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Q: How do I observe extended source objects?
A: Similar to point sources (see question above), but you must dither your object ON and OFF the slit. A quad would look like ON-OFF-OFF-ON, similar to an ABBA quad but with ~1/√2 times the S/B since you are not getting signal during OFF exposures. The pipeline will subtract the OFF frames from the ON frames to subtract the sky, dark, and bias.
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(S/N)corr = [ (S/N)sci-2 + (S/N)std-2]-1/2
where (S/N)sci is the S/N for your science target and (S/N)std is the S/N for your A0 telluric standard. It is clear that the smallest S/N will dominate the sum of (S/N)sci-2 + (S/N)std-2, thus would dominate (S/N)corr. Go to this page to estimate A0 star S/N or use the ETC.
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Q: What is the overhead time while observing?
Based on experience, allow the following amount of time for each science target:
- ~5-10 minutes to acquire a new target with telescope, make sure your pointing is good, and begin properly guiding. It is usually a good idea to have a finder chart ready ahead of time to speed this process up, especially for dimmer targets (see Kyle's IGRINS Observability program).
- ~1 minute between each exposure for readout, and dithering between AB or ON-OFF positions. Allow an extra 1-2 minutes when dithering between ON-OFF to reacquire guiding (~5 minutes per quad).
- ~20 minutes for an A0 telluric standard (depending on the star's magnitude, see this page for estimating S/N for telluric stds.)
Your typical observing time for each science target will be (Exp. Time)*(# of exp.) + 30 minutes.
You can sometimes cut down on overhead by sharing the same A0 telluric standard star between multiple science targets if they are near each other in the sky. Brighter targets are easier to find and guide on, so usually wll require lower overhead than dim targets.