Darren's words:
I know that f/8 is not unusual for a visual scope. (my 8" dob is f5.9)
But f/8 is still f/8 photographically (exposure-wise).
We've also switched subjects (more than a little) slightly when we are
talking about using the telescope as the lens to the camera. That's
called prime focus photography and it means that the projected image
will need to reach the DSLR sensor (with no lens attached). I'm not
wild about leaving my DSLR sensor exposed to the elements for any
longer than necessary, but that's what you are doing when you use it
on a reflecting telescope (as opposed to a catadioptric which is
sealed like an over-sized mirror lens). You also need to have a way to
really lock down that telescope focuser because it would be great
(not) if the focusing tube with camera body attached worked its way
right out of the focuser. And you are still back to the issue I
originally raised as to How Good/Accurate the tracking unit is on the
telescope you are mounting it on. Because now you are effectively
using the focal length of the telescope the same as you would a lens.
On my 8" dob that is 1200mm. Every step better be smooth or you are
going to have a vibration. You also need to be perfectly polar-aligned
or you will have issues with longer exposures.
In short, prime focus photography is not done cheaply or easily. You
should certainly "cut your teeth" on something easier and work your
way up as you learn the various pieces of the puzzle.
Well put. Some of those points I'm thinking about since december. the
support question - since the Starblast comes without rings I've been
thinking about a craddle, and imagined a support for the camera,
something able to take the weight out of the adapter and yet allowing
focusing. Not quite there yet, but some ideas are going to paper
tonight. Never gave the open air condition a tought until now - but
there is no way to avoid it, other than using a Barlow - not in my
plans, really... as for the tracking unit... as soon as I finish it,
we'll know. Will be an Isosceles mount, progressive tracking errors not
relevant since I intend to keep exposures under 5 minutes and start from
a closed unit. Do not plan to hand-turn it... and am very worried about
motor and train vibration.
FTR, the Starblaster is listes as 450mm, f/4 - not so long, not so dark
- if only it turns out good...
lf
--
luiz felipe
luiz.felipe at luizfelipe.fot.br
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