ACS Thermal Balance and Vacuum Calibration Plan at GSFC : Jun-Jul 2001

GOAL :

The general goals are to verify the flight worthiness of ACS and the functional operation of its components at temperatures beyond those expected in normal orbital conditions, and to measure its outgassing rate. The goals and procedures are described in detail in Thermal Balance/Thermal Vacuum Test Plan#3. Throughout the campaign, calibration activities will be performed by the STScI/JHU science team. These include the acquisition of dark frames for all three flight build detectors (HRC#1, WFC#4, SBC) over a range of temperatures, internal and external flat fields for the SBC to estimate count rates and characterize the delta flats, CTE measurements, and stability monitoring during transitions between hot and cold environments.

LOCATION AND DATE :

The TB/TV campaign will be conducted at GSFC over the period of May-Aug, including pre- and post-TV activities. A detailed timeline is given in the table below. ACS is in flight configuration in chamber 225 in building 7 and the science and SITS consoles and hardware are located in the nearby room 158.

INSTRUMENT CONFIGURATION :

The thermal balance fixture (TBF) holds ACS in square configuration in the vacuum chamber (the square configuration is chosen since there is no interface plate to mimic the Aft Shroud Cooling System). Photos of ACS with RAS/Cal before closing the chamber are shown in Figs 1-3. The TBF has six temperature controlled zones and with the chamber shrouds, can simulate the aft-shroud environment of HST. Diagrams of the configuration of the illumination sources for calibration purposes are shown in Figs 1 and 2. The light stimuli will be provided by a HeNe laser coupled to a single-mode fiber running through the chamber wall (see Figs 7-8) and feeding RAMP+RAS/Cal into the WFC aperture and by a PtNe backlight shining on a ~3" piece of spectralon in front of the HRC aperture. Both the PtNe lamp and the spectralon are mounted on a base plate attached to the TBF. The plate is offset so that it doesn't occult the WFC RAS/Cal beam and it (the lamp + spectralon) can be slided sideways to adjust the illumination at the HRC plane (a relatively uniform illumination is necessary for the SBC flats). The base plate can not be moved towards the instrument aperture but the lamp and spectralon can be lifted individually on the plate (with spacers under the PtNe lamp, for example). The PtNe power supply is located outside of the chamber and is nominally powered at 200 Volts and 20 mAmp for HRC coronagraphic spot tracking. The current must be adjusted appropriately for the SBC external flats to keep the global count at <3x10^5 counts/sec through different filters. We found that 14 mAmp for F165LP, 20 mAmp for F122M, and 9-10 mAmp for F115LP, are satisfactory (see Jul 18 session). Since RAMP can not be controlled remotely while in the chamber, RAS/Cal will remain aligned on the WFC for the duration of the campaign.

Before closing the chamber door, the fiber tip was aligned with the 4 um RAS/Cal pinhole and the flux of the PSF was adjusted appropriately with ND filters (see Jul 3 session). But for the first stability tracking session from TB#3 Anneal to TB#4 Hot Op, we discovered that the previous thermal transitions had offset the fiber and pinhole and that vibrations in RAS/Cal caused by the shroud blower resulted in an elongated PSF. Hence, for the Cold Soak CS#2 to Calibration transition (Jul 17) and for the orbital simulation (Jul 20), the fiber tip was illuminated and focused directly on the WFC subarray through the OPEN RAS/Cal aperture with the shroud blower turned off to minimize vibrations. The RAS/Cal tracking must be interrupted for several minutes when the temperature warms up and the blower must be turned on.

The predicted nominal detector temperatures are -77 C for WFC and -80 C for HRC. The default gain settings are 1 e-/DN for the WFC and 2 e-/DN for the HRC, with a bias offset of 3 for both detectors. The WFC will be read-out with all four amplifiers (ABCD) and the HRC with amp C. The SBC/MAMA can only be operated when the ACS internal pressure is <2x10-5 Torr. ACS will be powered on MEB side 2 for the duration of the entire campaign.

• Figure 1 : ACS is in square configuration in the vacuum chamber. RAS/Cal is located at the entrance aperture in the lower-right corner.
• Figure 2 : Same as Fig. 1.
• Figure 3 : Close-up side view of RAS/Cal.

