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Seeing shadow of secondary

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#1 Michael Rapp

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Posted 13 December 2012 - 06:11 PM

Hi all,

Been trying to figure this out, but can't seem to find the right answer. What influences seeing the secondary mirror when looking through an eyepiece on a Newtonian? Is it too large of an exit pupil?

#2 Starman1

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Posted 13 December 2012 - 06:41 PM

In a word, yes.
Let's say your secondary is 25% as wide as the primary.
That means, in a 6mm exit pupil, the shadow is 1.5mm wide. If your eye's pupil opens to 6mm at night, would you notice the shadow? No.
But let's say you wanted to use the scope in the daytime. Your pupil now opens to only 3mm. The shadow of the secondary is still 1.5mm wide, or half as wide as the pupil of your eye. Will you see the shadow? Yes.
How about at night when looking at the Moon? Well, there you probably have a pupil diameter similar to or just a tiny bit larger than your daytime pupil (your eye does a fairly good job of protecting itself from really bright light). Would you see the secondary shadow? Probably.
Let's say you experiment with eyepieces, and you try an eyepiece that yields a 10mm exit pupil. The shadow of the secondary is now 2.5mm wide. Would you see that? Well, if your dark-adapted pupil were 5mm you probably would.

The percentage of the secondary shadow stays the same as the exit pupil of the eyepiece shrinks, so it's only going to be at very low powers where the secondary shadow might interfere, even in daytime use, though, in daytime use you are limited by the atmosphere to low powers (which explains why nearly all spotting scopes are refractors, not reflectors--no secondary shadows).

Hope that answers your question.

#3 Jon Isaacs

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Posted 13 December 2012 - 11:06 PM

Hi all,

Been trying to figure this out, but can't seem to find the right answer. What influences seeing the secondary mirror when looking through an eyepiece on a Newtonian? Is it too large of an exit pupil?


Michael:

Don pretty much covered it, I just wanted to add that one rarely sees the secondary, it's not anything to worry about.

Jon

#4 dpwoos

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Posted 14 December 2012 - 07:59 AM

I am a self-avowed disbeliever of the eyepiece exit pupil dogma, but even so I have never seen the secondary shadow while observing at night. Add it to the long list of things that don't warrant worrying about.

#5 Jon Isaacs

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Posted 14 December 2012 - 08:52 AM

I am a self-avowed disbeliever of the eyepiece exit pupil dogma, but even so I have never seen the secondary shadow while observing at night. Add it to the long list of things that don't warrant worrying about.


If you want to see the shadow of the secondary at night, look at the full moon at low magnifications with a large exit pupil. That 55mm Plossl in an F/5 scope will bring it out but even with a more reasonable exit pupil, it can easily be seen.

Another way to see the shadow of the secondary is to just step back from the eyepiece, this works day or night, and view the exit pupil from a distance, best while viewing a brighter object. The circular exit pupil with a dark center is easily seen, the dark center being the secondary's shadow.

As far as "exit pupil dogma", I am not quite sure what you are referring to. The exit pupil is a very real thing, in many ways it is most real thing there is, it is after all, the only thing we ever really see. Magnification, aperture, focal ratio etc can be useful guides, but they are mere numbers, the exit pupil is what enters your eye.

It's worth understanding a bit about what you are actually looking at and how it relates to your entrance pupil and your eye's response.

Jon

#6 Michael Rapp

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Posted 14 December 2012 - 09:06 AM

Thanks Don (and all)! That explains it clearly. Also explains how I notice it in my 32 mm Plossl on my 8" when viewing solar. I just have to pull back from the eyepiece ever-so-slightly and I can see it.

#7 uniondrone

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Posted 14 December 2012 - 11:21 AM

Thanks Don (and all)! That explains it clearly. Also explains how I notice it in my 32 mm Plossl on my 8" when viewing solar. I just have to pull back from the eyepiece ever-so-slightly and I can see it.


Don's explanation pretty much covers it, but one thing more that's worth mentioning: you have probably noticed that you see the secondary's shadow in a defocused image. Pulling your eye away from the eyepiece (especially when the exit pupil is large) is in effect the same as looking at a defocused image, hence the visible shadow of the secondary.

When lunar viewing at low power with my C5, the shadow becomes immediately obvious when my eye is more than a couple inches aways from the eyepiece. It disappears, however, when my eye is at the focal point.






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