Afocal focus sensitivity

I have recently purchased the Orion SteadyPix camera mount for afocal
photography through my scope. How critical is the camera position relative
to the eyepiece in maintianing proper focus? Specifically, could I
acquire good focus on a bright object, swing the camera out of the way,
move to a nearby dim object, reposition the camera and still expect to
have good focus? The viewscreen on my digital is just not up to focusing
on dim objects.

Thanks,
Bryan (drop the X if you e-mail me)

wrote:
I have recently purchased the Orion SteadyPix camera mount for afocal
photography through my scope. How critical is the camera position relative
to the eyepiece in maintianing proper focus? Specifically, could I
acquire good focus on a bright object, swing the camera out of the way,
move to a nearby dim object, reposition the camera and still expect to
have good focus? The viewscreen on my digital is just not up to focusing
on dim objects.


Focus is pretty critical and the best approach depends on the camera.
If you are using it for astronomy, the object you use for focus should
not be too near (e.g., a mile or so away to be safe).

There are some good sources on the web for afocal astrophotography. For
example:

http://www.imaging-resource.com/ARTS/ASTRO/ASTRO.HTM

http://www.pk3.org/Astro/index.htm?a...ikon_cp995.htm

http://www.astrosurf.com/cidadao/camedia.htm

http://home.cfl.rr.com/aarp/Astrophotography.html

and and this barely scratches the surface.

Phil

In article , wrote:

I have recently purchased the Orion SteadyPix camera mount for afocal
photography through my scope. How critical is the camera position relative
to the eyepiece in maintianing proper focus? Specifically, could I
acquire good focus on a bright object, swing the camera out of the way,
move to a nearby dim object, reposition the camera and still expect to
have good focus? The viewscreen on my digital is just not up to focusing
on dim objects.

You want to position the camera so the entrance pupil of your camera
lens is at the exit pupil of the eyepiece, so the camera lens is filled
by the rays of light from the eyepiece. Otherwise you don't get all of
the field of view, just as when your eye is too far from the eyepiece.
If you can set your camera on infinity (many digital cameras have a button
setting for that) you can focus your telescope this way: first focus a
low power finderscope or binoculars at infinity by visually focusing on
a distant object or the Moon. Then aim your telescope at the Moon (for
example) and focus by looking through the eyepiece with the binoculars
(don't change the focus on the binoculars). This gives a good infinity
focus without your natural tendency to accomodation getting in the way.
Once focused you can put on the camera. The binocular trick works only at
low power because the effective magnification you get is the power of the
binoculars times the magnification of the telescope. Afocal photography
usually works with bright objects only. The effective focal ratio of
the system is equal to the focal length of the camera lens times the
magnification of the telescope - eyepiece combination divided by the
aperture of the telescope, so afocal photography usually is at slow
focal ratios. Most digital cameras can't do long exposures, either.

Phil Wheeler wrote:
William Hamblen wrote:
In article , wrote:


I have recently purchased the Orion SteadyPix camera mount for afocal
photography through my scope. How critical is the camera position relative
to the eyepiece in maintianing proper focus? Specifically, could I
acquire good focus on a bright object, swing the camera out of the way,
move to a nearby dim object, reposition the camera and still expect to
have good focus? The viewscreen on my digital is just not up to focusing
on dim objects.


You want to position the camera so the entrance pupil of your camera
lens is at the exit pupil of the eyepiece, so the camera lens is filled
by the rays of light from the eyepiece. Otherwise you don't get all of
the field of view, just as when your eye is too far from the eyepiece.
If you can set your camera on infinity (many digital cameras have a button
setting for that)

Note that many digital cameras with manual focus are not accurate at
infinity. With my Olys (C-2100UZ and E-100RS) I need to go to infinity
and back off a bit for accurate infinity focusing.

Phil

And my camera (Canon A40) does not even have a true manual focus. It has
Auto, Macro, Portrait and Landscape ("infinity") settings. But even using
the "infinity" setting, I notice it tries to do some limited auto-focus,
so I have to resort to letting it focus, lock it at that focus and then
fine focus with the telescope. The problem is, any adjustment of the zoom
(say to check if your focus is good) unlocks the focus.

Thanks for the tips everyone,
Bryan