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MCT Secondary Baffle Mod

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#126 PowellAstro

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Posted 30 November 2013 - 10:59 PM

I am so happy with the results, I have started pulling my very expensive 14" inch apart. I decided to put the C6 on hold until I complete my 14. People can say what they will but the only one I need to convince is myself. I did a lot of viewing between the modded C90 and my 80mm APO and the C90 has better contrast and a sharper image. This makes so much difference, I want my main scope, the 14, performing this way as soon as possible. And my copy of the 80mm Equinox is not subpar, it's correction is very good, around an 1/8 wave.

#127 Asbytec

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Posted 01 December 2013 - 12:12 AM

Yes, I agree with you totally. You and Ed got very similar results, which I find fascinating. And your testimony is motivating, as well. Thank you for adding so much to this thread.

#128 GlennLeDrew

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Posted 01 December 2013 - 04:06 AM

I hope someone at Celestron at least is watching this. Or this should be brought to their attention. I'm sure you'll find literature of theirs somewhere extolling the great performance of the C90. It's evident to even the half blind that there is room for considerable improvement, for not *too* much effort.

#129 Asbytec

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Posted 01 December 2013 - 06:38 AM

Glenn, I really appreciate your discussion of the exit pupil vs iris diameter in another thread and your discussion of linear brightness and contrast above.

#130 Starman1

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Posted 01 December 2013 - 05:11 PM

When I did my 8" SCT, the results were incredible (as I mentioned earlier in the thread). If you pay attention to EVERY possible source of scattered light, it helps!
I did discover that some eyepiece brands pay a lot more attention to controlling light scatter than do others.

#131 PowellAstro

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Posted 01 December 2013 - 05:23 PM

Hello Don,

I did do everything I could find, that might scatter light. I started on my Meade 14 and just now found how good this unit is built. It was hand picked by Scott and it looks different from what I see now, so I don't know if they are all this way. Mine has 50 knifeedge baffles in the primary tube. It has 6 knifeedge baffles on the inside of the seconday baffle as well as 6 on the outside. The ones in the primary have been ray traced, as they get smaller in ID the further back you look. I doubt there is anything to do to this one. I am shocked and happy with these findings.

#132 Starman1

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Posted 01 December 2013 - 05:30 PM

Hello Don,

I did do everything I could find, that might scatter light. I started on my Meade 14 and just now found how good this unit is built. It was hand picked by Scott and it looks different from what I see now, so I don't know if they are all this way. Mine has 50 knifeedge baffles in the primary tube. It has 6 knifeedge baffles on the inside of the seconday baffle as well as 6 on the outside. The ones in the primary have been ray traced, as they get smaller in ID the further back you look. I doubt there is anything to do to this one. I am shocked and happy with these findings.

This is what I discovered on the Meade 14 also. It was the last size designed, and the designers paid more attention to light scatter control. Everyone has said this size is the one with the best contrast--now you know why.

#133 PowellAstro

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Posted 02 December 2013 - 07:38 PM

Hey Guys,

I found something on the before and after images I never saw before. I was trying to look past the contrast issue and was just looking at sharpness as I re-centered the primary and was looking for any real improvement in better alignment, then I saw it. One of the images has a spider showing, can you find it?

#134 Ed Holland

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Posted 02 December 2013 - 08:33 PM

On the lst page, there was a query or two about the painted sandpaper.

1) I used standard Rustoleum flat black spray paint, applied as sparingly as possible, so as to preserve as mch of the sharp, angular light diffusing texture as possible.

2) This was an experiment, so I did not consider outgassing, beyond baking the painted paper in the oven to drive off the most volatile components. If I recall correctly, this was about 10 minutes at ~250°F

In my defence, I badly wanted to test the idea, and had no "proper" flocking material available, hence the experiment with a textured material that was to hand.

The intention with my Mak was to improve the contrast for the odd occasions it is used for daytime photography - I don't do much AP, and have not looked for improvements there. Daytime contrast was very disappointing under certain conditions, and was much improved by flocking. This mode of use really is a worst case scenario, since so much scattered or uncontrolled light is often available to reach the image plane.

In what little night time visual observing I hve done since, Jupiter looked rather good again, and I'm waiting for a night of decent seeing to really check it out...

It was very interesting to see PowellAstro's results on treating the C90 in a similar fashion - very similar, if not more striking.

Cheers,

Ed

#135 PowellAstro

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Posted 02 December 2013 - 08:57 PM

@ Ed,

Thank you sir! I don't know if you saw it in my post, but I did say I followed what you had done on the baffle with sandpaper. Same here, I used what I had and I only had 180 or smaller. I also just sprayed a coating as light as I could to keep the texture. It really works, maybe even better than flocking paper. I will have to get some and see. One thing I know, the paper should not come apart as I used Wet Sand paper.

#136 Asbytec

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Posted 03 December 2013 - 11:27 AM

Im going to find that spider.

Thanks Ed, for the reminder on the sand paper. I used readily available felt paper.

#137 Starman1

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Posted 03 December 2013 - 01:23 PM

This is the blackest, least reflective:
http://www.firstligh...g-material.html
This is second:
http://www.protostar.biz/flock.htm
Of all light suppressing materials, flat black paint is the worst suppressor and more reflective.
See:
http://www.cloudynig....php?item_id=85

#138 PowellAstro

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Posted 03 December 2013 - 01:38 PM

I wonder how it compares to black painted sandpaper? Just flat black paint is way, way more reflective at low angles than the painted sandpaper. I wonder about the flocking holding moisture and the possibility of fungus or mold hiding down inside the fibers.

#139 GlennLeDrew

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Posted 03 December 2013 - 02:34 PM

On not so large tubes I've often used fine sawdust with flat black paint. The highly irregular roughness really eats light! But it does tend to have an effectively greater thickness than most other options. And it's so darned easy to apply.

