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Problem focusing my camera with a T-ring.

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#1 Seldom

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Posted 23 November 2012 - 10:38 PM

Can't get my Panasonic Lumix DMC G2 to focus on the moon using a 1 1/4" Celestron T-adapter and a Fotodiox micro 4/3rds T-ring. Seems like 1 1/2" of focuser travel isn't enough to get close to focus, but all my EPs focus fine.

#2 lamplight

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Posted 24 November 2012 - 12:05 AM

try without your diagonal?

#3 Seldom

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Posted 24 November 2012 - 01:04 AM

It's a Newt, so losing the diagonal's not an option. I was hoping maybe somebody else had experienced this and that there was a hardware fix. The T-ring's about 1 1/2" thick and the T-adapter's another 1.5".

#4 Atl

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Posted 24 November 2012 - 07:08 AM

You need an extension tube.

#5 Maverick199

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Posted 24 November 2012 - 09:04 AM

Easy fix, use a Barlow.

#6 Seldom

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Posted 24 November 2012 - 08:01 PM

Thanks, Maverick199. That did it.

#7 lamplight

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Posted 25 November 2012 - 02:42 PM

I just noticed my reflector has the same issue with a new focuser.. in fact I found that the new Crawford style focuser doesn't QUITE come to focus with an eyepiece but remembered this thread so what did that worked was loosened the 2" to 1.25" eyepiece adaper and slid it out about 1/4" which allows me to reach focus on the star.. with a 2" eyepiece (I only have one) I just don't push it all the way in similarly, to reach focus. at the end of the night I tried my camera and it didn't focus so I'm assuming this Barlow trick will work but it was cold and very late and I was sick of waiting for the clouds to clear. I happened to notice in another thread while checking out a dob that looks like it comes with the exact same GSO focuser I bought, and the dob focuser COMES with an extension tube for the focuser. I guess this must be pretty common ? apertura/gso dob im thinking I'm just going to get the extension tube since it affects me visual and imaging.

#8 Seldom

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Posted 25 November 2012 - 03:08 PM

I guess this must be pretty common ?

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.

#9 Phil Sherman

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Posted 25 November 2012 - 05:43 PM

Pulling the eyepiece out a bit works fine if you can't raise the focuser enough but this rarely solves the problems with a DSLR. DSLRs have a much longer back focus than eyepieces and it's not uncommon that you can't run the focuser in enough to get everything in focus. A barlow could solve the problem but will work only if everything fits just right.

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.

Phil

#10 Seldom

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Posted 25 November 2012 - 05:51 PM

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.

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.

#11 lamplight

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Posted 26 November 2012 - 01:39 PM

I spoke with agena support who actually answered me on the weekend via email and they said that the camera focus issue is more commonly not enough focus travel IN rather than my eyepieces needing a little more OUT focus distance.. is that what you're saying Phil? i ordered a 2" extension tubes thats on sale from them for under $20 so we'll see.. id rather just slap that in than have something not quite pushed in all the way you know?but for now, nice sharp focus.

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)?

#12 Seldom

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Posted 26 November 2012 - 02:01 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.

#13 Jon Isaacs

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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.

Jon






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