What’s the mirror travel distance of a Celestron 6SE?

[PeteG]

Thread topic speaks for itself.
Cheers.

[Ukastronomer]

I don't know but why do you ask it would be nice to tell people in your question

[PeteG]

I did ask in a previous thread, but no-one replied, so I thought I’d simplify the question. I’m trying to work out how far to move my guide camera so that it’s in focus. I just need to know how far the focus moves for one turn of the focus knob. Any help would be greatly appreciated.

[mental4astro]

Not so simple.

It's not as easy as 10mm mirror travel equals 10mm equivalent focus travel. Unlike draw-tube focusers, mirror-shifting focus mechanisms are not linear, rather it is geometric. Mirror travel is actually very small but generates a long distance of focus travel.

Typical f/ratio of an SCT primary mirror is f/2. The convex secondary changes this to f/10. This is a factor of 5.
So, say the mirror moves just 2mm, the effective focus shift can be in closer to 10mm.

I think I understand what you want to do. One problem is there really isn't a zero starting point with the focuser of an SCT like with a draw-tube focuser. Then add to this the huge factor change created by the secondary and if you are out by small amount, the focus may be significantly out.

Only way around this is by installing an after market focuser onto the rear of the SCT to replace the need to focus via the scope's mechanism, and focus the scope solely via this second focuser.

I hope thiis goes some way to answering your question.

Alex.

[PeteG]

Thanks Alex. I thought it might not be that straight forward, as you say.
I’m using an Orion thin off axis guider, so a focusser at the back of the ota won’t help. The guidecam is ‘almost’ in perfect focus with the imaging camera, but a tiny tweak makes the guidecam (and phd2 response) spot on.
I think that with a few tweaks of the guidecam spacing I’ll get it better.
Thanks again.

[Sunfish]

I have an OAG with a focuser on top. That makes it easy to adjust. Perhaps you can add a helical focuser under your guide camera.

Although , if reducers and or eyepiece projection is used spacers may be required in addition.