A nicely wet coat of paint is laid down, followed immediately by a heavy application of sawdust; as much as the paint can hold. Immediately shake out that which does not securely stick. Let partially dry, then overspray until fully black; this bonds the works into a firm mat.

At edges like inner focuser tube openings, you can press down (or pluck) any protruding fibres, then give a light spray of paint again.

#140 Ed Holland

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Posted 03 December 2013 - 03:57 PM

It would be interesting to compare our favoured or improvised flocking materials.

I briefly tried fine grit (220 grade) and also the 100 grade upon which I settled for the photo tests. Texture is of equal or more importance than colour when considering the grazing angles of incidence at which our unwanted light arrives in this optical arrangement. Since a sharp, ring type baffle isn't practical in the constrained baffle tube, a surface with a "sharp" microstructure that has little or no area angled parallel to the optical axis serves well to disperse light at low angles of incidence, as well as absorb a good portion of it. In that way, we ensure multiple reflections must occur, with absorption at each, for stray light to reach the part of the OTA where it can affect the image. When I first looked, it was clear that the unpainted abrasive paper (3M aluminium oxide type) was better than black paint on a flat surface in its suppression of grazing reflection. The physicist in me wants to make photometric measurements :)

Ed

#141 Starman1

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Posted 03 December 2013 - 05:23 PM

I wonder how it compares to black painted sandpaper? Just flat black paint is way, way more reflective at low angles than the painted sandpaper. I wonder about the flocking holding moisture and the possibility of fungus or mold hiding down inside the fibers.


This has occurred to me about the use of velvet. I used it to line a dewshield and when it got wet it stayed wet for days. On the other hand, it dries really fast in most dobs I've seen it used in.
Flocking paper is much better in that regard, being thinner and less absorbent.
In terms of absorption of moisture, from worst to best:
--velvet
--felt
--flocking paper
--painted sandpaper
--flat black paint (or flat black paint on sawdust)

A friend used talcum powder instead of sawdust and built up 2 or 3 layers of paint, talc, paint, etc. It looked a lot like felt at low angle.

#142 GlennLeDrew

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Posted 03 December 2013 - 06:04 PM

One of the reasons I leaned toward a sawdust foundation so many years ago was that paint adheres to it so much better than non-absorptive materials like silica or carborundum.

An interesting comparison is afforded by a smooth surface painted flat black vs *shiny* aluminum given a sharp thread cut; the latter is often the better.

#143 Ed Holland

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Posted 03 December 2013 - 06:54 PM

^ the old trick for a low power laser beam-dump is proof of this. It consists of a stack of razor blades, perhaps an inch tall. If the edges are pristine, it looks dead black. Incoming light hits the chamfered edges and is reflected further into the ever closing gap between them. Really simple and very effective, since it has a high thermal mass, so can absorb useful amounts of power if required.

Sorry, thats a bit o/t.

Just looked at some leftovers from my sheet of 100 grit. The paint seems well adhered, and survives bending and the fingernail test. Moisture absorption could be reduced (if problematic) by painting both sides.

I'm now casting sideways glances at my C8 ;)

#144 PowellAstro

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Posted 04 December 2013 - 12:27 PM

@ Ed

That's what I did, painted both side and the edges after it was cut to size. I feel better using this to help prevent mold and fungus.

#145 Ed Holland

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Posted 05 December 2013 - 08:10 PM

I was playing around with the Mak today, in camera lens mode. Whilst mounting it up to the camera, I had another peek at the stray light issue. It seems there is another reflection, apparent as a sharp ring of light visible in reflection in the secondary mirror, just outside the perimeter of the image of the primary. It's rather hard to determine which surface is the culprit.

However, I wonder if this is really worth it... The tricky part I found earlier this year whilst attempting to photograph some birds, is getting accurate focus. The camera sensor struggles a bit with a lens fixed at f/12... and of course camera shake on a 1500mm lens isn't trivial even on a tripod - even the stabilised image sensor can only do so much. I don't care about contrast if the pictures are blurred...

#146 PowellAstro

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Posted 05 December 2013 - 08:59 PM

I saw something like what you described in mine. This is why I blacked the edge of the primary. I don't know if that's what you are seeing but mine went away after that.

#147 Asbytec

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Posted 08 December 2013 - 02:15 PM

I want to go on record changing my opinion of there being a difference with the primary baffle flocked. There is a difference, at least with some indirect lighting around.

Doing some maintenance on my scope, I pulled the primary baffle flocking. Looking around the sky at night, I could clearly see the FOV brighten a bit as I swept somewhere near a street light. I suspect this light was falling directly on the baffle interior and is not being attenuated without the flocking paper.

I guess I had just been using the flocking paper so long and early since the scope's purchase I just hadn't noticed or remember noticing. Plus, the lighting is a little different since I moved to the suburbs (from the country, silly me.) So, yes, there is a difference with the primary baffle flocked.

#148 PowellAstro

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Posted 08 December 2013 - 02:21 PM

Day time shows this the most, but the moon also really shows the change as does any bright star or planets not in the center of the field.

#149 Asbytec

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Posted 08 December 2013 - 08:46 PM

I observed Jupiter last night without the flocking in replaced. I'll admit there was something intangible lacking from the view, whether it was flocking or not is to be determined. I'll observe again, or course, with the flocking replaced and will report back on any changes. Sure, day time and any bright source that can put light into the baffle is a risk. I'm a believer, your and Ed's images show this effect so well.

#150 jrbarnett

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Posted 09 December 2013 - 06:48 PM

Is there a shot of hood alone - no flocking?

Regards,

Jim


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