Description of Activities : Ambient Ground Calibration at GSFC (Mar-Aug 2001)

Note : A detailed description of the CDS and RAMP+RAS/Cal is available at Calibration Facilities, Test Apparatuses, and Stimuli.

Filter Wheel Repeatability Improvement

The resolver position vs step position of each filter wheel will be mapped with finer steps than done previously. The SMSs and analysis are from T. Wheeler (STScI). This activitiy was executed on Apr 2, 4, 5.

Flat Fields

The flat field calibration activities can be divided into three items :

1. Deep internal flat fields (T2/WFC and T4/HRC) will be acquired on each detector through the F435W, F625W, and F814W filters as a comparison and stability check with the baseline flats of Mar 1, 2001 (HRC : ID 26994-27007) and Mar 2, 2001 (WFC : ID 27082-27091) acquired during the RAS/HOMS campaign at BATC and with the baseline flats of Apr 3, 2001 (HRC : ID 28282-2297) and Apr 4, 2001 (WFC : ID 28314-28325) from the current RAS/Cal campaign at GSFC.

2. The "dust search" SMSs will be executed to assess the internal count rates and to make a final inspection for filter defects. These SMSs primarily make use of WFC/T2 so supplemental observations must be made in real-time to fully characterize the count rates for all other filter/detector combinations, in particular HRC/T4. Additionally, observations must be made with the secondary tungsten lamps T1 and T3 from the blue to the red to compare and ratio with the count rates of the primary lamps and to compare with the models.

3. Broad-band flat fields from an external illumination source will be acquired in the "blue" filters with HRC and with redder filters for comparison with RAS/HOMS flats (BATC, Feb-Mar 2001). A reflective diffuser is inserted behind the RAS/Cal pupil mask on the conic mirror. For blue flats with HRC below 4000 Ang, the illumination is provided with the Hamamatsu D2 lamp and at longer wavelengths, for comparison with RAS/HOMS, by a QTH fiber lamp.

This item was executed on Apr 10. A diagram of the final instrumental setup is shown in Figure 4. A ~4" piece of teflon served as the diffuser. Ideally, a piece of spectralon, which is simply teflon with a matted finish for a more uniform reflection, should have been used, but none was available with the proper dimensions. To get sufficient flux at the asphere, the D2 lamp had to be focused midway between the aperture wheel and the asphere through the largest hole ("More OPEN"). Blue flat fields were acquired for F220W, F250W, F330W, and F344N. The QTH fiber lamp (consisting of a fiber light source and a projector bulb) was then connected to the OPEN hole with a fiber bundle which was taped at the source and flats in F435W and F625W were obtained for comparison with RAS/HOMS flats. Note that ideally, the light source would shine directly on the diffuser, bypassing the asphere and the aperture wheel, but space constraints in RAS/Cal do not permit this setup. In Figure 5, we show the initial setup which gave insufficient flux at the asphere, thus resulting in the final modifications of Figure 4.

A preliminary comparison of the shape of the RAS/Cal F435W and F625W flat fields with those from RAS/HOMS indicates that they differ by +/-10% and the dust motes show different profiles (R. Bohlin). This is likely due to the different illumination between RAS/HOMS and RAS/Cal and specular reflection off the teflon. This test will need to be repeated in Aug 2001 with a piece of spectralon. Also, flats in F435W will be acquired with both the D2 lamp and the QTH lamp to cross-calibrate the flat field shape between these two wavelength regimes and with the RAS/HOMS F435W flats.

• Figure 4 : Final configuration for external flat fields. The light is focused midway between the pinhole aperture and the asphere, giving sufficient flux at the asphere.
• Figure 5 : Initial configuration for external flat fields. The light is focused on the aperture plane resulting in weak flux at the asphere.

CCD Characteristics

1. With the internal tungsten lamps, the basic properties of the detectors will be measured : gain, linearity, noise, and bias shape/level/stability, for all supported gain settings at the expected flight operational temperatures. This activity was executed on Apr 3, 4, 9 for gains of 1, 2, 4 (see WFC#4 Gain, Linearity, Saturation and HRC#1 Gain, Linearity, Saturation).

2. The CTE vs exposure level will be measured using EPER and FPR timing patterns with the internal tungsten lamps. These measurements will serve as a comparison baseline for on-orbit measurements. Note : This activity was deferred to the the final thermal vacuum campaign (TB/TV 3) (see also the SMS Master List).

