CCD setback

[harry page]

Hi

Often when imaging I use reducers/flatterers and many of these need critical

spacings.

The ccd setback on my6sx cameras is a little variable and got feed up with

trial and error , so with a little help from terry platt from starlight xpress , here is how I found my set

back of 16.6047mm -- should good enough for most

http://www.harrysastroshed.com/ccdplate.html

Might be of help

Harry

[ollypenrice]

Rather nifty, Harry! Thanks for passing this on.

The variable, though, is the effect of any filters behind the reducer or flattener. The rule of thumb you come across is to add a third of the thickness of the filter to the chip distance. How accurate this is I don't know but I suspect it will vary with the angle of the light cone. With OSC your method will presumably be bang on.

Olly

[harry page]

Hi

Personaly I do not think it does otherwise the light entering the filter straight on ( i.e in the middle of the filter ) would be refracted at different rates causing all sorts of strange effects

I could be wrong of course

Harry

[andrew s]

Acording to Rutten & van Venrooil "Telescope Optics" the displacement is t(n-1)/n where t is the thickness and n is the refractive index and this displacement is dependant on the cone angle. However, I think for most normal filters and typical focal ratios it is negligible. Plates can also introduce aberations but again these will be samll. I think you would need to do ray tracing to see if it was an issue in a particular setup.

Regards Andrew

[harry page]

Hi

Excellent information , many thanks --- but in real life I think the difference is that small its not worth taking into consideration

Kind regards Harry

[Coco]

Clever stuff,.. Now if only you could work out chip face to reducer distance as accurately as this..

By this I mean fine tuning not makers recommendations.. I'm presuming this is for discovering what the CCD to nose is and not CCD to rear reducer element of the flattener/reducer is, now that really would be a time saver.