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Blackening mirror edges?

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#26 Sarkikos

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Posted 06 March 2013 - 01:08 PM

I've never had an optics-damaging accident when removing, cleaning, etc. mirrors in my Newts. Never dropped an eyepiece. Never damaged a lens or mirror. You just need to be careful and aware of what you are doing and aware of your surroundings. No quick, hyper movements. Maybe cut back on the coffee beforehand. :twitch: Enter a Zen-state of relaxed concentration. :praying:

:grin:
Mike

#27 Sarkikos

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Posted 06 March 2013 - 01:19 PM

Jason D,

In my case, I did install a fan baffle and the ring mask I have shown in the earlier post in this thread. I used this setup for a while but I have not been happy with it. When I did some analysis, here is what I noticed:

1- I noticed some air was escaping around the fan. I did my best to cut a baffle with an almost perfect fit but the pressure building between the mirror and the fan was too high and managed to push air through crevices between the baffle and the scope.

2- As far as the air that was redirected across the mirror by the ring mask, it was causing too much air turbulence above the mirror surface. I can see that clearly by defocusing a bright star. When the fan in turned on then the “boiling” effect got too severe.

I ended up removing the baffle and the ring – back to the original scope setup – and things got better. The turbulence was minimal when the fan was turned on.


I did something similar for my 8" f/6 Newt. At first, I attached a thin baffle under the mirror clips as you did. But I think this might be too close to the mirror, or as you experienced, will not allow a good seal. Maybe good to cover a TDE, but not so good for directing air currents. The effect was either neutral or a bit worse than without the baffle. (I have a fan suspended in a baffle below the primary, blowing air up against the bottom of the mirror.)

But then I constructed an annular baffle from black foam core and positioned it directly above the mirror clips. The baffle is snug tight against the inner wall of the OTA. This time I noticed a circular motion of the thermals in a defocused star or planet. At least I was having some effect on the image - the boiling effect was replaced by a circular motion - showing that the thermals were being moved differently than without the baffle.

Recently I drilled a line of several lateral holes just above the edge of the primary, below the baffle. Hopefully this will allow the thermals to escape more easily from the solid-tube OTA and improve the image.

Others have done similar things and gotten mixed results. I've yet to get a chance to retest the scope under good seeing. If it works out well, I might do the same thing for my 10" solid-tube Newt.

Of course, back on the topic of blackening mirror edges, an annular baffle would cover the bevel - and perhaps a TDE, if the baffle is wide enough - and negate any need for blackening the bevel.

Mike

#28 Jason D

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Posted 06 March 2013 - 02:01 PM

But then I constructed an annular baffle from black foam core and positioned it directly above the mirror clips. The baffle is snug tight against the inner wall of the OTA.

Sounds like your solution placed the mask few millimeters higher than mine and sealed it against the OTA wall. I placed mine right above the clips.

Recently I drilled a line of several lateral holes just above the edge of the primary, below the baffle. Hopefully this will allow the thermals to escape more easily from the solid-tube OTA and improve the image.

I am aware of this mod – including mounting in/out fans outside the holes – but I do not have the guts to mutilate my OTA yet.

Of course, back on the topic of blackening mirror edges, an annular baffle would cover the bevel - and perhaps a TDE, if the baffle is wide enough - and negate any need for blackening the bevel.

Another important observation I noticed is the effect of the mask on the defocused star image. I felt the in-defocus star image showed a more scattered outer ring. After all, the mask edge is on the light path and will have some sort of a diffraction impact. The in-defocused star image improved after removing it. Of course, if someone has a TDE problem then the mask will be an improvement but when used with premium optics I believe the mask will make it worse. This is based on my observation and I am not generalizing it.
Bottom line, I believe the baffle and mask positive/negative impact depends on the scope, optics, and the local weather. Results could vary.

Jason

#29 nevy

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Posted 06 March 2013 - 04:17 PM

Thanks Jason , that's very helpfull as usuall :waytogo:
Staying on the subject of blackening the bevel , I haven't done mine and don't intend too but I have painted the secondary back & sides with blackboard paint.

#30 Jason D

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Posted 06 March 2013 - 06:44 PM

In my case, I applied a piece of a flocking paper to cover the edge of my secondary mirror visible from the focuser as shown below. When I made the cut, I left two notches for the safety string.
Posted Image

You can see how “blackish” the flocking paper as seen from the focuser in the following photo:
Posted Image

I cut a perfect fit, I did the math as shown below:
Posted Image

Jason

#31 Sarkikos

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Posted 06 March 2013 - 09:05 PM

My secondaries are in housings. I flocked the entire outer surface of the housings with ProtoStar. I didn't bother with math to figure it out. I just measured carefully and cut to shape. Fits perfectly.

