Help in Newt. design please...

I'm about to build/ modify a Dob around a wonderful, new mirror from Cary at Woden Optics.

I hope you can help me. Where does your ToUcam Pro come to focus in a 5x powermate as compared to an eyepiece? Do I need move the image/ focal plane outward in order to be able to focus my webcam... or is it going to focus in about the same position as one of my eyepieces?

Thanks!

Joe

Has anybody read this?

Hi Joe,

All I can tell you is that with my 18" F/4.3 Starmaster the eyepiece and camera do not focus at the same position (it's close though), you might try this on the Equipment talk forum.

Thanks,

Wes Higgins

Joe,

An excellent question... in particular because I am also building a new scope, and I have both 4x and 5x powermates so I want to know the answer too!

Now it just so happens that I have a focusser with digital position readout, so it was only a matter of an hour or so to do the experiment tonight.

Rather than setup the camera, I used eyepieces for the comparisons, I'm going to assume that the relative positions are pretty close to what your camera would also see :-)

I tried 3 sets of eyepieces:

1. Celestron ultima 30mm
2. garden variety plossl 15mm
3. Vixen LV 9mm

And I tried 3 setups:

a) EP on it's own. with a 2" to 1.25" adapter that adds about 10mm
b) EP through a 4x powermate with the same 2"-to-1.25" adapter
between the powermate and the eyepiece.
c) EP through a 5x powermate, no adapter needed.

What's important in the numbers below are the *relative* focus positions, not the absolute positions. I'll give the absolute position readouts anyway so you can see the raw results.

Here we go:

1a: 38.87mm *
2a: 40.78mm *
3a: 39.13mm

1b: 60.04mm *
2b: 60.22mm *
3b: 60.05mm

1c: 62.68mm *
2c: same as 1c *
3c: same as 1c

* Add 10mm for 2" to 1.25" adapter height to get "real" position.

So, across the board I see roughly 20mm extra travel required to bring all of these eyepieces to focus with both a 4x powermate and 10mm with the 5x powermate after correcting for the added height from the 2" to 1.25" adapter.

Now, lots of caveats... This test was done with a 10" f/6 scope - it may be that the f-ratio has some effect on this extra distance, or maybe the 1500mm focal length of my scope has an effect. Trust these numbers at your peril, and be prepared to drill new holes :-)

But it looks like you should allow an extra 20mm travel at the minimum, and probably 30mm to make sure you're safe.

regards, Anthony