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RACI FInders verses RA Reversed image : lim Mag

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#1 mark379

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Posted 26 August 2013 - 09:25 AM

Hello,
I'm just a bit curious on the limiting magnitude difference between the reversed image and correct image finders. Both are right angle, one is 8x50 reversed, the other 9x50 RACI
How much light does the prisim in the RACI "eat up" when it comes to faint stars?

#2 GlennLeDrew

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Posted 26 August 2013 - 09:37 AM

I would expect you should see no difference, really. If the Amici prism is coated, and the other finder's prism is too, or if the other finder uses a standard aluminum coating (most likely) mirror, the transmission difference should be no greater than about 5%, which is about 0.05 magnitude. Even a 10% difference, which would apply if a mirror were dielectric, is still 0.1 magnitude.

#3 Jon Isaacs

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Posted 26 August 2013 - 09:38 AM

Hello,
I'm just a bit curious on the limiting magnitude difference between the reversed image and correct image finders. Both are right angle, one is 8x50 reversed, the other 9x50 RACI
How much light does the prisim in the RACI "eat up" when it comes to faint stars?


My thinking:

Not enough to worry about and certainly not enough to make up for the advantage of the 12.5% greater magnification...

Jon

#4 GlennLeDrew

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Posted 26 August 2013 - 09:52 AM

Indeed, the magnification difference will almost certainly be the dominant factor. A smaller exit pupil darkens the sky, thus allowing slightly fainter stars to shine through.

#5 Jon Isaacs

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Posted 26 August 2013 - 10:01 AM

Indeed, the magnification difference will almost certainly be the dominant factor. A smaller exit pupil darkens the sky, thus allowing slightly fainter stars to shine through.


Glenn:

The other night I was comparing two straight-through finders, both Synta, one was an 8x50, one was a 9x50. There was a clear difference in what was visible. The 9x50 also had a wider field of view so the 9x50 stayed on the scope.

Jon






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