Bright Object Tool

The Bright Object Tool is available for WFPC2, NICMOS, and ACS observations. For NICMOS, the tool does NOT check for persistence caused by bright objects; see Limitations below for more details.

What is the Bright Object Tool?

The Bright Object Tool allows you to check for field objects that may be detrimental to the science data quality of an observation (e.g. bleeding of the CCD) or even physically damaging to the detector (possible with the ACS SBC and the FGSs).

All objects that could potentially damage the instrument must be placed outside the field-of-view of the detector, or must be shown to be safe (see How to resolve problems below). Objects which could degrade the scientific utility of the observation will be dealt with at the discretion of the proposer.

Note that you should check the primary science target (where you have detailed knowledge of the object) via the Exposure Time Calculator, because the Bright Object Tool uses simple and conservative assumptions about the objects (see Limitations below).

Why use the Bright Object Tool?

This tool will be run by an Instrument Scientist when reviewing your Phase II submission to determine if there are any potentially damaging objects in the field of view. By running this tool prior to submission, you can adjust any exposures that have problems early, and thus avoid any potential scheduling delays due to having to fix the problems after the Instrument Scientist review. Also note that Instrument Scientists will generally not be checking for targets that could degrade the science, so this is your only opportunity to correct these problems.

How the Bright Object Tool works

Based on your target position and detector/aperture selection, the Guide Star Catalog 2 (GSC2) is searched for all objects that could potentially fall in the field-of-view; the area searched is larger than the aperture you are using. The GSC2 magnitudes are then coverted to the standard V and B-V colors (note that some objects in the GSC2, such as those from the Tycho 2 catalog, have direct V and B-V colors), and the B-V is used to determine the spectral type of each object assuming it is a main sequence star (however, see Limitations below for important caveats). Based on exposure information (e.g. spectral element/filter and exposure time) you provide, the count rate and total counts for each object are then determined (via an Exposure Time Calculator based lookup table), and compared with the health-and-safety (i.e. Bright Object) and scientific (e.g. saturation, bleeding, etc.) limits. All objects that exceed these limits are then marked on your DSS image, and a table giving relevant information for each problem object is generated. The interpretation of these outputs, as well as ways to resolve any problems, are discussed below.

How to use the Bright Object Tool

The following steps should be followed to run the tool:
  1. Select an exposure specification with the VTT as the current tool.
  2. Turn on the Bright Object Tool (push the " Bright" button on the top tool bar)




  3. Complete the relevant information on your exposure in the dialog box. (Note that exposure time should be entered as just a number with implied units of seconds.) Also note that not all instrument configurations are supported for bright object checking




  4. Click on the "Apply" button to initiate the checking




Note that if you change the information in the Bright Object dialog box or move th exposure, you will need to rerun the tool by hitting the "Apply" button. A red warning message to this effect is printed on the bottom of the dialog box.

Interpretation of output

There are two types of output products to allow you to see the results of the checking. On the image, the GSC2 objects that are potential problems (and/or the safe objects depending on what you selected from the dialog box) will be marked. If no objects are marked, it means that there were no objects that met the criteria.

To see the details of what problems each object had, click on the "Details" button in the dialog box. This will bring up a window that will present the following information for each problem object: GSC2 ID, coordinates, F and J magnitudes (the GSC2 system), V and B-V colors, the derived OR ASSUMED (O5) spectral type, the derived signal (counts and count rates), and the reason the object is a potential problem. If you have opted to see stars that are safe, then the dialog box will present the detailed information for those stars (which you might use to determine how close they are to being problems).





There are three special cases that require further explanation. Some objects in the GSC2 do not have complete information, and therefore cannot be checked. These objects are listed as "unknown", and it is up to the proposer to gather further information on these objects (for ACS SBC observations) to prove they are not a health-and-safety concern if they are to remain in the field-of-view. To assist ACS SBC users, if there is an F (red) but no J (blue) magnitude, the tool will search a larger area around the target to find the faintest star detected in J. This limiting magnitude will be assumed for the target under investigation, and normal processing will continue. Since most stars of this type will be red and faint (and thus not likely to be a health-and-safety concern), this technique should resolve most such cases.

Many faint objects in the GSC are not classified as stars due to the poor quality of the point spread function, and the tool does not check them. These are listed as "not a star", and it is up to the proposer to gather further information on these objects (for ACS SBC observations) to prove they are not a health-and-safety concern if they are to remain in the field-of-view. To assist ACS SBC users, if the object is fainter than some (detector dependent) cutoff magnitude, the tool will assume the objects are stars, and process them normally. Since these objects are the faintest objects in the GSC2 (and thus not likely to be a health-and-safety concern), this technique should resolve most such cases.

For ACS SBC observations, the global countrate for an observation is not just the rate for the primary target, but is the combined rate for all objects in the field-of-view. Therefore, a "field check" is performed, and if the combined countrates for all objects exceeds the checking limit, a message is printed to that affect. Note that the field check message is printed for all exposures where an individual target exceeds the limit, and that it is possible for all objects in a field to be safe, while the combined countrates exceed the limit.

How to resolve problems

If you have objects that are potential health-and-safety concerns, you must do one of the following:

Limitations

Note that this tool has limited information (only GSC2 magnitudes, yielding one color) on each target, and uses conservative assumptions in determining the spectral type of each object. It is therefore quite possible that flagged objects may really be safe to observe (although we tried to use rules that would not result in an excessive number of "false positives"). To convert from V and B-V to spectral type, the tool assumes the following: