Can you double check my Numbers please:

[Earl]

Im specifying a custom adapter for a Takahashi Sky 90 reducer to my QSI

The Takahashi Sky 90 Reducer / Flattener has a spacing distance of 72.2mm taken from :

http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=8&ved=0CHgQFjAH&url=http%3A%2F%2Fwww.takahashiamerica.com%2Fmanuals%2FCCD%2520SET%2520UP%2520SCENARIOS.doc&ei=ME6vUuqlJtOThQeG5oGQBg&usg=AFQjCNGouU2M5nU-f5UB7Wr1VuC6Wqq85g&sig2=GdhrEj3J3OLgQB38F7tJYA&bvm=bv.57967247,d.ZG4&cad=rja

The QSI 683wsg has a confirmed back focus of 50.17mm and states .7mm needs to be subtracted for a Baader Filter, giving the camera with filter a Back focus of 49.47mm

http://www.qsimaging.com/downloads/QSI-500-600-Series-Back-Focus.pdf

This leads me needing a spacer of 22.73mm.

IF anyone could confirm this for me i would be very grateful.

[dmahon]

I'd check where the 72mm is from. It might be from the glass, not the thread.

[Earl]

I'd check where the 72mm is from. It might be from the glass, not the thread.

Thats a good point, I have not turned any info on that yet.

[steppenwolf]

The QSI 683wsg has a confirmed back focus of 50.17mm and states .7mm needs to be subtracted for a Baader Filter, giving the camera with filter a Back focus of 49.47mm

This is not my understanding - I think you should ADD the 0.7mm. I can confirm that the 72.2mm is from the mounting face not the lens surface.

[Earl]

This is not my understanding - I think you should ADD the 0.7mm. I can confirm that the 72.2mm is from the mounting face not the lens surface.

I thought it should be added also but is specifically states minus on the QSI document.

[steppenwolf]

Here's a quote from a QSI source (unverified):-

"back focus in this document is discussed from the perspective of the camera. Adding glass to the optical path pushes the focus further away (toward the camera) reducing the effective optical back focus of the camera by 1/3 the thickness of the glass."

Here's a quote from Don Goldman (Astrodon):-

"Camera manufacturers show a smaller optical backfocus than mechanical backfocus because they are measuring the distance from the imaging CCD focal plane. This includes detector chamber window and sometimes coverslips on the detector.

HOWEVER, that is not where WE measure backfocus from. We want to know how much space we need to add from the metal back of a telescope or from a field corrector/reducer to the imaging focal plane. This is how we select spacers, etc. So, our starting point is the scope.

THEREFORE, the addition of a 3 mm filter ADDS 1 mm [t * (n-1)/n] of backfocus between the scope and your camera (t is the filter thickness and n is the refractive index of the substrate of the filter - typically 1.5) So it ends up being t / 3. 3/3 = 1mm in this discussion. You need to ADD 1mm of space between your scope and your camera.

This is often confusing. When you place a filter in a beam of light that converges from left to right, the focus is extended FURTHER right INCREASING the backfocus distance as measured from the scope.

All Astrodon filters are 3 mm thick, so the same 1 mm must be ADDED."

[Earl]

Im dropping QSI an email to confirm

[Earl]

QSI have email me back

Earl,

Your calculations of the QSI back focus are correct. With a Baader filter, the back focus of a QSI 683wsg with the standard T-mount adapter will be (50.17mm - 0.7mm) 49.47mm. Subtracting that from your desired focal depth of 72.2mm yields 22.73mm.

Regards,