[AZ-Observing] Re: Happy Jack site

  • From: "Tom Polakis" <tpolakis@xxxxxxx>
  • To: "Reply-To:az-observing"@freelists.org
  • Date: Fri, 12 Mar 2004 15:36:14 -0700

Thanks for the information about the site's seeing Brian.  Not shown in
the histogram is the seasonal variation.  Are there known trends in seeing
quality throughout the year at established sites up your way?

Back when I was imaging with my 13" and ST-7 setup in Tempe, "good" seeing
was a bit below 2 arcseconds, and typical nights ranged from 2" to 3". 
Just based on visual observations of planets, I haven't found our popular
desert sites to be any better than my backyard.  As Brian has said so
often, setting up on a small rise pays off.  Unfortunately, we get often
into problems with space, access, and wind.

Folks might be interested in how the Happy Jack seeing measurements were
made with very simple, off-the-shelf hardware.  They use a differential
image motion monitor (DIMM).  Typically, an SCT with twin aperture stops
(one with a prism to offset the image) sends the twin images of a bright
star through two separate light paths.  Then the _relative_ motion of the
two star images is measured to determine the seeing.  The setup works
remarkably well even when wind is buffeting the poorly mounted scope,
since only the relative location of the two images is required.  There's a
good explanation and illustration at:


I saw this one in action at Apache Point Observatory.


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