[AZ-Observing] Re: MM date limits (again)

  • From: Brian Skiff <Brian.Skiff@xxxxxxxxxx>
  • To: az-observing@xxxxxxxxxxxxx
  • Date: Thu, 18 Mar 2004 14:34:49 -0700 (MST)

     Except for seeing the very faintest things, astronomical twilight
is more-or-less irrelevant.  It is nearly as dark as it's going to get
within 10 minutes of the time of nautical twilight, and for things
such as picking up M30 next Sunday morning, I wouldn't be worried
observing well into the interval between the end of nautical and the
beginning of civil twilight---something like that not-quite-arbitrary
10-deg figure Tom P used.  For ordinary deep-sky observing I'd
certainly start work shortly after nautical in the evening.
     In a lot of CCD-related observing (professional or otherwise),
the time of sunset is maybe at least as important.  With our 72-inch,
you start taking twilight flat-field images in the B filter about 10
minutes after sunset (it would be even earlier with a standard U filter);
or start taking spectra (2" entrance aperture) with the Sun about 
4 deg below the horizon.  Dave Schliecher, who does comet photometry
at very short ultraviolet wavelengths, starts taking his flat-fields
10 minutes _before_ sunset!

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