Focus question - how long is a 1.25" diagonal?

[Kaptain Klevtsov]

Checking out some 'scopes, I read that one in particular (TAL 200K) suffers from a lack of focus travel, but in the pictures it is fitted with a diagonal. Seing as how I will be sticking the camera on without the diagonal, I expect that some light path will be recovered, so it might be OK.

The bottom line is, where, in relation to an eyepiece, is the focus point of the 'scope? I think that it's somewhere in front of the EP, by the focal length of the EP, measured from somewhere in the glass of the EP.

Anybody know / have a link?

TIA

Captain Chaos

[GazOC]

I read in a binoviewers thread on another site a while back that a 1.25" diagonal adds 20mm optically. No idea if this is correct though.

[Kaptain Klevtsov]

Mechanically, I'd guess at nearer 50mm from the end to the bottom of the hole where the EP goes in, must go and measure one.

Captain Chaos

[GazOC]

Does it matter how big it is mechanically? Or I am missing the point?

[GordonCopestake]

CC, dont forget that the eyepiece sits inside the diagonal like it sits inside the focuser, and the diagonal barrel sits inside the focuser. I would say that 20mm is about right.

[Kaptain Klevtsov]

My point is, if you don't use a diagonal but put the EP in the back of the 'scope, you'd have to wind the focuser out a long way to compensate for the lost light path. Just how much that is I don't know. My own diagonal adds 55mm ish to the light path, which is about the distance from the chip to the other side of the filter wheel. Come to think of it, 50mm ish is the normal T mount ring to film plane distance so it ought to kind of work maybe, if a DSLR works

Gorgon, I was measuring from the end of the shiny tube on the diagonal to where the end of the shiny tube on the EP would be at the bottom of the hole in the diagonal. I think that is how much extra path is involved, as its the measurement from the same point on the EP with and without the diagonal bit, if that makes sense.

Captain Chaos

[GordonCopestake]

Hmmm. Dunno then. I guess the best way of measuing it would be to focus with the diagonal in place, then focus without the diagonal and see how much more out focus you need.

[GazOC]

Those sound like the right points to measure, I'm very surprised its ~55mm.......Surely you still need the optical length, not the mechanical. ie. prisms and diagonals will be the same mechanically but different optically?

EDIT: FWIW I get ~35mm

[Kaptain Klevtsov]

Taking on board Gordon's advice, I pointed the ED80 at my favourite pylon and focused using my diagonal and a 20mm Plossl EP. Then I took out the diagonal and stuck in the Atik plus filter wheel. I had to wind the focuser out an additional 36mm, so the lack of in travel myth has been truly busted. Looks like the big TAL will work fine with the Atik, even with the filter wheel on. Just for a giggle I tried the Nikon DSLR. 20mm further out than the diagonal and EP combo. The object of the exercise was to find out if the lack of in travel of the TAL focuser would be a problem for imaging, it turns out that the focus point for both a CCD and a DSLR are further out than a diagonal and eyepiece. Bizzare or what?

Are there other kinds of diagonal that have magic properties, or was the guy in the review trying to use the DSLR and diagonal?

It does mean though, I think, that any 'scope that can use a diagonal and EP can work with the cameras that I have. Just, in fact, what Gordon said the other day.

just for information, I tried the Plossl without the diagonal, 75mm further out (65mm of focuser plus 10mm of EP hanging out of the extension that was in there all the time).

Captain Chaos

[GordonCopestake]

Wow, I was right? How odd! That doesn't happen all that often, let me bookmark this page....

[GazOC]

Weird, reviewer might have been using a 2" diagonal. Seems strange he didn't just take it out though....

[barkis]

The focal point of a point source at infinity will be at a fixed place. If you place your diagonal in the drawtube and place a knifedge across the top of the diagonal minus the eyepiece of course,. Rack the focuser in and out whilst tracking a fairly bright star. If inside focus you will see the knifedge cross the light in the same direction. Outside of focus, from the opposite side. Bang on focus, then the the light will darken all over instantly. That then is the focal point of the scope pointing at any object at infinity. Providing you don't alter the position of your mirror.

All your other devices can be placed according to that point of focus. You can do it without the diagonal in place. I used the method for focusing my SLR camera by placing the knifedge on the film rail. Of course it was awkward putting the film in afterwards, without moving anything.

After all this CC, I am not sure I have answered the question you asked. If not ... SORRY.

Ron.