Problem focusing my camera with a T-ring.
Posted 23 November 2012 - 10:38 PM
Posted 24 November 2012 - 01:04 AM
Posted 25 November 2012 - 02:42 PM
Posted 25 November 2012 - 03:08 PM
Common enough to be mentioned in the Backyard Astronomer's Guide as a problem. But they didn't mention Mavierick199's easy fix. Actually, one Celestron T-adapter comes with a built in Barlow. If I'd gotten that one, I'd never have known about the problem.
I guess this must be pretty common ?
Posted 25 November 2012 - 05:43 PM
The best hardware solution to try is a Baader MPCC. This Multi Purpose Coma Corrector (MPCC) adds 10-15mm of back focus without changing anything else about your optical system. If works best with f/5 to f/7? reflector scopes and is designed to fix the off axis coma that's inherent in a parabolic mirror. T adapters present a fixed distance from the front of the T flange to the imaging chip which is why they are different for each camera. The MPCC is designed to work when mounted at that distance from the sensor.
The MPCC does require a 2" focuser drawtube. If your scope has only a 1.25" then you'll need to upgrade your focuser. Getting a low profile one may allow you to use the camera without an MPCC. Another alternative is to move the mirror 2-3cm up the tube but you could loose some light from the primary if the secondary isn't large enough to intercept the full light cone after the mirror has been moved.
Posted 25 November 2012 - 05:51 PM
When I tried using a Paracorr on my f4.2 Dob I couldn't focus. I assume the MPCC wouldn't work either? When I pulled the Paracorr and put in the Barlow, that worked, but only with the Barlow alone.
If works best with f/5 to f/7? reflector scopes and is designed to fix the off axis coma that's inherent in a parabolic mirror.
Posted 26 November 2012 - 01:39 PM
ill probably get a chance to try the barlow trick for my camera side of the problem, before it comes in..
i have seen that seldom (built in barlow). i have a 2x and 3x. it must change the magnification as it does with an eyepiece? or is it just moving the focal plane sort of (if thats the right term)?
Posted 26 November 2012 - 02:01 PM
Posted 26 November 2012 - 02:32 PM
I'm pretty sure the Barlow effectively multiplies the focal length of the scope by spreading out the rays so you only see a reduced angle. Similar to the thing that happens when you buy your first DSLR, and find all of a sudden your 200mm lens is a 300, and your 20mm lens is a 30. Don't know how this affects the focal plane, but I guess it moves it some.
Typically Newtonians do not have enough inward focuser travel to come to focus with a camera. Some of the old Newtonians actually had two sets of mounting holes in the OTA for the primary mirror, one for visual, one for photography. My old 12.5 inch F/6 is done that way.
The Barlow solution changes the effective focal length and focal ratio of the scope, this is not always a good thing. And whether the Barlow works for this depends on the Barlow, most Barlows actually require additional inward focuser travel (called "Backfocus") but using just the Barlow optics alone can be helpful.