Imaging must be planned with several factors in mind. Taken together, these factors require one to make trade-offs in selecting the number and duration of exposures, the placement of exposures on the detector, and the observing configuration.
Read noise: Read noise can be significant in observations of faint objects. Fortunately, read noise is less for CCDGAIN=1 than for CCDGAIN=4. On-chip binning has been recommended to reduce the read noise by the binning factors. Binning, however, also increases the percentage of the image affected by hot pixels (which are becoming more numerous) and cosmic rays. Binning thus becomes less desirable with later epoch and longer exposure time.
Hot pixels: The number of hot pixels has been steadily increasing, making on-chip binning less favorable and dithering more desirable. If exposures are dithered by a few pixels, hot pixels can be rejected like cosmic rays when the images are registered and combined. Dithering can be implemented in Phase 2 with the use of patterns.
Cosmic rays: Two or more exposures should be taken, either with CR-SPLIT or in combination with dithering, so that cosmic rays can be rejected when the images are combined. Dithering can be implemented in Phase 2 with the use of patterns.
Charge transfer efficiency: CTE of the CCD detector steadily decreases with time in orbit due to radiation damage. Charge transfer is more efficient for greater count levels in the target and in the background, so longer exposure times and filters with broader passbands can reduce signal loss and image distortion. Losses are less for sources in rows closer to the readout amplifier, so specifying a subarray high on the detector can also reduce loss.
Saturation: Saturation occurs at a higher level forCCDGAIN=4 than for CCDGAIN=1. Nonlinear amplification occurs at levels somewhat below saturation for CCDGAIN=1. Saturation does not damage the detector, so it can be tolerated if it does not occur at locations of interest or bleed charge into them along columns.
PSF sampling: The CCD marginally undersamples the HST PSF at optical wavelengths. Some improvement in spatial resolution can be gained by subpixel dithering. Dithering can be implemented in Phase 2 with the use of patterns. Visit the HST Focus web site for further information on PSF variability.
Bright object limits: Local and global count rate limits must be met for the aperture area at any position angle allowed for the exposure, and within 5 arcsec beyond that area. This requirement applies to the combined four quadrants of the F25NDQ neutral-density filter. Less severe limits must be met within an additional buffer zone. There are several strategies for reducing the flux received from a bright target or bright companion object. Additionally, you may be able to exclude a bright companion object from the critial area by selecting a limited position angle range (ORIENT range) for the aperture. Application of MAMA bright object limits to solar system observations can be found in the STIS Instrument Handbook.
Time resolution: Time-resolved observations can be made either in ACCUM mode or in TIME-TAG mode. The minimum sampling time is shorter in TIME-TAG mode, but other timing constraints can also affect the choice of mode . For TIME-TAG mode, one must choose the BUFFER-TIME to control the frequency of data transfer. TIME-TAG analysis is described in STIS ISR 2000-02. Errors in the initial processing of TIME-TAG data are described in the STANs for March 2001 and June 2001.
Saturation: The STIS internal buffer limits MAMA images to 65,536 photons per pixel.
PSF sampling: PSF sampling can be improved by using the highres mode image, at the cost of flat field accuracy and signal-to-noise. Native format pixels marginally undersample the HST PSF. Some improvement in spatial resolution can be gained by subpixel dithering. Dithering can be implemented in Phase 2 with the use of patterns. Visit the HST Focus web site for further information on PSF variability.