The IR channel likewise has no bright-object constraints that are imposed due to instrument safety concerns. However, as discussed in Section 7.9.4, observers should bear in mind that there is a potential for image-persistence effects that could compromise observations made immediately following an exposure on a bright target. Such observations contain “afterglow” images at the location of the overexposed object, which gradually fades away over an interval of several hours. Observers are not expected to be able to mitigate the effects of persistence from previous visits of other investigators. The Institute is minimizing the effects of persistence due to other observers as part of scheduling, and is prototyping tools designed to reduce the effects of persistence. Observers are responsible for assuring that persistence within a visit is not harmful to the science requirements of their own observations, for example by taking short exposures before long exposures in a given visit and by considering the consequences of dithering.
As described in Chapter 9, the WFC3
Exposure Time Calculator (ETC) can be used to estimate the count rate in the central pixel for a given astronomical source. However, as a rough guideline, below we present tables of the count rates for two cases: a “hot star” with spectral type O3V,
Teff = 45,000 K, [Fe/H] = 0, and log
g = 4.0; and a “cool star” with spectral type M5V,
Teff = 3500 K, [Fe/H] = 0, and log
g = 5.0.
Tables D.1,
D.2,
D.3, and
D.4 give the results for the cases where one normalizes to Johnson
J, K, V, and Bessel
H, as stated in the table captions. In each case a magnitude of 15 in the respective bandpass is assumed. The count rates are given in e
-/s for the central pixel of a star centered in a WFC3 IR pixel. These tables give the most reliable results when normalizing to a ground-based bandpass that overlaps with the WFC3 bandpass, regardless of the assumed spectral energy distribution. However, when normalizing to Johnson
V, one must know the underlying spectral energy distribution to high accuracy in order to predict the count rate in the WFC3/IR bandpasses.
The Bright Object Tool (BOT) in the Astronomer’s Proposal Tool (APT) can provide a list of saturated objects for a potential WFC3/IR observation, given a Phase II proposal. Because the 2MASS survey is sufficiently deep for objects that would severely oversaturate the detector (by more than a factor of 100), the BOT uses 2MASS data where they are available, and the GSC2 where no 2MASS data are available. To use this feature, display a visit with the Aladin tool, loading the DSS image. Then, click on the BOT button in the main tool bar, which will bring up the Bright Object Tool. At this point you press the “Update 2MASS Display” button and stars likely to cause persistence problems are indicated in the Aladin Window, and can be looked at individually there, or shown as a list with the “Get Details...” button. The tool lists stars with different levels of saturation, computed using the time between pixel resets, which can be significantly longer than the exposure time for subarray exposures (
WFC3 ISR 2011-09). One should probably be more concerned with the numbers of stars listed as saturated and their locations than with the crude categorization by saturation level, but keep in mind that extremely bright stars saturate not only the central pixel, but also pixels in the psf wings.
