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Hubble Space Telescope
Chopping & Dithering

Q.: What filters require separate measurement of the background (usually by chopping) to remove the time-variable thermal component?
A.: When chopping or dithering is required to measure the temporally variable thermal background on NICMOS is a function of the brightness of the target you are observing and the science you are trying to do. Thermal background is expected to dominate the total background at wavelengths of 1.7 um and longer. Therefore, for filters with a significant portion of their bandpass at wavelengths longer than this, chopping should be considered. At shorter wavelengths, zodiacal background dominates. Zodiacal backgrounds are not rapidly time variable.

One example of a conservative criterion is a requirement that the total number of expected background counts not exceed the expected read noise (30 e-). For an (arbitrary) integration time of 300 s, this yields a background count rate, B, of B </= 0.1 e-/sec. For this specific example, it would be possible to observe in filters F187N and F190N using NIC1 without chopping. However, the same two filters with NIC2 or NIC3 would exceed the criterion used in this example.

It must be emphasized that no single formulation is capable of determining when background subtraction by chopping will be desireable for all possible observations. The table in the NICMOS Instrument Handbook should be used as a guidline.

Q.: Will it be desirable to chop with coronographic observations? Does this depend on the chop throw?
A.: As long as guide-star fine lock is maintained, it should be possible to move off the coronographic spot to measure backgrounds and return with sub-pixel accuracy. To be sure that guide stars are not lost during this telescope motion, the chop distance should be less than 60 arcsec in most cases. Sometimes even this will not be possible with a particular pair of guide stars. Alternatively, for relatively compact sources, a dither over a smaller offset may be used.
Q.: When is dithering enough? How far of a chop/dither is needed?
A.: In general, we recommend that a measurement of the background be taken at a minimum distance of several source radii from an extended target or several PSF FWHM for point sources. For individual cases this will depend on the nature of the source and nearby objects that may contaminate the background. For compact sources it may be beneficial to chop or dither a fraction of a camera field of view away so that the object appears at a different position on the array in the "background" frame.
Q.: If I want to chop between a target and the background, what is the overhead I have to account for?
A.: If the background is located less than 2 arcmin from the target, then the slew time is (x+20) seconds, EACH way (target to background and background to target) and for EACH chop slew, where x is the distance between target and background positions in arcseconds. For such small slews, the same guide stars can be retained. If the background is located more than 2 arcmin from the target, the slew time is (x+31) seconds, EACH way and for EACH chop slew, where, again, x is the distance between the target and the background in arcseconds. If long on-target exposures are necessary, then after slewing back on the target, a guide star re-acquisition (6 minutes) should be requested. For short exposures, on-target guide star re-acquisition may not be necessary, and the observations following the first acquisition will be carried out on gyros, if a pointing uncertainty of about 1 milliarcsec/second due to telescope drift is acceptable. For large slews (e.g., the background is located more than a few arcmin away from the target), the user may consider the use of Type 2 Slews. The overhead for a Type 2 slew is 2.5 minutes for a move of less than 1 degree + 9 minutes for guide star acquisition. In this case, the background is treated as another target. The downside is that the observations obtained in this way will be not treated as "associations" by the calibration pipeline, and will be regarded as individual images.