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Does an off-axis aperture equal an Apo of same size?

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#26 siriusandthepup

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Posted 17 April 2019 - 01:52 AM

Usually the off-axis stop Newt will incur an image brightness penalty of about 15-20%.

 

Given a guess of about 90% reflectivity for the average mirror coatings, you have 90% x 90% = 81% total reflectivity for the primary and secondary of your Newt. Enhanced coatings will of course improve the numbers. Old coatings will degrade the numbers.

 

The usual modern APO (multi) coatings are more like 95% or better overall for all 4 (or 6 for triplets) surfaces. Better for oiled objectives - only two surfaces involved. Even old style Mag fluoride coatings will get you approx. 1.5% loss per surface. No coatings? About 4% loss per surface ~ 96% transmission each. .96 x 0.96 x 0.96 x 0.96 = 85%  - so old style, uncoated doublet is a closer match for your average off axis stop Newt brightness. Kinda makes you think about the importance of good coatings in your favorite eyepieces, huh?


Edited by siriusandthepup, 17 April 2019 - 01:54 AM.


#27 ngc7319_20

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Posted 17 April 2019 - 04:18 AM

 

The biggest benefit of the apodizing mask will be seen on double stars, but planetary views can also benefit.  

And this.  Why ask the question? Cardboard is cheap and finding a suitable size circle to mark your aperture should be easy.  My advice?  Try it and see if you like it.

 

The apodizing mask is harder to make, but having used both, my own belief is that of the two approaches the apodizing mask might be the better way to go. While an apodizing mask is a bit harder to construct, it is by no means difficult and can be made for less than $15.  

 

 

I tried an apodizing mask in an 8" Newtonian on Jupiter over several nights after reading an article about it in S&T years ago.  Results were unimpressive.  Improvement over full aperture was marginal at best.  Probably it was better than a 3" off-axis mask (I did not try it).  One objectionable feature is that the field of view was filled with bright criss-crossing diffraction patterns from the window screen -- though a small area right around Jupiter was free from the spikes.  Yes, maybe it would work best on double stars.




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