Space Telescope Science Institute   7.2.1 How Do You Determine if You Violate a Bright Object Limit for SBC Exposures?  7.2.3 Bright-Object Protection for Solar System Observations

7.2.2 Policy and Observers' Responsibility in Phase I and Phase II


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.

Policy on Observations Which Fail Because they Exceed Bright-Object Limits

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.

Prism Spectroscopy

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,


Table 7.3: Absolute SBC count rate screening limits for nonvariable and variable objects.
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.

The limits for irregular variable sources are a factor 2.5 more conservative than for sources with predictable fluxes. Predictable variables are treated as nonvariable for this purpose. Examples of sources whose variability is predictable are Cepheids or eclipsing binaries. Irregularly variable sources are, for instance, cataclysmic variables or AGN.

Table 7.4: Bright limit V-band magnitudes for observations with the SBC filters and prisms (no reddening).
Spectral type
Log Teff
F122M
F115LP
F125LP
F140LP
F150LP
F165LP
PR110L
PR130L
O5 V
4.648
16.1
19.2
19.0
18.5
18.0
16.6
15.8
15.5
B1 V
4.405
15.2
18.4
18.2
17.7
17.1
15.7
15.0
14.7
B3 V
4.271
14.3
17.6
17.5
17.0
16.4
15.2
14.2
14.0
B5 V
4.188
13.5
17.0
16.9
16.5
15.9
14.7
13.7
13.4
B8 V
4.077
12.0
15.9
15.9
15.5
15.0
13.8
12.7
12.5
A1 V
3.965
9.4
14.0
13.9
13.8
13.6
12.6
11.1
11.0
A3 V
3.940
8.2
13.0
13.0
12.9
12.8
12.2
10.5
10.3
A5 V
3.914
6.8
11.9
11.9
11.9
11.9
11.6
9.9
9.8
F0 V
3.857
4.8
10.0
10.0
10.0
10.0
9.8
8.2
8.2
F2 V
3.838
4.0
9.0
9.0
9.0
9.0
9.0
7.5
7.5
F5 V
3.809
2.1
7.4
7.4
7.4
7.4
7.3
5.9
5.8
F8 V
3.792
0.9
6.2
6.2
6.2
6.1
6.1
4.7
4.7
G2 V2
---
2.2
5.6
5.2
5.1
5.0
4.7
3.1
3.1
G8 V3
---
---
5.1
5.0
4.8
4.6
3.9
2.1
2.0
K2 V4
---
---
5.1
5.1
4.8
4.5
3.6
3.1
3.0
KM III5
---
---
3.7
3.7
3.2
2.8
1.8
1.6
1.1
Double6
---
13.4
16.5
16.3
15.8
15.3
13.9
13.1
12.8
AG Peg7
---
12.9
16.0
15.8
15.4
15.0
13.3
12.4
12.1
1. O5 V through F8 V values are based on Kurucz models.
2. The magnitudes listed for G2 V are for the Solar template in the ACS Exposure Time Calculator.
3. The magnitudes listed for G8 V are from IUE data for the star Tau Ceti.
4. The magnitudes listed for K2 V are from IUE data for the star Epsilon Eri.
5. The magnitudes listed for KM III are from IUE data for 9 stars of these types.
6. System made of a main sequence late-type star with an O5 V star contributing 20% to the total light in the V band. In the UV, the O5 component dominates and sets the same magnitude limit for companion types A to M.
7. A star with a flux distribution like the symbiotic binary AG Peg.

Imaging

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 II. However, observers should be prepared for the possibility that under exceptional circumstances GSC II 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.


 7.2.1 How Do You Determine if You Violate a Bright Object Limit for SBC Exposures?  7.2.3 Bright-Object Protection for Solar System Observations
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