Handbook Reference
Guide
The proposal instructions provide the details of the syntax for preparing an HST observing proposal, but it is this handbook which provides the details necessary for putting together a feasible program. Unfortunately, it is a difficult task to keep all the details of the FOC in mind when preparing a proposal. The following summary describes the most common problems in using the FOC and where the problems are discussed in the handbook.
- Bright Target Acquisition: The FOC must take great care not to be exposed to an excessive amount of light. The specific bright limits are described in "Detector Overload" on page 79. Bright targets, defined as any target of magnitude 9 or brighter, must be observed using the new Bright Object Acquisition Procedure (described in "Target Acquisition Modes" on page 56). This procedure checks to make sure that proper filters are in place to assure a low enough count rate from the star. The filter combination, however, must be verified by an FOC Instrument Scientist prior to execution of the proposal, necessitating a careful simulation of the expected observation.
- Photometric Linearity and Image formats: The FOC offers a wide range of formats for imaging, each with its own photometric characteristics. As a rule, the smaller the format, the higher the count rate can be and still maintain a photometric linearity in the image (see "Dynamic Range" on page 66). The smaller formats are best suited for imaging brighter objects that are not very large, while the largest formats provide the best field of view for imaging faint sources. In general, though, point sources should not exceed 1 count sec-1 pixel-1 for the central pixel ("Non-Uniform Illumination" on page 69) and extended sources should not exceed 0.15 counts sec-1 pixel-1 in the 512x512 format ("Uniform Illumination" on page 66 and Table 6.2) in order to obtain a photometrically linear image.
- Small Image Format and INT ACQs: Sometimes it is necessary to use the smallest formats, such as the 256256 format. In these cases, it becomes critical to have pinpoint target acquisition since the field of view is only 3.7"3.7". An error of even 1" in the target coordinates could result in the target missing the field of view, since guide star errors are typically 0.6" and the FOC apertures positions are only known to about 0.2" (see "Calibration Accuracies" on page 120). Therefore, anytime a format with a dimension less than 512 pixels is used, an INT ACQ must be taken to insure that the target is acquired within the small format.
- Inaccurate Target Coordinates: The default acquisition mode is a blind pointing, described in Section . The small field of view for the FOC places strict constraints on the accuracies for the target coordinates. Even errors as large as 1" can seriously jeopardize the observation given the small fields of view for the FOC. Therefore, all FOC target coordinates should be measured using GASP in order to provide the most consistent set of coordinates for this blind pointing.
- Wrap-around in 8-bit formats: The largest format for the F/96 relay provides a field of view of 14.7"14.7". Unfortunately, due to memory constraints (Section 4.18), the largest formats (512z x 1024 and 512 x 1024) only have 8-bit deep pixels, i.e. they can only count up to 255 counts per pixel (Table 5.1). For a point source with a count rate of 0.25 counts sec-1 pixel-1, the peak pixel of the star will count up to 255 in 1000 seconds then reset to 0 on the next count. The image has lost track of those counts, resulting in a loss of photometry. In order to use this format, short exposures should be taken, then co-added, as long as the sources in the image maintain count rates that are linear (see Table 6.2 for extended sources, "Non-Uniform Illumination" on page 69 for point-sources).
- Format sensitivity: Each FOC format has a different photometric sensitivity ("Format-dependent Effects" on page 75). This usually causes no problem, except in cases where images taken in different formats are compared. If different fields are to be compared, then taking the images in the same format will insure a consistent photometric response due to the image format, eliminating one more source of error in the photometry.
- PSF Artifacts: A great advantage of the FOC is to be able to discern detail on much smaller scales than with any other instrument on HST. However, for the smallest details, differences in the shape of the PSF must be known in order to differentiate source structure from instrumental structure. A description of some of the most obvious features of FOC PSFs is given in "The Point Spred Function" on page 59, including a jet-like feature in the F372M PSF and some degradation in the F320W PSF. A couple of other filters (F501N and F502M) both show faint ghost images, as well. These might be important for programs trying to detect faint sources near brighter ones. A full library of observed PSFs can be retrieved using the FOC WWW pages (Chapter 9) to provide a better understanding of them prior to selecting the filter for use in the proposal.
- Background noise, stray light and flat-field features: Although the FOC benefits from having very low background noise ("Detector Background" on page 76), there are still limitations as to what can be detected in that faint background. Detection of very faint, background-limited sources relies heavily on knowing the flat-field response of the FOC ("Uniformity of Response (Flat Fielding)" on page 80 and "Flat Fields" on page 121) and the effect of the geometric correction on the background ("Geometric Distortion and Stability" on page 87). In addition, requirements can be specified in the proposal to minimize the effect of stray light ("Stray Light" on page 76) in images of extremely faint sources.
- Overhead times for images: The use of RPS2 in preparing observing proposals now allows observers to more efficiently schedule their observations in each orbit. This system automatically incorporates all the overhead times for taking FOC images ("Overhead Times and Multiple Exposures" on page 79). One overhead which can be minimized is the selection of the filter for the image. By selecting the filters one after the other as they are situated on the filter wheels (see Tables 4.1 and 4.2 for filter wheel positions), the time spent selecting the filters can be minimized, allowing for more exposure time.
- Visible Leaks for filters: Selecting the proper filter for the exposure usually focuses on obtaining a good count rate. However, each filter has its own transmission characteristics ("Bandpass and Neutral Density Filters" on page 37) with their own visible leak concerns. For some observations, a small percentage of light from the visible (for a UV filter) does not make a difference. For other science, the precise amount of light from outside the desired passband can seriously affect the results. This problem is addressed in "Visible Leaks" on page 85, with FOCSIM also providing a means of judging how serious this problem would be for a given observation ("FOCSIM" on page 106).
- Geometric Stability: Long series of FOC images have been noticed to suffer from small, residual variations in the geometric stability, as described in sections "Geometric Distortion and Stability" on page 87 and "Plate Scale" on page 90. This can present some problems from programs needing astrometric conditions for their science. The accuracies in the distortion which can be expected are discussed in "Geometric Distortion" on page 121, along with recommendations on how to best account for this in the proposal.
We advise all the users who finally decide to write a proposal using the FOC to examine all the items presented in this checklist. This will help them to optimize the scientific return from the instrument, and to obtain excellent images.
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