Backfocus for Camera

[AlistairW]

Hello,

How important is getting the exact backfocus - I am fretting over 0.5mm at the moment. I know the test is to take an image and look at the star corners, but the weather isn't playing ball at the moment.

Alistair

[Alien 13]

If the backfocus is from camera to a FF or similar the you might need 0.1 mm accuracy, it certainly is like that with my scope.

Alan

[AlistairW]

Alan, that's exactly my situation. It amazes me it needs such accuracy.

[Aidan]

When i was struggling with my FF, i got some fine tuning spacers which in conjuction with various spacers got me to where i needed to be... I think i got them from FLO

[Alien 13]

I did a lot of trial and error with various types of adapters and spacers (some home made) to get it right but once found the adapter combo was never disassembled again.

Alan

[Davey-T]

I use Baader VariLocKs, infinitely adjustable.

Dave

[pete_l]

9 hours ago, AlistairW said:

How important is getting the exact backfocus - I am fretting over 0.5mm at the moment. I know the test is to take an image and look at the star corners, but the weather isn't playing ball at the moment.

The precision needed will depend on the size of your camera's sensor and also the rate of convergence of the light cone, i.e. the F-ratio.
That is why the only true test is to take an image.

[Notty]

Stupid question: is it possible to determine whether or not you've got the back spacing correct on a daylight subject?  Otherwise can anyone tell me exactly what you're looking for in order to determine if you've got it right?

[AlistairW]

Notty,

Hopefully the pictures attached show what to look for with a star image ( I take no credit as i got this from a thread Sara Wager was on). No idea about daylight, unless you can use an artifical star (?)

[AKB]

On 06/06/2017 at 23:17, Davey-T said:

I use Baader VariLocKs, infinitely adjustable.

I've just been introduced to the joy of these... exactly what I needed to make my flip mirror eyepiece parfocal with the camera... also to replace a camera adapter on a Hyperstar with a Baader UFC filter system.  Now there you do need sub-millimetre (not to say micron) accuracy!