[Aavso-photometry] Re Alt-AZ imaging

[Wolfgang Renz]

The meridian isn't bad in general. Actually its the only place where
no alt corrections are necessary for tracking stars.

But crossing the meridian close to the zenith is. There in alt/az the az
has to change by 180° within a short time that can't be handled by the
az motor anymore if its very close to the zenith. There the field rotation
is the largest too and will rotate the image by 180°.



Clear skies
 Wolfgang

-- 
Wolfgang Renz, Karlsruhe, Germany
Rz.BAV = WRe.vsnet = RWG.AAVSO



----- Original Message ----- 
From: "Jeff Hopkins" <phxjeff at hposoft.com>
To: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Sent: Thursday, April 19, 2007 6:28 PM
Subject: Re: [Aavso-photometry] Re Alt-AZ imaging


> 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
> 7812 West Clayton Drive
> Phoenix, Arizona 85033-2439 U.S.A.
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