10.8 Bright Object Protection for Solar System Observations

Observations of solar system objects with COS require particularly careful planning. To determine if a particular observation of a solar system target is safe, tables in Section 10.4 can be used for an initial check. Ultimately, observers must use the COS ETC. 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 – 1 arcsecond) solar system objects the local count rate limit is the more restrictive constraint, while for large objects (> ~ 1 – 2 arcseconds) the global limit is much 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, tables in Section 10.4 can be used to estimate whether an observation with a particular COS grating and aperture 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 are almost always < 1 (meaning that the flux of the object will be less than that of the assumed solar spectrum at UV wavelengths where the bright object limits apply). 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/arcsec2 as follows:

V / arcsec2 = Vo – 2.5 log ( 1 / area ),

where area is the area of the target in arcsec2. This V/arcsec2 and the diameter of the target in arcsec can then be input into the ETC, using the Solar Spectrum template in the HST Standard Star menu for the spectral energy distribution, to test whether the bright object limits can be satisfied.

Alternatively, an observed spectrum obtained with a known slit or aperture size can be used as input to the ETC. Most calibration techniques produce units of flux (e.g., ergs/s/cm2/Å), even for extended targets. Such a calibration implicitly assumes a flux per solid angle (i.e., the angle subtended by the observing aperture or object, whichever is smaller), and it is more appropriate to convert to units of surface brightness (ergs/s/cm2/Å/arcsec2) by dividing the calibrated flux by the appropriate area (aperture size or object size, whichever is smaller). If such a spectrum is available, it can be passed to the ETC as a user-supplied spectrum. If the ETC result exceeds the local or global limit, the observation cannot be performed.

Field Safety Checks for Moving Targets

Multiple fields along the path of an object must be cleared for the safety of moving target observations. Of course, this cannot be accomplished until scheduling windows are assigned, and the fields may have to be checked again if the windows change. An automated capability to do that has been added to the APT/BOT. Your Contact Scientist will advise and assist with this procedure.