Re: Autoguider correction frequency


Kurt Mihalco <mihalco@...>
 

Hi Murray,
To answer your question, the main reason for allowing short exposures
is for non-astro imaging. The cmos imaging and associated driver
chips are general purpose and marketed for all sorts of applications,
many of which are video surveillance. For these uses, the short
exposures are necessary to avoid saturation in daylight
applications. Orion happens to use this imager and chipset in the
Starshoot autoguider primarily because it is cheap relative to the
CCD imagers and support chips. It is also less sensitive, but who
cares - you simply take a 1 sec exposure instead of 1/2 second. For
this application, it really doesn't matter. Again, the autoguider can
only digitize and deliver a full frame to the computer every 1/15
second - even if the exposure is 1/1000 a second (or less).
Regarding the mount response, others have already commented. Trying
to move the mass of the tube 15 times/second is pretty much like like
tapping on the side of the tube - it takes a while for the impulse to
settle out. If it is constantly being done, it never reaches
equilibrium. That's why those that want the fast corrections (that
take into account atmospheric seeing) use the SBIG AO hardware.
These can be driven at high rates (up to 60 corrections/sec) due to
their low mass. Hope this helps.
Regards,
Kurt


--- In ap-gto@..., Murray Hammick <mphammick@...> wrote:

Kurt,

I do not have a problem with the maths - that is a straightforward
calculation.

What I do have a question about is why the software in the
autoguider has the ability to sample as often as up to 20 times per
second if there is no real requirement to do so. According to others
on this site (including Roland) you should be looking at corrections
every second or slower. So what sort of mount is able to make good
use of those 1/20th or a second exposures which imply a correction
rate of up to 20Hertz. I say "up to" because of course if you have a
good mount then you might only be correcting very 20, 30 or 100
exposures or whatever.

Perhaps this is a redundant thread for an AP site as the view is
that a setting of 1 sec or slower is what is needed for mounts like
the Mach 1, 900, 1200 etc. and this will avoid over-correcting and
bouncing the mount around as it tries to "chase its tail".

Murray

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