I flock or paint virtually every surface inside the OTA except the optical surfaces. :grin:

Mike

#32 Binojunky

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Posted 07 March 2013 - 10:43 AM

I've never had an optics-damaging accident when removing, cleaning, etc. mirrors in my Newts. Never dropped an eyepiece. Never damaged a lens or mirror. You just need to be careful and aware of what you are doing and aware of your surroundings. No quick, hyper movements. Maybe cut back on the coffee beforehand. :twitch: Enter a Zen-state of relaxed concentration. :praying:

:grin:
Mike

That reminds me of the driver who.s never had an accident or the gambler who always claims he wins and never loses,having more than once seen mirrors damaged from removal, it happens, simple as that, cleaning a mirror is a neccesary evil because dirt over time can effect performance however removing optics to blacken edges is a risk that can be avoided,DA.

#33 Sarkikos

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Posted 07 March 2013 - 11:15 AM

Gambling isn't a good analogy to driving, and driving isn't a good analogy to cleaning telescope mirrors. Among these situations, we have more and more control over the variables involved. In gambling, the outcome is mostly a matter of chance. In driving, we have more control over the outcome, but we cannot control the weather or the other drivers. For cleaning telescope mirrors or blackening mirror edges, unless something out-of-the-ordinary occurs - such as an asteroid strike or the house blowing up - we are pretty much in control of the entire situation, beginning to end.

Mike

#34 Binojunky

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Posted 08 March 2013 - 04:09 PM

Gambling isn't a good analogy to driving, and driving isn't a good analogy to cleaning telescope mirrors. Among these situations, we have more and more control over the variables involved. In gambling, the outcome is mostly a matter of chance. In driving, we have more control over the outcome, but we cannot control the weather or the other drivers. For cleaning telecope mirrors or blackening mirror edges, unless something out-of-the-ordinary occurs - such as an asteriod strike or the house blowing up - we are pretty much in control of the entire situation, beginning to end.

Mike


The end result in either is the same though, be it a damaged car, lost money, or a damaged mirror that could have been avoided, lots of cursing under ones breath and"why didn,t I leave well alone?",DA.

#35 Scanning4Comets

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Posted 08 March 2013 - 04:52 PM

But then I constructed an annular baffle from black foam core and positioned it directly above the mirror clips. The baffle is snug tight against the inner wall of the OTA. This time I noticed a circular motion of the thermals in a de-focused star or planet. At least I was having some effect on the image - the boiling effect was replaced by a circular motion - showing that the thermals were being moved differently than without the baffle.


Mike,

I also had the same circular motion with AND without the baffle. Now I just leave the fan suspended on elastics. This way, if my batteries go kaput, I can at least have the mirror exposed to the air. I don't like the idea of having a bunch of extra D-Cell batteries lying around in case they sit too long and get wasted. Another good reason I have my fan on elastics and the mirror exposed is for a faster warm-up, when I get in the house.

Gambling isn't a good analogy to driving, and driving isn't a good analogy to cleaning telescope mirrors. Among these situations, we have more and more control over the variables involved. In gambling, the outcome is mostly a matter of chance. In driving, we have more control over the outcome, but we cannot control the weather or the other drivers. For cleaning telecope mirrors or blackening mirror edges, unless something out-of-the-ordinary occurs - such as an asteriod strike or the house blowing up - we are pretty much in control of the entire situation, beginning to end.

Mike


Yes, but nobody is perfect and accidents DO happen, kinda like the spelling of the words "telescope" and "asteroid" in your last post. ;)

Cheers,

#36 Sarkikos

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Posted 08 March 2013 - 05:22 PM

Seen and corrected. A little typo now and then is not equivalent to damaged optics. I'm much more careful when I clean my mirrors. :ubetcha:

:grin:
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#37 Scanning4Comets

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Posted 08 March 2013 - 06:11 PM

C'mon Mike, You're not perfect, and neither am I. I was just saying that accidents do happen. I am extra careful myself but once had a 12.5" mirror fall right out of my telescope tube because I had FORGOT to put the screws back in the bottom after an inspection. I had to use silicone adhesive to get it back onto my cell when it did happen.

I was thankful that it didn't break when it had hit the floor. I was nowhere near it when it happened.

Just sayin'

Cheers,

#38 Sarkikos

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Posted 08 March 2013 - 10:55 PM

Nope, never had an optical accident. My typing could stand some improvement, however.

Mike

#39 Jason D

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Posted 08 March 2013 - 11:21 PM

Why doesn’t everyone leave the decision whether to take the mirror out or not to scope owners!
Some are careful and some are clumsy. No one can generalize.

#40 Sarkikos

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Posted 09 March 2013 - 12:52 AM

Sounds like a good idea to me.

Mike

#41 Binojunky

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Posted 09 March 2013 - 01:13 PM

Agreed :),DA.

#42 FineArt

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Posted 09 March 2013 - 02:05 PM

If you use black butyl caulking instead of paint, you can peel it off when it comes time to re-coat the mirror.

#43 Sarkikos

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Posted 09 March 2013 - 02:15 PM

I'll have to check into that.

Mike






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