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Different Back Focus Numbers (ASA 10"N Wynn 3")

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#1 YahooGroup



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Posted 19 May 2013 - 12:43 PM

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,


#2 wemartin



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Posted 15 July 2013 - 10:38 AM

Gerhard, Doug


A 4mm thickness of glass with n=1.5 is the equivalent of 6mm=4x1.5 of air so the delta thickness of the path in air is 2mm, not 1.32mm.



#3 GerhardB



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Posted 15 July 2013 - 03:59 PM

Hi Bill,


The focal shift Delta can be computed with this formula:

Delta = (1 - tan alpha'/tan alpha) * D,

D = thickness of filter
alpha = angle of incoming light from vertical direction
alpha' = angle after refraction in glass

For small angles ( like in our case) you can make the simplification:

tan alpha' ~ sin alpha'
tan alpha ~ sin alpha

Snellius refraction law:

sin alpha'/sin alpha = 1/r

which leads us to

Delta ~ (1 - 1/r) * D

For r = 1.5 we get Delta/D = 0.333


Best regards,


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