• Figure 4 : Schematic of the illumination sources for calibration
• Figure 6 : Schematic of the PtNE lamp and the spectralon on the base plate
• Figure 7 : The HeNe laser (bottom-left) feeds the pink fiber through the chamber wall. The ND filters and microscope objective are also visible.
• Figure 8 : Side view of the laser and its power source, the ND filters, and the microscope objective.

CALIBRATION ACTIVITIES :

The calibration activities are summarized in the table below as well as in sections 8.5.1-8.5.9 of the TB/TV test plan. The PtNe lamp's main purpose is to provide sufficient flux for external flats through the SBC filters/prisms in reasonable integration times and for spot tracking with HRC+Coronagraph in F606W (from the red Ne lines). Of course, the light path is not OTA-like so the low-frequency flats will be different but the high-frequency delta flats should be comparable to those acquired in the first thermal vacuum campaign (Feb 1999). Stability measurements during thermal transitions will also be performed with RAS/Cal and the HeNe point source at the WFC/F625W field centre. The measurements with the HRC+Coronagraph are the most reliable since for the WFC+RAS/Cal, the relative differences in temperature between ACS and RAS/Cal can result in spurious motions of the HeNe PSF (RAS/Cal will be moving with respect to ACS), even more so during thermal transitions, and the motions in ACS only can therefore not be isolated. The square configuration will result in severe astigmatism in the PSF but this is not important for monitoring its centroid. The use of the internal D2 lamp will be limited because of its quick degradation. We will use it to estimate count rates through the blue filters/prisms (SBC, HRC) and for some low-quality flats. The high-quality, deep flats will be obtained with the external PtNe lamp. The thorough CTE SMSs will be executed at the nominal calibration environment and detector temperatures and the CTE monitor SMSs at slightly different detector temperatures (-75 C for WFC and -83 C for HRC).

Notes :

• RT : Real time commanding with CCL.
• T1, T2 : WFC internal tungsten lamps. T2 is the default.
• T3, T4 : HRC internal tungsten lamps. T4 is the default.
• D2 : Internal deuterium lamp for the SBC filters/prisms or HRC blue filters.
• In calibration item 8, the deep external and internal SBC flats were acquired with the F115LP filter instead of F125LP because of its higher throughput (see Jul 18 session below).
• In TV1, the internal SBC/D2 count rates were measured with a ND2 filter which was later replaced with an ND1.3, so all the count rates need to be updated.

TIMELINE :

The campaign consists of a Thermal Balance (TB) session followed by a Thermal Vacuum (TV) session. In the TB program, ACS will be subjected to four different orbital environments to be encountered in the HST aft shroud (TB#1, #2, #3, and #4). The TV session includes two worse case cold (cold soaks #1, #2) and hot (hot soaks #1, #2) environments at -32 C and +4 C, respectively, approximately 10 C more severe than the expected orbital temperatures. The bulk of the science calibration activities are scheduled in the last ~5 days of TV at a nominal aft shroud temperature of -8 C environment but some (such as image stability) will be executed whenever the schedule allows. In the following table, we list the test procedure of the campaign as given in the TB/TV plan and its timeline, and the possible calibration activities associated with each. Of course, the scheduling of the activities is subject to change as the campaign progresses.

Note :

• In the Thermal Balance section, the duration column lists the start and end times of the transition from the previous item to the current item (column 1). Stabilization of the current item is declared at the end time.
• The SBC dark rate and D2 lamp count rates of this TV3 campaign have been measured. The outdated SBC dark rate and D2 count rates from the TV1 campaign with the ND2 filter are also available.

MISCELLANEOUS :

• The thermal vacuum plan is detailed in Thermal Balance/Thermal Vacuum Test Plan #3 (Montt de Garcia, K. et al., May 2001, Doc. No. P-442-2796) and its associated figures.
• timeline of the campaign from the TB/TV test plan
• the calibration plan for the first thermal vacuum campaign (TV1)
• the SMSs for the thermal vacuum campaign are tabulated in the SMS Master Table
• the SBC dark rate and D2 lamp count rates of this TB/TV 3 campaign have been measured
• the SBC dark rate and D2 lamp count rates with ND2 of the TV1 campaign are now outdated
• a general description of RAS/Cal is available