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Old 23-05-2005, 08:24 PM
blackpearl
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newby question re an SLR

Very basic question -I have a SLR Pentax and the pentax adapter for my 12" Dob. I have connected it up and tried to take pictures of the moon however, all I get is the image of the viewing screen of the inside of the camera. I suspect that the focal length is incorrect. How do I adjust/modify the equipment or find out how to do it?
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Old 23-05-2005, 09:21 PM
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[1ponders] (Paul)
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Hi blackpearl. We haven't heard from you for a while. That could be the problem. I'm not a newt user so i can only quote what I've read/heard.
This comes from Michael Covingtons "Astrophotography for the Amateur"
quote:
In order for prime focus photography to work, the telescope has to provide adequate back focus...................with Newtonians there may not be enough back focus. The symptom is that you can focus on nearby objects, such as trees, but not on the sky.

To overcome the problem, you can modify a Newtonian telescope by moving the morror mount a few centimeters forward from its original position in the tube. When this is done, it will then be necessary to insert a small extension tube when using eyepieces, to get them out to the new psoition of the image plane; a suitable extension tube can be made of 1 1/4 inch(35 mm ) sink trap plumbing parts, available at any hardware store. It may also be necessary to enlare the diagonal mirror to get full edge of field illumination with low power eyepieces. You can also replace the focusing mount with a special low profile focuser that takes up less space, thus giving more back focus. On the whole, though, it may be more practical to leave the telescope unmodified and use one of the optical configurations that do not require much back focus, such as afocal or positive projection.

End quote.

I'm sure someone else around here will have an alternative solution
clear skies
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Old 24-05-2005, 06:04 AM
gbeal
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Not being clever, but have you taken off the lens?
Gary
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Old 24-05-2005, 04:35 PM
rowena
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what exaclty are you connecting between the camera and the telescope?

you should have a t-adapter, plus the pentax converter(so that sits on the camera and hooks up to the t-adapter).

Also i found prime focus an abosulte pain! what i have done is hooked my t-adapter straight onto the back of the scope, and then the converter to t-adapter and then that to the SLR. You should then be able to look through your cameras view finder, and then focus the camera by using the scopes focusing nobs. you won't need to do prime focus using a lens, because the moon should pretty much fill most of your view anyhow. (i found this when using a 10" lx200, at the sydney observatory.)

i would use iso 100 film, and around a 1/180 of an exposure depending on what your lens F appeture is aswell.
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Old 24-05-2005, 10:29 PM
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If only it was that easy rowena. When it comes to prime focus Newts are a bigger challenge than SCTs, unless specifially designed for it or modified to do it. Its not so much the connecting as not having enough back focus. SCTs and refractors generally have a lot of focal travel (I have nearly 15 cm on my refractor). Newts focal travel is much much shorter. If I understand it correctly you just can't rack the focuser out far enough to get a clear image to form on the film/ccd surface. Hence the need to shift the mirror or change the focuser design.

Hope someone comes along soon and answers this better than I can. Newt un-genius here.
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Old 25-05-2005, 06:45 PM
rowena
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ponders.... that sounds very frustrating
one always learns something new everyday on here!

glad i have my schmidt-cassegrains!
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Old 25-05-2005, 07:14 PM
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Me too
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Old 25-05-2005, 07:46 PM
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Gary's comment is relevant, if you still have the lens on the camera and are photographing through the eyepiece then you need to set the SLR focus at infinity and focus using your OTA. Also googled and found a some stuff on photography using newts.
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
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Old 25-05-2005, 08:27 PM
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I thought Margo was setup in prime focus Phil, maybe not.

Margo are you doing afocal or prime focus? I read SLR and adapter and immediately though prime focus. Are you taking the Pentax lens off and attaching the camera directly to the focuser on your dob or are you leaving the pentax lens on and shooting down the focus tube. If your leaving the lens on, you are trying afocal and for that you need to leave the dob eyepiece in. If afocal, what is happening is your camera is capturing an image of the secondary, off the primary, off the secondary off the inside of the camera, (if that makes sense). The same as when you take an eyepiece out and look down the focuser tube you can see your eye, the camera can see itself. Assuming of course that's what your doing (afocal)
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Old 26-05-2005, 11:10 AM
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Hello Margo,
I own a schmidt newtonian so I guess that is a little closer to a newt than a cass is. I have an old olympus SLR camera that I've manage to attach to my scope. For prime focus, which is what it sounds like you are describing, my camera body (no lens) connects to the T-adapter which connects to a T-ring (this should be supplied with your scope), which then connects to the focusing tube of the scope (for my scope, I have to take off the 1.25 inch focusing adapter tube). From memory, and I will get back to you on this if I'm wrong, I usually have to travel a fair bit inwards to focus properly.
We really need more information about your setup to determine what is going wrong but I hope what I've said has helped a little.
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Old 26-05-2005, 09:55 PM
blackpearl
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Thank you all for your interest in this question
I have an SLR Pentax with the lens removed, the adapter for the telescope and camera and then my clever cousin made an extension to allow us to rack the focusser out far enough for us to focus through the eyepiece of the camera! Attached is a photo of the whole contraption - we will know tomrrow when the film is developed whether we have photos of the moon or the inside of the telescope
Attached Thumbnails
Click for full-size image (camera 005.jpg)
70.5 KB51 views
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Old 26-05-2005, 09:59 PM
blackpearl
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Sorry
I changed the image to 800X600 which made it about 68 Kbs - but something is wrong -
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Old 26-05-2005, 09:59 PM
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iceman (Mike)
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Nice one, well done!

This is through your 12" dob I assume? If so, you should check out the article I wrote that may give you a bit of a leg up..

Astrophotography with a dob
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Old 26-05-2005, 10:00 PM
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72k is ok, don't worry about it.

btw my article doesn't have a lot of info about SLR's in particular, but there's other hints and tips you'll hopefully be able to use.
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Old 27-05-2005, 05:53 AM
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Margo,
the picture helps.
If you are using the Pentax, and the dob (no eyepiece) then I venture to suggest you are too far from the focus point, sorry clever cousin.
If you are using an eyepiece try focusing it by eye, and without the camera in place, if possible. Then attach the camera behind the eyepiece, and it should be close to focus, with afew tweaks needed to make it better/perfect.
Gary
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