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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.
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