Observations of planets with the SBC require particularly careful planning due to very stringent overlight limits. In principle, Table 7.3 and Table 7.4 can be used to determine if a particular observation of a solar-system target exceeds the safety limit. In practice, the simplest and most straightforward method of checking the bright object limits for a particular observation is to use the ACS Exposure-Time Calculator
. With a user-supplied input spectrum, or assumptions about the spectral energy distribution of the target, the ETC
will determine whether a specified observation violates any bright object limits.
Generally speaking, for small (< ~0.5 to 1 arcseconds) solar-system objects the local count rate limit is the more restrictive constraint, while for large objects (> ~1 to 2 arcseconds) the global limit is more restrictive.
As a first approximation, small solar system targets can be regarded as point sources with a solar (G2 V) spectrum, and if the V magnitude is known, Table 7.3 and Table 7.4 can be used to estimate whether an observation with a particular ACS prism or filter is near the bright-object limits. V magnitudes for the most common solar-system targets (all planets and satellites, and the principal minor planets) can be found in the Astronomical Almanac. This approximation should provide a conservative estimate, particularly for the local limit, because it is equivalent to assuming that all the flux from the target falls on a single pixel, which is an overestimate, and because the albedos of solar-system objects in the UV are almost always significantly less than their values in the visible part of the spectrum. A very conservative estimate of the global count rate can be obtained by estimating the peak (local) count rate assuming all the flux falls on one pixel, and then multiplying by the number of pixels subtended by the target. If these simple estimates produce numbers near the bright-object limits, more sophisticated estimates may be required to provide assurance that the object is not too bright to observe in a particular configuration.
For large solar-system targets, checking of the bright-object limits is most conveniently done by converting the integrated V magnitude (Vo, which can be found in the Astronomical Almanac) to V magnitude/arcseconds2 as follows:
where area is the area of the target in arcseconds2. This surface brightness and the diameter of the target in arcseconds can then be input into the ETC
(choose the Kurucz model G2 V spectrum for the spectral energy distribution) to test whether the bright-object limits are satisfied.
Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |