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Need Help Calculating my Focal Plane

Started by mccarlp, 04/27/2002 03:51PM
Posted 04/27/2002 03:51PM Opening Post
Well I am nearing that oh so critical point in my truss tube conversion. I am almost ready to cut the tubes. Yikes!
My problem is not in calculating the distance to the focal plane for eyepieces, I have that down, I think. My actual measurements from the current sonotube setup are matching my calculated numbers, so I think I am ok there. The problem is I also want to be able to take pictures with my SLR Canon EOS camera. And I learned a long time ago that there is a big difference between a focuser for an eyepiece and one that works with my camera, even with the T-ring flush against the draw tube. My focuser on the sonotube doesn't let me reach focus on distant objects, although I can take some nice shots of the birds in my neighbor's trees. 8^) So I can't really measure accurately the difference in the focal planes. Now I have measured the difference using my short tube 80 refractor, and it was just over an inch different. So my question is will the relative distance between focus for a particular eyepiece and my camera be the same on my 17.5" 2000mm fl dob as it is on my 400mm fl ST80? if not, how do I calculate it? Any help will be appreciated.


Paul McCarl
Posted 04/27/2002 04:04PM #1
I've never taken a prime-focus astrophoto, but if I have the right picture in mind, i.e. just a the camera body and adapter with no lenses, then the difference in focuser position should be the same as with the ST80.
Posted 04/27/2002 05:20PM #2
I tried a little test using a Pentax K1000 camera. I focused on a distant object using a Meade 6.7mm UWA eyepiece. I then focused on the same object with the camera. I needed 2.7" in travel to focus the camera. The camera adapter is 0.95" thick. It is a 2" to T thread adapter coupled to a T thread to Pentax bayonet adapter. The 2" to 1 1/4" eyepiece adapter is 0.25" thick so there is a difference in adapter thickness of 0.7" and the actual in travel from the eyepiece rim to the lens mounting plate surface is 2". I tried the same test with a Meade 3000 Series 25mm eyepiece and got a difference of 1.9".

Your in travel requirements will depend on the distance between the film plane the lens mounting surface of your camera and the thickness of your camera adapter. The distance between the lens mounting surface and the film plane on the Pentax is 1.8".

Hope this helps.

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