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blacking out the primary edge?

Moonshane

By Moonshane
in DIY Astronomer

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Moonshane

Moonshane

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hi all

I am building my 16" dob and have just blacked the secondary edge and back as I would always do with my scopes.

having read about blacking the edges of primaries in refractors, I wondered if there's any benefit to doing so with reflectors?

logic says no but while I am at it!

cheers

Shane

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acey

acey

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The only reason to blacken a primary edge would be if a ray of light could reflect off it, then reflect off a surface sufficiently reflective to send a significant amount of light directly into the eyepiece. But the only way a ray could reflect from the primary edge up to the top end of the tube would be if it were incident from the bottom end, i.e. there was an open tube bottom (e.g. for ventilation) leaving a visible gap around the primary. But then the issue would really be direct light coming up from the bottom end, not light reflected off the primary. See this S&T article by Gary Seronik about dealing with stray light:

http://www.urania.be/forum/download.php?f=1&file=baffling.pdf

Good luck with the project, it sounds great.

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HD-AP

HD-AP

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cheers matey.

I'll leave well alone then

Hi

Blackening the edge of a primary mirror would probably slow the primary's rate of cool down not something to encourage with a large mirror. And would have zero effect to system stray light.

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Moonshane

Moonshane

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probably one of those things that's never really been tested. I think I might make a removable 1" wide matt black baffle for the back end. this way, air can still circulate and it prevents any chance of 'loose reflections' hitting anything important.

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acey

acey

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"Loose reflections" are only important if they're sending light into the eyepiece. The advice in Seronik's article is to look at the reflection of the primary in the secondary (looking through the drawtube with no eyepiece). If you can see significant light around the (reflected) primary, e.g. from part of the tube wall above the primary, then it could be worth worth flocking that visible part or inserting an annular baffle above the primary. What you certainly won't see in the secondary is the edge of the primary, so blackening it won't do anything. I should think that any difference to the thermal properties of the mirror would be negligible so it would do no harm but no good either.

In a refractor you're additionally concerned with light scattering as it passes through the objective, hence the use of edge blackening, but that's not relevant in a reflector.

Edge-Blackening - Edmund Optics

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HD-AP

HD-AP

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Plus besides providing no real benifits ,blacking/painting may cause problems when it's time to re-aluminise /re-coat the primary.

like the flocking band experiment idea as it does no harm ,will it make a difference ?

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Moonshane

Moonshane

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The only reason to blacken a primary edge would be if a ray of light could reflect off it, then reflect off a surface sufficiently reflective to send a significant amount of light directly into the eyepiece. But the only way a ray could reflect from the primary edge up to the top end of the tube would be if it were incident from the bottom end, i.e. there was an open tube bottom (e.g. for ventilation) leaving a visible gap around the primary. But then the issue would really be direct light coming up from the bottom end, not light reflected off the primary. See this S&T article by Gary Seronik about dealing with stray light:

http://www.urania.be/forum/download.php?f=1&file=baffling.pdf

Good luck with the project, it sounds great.

hi Acey

finally got round to reading this article in depth.

it's excellent and I will be following many of the recommended mods to my scope (and in fact have already covered many of them.

One thing that will help me is that I have established that my 'light shield' (I have a foot long dew shield which is really only needed to block stray light) can be reduced in length to about 3-4 inches to do its job. I may just trim it to 6" as I have just added a 9x50mm finder and this makes the scope sag at lower elevations so any weight savings at the top end will help this.

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sbooder

sbooder

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I would have thought that flocking the OTA wall opposite the primary edge would be a safer bet even if you do not flock the whole tube. Any light that may reflect off the edge of the mirror would not reflect off the flocking and therefore not cause a problem?

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