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Re: [APML] 35 mm Camera & Focuser
Serge Theberge wrote:
Welcome !
> - I have also ordered the Vixen RS200 8" f/4 Newtonian with coma corrector
> at John's recommendation for deep-space photography.
Cool ! :^)
> Next, and this is why I am seeking your advice, is the 35 mm camera and
> focuser issue.
>
> - I have an old Canon AT-1 manual camera, but nothing on it is
> removable/interchangeable.
I don't see this as a hardship. Others do. But use what ya have for now
:^)
> One option with it would be to get the Stiletto
> Series IV focuser. However, I am concerned about getting the focus error
> within tolerance, which, according to Covington's book, should be within
> .04mm to 08mm for an f/4 scope.
Okay .... here goes a rather long winded story of my recent experiences
with a 253mm f/4. I have both a Stiletto and a Hutech. And I am using a
Nikon F which the factory supplied specification of lens flange to the
film plane is 46.5 mm. Over the past year or so my results have been
anything other than exemplary. Oh I get lucky every now and then but
even a broken watch is right twice a day. I have been plagued with
bloated stars and even sometimes the central obstruction is clearly
visible. Measurements of the Hutech and Stiletto confirmed that both
were set correctly to the Nikon Spec. Measuring the Nikons was a bit
less precise but from memory:
Lens flange to inner film rails 46.46mm (may be a bit imprecise due to
difficulty measuring it)
Lens flange to outer film rails 46.66mm
This leaves .2mm for the film guide which seems reasonable given:
Kodak E200 - .13915mm
Kodak Tech Pan - .1012mm (ever wonder why we shim the pressure plate
when using Tech Pan ? )
Kodak Supra 400 - .13915mm
Kodak T-Max 400 - .1265mm
So I threw away the "factory" setting and recalibrated the Hutech to
46.48mm (SWAG) and took an image using TMAX 400. Looked better - no
central obstruction but still bloated. Tried 46.46mm and stars looked
better but diffraction spikes indicated that there was further room for
improvement. 46.44 revealed what was a good looking negative under a 10x
loupe. Reshot one more time using Tech Pan and focus still looked good
as did another image using Supra 400. So the moral of this story is that
Mr. Covington is correct (as is Walls and Provin) when he/they discuss
the critical focus required. None of my imaging friends experience
anything like this because they are at f/7 which is much less of a
problem. I have also confirmed that this measurement still hold at f/6
as well.
Whatever focuser you decide to use be prepared to calibrate it to your
system.
> I am concerned (maybe wrongly) that it may
> be a bit much to expect everything to stay put within 40-80 microns while
> unmounting the focuser and mounting the camera.
Heh ... no problem if you pay strict attention to details.
> - Another option would be to buy a used Olympus OM-1 for which there seems
> to be more options. I found a used one that comes with 2 shutter release
> cables and a Beattie IntenScreen for $299CDN. I have no idea as to whether
> this Beattie IntenScreen will be of any help for deep-sky observing. John
> suggested that if I take the Olympus road, I should consider getting the
> Hutech circular knife edge. But again, it means unmounting and remounting
> the camera.
John has offered up some good advice. The OM series is lightweight and
has mirror lockup which I find very useful. The viewfinder I wouldn't
lose a bunch of sleep over because you are only going to be using it to
compose your picture anyway. Focus should be handled by a knife edge
that is calibrated to your camera. Changing the knife to the camera
isn't that big a problem as long as you are careful.
> - John also suggested to consider getting the 6x30 Lumicon "critical focus
> magnifiers" which can be mounted on the Olympus viewfinder by gluing the
> Lumicon to the metal frame of the eyeguard, thus enabling it to be fixed to
> the camera. I must say that I find this non-swapping approach more
> attractive.
Good luck with this. My opinion is that focusing through the viewfinder
at f/4 is a crap shoot. Better solutions exist for this.
> Finally, on a line of thought similar to John's Lumicon, the last approach
> I found was from Scott Tucker's web site where he uses a Peake 22x loupe
> mounted on a camera that has a removable finder. Neither my Canon or the
> Olympus OM-1 have removable finders, which would therefore leave me to buy a
> camera body such as a Nikon F series. Snooping around the APML archives, I
> found a thread where Wil Milan states that "The F1 can work, the F2 and F3
> are probably the best for pure astro work, the F4 can work well but is very
> heavy and very expensive, the F5 would not be a good choice because it has
> no manual (i.e., no-battery-drain) time-exposure mode as do the F1 - F4."
There is also nothing wrong with a Nikon F and they can be had for not
much money. I did change out the focusing screen and even found a DW-2
magnifier for it but still the knife edge is the way to go - in my
opinion.
>
> So this is where I stand right now in setting myself up for
> astrophotography.
>
> I am looking forward to hearing your advice on this 35 mm camera and focuser
> issue.
One more thing ..... I hope that you are planning on guiding the f/4
off-axis ? Seperate guidescopes have been done but most use an off axis
guider. I tried the seperate guidescope and it just didn't work on
anything over 10-15 minutes. In a one hour photo there is too many
things that can go wrong to trust that there won't be any movement
between the guiding scope and the imaging equipment. Most of this
movement I found to be in the secondary holder. Not the spider. Which I
found somewhat surprising.
Sorry for the long windedness :^)
Regards
Bill
--
William R. Mattil |
wrmattil@ix.netcom.com | http://celestial-images.com
(972) 399-4106 |
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