Swift 831 Drawtube

[Mikel56]

Both photographs have the focus control knob in the same position. The first photograph has the eyepiece fitted directly to and in line with the direction of the telescope. I’ve focussed towards an object about 500 m. distant. Using only the draw tube for focus I have had to extend the draw tube in order to focus the object at approximately 500m.

The second photograph has the same eyepiece fitted to a diagonal. In order to focus the same object at 500m. I have had to close the draw tube quite a bit. I noticed last night whilst focussed on Polaris (I think) with the diagonal the draw tube was  closed. After focussing with the focus knobs I still had some room for movement with the focus knobs but not that much. Although the telescope was focussed at infinity with a couple of turns left to go I was surprised that the diagonal physically pushed the focus control (draw tube plus focus knobs) to near limit at infinity. Does a diagonal have such an adverse effect ?

Mike

[Mandy D]

All that the focuser does is move the eyepiece back and forth from the objective to obtain focus. Putting a diagonal after the focuser does the same thing adding it's extra light path length to the focuser, so yes it will have this effect.

[Cornelius Varley]

7 minutes ago, Mikel56 said:

Does a diagonal have such an adverse effect ?

Yes it will. The distance the draw tube has to move to regain focus with the diagonal in place will be the same as the effective length of the diagonal.

[Mikel56]

Thanks Mandy and Peter. I was concerned but not now ! Maybe I need a smaller diagonal.

[Carbon Brush]

Is your diagonal a mirror or prism?
A prism can mean a smaller extension to the light path than a mirror diagonal. Depending on the diagonal construction.
Take look at the link to get you started.

If you are running out of travel, consider changing the adapter from the focus tube to 1.25" for something lower profile.

HTH, David.

https://www.google.com/search?client=firefox-b-d&q=prism+diagonal+light+path#vhid=gjh9-cm9TC17UM&vssid=l

[Mikel56]

It looks to me very much like a mirror. The back has “Celestron Star Diagonal 94115 - A. I take your point on the adapter.

Mike

Edited August 18, 2023 by Mikel56

[Mikel56]

The photograph below has the diagonal I was using as above. I think it’s a mirror. I just found the one on the right in a box. It looks like a 45 and according to your link that might be a prism? When looking inside the diagonal on the right there are multiple angled flat surfaces appearing on the circumference. Probably a prism ?
 

Mike

Edited August 18, 2023 by Mikel56

[Mandy D]

9 minutes ago, Mikel56 said:

It looks to me very much like a mirror. The back has “Celestron Star Diagonal 94115 - A. I take your point on the adapter.

Mike

It is a prism.