[Aavso-photometry] Re Alt-AZ imaging

[Jeff Hopkins]

The big problem with using an Alt-AZ scope for photometry is that 
photometry is best done close to the meridian. And that is where the 
Alt-AZ mount is poor.

It is always best to plan your observing (if you can) to catch the 
object just before it crosses the meridian while it is the highest in 
the sky (lowest air mass) and continue observing at and past the 
meridian. A fork mounted in a polar configuration works extremely 
well for this. This is how I do most of my photometry. In cases you 
need to observe far from the meridian, the errors go up.

I suspect tracking with the newer telescopes is not a problem, but 
the field rotation is a pain and the median flip a problem that gets 
old fast. I highly recommend investing in a polar mount.

Jeff

At 09:10 -0700 04/19/2007, Jim Roe wrote:
>Brad Walter wrote:
>>  There is another issue besides field rotation that I have noticed with any
>>  tracking system for dob (I looked at the website in your e-mail). The alt-az
>>  corrections are not smooth and continuous. The corrections are made in small
>>  but clearly visible steps. This is just fine for visual work, but even with
>>  a field de-rotator, it won't work for imaging.
>
>That might be true for some alt-az drives (and the tscope in
>particular), but it is not true of all.  My experience with LX-200s in
>alt-az mode shows no jumping and I've seen published images from other
>alt-az scopes that look just fine.  See
>
><http://www.ghg.net/akelly/2292larg.jpg>
>
>for an example with 19 (nineteen!) four-minute exposures (in various
>filters) stacked.
>
>Jim Roe
>Wentzville, Missouri
>
>Incidentally, that 80 cm telescope is about to be put back in service in
>Missouri. :-)

-- 
Jeff Hopkins
HPO SOFT
Counting Photons
http://www.hposoft.com/Astro/astro.html
Hopkins Phoenix Observatory
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