Is there a lookup table for telescope back focus?

[uk_friendly_fire]

Can't find back focus spec for Altair Astro Starwave 66 ed-r on the net. Is there such a source?

[PhilB61]

Back focus distance is a term usually used to describe the spacing from a flattener/reducer to the camera sensor, which is critical for good star shapes. This spacing is set by the manufacturer of the flattener/reducer not the telescope manufacturer although of course they could be the same. What are you using the term for in relation to this telescope?

[michael8554]

The Altair Astro Starwave 66 ED-R is a Refractor with a focal length of 400mm.

Measure 400mm from the doublet front element.

That's where the scope focuses, that point does not move when you alter the focuser.

The focuser merely moves the camera or eyepiece to that focus position.

You may need Extensions to place your camera or eyepiece at that focus point, if the focuser runs out of range.

Michael

[uk_friendly_fire]

20 hours ago, PhilB61 said:

 What are you using the term for in relation to this telescope?

Phil - I thought 'back focus' was a measurement taken from the end of the focus tube (racked fully in) to a point where the telescope produces the focal point.

As I understand it, introducing a field focuser or other optic device such as a barlow lens alters that distance.

What I want to do is focus my ccd more easily in the field rather than having to go to and fro making focal adjustments to try and get some sort of image to then carry out final adjustments.  I do not currently use a field flattener or corrector.

[uk_friendly_fire]

9 hours ago, michael8554 said:

Measure 400mm from the doublet front element.

OMG - Thanks Michael8554 - I feel such a numbskull for overlooking the obvious.  Obviously that works fine for a refractor as in this case but a bit more tricky with those scopes using a mirror.

Thanks for your answers!

[PhilB61]

You could do some rough focusing on a distant object in the daytime,  once you have decent focus you can just make a note of the position. The focus position for astronomical objects will be slightly inwards of this position. If there isn't a scale on the drawtube it's simple enough to neatly measure and draw something out on a small piece of paper and attach to the drawtube with clear tape. I did this for a SW 72ed.

[uk_friendly_fire]

Thanks PhilB61 that's a practical idea.

I have the same scope amongst my collection.

I also have a Brother handheld electrician's printer for use with pvc adhesive tape which should do the trick.