Re: [APML] Best Scope and mount for $2000 was Scope Question

[George Anderson <georgea@cam.org>]

Rob Miracle wrote:
> 
> Well, I've tried pointing the scope at other objects and frankly a lot of
> nebulas and deep sky objects are just fuzz balls unless you are
> photographing them.  The eye can't capture enough photons to make a good
> visual.  Thats why we need to take long exposures, so we can capture them, no?

That is correct, you need a long accurate exposure (read precise
tracking).


> 
> >>But for deep sky work, and shooting a few galaxies here and there would
> >>be cool, the 8" from what I've read should be a good sized scope.   Of
> >>all the scopes, other than a refractor, its the best when dealing with
> >>camera's attached.

The light gathering of the 8" is good, the focal length starts getting
long so your field of view is more limited.

> Well, my main source of information is Covington's Astrophotography for the
> Amateur.


Also have a look at Robert Reeves "Wide Field Astrophotography", and
vacuume up all of the advice you can get from many of the published (and
soon to be e-published) people on this list.
> 
> >>   I still have trouble understanding why a scope with a GOTO computer
> >> that supposed to be able to track cant track without an Equatorial
> >> mount.   If it can point to object A in the sky, it should be able to
> >> point to it in the sky 5 minutes from now.

It will put the object in the FOV, but will it be within sub-arcseconds
of the position that it was in before? That is where inexpensive goto
dives fall down. It is also the reason that Losmandy and AP mounts cost
so dear, they have the required precision to accurately keep the object
in the exact same spot for a long time (even longer when guided). This
assumes accurate polar alignment.

> 
> I'm a computer programmer by nature.   If I can point at object A now, why
> can't I point at it now+5?  I understand the rotational stuff, but if the
> software is smart enough, it should be able to adjust for the
> rotation.   The problem, I assume is the software isn't smart enough.

The software is smart enough, it is the mechanical side that is going to
let you down. The slop in the gear train is too high. To make an analogy
your gear train is an 8 bit analog to digital converter with an error of
+/- 1/2 LSB producing integers, for astrophotography you need 32 or even
64 bit real numbers. Even with interpolation your 8 bit A to D will
still be way out as the conversion time is 25 milliseconds and you
astrophotography work wants 10 microseconds between conversions.

> 
> >If you don't understand this, then you are not ready to take pictures with
> >an equatorial mount...
> 
> Possibly not, but I'm not getting anywhere new with fixed tripod shots
> either.   I shot some stuff this week.  I got some good 8-15 second
> exposures of constellations, on my D1 (it was cold enough that the dark
> current noise wasn't too bad).  I shot some 3-5 minute exposures pointed at
> Polaris on film so I have to wait and see what came out there.  I'm hoping
> for some nice star trails.
> 
> I guess I need to read Covington some more.

Spend $20 at the hardware store and put togeather a barn door tracker,
your exposure time will open up to 5 minutes with a type 1 using 50mm or
less. From there you will pick up just how critical things are and how
unforgiving film is compared to the eye.

George Anderson
Montreal Canada

Clear skies and good health

ps: I could be out to lunch on this, but I am sure the others will let
you know

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