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Newt. wont reach focus with DSLR


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OK so I've been around the block a bit with this so here's the summary (though its quite wintery outside).

Kit is Skywatcher 200mm f/5 Newt. with the standard stuff. Camera is a Nikon D50 DSLR. Here's the question, if I can describe it properly:-

The scope comes with two interchangeable eyepiece holders, one for 1.25" and one for 2" eyepieces. The smaller unit has a detachable tube which is the bit where the eyepiece actually fits and the tube section is removed when using the DSLR. When the tube is removed it is possible to fit a T ring onto a thread on the stub which is left.

With the stub section the DSLR will reach focus, but there is no place to put a filter, as the stub has no thread to accept one.

If I use a T adapter on the DSLR which has a 1.25" nose and which will accept filters, I can fit it to the stub and tube assembly but the focus travel is insufficient for it to reach focus.

Astro Ant described this as being fixable by adjustment of the collimation of the primary mirror in another thread, though he didn't elaborate on which flavour Newt he had cured.

I would like to be able to use filters without resorting to using a Barlow lens, as sometimes this just gets too close in for the image that I want.

I would also like the option of using a 2" adapter to get around the vignetting that I get on all the images with the DSLR, so this "cure" could mean that I could use the 2" one piece adapter and a T-mount 2" nosepiece on the DSLR.

I'm hoping for a BTDT reply which would show me the way to go from here.

TIA

Captain Chaos

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OK so here are the bits:-

image.jpg

on the left are the 1.25" bits, on the right is the 2" bit. Either the 1.25" or 2" bit will fit on the 'scope via the round dovetail on the right of both parts (top one on the left).

image.jpg

This picture shows the 1.25" 'scope adapter and nosepiece/T-ring/DSLR bayonet all fitted together. This combination will not acheive focus.

The part on the right ('scope adapter) seperates from the tube with thumbscrews by unscrewing.

image.jpg

The T-ring and dissassembled 'scope adapter shown seperately.

image.jpg

Finally the T-ring is fitted to the 'scope adapter - this is OK for getting focus, but has no filter thread anywhere.

Hope this has made it a little clearer?

Captain Chaos

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You have exactly the same set up as me, except for the camera adapter...

Mine is shorter..However doesn't the 1.25 nosepiece have a thread for filters, my BC&F one does. I just use the 1.25 adapter staight onto the camera and onto the scope.

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Arthur, the 2" and 1.25" adapters are the same length and so would not work.

Phattire, the thing in your picture is the bottom left part in picture 1 that I posted. I have the T-ring nosepiece and it does have a filter thread. BUT it won't reach focus.

A shorter version wouldn't poke into the 'scope as much but the DSLR focal plane would be just as far from the end of the 'scope socket.

I think that I need to cut the bit with the thumbscrews in half to allow the nosepiece to go in further. Then I would need to do something about the lack of the thumbscrews.

Captain Chaos

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What I need to do is to be able to wind te DSLR closer to the 'scope tube than the focusser will currently allow me to do with the 1.25" or 2" EP adapter. If either were shorter, then I could acheive focus, either with a 1.25" or 2" T-mount camera nosepiece. As it is both stand the camera off from the 'scope tube by too much as they are both too long.

I'm going to try zapping Saturn and the changing shadows on the moon now with my newly bodged manufactured eyepiece projection adapter.

So Damo, how would a diagonal help? Surely it makes it further between the 'scope and the DSLR when I need closer. VERY intrigued now (will have a bash maybe if the clouds stay away) So much to do, so little time.

Next question is how to set up the SC3 webcam thingy, so brace yourselves guys.

Captain Chaos

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It might be a case of a "Low profile" focuser required?

Thats what I am thinking.

Or, accept the need to use a barlow.

Or, set your mirror an inch or two further up the tube!

The simple answer would be to use a Diagonal...Then focus is achievable.

How would that work?

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I was having trouble achieving focus with my DSLR through a refractor. Added the the diagonal, and Hey Presto. :lol:

Damien,

Refractors tend to have the opposite problem to Newtonians - the focuser won't rack out far enough. Thats why the diagonal helped in your instance but I doubt that it will solve CC's problem.

At least thats my understanding, mind you, I am not an imager so what do I know :roll:

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Update on the focus thing.

Last night I was playing with the new SC3 webcam thing and I had just the same problem. The webcam wouldn't focus either.

Interim cure was to unscrew the eyepiece holder tube assembly (top right in pic. 1 above) and screw the tube in the wrong side of the bit that goes into the focusser. I took the thumbscrews out as a precaution as I didn't want any chance of them dropping inside the 'scope. With this arrangement I was able to get the thing to focus and get it to work.

WRT re-collimating the Newt. to get the focus to work with the normal arrangement, I don't think I'll bother. If I use the 2" EP adapter I have to fit the 2" EP only part way inserted otherwise the focus won't come out enough. As it stands I can use both extremes of the focusser travel but with minor bodges.

Looking out now for some nylon screws to use in the EP holder tube to clamp the webcam or DSLR. they'll have to be short or they won't fit inside the focusser. I'm guessing that they'll be easy to cut to length though.

Captain chaos

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