[Aavso-photometry] Re Alt-AZ imaging

[Roland Roberts]

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. 
>   
I'm not clear that this is generally true.  Some drives use steppers, 
some use servos.  The former are necessarily discrete and just how jerky 
depends on the gear ratios used and what part of the sky one is 
imaging.  The latter can be quite smooth---keeping track is discrete due 
to the encoders.

I've certainly seen quite a few "pretty pictures" taken with alt-az 
mounts and, lets face, major professional telescopes are often alt-az.  
I'm trying to figure out if I can get decent results without a major 
professional budget :-)

The bigger concern which has occurred to me is that of getting good 
flats.  If I am imaging through a range of position angles, I'm not sure 
I can get good flats.  Dust donuts and the like will be in fixed 
positions, but unless my illumination is radially symmetric around the 
center of the chip, I might be in trouble....

regards,

roland

-- 
                       PGP Key ID: 66 BC 3B CD
Roland B. Roberts, PhD                             RL Enterprises
roland at rlenter.com                            6818 Madeline Court
roland at astrofoto.org                           Brooklyn, NY 11220