Advice on Extension Tubes

[bokeh]

So I have a Canon EOS 1100d and I just bought a T ring and 1.25" adapter so I can fit it onto my Celestron Astromaster 130EQ (which is a Newtonian). Everything fits well, however, I seem to have run into the problem which according to many other sites, many people with Newtonian telescopes have faced. You've probably heard this before, but I can't focus the image I see when I have the camera in the scope.

I've been told that an extension tube will solve my problem. To my understanding, an extension tube attaches to the focusing tube of the telescope as well as the T ring of your camera, and also allows you to insert an eye piece into it?

I'm guessing (if this is true) that the focusing of the eye piece would then focus the image onto the camera sensor, solving my problem since I'll end up with a clear, focused image? Am I right?

Please help, I don't want to splash out on another piece of equipment and see it wasted.

Thank you in advance

[atmenterprises]

I don't know about your specific Newt, but I have the 200p Newt and I went through this. An extension tube didn't help because I needed to get the camera sensor closer to the tube, not further away as an extension tube would do. I had to move my primary mirror up a couple of inches (200mm maybe for you Brits?). This solved the focus problem with my dSLR.

However...

Once I moved the mirror up some of my EPs wouldn't focus, so I needed an extension tube for them.

I'm not sure how to move the primary in your particular Newt, but hopefully someone will come along that can speak to it specifically.

It wasn't difficult to do. $10 worth of bolts and nuts and about 30-45 minutes of time.

[atmenterprises]

Oh, and an extension tube doesn't allow eyepieces. You're talking about an eyepiece projection tube. They look similar but the locking screws are in different places + the eyepiece projection tube is threaded for your t-ring.

[Josh Wilson]

I had to move my primary mirror up a couple of inches (200mm maybe for you Brits?).

I'm not british but I know that 200mm is equivalent to 8 inches. You had to move the mirror up 8 inches? I know you said maybe so you're just taking a guess, but, depending on the scope, you should only need to move the mirror up about 3 to 4 inches unless you don't want the the focuser drawtube inside the optical path to get the most aperture; in which case you should move it up about 5 inches or however many you need to. 3 inches is equivalent to about 80mm, 4 inches= about 105mm, 5 inches= 125mm.

[JKB]

Hi

my friend had same scope and same problem. If I recall he used a 2x Barlow with t a nose piece adaptor on the dslr. This gives the desired in-focus. http://www.firstlightoptics.com/adaptors/flo-125-inch-t-mount-camera-adapter.html I think is the one he bought.

hth

cheers

Jamie

[atmenterprises]

I'm not british but I know that 200mm is equivalent to 8 inches. You had to move the mirror up 8 inches? I know you said maybe so you're just taking a guess, but, depending on the scope, you should only need to move the mirror up about 3 to 4 inches unless you don't want the the focuser drawtube inside the optical path to get the most aperture; in which case you should move it up about 5 inches or however many you need to. 3 inches is equivalent to about 80mm, 4 inches= about 105mm, 5 inches= 125mm.

Thank you! That was a bad typo. It wasn't 200mm! I should never, ever try to do American-to-British conversion.

[JamesF]

As atmenterprises says, the usual problem here is that the 44mm flange to sensor distance on the camera means you can't get the sensor close enough to the focal plane of the telescope to get the image into focus. There are a number of possible workarounds for that problem, the easiest of which is probably to use a barlow with the camera.

Moving the mirror up the tube is sometimes possible but depends on the design of the OTA. This moves the focal plane out of the focuser by the same amount that you move the mirror up. It's not necessarily a perfect solution however, as you may then need an extension to get eyepieces to come to focus. It can also mean that you lose light because the secondary intersects the light cone at a wider point and is no longer big enough.

Another option is to replace the focuser with a low-profile model, allowing the camera to move further in. This is likely to be expensive and may mean modifying the OTA to fit the new focuser.

My gut feeling is that unless you have the tools and feel confident then replacing the telescope entirely with something that will fit your requirements may well be a world less pain than the latter two options, in which case buy a copy of "Making Every Photon Count" first

James