Author Topic: "Shadow breakout" -- when?  (Read 66 times)

Michael Burney

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"Shadow breakout" -- when?
« on: January 23, 2018, 02:53:39 AM »
One of the things that's always puzzled me is the concept of "shadow breakout". Suiter talks about it in the context of the star test but never really explains it. So I'd like to ask the experts here: in the below simulation of a 200mm 33% CO aperture, when is the shadow considered "broken out"?

I've simulated all the way till 8 waves defocus (the 2-wave defocus is erroneously marked as 1.0, ignore this). Is the 3-wave point it? 4-waves? Unless there is an unambiguous definition of "shadow breakout", I can't imagine it's any use in the star test.

Tanveer.



pernogori

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Re: "Shadow breakout" -- when?
« Reply #1 on: January 25, 2018, 11:06:54 PM »
The shadow has definitely broken out by 3.0 and is showing the first signs of breakout at 1.0 although it does not quite qualify as broken out yet.

Matthew Danielson

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Re: "Shadow breakout" -- when?
« Reply #2 on: January 26, 2018, 04:26:37 AM »
I thought shadow breakout occurred with too large an exit pupil.

dehimater

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Re: "Shadow breakout" -- when?
« Reply #3 on: January 31, 2018, 11:21:18 AM »
Quote
I thought shadow breakout occurred with too large an exit pupil. 


that's different.

to star test optical quality you look at difference in defocused stars and the 2ndry breakout in both intra/extrafocal images.

https://www.youtube....h?v=MFmFpuST67M

you talking about actually seeing the shadow while focused.

Michael Shen

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Re: "Shadow breakout" -- when?
« Reply #4 on: February 03, 2018, 04:49:11 AM »
OP, what you want to do is compare the appearance of the unobstructed to that with obstructions. Then the appearance of the "shadow" is more obvious:

Left is 0 obstruction, right is 30%, all for 8" f/5 (with no other errors so inside = outside).

The first is 1 wave defocus :
2 wave defocus:
3 wave defocus:
4 wave defocus:
5 wave defocus:
What you're looking for/at is where the "shadow" appears and then sticks around, which is around 2 waves defocus here, to my eye anyway. In any case, in looking for putative "shadow" breakout you're looking for the same appearance of the "shadow" each side of focus, so where it actually "happens" isn't that important.

That said, these examples are for otherwise error free optics, and relying on "shadow breakout" either side of focus to judge optical quality of real optics is, IMHO, a bit of a fools game, due to the optical factors that control when such a subjective "event" occurs - but have little effect on actual performance. Better to just study the overall star pattern at small defocus on either side. Experience there can show you a lot more. That's why I put "shadow" in quotes everytime...

Read what Roland Christen has to say about some of this: http://www.csun.edu/.../startest2.html