Re: [APML] 8" Reflector decent enough for prime focus AP?

[Roland Roberts <roland@astrofoto.org>]

>>>>> "Rex" == Rex Moore <RMoore@fool.com> writes:

    Rex> The introductory email said all levels of experience are
    Rex> welcome, so I hope you don't mind a question from a rank
    Rex> beginner!

I'm barely past the newbie stage myself!

    Rex> This Orion SkyView Deluxe 8 EQ Newtonian is about at the
    Rex> limit of what I can afford to purchase right now:

I think you will find that this setup is no good for prime focus; the
mount is pretty much at it's limit with that scope.  As you have
guessed, you may not even be able to achieve focus at with the
supplied focuser; it is not uncommon for that to be the case.

Like most others, I'll recommend you start by trying piggyback
photography for wide-field work.  That's where I'm at and I'm hoping
to acquire a nice collection of constellation portraits as well as
some "close-ups" of various things.  If the weather will cooperate,
I'm hoping to get Sirius and M-41 in a single shot with a 135mm lens
this weekend, but the weather man is giving me bad news :-(

Seriously, there is plenty to learn just doing piggyback photography
at focal lengths of 50-200mm.  At 50mm, I discovered I could do a
pretty crude polar alignment.  My first attempt at Cygnus, with
nothing more than a rough alignment via the polar alignment scope,
yielded a nice shot with a 4-minute exposure on Elite Chrome 200
(which was really too short, but the North America nebula *did* show
up).  At 135mm and longer exposures (e.g., 10-20 minutes), getting the
polar alignment was more important and I had to learn how to do a
drift alignment.  I also learned that bumping the tripod is a "very bad
thing" :-(  I have some nice shots of M42 that look like they were on
a fixed tripod as a result.

All of the above is stuff that you have to learn by doing and, when
you go to doing prime focus photography, you will need to be
comfortable doing well.  Doing it for piggyback photography is easier
and gives you a chance to get good at it.

Alternately, you could use a webcam at prime focus with that scope,
but you will be somewhat limited with how deep you can go.  And the
SkyView Deluxe 8-inch is f/4 which means to do planetary shots with
the webcam you wil need at least a 3X barlow.  I've actually taken
some shots of Mars from last summer with the Orion XT8 (yes, a Dob!)
and a web cam at f/12.  Ideally, you want an even longer focal length
so you can reduce the resolution on the web cam and capture images
faster.  At 640x480, a USB camera can only put out about 1 frame/sec
uncompressed.  At 320x240, it can put out about 5.  But your 8-inch
need to be at f/24 or higher to get decent resolution.


roland
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Roland B. Roberts, PhD                             RL Enterprises
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