HRC/SBC Confocality

As for the Sep 15, 2000 confocality test, the optimal focus will be measured at the common field center point of both HRC and SBC with RAS/Cal illuminated with an Hamamatsu D2 lamp. This item was completed on Apr 5 and a diagram of the instrumental setup is shown in Figure 6. The optimal HRC focus was found to be 2082 and for the SBC, 2099, a difference of ~1%.

• Figure 6 : Configuration for the confocality measurements

Grism and Prism Dispersion

1. With RAMP+RAS/Cal, the dispersion of the grism (5000-11000 Ang) at six field points will be measured on both the HRC and WFC with dispersed and undispersed spectra of an Ar lamp. Continuum spectra from a QTH lamp will also be acquired at the same field positions to verify the fringing model. This activity was performed on Apr 6 and the instrumental configuration is shown in Figure 7. Details of the method, reduction, and results are presented in Grism Dispersion (Apr 2001).

• Figure 7 : Configuration for the grism measurements

2. With RAMP+RAS/Cal, the dispersion of the prism (1600-5000 Ang) at six field points will be measured on the HRC with dispersed spectra of an Hg lamp. In addition to the full Hg spectra, the Hg 5461 line will also be isolated with a narrow-band filter to facilitate fitting of the dispersion curves. This activity was performed on Apr 9 and the instrumental configuration is shown in Figure 8. Details of the method, reduction, and results are presented in Prism Dispersion (Apr 2001).

• Figure 8 : Configuration for the prism measurements

Absolute Throughput

The absolute throughput of each ACS filter is measured at the field centre of HRC#1 and WFC#4 at the central wavelength of each filter bandpass and at several positions along each segment of the ramp filters. The polarizers and dispersers are included. The throughput of the small filters are measured on the WFC1 (amp B) quadrant only. The light source will be the QTH and D2 lamps coupled to the monochromator. A photodiode will provide the absolute flux calibrations. This item was completed on Aug 9-15.

Polarimetry

1. The instrumental polarization will be measured by illuminating a reflective diffuser, such as a piece of spectralon, with an unpolarized white-light QTH lamp through three broad-band filters, from the blue to the red. The results will be compared with the RAS/HOMS measurements of Mar 2, 2001 acquired in F435W, F555W, F775W, and F814W (WFC : ID 27051-27066; HRC : ID 27067-27081). A diagram of the instrumental setup is shown in Figure 9. An improvement could be made by using a transmissive diffuser to avoid any spurious polarization from the reflective diffuser. We note that RAS/Cal does not provide an effective method of measuring the instrumental polarization because the turning mirrors 1 and 2 might introduce some polarization. (The mirrors are offset by ~90 deg, canceling out their polarization, but even a slight misalignment would contaminate the dataset). Note : This item was not executed at GSFC so only the March BATC data is available.

• Figure 9 : Configuration for measurement of the instrumental polarization

2. The polarimetric efficiency of each polarizer filter will be measured at several wavelengths using RAS/Cal with a rotating Glan-Thompson prism polarizer, at the center field point as well as others. For each UVPOL and VisPOL filters crossed with F625W, the count rates on HRC will be measured at 12 orientations of the Glan-Thompson polarizer at 15 degree increments, resulting in 12x90/15=72 HRC full frames. This will be performed at five field points (centre + four corners) on HRC for a total of 360 images plus some bias frames. The procedure will then be repeated with the F435W and F814W filters but only at the field center point. For WFC, the efficiency and polarization angle will be measured only through F625W on the WFC1 and WFC2 apertures, for a total of 26 subarrays. This item was executed on Aug 15-22.

3. The polarization angle will be measured in the V2-V3 plane of each polarizer at one wavelength with RAS/Cal. (For the WFC, the +V2 axis runs along the amp A-B axis and +V3 along the amp A-C axis).

FLASH

High S/N ratio images of the flash LED illumination will be obtained for the HRC and WFC using both shutter blades and both sides of the instrument (MEB 1 and 2). To test the LED repeatibility, a series of exposures will be acquired at the shortest expected integration times with one shutter blade for the HRC only. The SMSs are JGCH35A and JGCW35A. This item was deferred to the RAS/HOMS campaign at BATC (Oct-Nov 2001) and the results have been reported.