Furthermore, we derive flat-field cubes that provide pixel-to-pixel information as function of wavelength to an accuracy of about 1%. These flat-field data is based on narrow band (100å) illuminations of the full detector area in steps of 200å. Updated sensitivity calibrations will be derived when data from TV3 (March ­ April 2008) with the new detectors becomes available.

References
Larsen, Bushouse, & Walsh, 2005, ST-ECF Instrument Science Report, WFC32005-07 KÝmmel, Kuntschner, Walsh, 2007, ST-ECF Newsletter, 43, 8

19