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It is the responsibility of the observer to ensure that observations do not exceed the bright-object count limits stated in Table 7.3. |
It is your responsibility to ensure that you have checked your planned observations against the brightness limits prior to proposing for Phase I. If your proposal is accepted and we, or you, subsequently determine in Phase II that your source violates the absolute limits, then you will either have to change the configuration or target, if allowed, or lose the granted observing time. We encourage you to include a justification in your Phase I proposal if your target is within 1 magnitude of the bright-object limits for your observing configuration. For SBC target-of-opportunity proposals, please include in your Phase I proposal an explanation of how you will ensure your target can be safely observed.
STScI will screen all ACS observations that use the MAMA detector to ensure that they do not exceed the bright-object limits. In Phase II, you will be required to provide sufficient information to allow screening to be performed.
If your source passes screening, but causes the automatic flight checking to shutter your exposures or shut down the detector voltage causing the loss of your observing time, then that lost time will not be returned to you; it is the observer's responsibility to ensure that observations do not exceed the bright-object limits.
To allow screening of your target in Phase II for spectroscopic MAMA observations, you must provide the following information for your target (i.e., for all sources which will illuminate the detector during your observations):
If you wish to observe a target which comes within one magnitude (or a factor of 2.5 in flux) of the limits in the magnitude limit table ( Table 7.4) after correction for reddening, but which you believe will not exceed the absolute limits in Table 7.3, you must provide auxiliary information to justify your request. Specifically,
Target |
Limit type |
Screening limit |
---|---|---|
Nonvariable |
Global |
200,000 counts/second |
Nonvariable |
Local |
50 counts/second/pixel |
Irregularly variable1 |
Global |
80,000 counts/second |
Irregularly variable1 |
Local |
20 counts/second/pixel |
1Applies to the brightest state of the target. |
ACS Exposure Time Calculator
.The SBC imaging bright-object screening magnitudes in Table 7.4 are very stringent, ranging from V = 15.5 to V = 19.2 for an unreddened O5 star in the different imaging apertures, and apply to all sources imaged onto the MAMA detector (i.e., not just the intended target of interest).
STScI has been using the second-generation Guide-Star Catalog
(GSC II) to perform imaging screening for objects in the field of view other than the target itself. The GSC II
contains measurements from photometrically calibrated photographic plates with color information for magnitudes down to at least V = 22 magnitudes. This information is used to support bright-object checking for fixed and for moving targets. STScI will make a best effort to perform the imaging screening using GSC I
I. However, observers should be prepared for the possibility that under exceptional circumstances GSC I
I may be insufficient. For instance, fields close to the Galactic plane may be too crowded to obtain reliable photometry. If for any reason the screening cannot be done with GSC II
, the observer is responsible for providing the required photometry. In the case of moving targets, STScI will identify "safe" fields, and the observations will be scheduled accordingly.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |