ACS Thermal Vacuum Calibration Items

A. CCD Properties : The basic CCD properties are the gain, readnoise, dark current, and charge transfer efficiency (CTE). The latter requires specialized tests and is a subject of a seperate item. We will measure the other properties as a function of CCD Temperature (T_CCD). This must be determined at the lowest stable T_CCD at the hottest environmental temperature and at a few other higher T_CCD settings.

B. Charge Transfer Efficiency (CTE) : A full CTE versus exposure level curve must be made for each CCD at the nominal T_CCD. In addition we will explore how CTE varies with T_CCD at a few well chosen exposure levels. Both parrallel and serial CTE will be measured. For HRC, both of these will be measured with Mike Jones's First Pixel Response (FPR) method. For the WFC, FPR will be used to measure serial CTE and the Extended Pixel Edge Response (EPER) method will be used to measure parrallel CTE. Different special read-out methods are required in both instances.

C. Detector Defects : The number and location of hot and unresponsive pixels, and detector columns must be monitored as a function of time and T_CCD (for both HRC and WFC). This will be done using combinations of dark frames and flattfields.

D. Dark Structure : Long duration dark frames are required to determine if there is structure in the dark current, and to provide more precise determinations of the average dark current. These should be taken at various T_CCD covering the full commandable range of the TECs, preferabley in very fine T_CCD steps (< ~2 C).

E. Image Stability : We need to determine whether thermal variations cause any mechanical stresses on the instrument that may effect image quality. This will be done by deploying the coronograph arm and then monitoring HRC or SBC images while the instrument is cycled through various environmental temperatures. The images will be analyzed to look for changes in the cornographic spot location. The arm must not be moved during this portion of the test.

F. Internal Calibration System Verification : We will try out all calibration lamps to check whether they work in the TV environment, if adequate exposures can be obtained, and to verify whether or not the model count rate calculations are valid. Of special concern is whether the filter in front of the SBC D₂ lamp is optimal.

G. Flat-Field Uniformity : We will test how flat the flat fields are, and how flat field shape varies with wavelength (e.g. can a flat field obtained at 6000 Ang adequately flatten an image taken at 4000 Ang ?). A full suite of high S/N flats will be obtained for the SBC (high S/N ground flats for the other detectors will be obtained in ambient and/or N₂ purge environments). The SBC flats require ~ 10+ hr long integrations with an external source (STUFF - STimulus for Ultraviolet Flat Fields).

H. Flat-Field Repeatability : For the CCDs we will test whether the flat field shape varies with time and T_CCD. We are particularly interested if pinholes become a problem at any T_CCD. For the SBC MAMA we will test whether the flat field level correlates with the monitor diode/photometer reading, as well as checking for temporal variations.

I. Flat-Field Illumination : Illumination of the detectors by internal flatfield sources will be different than by astronomical objects: different ray angles are allowed. Effectively internal flatfields and diffusing screens in front of the instrument have lower f/ratios than distant sources seen through the HST OTA (or simulator like RAS/HOMS). For the SBC we will test the importance of the beam f/ratio using STUFF. The flatfields obtained for items G & H, which have a very low f/ratio will be used to flatfield the pin-hole images, which have a large f/ratio set by the geometry of the double-pinhole design.

J. SBC Throughput : Using STUFF, the external UV stimulus being built at GSFC, we will determine the absolute throughput at two monochromatic wavlengths 1236 Ang (Kr) and 1469 Ang (Xe), through all filters and prisms that pass these wavelengths.

K. UV Contamination Buildup/Throughput Degradation : We will monitor the UV throughput of the SBC at two wavelengths (1236 Ang and 1469 Ang) to look for indications of contamination build-up and/or changes in the throughput.

Original version by : G. Meurer