Afocal imaging in a Dob

[Straylight]

Now, I was thinking of putting my 16" on an equatorial table and having a go at some widefield imaging.

However, just out of interest, I got an adapter for my 400d and found I cannot get the camera into focus, as I suspected would happen.

Question: Is there any way I can get the camera into focus WITHOUT moving the primary lens up the tube? I still want to use the Dob (16" LB) for visual observing.

[iceman (Mike)]

Hi there.

There's an article in the projects section about how to move the mirror up the tube, but you could also try a low-profile focuser which might give you enough in-focus.

[Straylight]

Hmm. Even with my current focuser in as far as possible, the focus is still waaaay out. So far out, I don't think a low profile focuser will work.

Is there a way to work out where the focal point is mathematically? I can supply the specs if someone wants to work it out.

Looks like an imaging 'scope is on the cards at some stage then. Got to learn where things are in the sky first.

[Zuts]

Originally Posted by Straylight

Now, I was thinking of putting my 16" on an equatorial table and having a go at some widefield imaging.

However, just out of interest, I got an adapter for my 400d and found I cannot get the camera into focus, as I suspected would happen.

Question: Is there any way I can get the camera into focus WITHOUT moving the primary lens up the tube? I still want to use the Dob (16" LB) for visual observing.

Why cant you move the mirror up the tube so you can use it for photography and then get an eyepiece extender for visual use?

Paul

[Straylight]

Why cant you move the mirror up the tube so you can use it for photography and then get an eyepiece extender for visual use?

I don't see why not. I am relatively new to astronomy though, and doing surgery on my only scope is not what I had in mind first up. I'll check out the DIY article and see how hard it is.

[iceman (Mike)]

Originally Posted by Straylight

Is there a way to work out where the focal point is mathematically? I can supply the specs if someone wants to work it out.

I'm not sure about mathematically - probably there is, but I don't know how. But you could work it out by taking off the focuser, and moving your DSLR closer to the secondary until you find the point that it's in focus.

Does your dob have tracking?

What sort of photography did you want to do?

[bkm2304 (Richard Brown)]

HI all,

If this is a 16" Lightbridge you only have to get a spacer or extender for the focusser. Point the scope at the moon, point your lensless camera down the focusser barrel and slowly draw your camera away from the focusser. If it's like mine it needs a spacer to extend the focusser length by about 35-50mm. Then it should be fine! Bintel or Andrews Communications should be able to help you.

Richard.

[Straylight]

Wow, OK. I thought the focus was going to be inside the tube, not further out than the focusor could take me.

Will experiment in 3 months time, when I get my first cloud free sky.