RE: [APML] Focusing and temperature change issues during an exposure- advice wanted

["Gene Horr" <genehorr@houston.rr.com>]

-----Original Message-----
From: owner-astro-photo@seds.org [mailto:owner-astro-photo@seds.org]On
Behalf Of Alan Voetsch

> I want to find the most effective way to deal with the
> above issues during an exposure. Seeing as how most
> exposures now run in the 2-5 hour range, I think you
> can see how this would become a significant issue.

Well, first of all there are a couple of bolt on Crayford focusers that
once calibrated will adjust for temperature.  Downside is that I think
all are limited to 2" apertures.

The second idea would be to add a Robo-Focus to the focus knob.  They
claim that they can compensate for backlash.  Downside is that I'm
not sure that the temp. probe can be mounted to the OTA wall and
may be measuring ambient rather thath tube temp.

The cheapest solution is to add a cheapo motorized knob focuser and
do your own calibration run with a thermometer strapped to the tube.
Refocus through the night and record how far you have to move the
focuser knob each time.

Gene Horr
genehorr@texas.net


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