Secondary size

Wanna make a scope? Or better still, grind a mirror yourself. Or, you have some good tips in making a really useful accessory? This is the place to show what your hands can do...

Vote for best secondary mirror size

2.14"
2
50%
2.60"
2
50%
 
Total votes: 4

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MooEy
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Post by MooEy »

portaball 8" uses 1.3", 16% only

~MooEy~
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Canopus Lim
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Post by Canopus Lim »

Today I had time in office to use Zemax to simulate the vignetting on my scope.

Internal diameter 13.25"
Secondary mirror slightly less than 2.17" (taken account of the lip of the holder)

Diagonal to focal plane distance 8.6" (I guess.. probaby shorter than that. If shorter, it would have better illumination to the edges).
Primary 12.5" f/5

The Vignetting was not that serious. It was above 65 percent at 1.5 degree TFOV (using a 31 Nagler). The drop in vignetting was not that bad too.. rather gradual and linear drop. It will not be easy to see the vignetting. Hence I really see no issue with the diagonal size and the UTA size for visual observing including DSO. For my 17 Nagler, the illumination is good till the very edge where it drops a bit.

Also, by adjusting the size of the UTA..there is really slight improvement.... and I rather trust Zemax than Mel Bartel calculator. :P Zemax plots out the vignetting and it is the world's most popular Optical Design software. Furthermore by making a larger UTA you are sacrificing inward travel....and also needing to push the primary mirror nearer the secondary mirror and causing more vignetting in the process. Lastly with a bigger UTA, there would be more stray light entering the scope...some Newts don't even have a primary mirror baffle. Last but not least, a big UTA is a trouble carrying around. An optimized small UTA and appropriate sized diagonal does the job fine.
AstroDuck
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weixing
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Post by weixing »

Hi,
I rather trust Zemax than Mel Bartel calculator.
Err... I key in the information you gave in the Mel Bartel calculator and it also give similar result.... 69.77% at 20mm off axis.

Anyway, the Mel Bartel calculator is not a scientific program, it just a guide to make sure you'll have the desire 100% illumination zone in a quick way.

Have a nice day.
Yang Weixing
:mrgreen: "The universe is composed mainly of hydrogen and ignorance." :mrgreen:
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Canopus Lim
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Post by Canopus Lim »

Ha. Actually the main thing was not about the Mel Bartel's calculator. It is just that I find what some people on the net say about the illumination in some newtonians with the 'undersized UTA and undersized secondary mirror' size rather ridiculous. They claim that the 35 Panoptic on such a 12.5" f/5 scope will have serious vignetting..and that the edge of field can be about only 50 percent illumination on such eyepiece;it is wrong... it is more than 65 percent (close to the 'so called' ideal 70 percent).They also claim that the UTA is too small..... and I do not see such things on what I have simulated. They also claim that by having a large UTA will improve the vignetting significantly and there was one person who even changed his UTA to a bigger size one... which is wrong too. The improvement is so slight with a bigger UTA. I never downloaded that Mel Bartel's calculator, but it makes me wonder how did people arrive in such a conclusion using that calculator. Perhaps it is the user's wrong interpretation of the results. Beats me.
AstroDuck
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ariefm71
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Post by ariefm71 »

Maybe because the guy wants to fully utilize his 12.5" mirror for any type of observing and not cut down to 12" due to vignetting. I will have some vignetting on my scope as well as I can only go down to 13.5" ID, not the ideal 14" ID, with my UTA.

Of course, no one really can detect a 0.5" aperture difference, same as no one can detect the difference between 95% and 99% strehl optics :)
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