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Flocking a 6SE


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If I'd thought of it at the time I might have mentioned it. Never mind. Someone else can have that fun. I'd be quite happy to do it when I get that far. Not sure how to objectively measure the light scatter though. Perhaps measure the brightness of a given length exposure of a matt black surface under fixed lighting conditions?

James

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Well, all the matt black painted surfaces (in the Celestron at least) do appear to reflect quite a bit more light than the flocking material ..

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If I hadn't flocked the entire scope then I would be forever wondering how much improvement could be obtained from doing the whole thing, so I thought I may as well get it over and done with whilst I'm at it and just go for it there and then.

Although I didn't flock the outside edge of the main baffle that goes through the primary mirror, still thinking about doing that, from the tip to nearly as far as where the primary mirror goes that is. As shown with the arrow here ..

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Such is the life of a dark sider lol, never satisfied.

Edited by Cath
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Just last weekend I had the chance to check a Celestron SC 6SE OTA. It was returned because the costumer found the mentioned halo rings from bright stars near the edges of image - now he's a happy owner of a 5" f/9 achromatic refractor. I tested the telescope with several eyepieces and star diagonals. The halo rings appeared with every star diagonal I tried, from the original 1,25" prism supplied by Celestron to 2" SC-threaded dielectric mirror and 2" refractor-type dielectric mirror, using 2" wide angle eyepieces from 22 mm to 40 mm and also the 6SE's standar 25 mm 1,25" Plössl.

By the way, I believe I can recall the same type of halo rings in the 8" SC OTA of my CPC 800 when looking through some long focal lenght wide angle eyepieces. I'll test this again as soon as the sky gets clear.

Well, I've been testing the CPC 800 today. I've used a 2" SCT mirror diagonal and several wide angle eyepieces. The halo rings have appeared with 32mm 70º eyepiece and larger, which roughly agree with the under 70x magnification halo rings in the 6" f/10 SCT (22mm eyepiece and larger). I could take a picture of Arcturus near the edge of the field of view of a 40mm 68º eyepiece, showing the halo ring towards the inside.

halo_sct.jpg

I'm determined to flock my 8" SCT OTA if it makes the damned halo rings disappear. My main concern about flocking the tube is to leave the mirrors and the corrector plate perfectly clean after the work - then I would like to close the tube with a skyligth filter to keep the dust out forever.

In addition, I've reported the matter to the official Celestron support in Spain.

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I expect Celestron know all about the problem, but as most people are happy they leave things as is. Which unfortunately is how companies work these days :(

I had the chance to get my scope out night before last, so decided to see if I could get the halo rings again after flocking by moving Arcturus across the field of view, moving it just outside of view etc, but try as I may I couldn't see any sign if the problem what so ever - this was with the normal diagonal and normal 25mm eye piece that came with scope (60x magnification - f10). So it looks good!

I'll try again when the sky is clear and the moon is out - this may take sometime lol.

I expect the problem to be with a single part of the scope, but I can't say which area that might be as I've flocked it all.

Edited by Cath
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I expect the problem to be with a single part of the scope, but I can't say which area that might be as I've flocked it all.

My guess is the primary baffle tube. Anyway, whichever it is the origin of the halo rings, the fact is that flocking the whole thing will increase contrast and, hence, resolution.

Thank you, Cath. I hope Celestron's engineers find this information useful.

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My guess is the primary baffle tube.

That would be my guess too.

Any light entering the front (from a 180deg cone) can easily be defused/scattered around off the sides of various parts and eventually find it's way down the primary baffle and so reduce contrast. So flocking it has certainly made the blacks blacker for me (less internal ambient scattered light).

Thank you, Cath. I hope Celestron's engineers find this information useful.

One can hope, but I doubt it they'd act upon it, I think they would have already done so if they had any intention of doing so.

Edited by Cath
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Certainly, Celestron is more concerned about selling the EdgeHD optics now. Nonetheless, in the real world, most people are happy with a little of field curvature and coma off-axis in the cheaper classic SCT.

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  • 3 weeks later...

Hi again!

I haven't done any flocking yet, but I've tested a Meade 6" ACF - specifically, a LigthSwitch telescope - with Arcturus, in search of the well known halo rings. In comparison to the Celestron's SCTs, the halo rings of the Meade ACF appear into a narrower band and are quite dimmer. With the 40mm 68º eyepiece, the limited zone where halo rings appear starts and finish loosely inside the field of view. With the 22mm 70º eyepiece, the halo ring only appears when you place a bright start into a narrow band outside the field of view.

Regards,

Leo

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Hi Leo

Sounds like the Meade doesn't suffer with them as much as the Celestrons do, which is good!

A good test to might to roll a length of thin felt/velour up for a nice fit into the rear exit hole (the primary mirror baffle) and try the test again?

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A good test to might to roll a length of thin felt/velour up for a nice fit into the rear exit hole (the primary mirror baffle) and try the test again?

Yes, I have a roll of adhesive matt black velour, so I'll try to stick a perfectly fit piece of it into the primary baffle tube of the 6SE OTA and test again tonight. Then, I will come back to report. ;-)

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Well, my NexStar 6SE is now ready to test under the stars after flocking. I've flocked the rear cylindrical stretch inside of the primary baffle tube with adhesive velour. I've taken some photos of the baffle tube before and after flocking, with te telescope pointing towards a white lamp. We can see that the reduction of internal glare is plainly drastic.

6SE's rear long baffle before flocking (left) / 6SE's rear long baffle after flocking (right)

6se_baffle_glare.jpg6se_baffle_flock.jpg

I've also photographed the primary baffle tube of the CPC800 and LS6, both without any flocking yet. We can confirm the 6SE has the brightest baffle, while the Meade LS6 has the darkest baffle.

6SE's rear long baffle before flocking (left) / CPC800's rear long baffle before flocking (center) / LS6's rear long baffle before flocking (right)

6se_baffle_glare.jpgcpc800_baffle_glare.jpgls6_baffle_glare.jpg

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That's a big difference on the 6SE! .. which is about the same as I found too. It appears to be a definite problem area with cassigrain scopes, and well worth fixing by flocking the inside of the tube, I think.

Great research!

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Yes, definitely we can conclude that the outlying halo rings in SCTs can be completely suppressed by simply flocking the inside of the cylindrical section of the primary mirror baffle tube. This can even be done without disassembling the rear cell, but you must be extremely careful to cut out the velour piece with the exact dimensions to fit the tube, and to stick it well in order to keep a perfectly circular exit pupil. As you can see, I didn't manage to stick the velour piece perfectly towards the interior end of the baffle tube, so I'll have to remove the rear cell to perfect the work in the end, once the efficacy of flocking has absolutely been proved, because there isn't any sign of halo ring from bright stars left.

Regards,

Leo

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Yes well done Leo :)

How did you manage to get the velour in the tube and stuck down I wonder?

I cut mine very accuractly and left the backing on and just inserted it into the tube. It's a small diameter tube and looong, and the velour is very sticky. Getting a very sticky velour down into a narrow long tube is not easy lol

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How did you manage to get the velour in the tube and stuck down I wonder?

Well, I rolled the piece of velour to conform a thin tube with the back paper on. Then, I removed the paper to insert the thin roll in the baffle tube and used the stick of a brush to press the velour against the tube. I had to do several attempts and wasted a piece o velour.

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It's when I'm imaging the effect occurs, no barlow, just camera directly on back of scope.

Cath, what camera do you use? Is it a full-frame digital SLR? I'm asking because I didn't find halo rings with my EOS 550D, even before flocking.

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