YahooGroup Posted May 19, 2013 Report Share Posted May 19, 2013 This is a transfered topic from the ASA Yahoo Group. Posted By: doug_landmann Thu Apr 18, 2013 5:27 pmWhy is it that get two different answers for the thickness of the adapterrequired if I use the ASA calculator or and if I take the optical backfocusnumber from SBIG and subtract it from the ASA backfocus requirement?ASA 10"N Wynn 3" corrector f/3.8 backfocus from Chart A = 57.29mmWhich is the same number that comes up in the spreadsheet calculator for the 10Nwith Wynn 3"corrector at f/3.8Using the calculator and accepting the 1mm glass thickness over the CCD sensorand 3mm filter thickness and inputting 44.74mm for the backfocus for the SBIGSTXL-11002M with standard filter wheel. The calculator returns the adapterthickness of 13.87mm.Using SBIGs Optical backfocus number (42.77mm) and simply subtracting that fromthe 57.29mm total backfocus the result is 14.52mm. a difference of 0.65mm. Isthat number significant? Should I not worry about it Would you error in favorof 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 pmHi Doug,The formula for the path increasement of light in a media is (r - 1)/r, r beingthe 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 = 13.87mmThe ASA calculator is right, SBIG's specified optical length is wrong.Best regards,Gerhard Link to comment Share on other sites More sharing options...
wemartin Posted July 15, 2013 Report Share Posted July 15, 2013 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. Bill Link to comment Share on other sites More sharing options...
GerhardB Posted July 15, 2013 Report Share Posted July 15, 2013 Hi Bill, The focal shift Delta can be computed with this formula: Delta = (1 - tan alpha'/tan alpha) * D,D = thickness of filteralpha = angle of incoming light from vertical directionalpha' = angle after refraction in glass For small angles ( like in our case) you can make the simplification: tan alpha' ~ sin alpha'tan alpha ~ sin alphaSnellius refraction law:sin alpha'/sin alpha = 1/rwhich leads us toDelta ~ (1 - 1/r) * DFor r = 1.5 we get Delta/D = 0.333 Best regards, Gerhard Link to comment Share on other sites More sharing options...
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