From: "James Lerch" 
To: 
Cc: "ATM List" 
Reply-To: "James Lerch" 


----- Original Message -----
From: "planetmanwest" 

> Amen, Jerry!
>      Whew! That was some set of threads.
>      Thank goodness for a polar axis and AC synchronous (or even DC)
> motors. Simple, cheap, dependable, accurate, and no image rotation.
> (Look Ma! No computer!)   :)
>      But you've got to admit that alt-alt-az idea makes a lot of
> sense for tracking satellites across a pre-determined arc. Ingenious!
> Though for astronomy I fail to see any advantage any of those alt-az
> spinoffs could have over an equatorial - especially in an application
> where portability is not an issue. I guess I'm just dense.
>
> - Jim S.

Hi Jim,

Here's the case where I find the Alt-Alt-Az of Mel's exciting, Portable
Astro-Photography, especially if you have only one or two nights to 'get
the job
done'

I my case, due to my CCD equipment, I'm limited to ~30sec exposures.  To
get decent images means taking lots of images and stacking them.

To date all my work's been done on a simple motorized EQ mount.  The
problem I keep running into is 'Poor' polar alignment.  Specifically, the
drift that results over the course of an hour or so.  The problem with the
drift is it causes field rotation.  While you can't see the field rotation
in any one still frame, when you go and stack 120 frames from over an hour,
the field rotation pretty much kills ya.

While there is limited software that will Stack and De-rotate, I have
another problem with drift over time, I use eyepiece projection for a lot
of my imaging.
With eyepiece projection I get Pin-Cushion.  The Pin-Cushion effect means
as a star drift across the field, the apparent distance between adjacent
stars changes.  When I finally get everything aligned, de-rotated, and
stacked, the stars near the outer edge of the image no longer align with
each other... Makes
for a pretty ugly experience...

What really kills me, is its hard to notice the drift while your in the
field. It's not till the next day when you goto process them that the
faults show up. All this leads to a most discouraging set of events!

Now a proper, perfect polar alignment would solve all these problems.  However I
seem to be unable to create such a thing while in the field!

The attraction of an Alt/Az + field de-rotation, or Mel's Alt/Alt/Az is you just
plop the sucker down on the ground, do a multi-star initialization, and let
the computer figure out all the hard stuff.  In theory this would solve the
problem of all those wasted nights of imaging.

Here's a for instance image:
http://lerch.no-ip.com/atm/M31.jpg (116KB)

Notice the stars in the upper right and lower left corners. See how they
are stretched out?  That's a result of the combination of Pin-cushion and
Drift. There is no way to 'Fix' it that I know of.  The stack of images is
over the course of several hours, and even AFTER de-rotating each image
manually!

Consequently a scope I can just plop on the ground, precisely point at a
couple stars, and have fairly high confidence that my images will turn out
ok, is VERY exciting!!!!!!!!


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction,testing, and coating site)

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