**This is a transfered topic from the ASA Yahoo Group.**

**Posted By: doug_landmann Thu Apr 18, 2013 5:27 pm**

Why is it that get two different answers for the thickness of the adapter

required if I use the ASA calculator or and if I take the optical backfocus

number from SBIG and subtract it from the ASA backfocus requirement?

ASA 10"N Wynn 3" corrector f/3.8 backfocus from Chart A = 57.29mm

Which is the same number that comes up in the spreadsheet calculator for the 10N

with Wynn 3"corrector at f/3.8

Using the calculator and accepting the 1mm glass thickness over the CCD sensor

and 3mm filter thickness and inputting 44.74mm for the backfocus for the SBIG

STXL-11002M with standard filter wheel. The calculator returns the adapter

thickness of 13.87mm.

Using SBIGs Optical backfocus number (42.77mm) and simply subtracting that from

the 57.29mm total backfocus the result is 14.52mm. a difference of 0.65mm. Is

that number significant? Should I not worry about it Would you error in favor

of the ASA calculator or the SBIG optical focus number?

Thanks in advance for your input.

Doug Landmann

_____________________________________________________**Posted By: portaball2001 Fri Apr 19, 2013 1:14 pm**

Hi Doug,

The formula for the path increasement of light in a media is (r - 1)/r, r being

the refraction index.

For glass r = 1.5, which gives a delta of 0.33*thickness_of_media.

In your example(thickness_of_media = 4mm) delta is 1.32mm.

Necessary adapter length: 57.29mm - 44.74mm + 1.32mm = 13.87mm

The ASA calculator is right, SBIG's specified optical length is wrong.

Best regards,

Gerhard