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what is causing this glare in my view?

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#1 shams42

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Posted 13 September 2009 - 12:23 AM

Lately, while viewing Jupiter with my 6" SCT, I've been seeing this strange circular / elliptical glare in the eyepiece. The glare intersects Jupiter and goes off to the side. I've attached a very crude pic to illustrate what I'm talking about. The gray part is the glare. As I slew the scope, moving Jupiter through the AFOV, the radius of the glare changes. By placing Jupiter in a certain part of the field of view (near the center, but not exactly), I can make the glare go away.

I have confirmed that it is not caused by the dew shield or the diagonal. The glare shows up in all of my eyepieces. I collimated the scope last night, which seemed to make the glare go away. Tonight the glare was back, though a star test indicated that the scope was still very close to perfect collimation.

What is the cause of this? Anything I can do about it?

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#2 Eddgie

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Posted 13 September 2009 - 09:27 AM

Have not seen this before, but since it is circular and is happening with all of your eyepieces, then my best guess would be that there is a reflection off of a bright surface somewhere.

Check to see that there are no bright rings inside the central baffle. There have been some cases where people have said that they had an SCT where the inside of the baffle was not painted properly.

When you say that you illiminated the diagonal, did you do this by going straight through (eyepiece inserted directly into visual back)?

Next, do a star test. Defocus both inside and outside of focus to the point that about 15 rings are visible between the bright inner and outer ring. Check to see if all of the internal rings seem to be evenly spaced and equal in brigtness. If you see a dark or bright set of rings, this is a zone. Zones usually do NOT give this kind of symptom, but it is the only thing I can think of that would be wrong with the optics themselves that could even conceivably cause your symptom.

Also, if you have cleaned the corrector, check it with a flashlight to see if there is a residue.

And finally this. I know it might sound silly, but you never know with such a strange symptom... Try your other eye. Sometimes your eye can do funny things. I am surprised by how many people overlook this. It took me years to realize that while I am right handed and right eye dominant, my left eye actually seems to have better visual aquity and seems to be free from more serious defects.

Good luck, and be sure to let the forum know what you discover.

#3 Midnight Dan

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Posted 13 September 2009 - 09:43 AM

Definitely seems like an internal reflection to me. How did you confirm it's not the diagonal? Did you try using an eyepiece directly in the visual back without the diagonal? Also, what's the history of this? Is it a new effect? Is the scope new/used? How long have you owned it? Did you add any new components to it like a new visual back?

-Dan

#4 shams42

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Posted 13 September 2009 - 10:53 AM

It's a used scope, I've had it for about 2 months. I do have a new 2" visual back and 2" to 1.25" adapter. The inside of the visual back is glossy black - I wonder if this is the cause? I will try looking tonight with the old visual back if we are fortunate enough to have a third clear night in a row.

Yes, I removed the diagonal and placed the eyepiece directly in the (new) visual back. Actually I placed the eyepiece and the adapter in the visual back to rule out the diagonal.

I haven't seen any sign of zones in the star test. I'm no expert, but the optics seem excellent on this scope. Once it is cooled down and collimated I am able to see extraordinary detail on Jupiter. I've attached a pic I made two nights ago. The view at the eyepiece was nearly this detailed during moments of still air.

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#5 Mert

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Posted 13 September 2009 - 11:22 AM

Might even be the inside of the tube that holds
the primary mirror!
I also have some problem with this but only on
bright ( very ) objects.

#6 Midnight Dan

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Posted 13 September 2009 - 12:30 PM

The glossy black interior of the visual back is a good possibility. Seems odd that it should be glossy.

-Dan

#7 MitchAlsup

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Posted 13 September 2009 - 02:01 PM

A: try taking out the diagonal and using the scope the hard way. This will isolate the diagonal from the optical train in question.

B: with the eyepiece out; center your eye on the optical axis and then pan your eye around in the FoV (image plane). Anything that you can see that is neither a mirror nor blackened can cause a reflection such as the one you are describing.

#8 mclewis1

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Posted 13 September 2009 - 02:05 PM

Matt, that image of Jupiter is exceptional so I think you're correct in your assessment that your scope has good optics.

The inside of the visual back is usually completely covered by the barrel of the diagonal or eyepiece or whatever is inserted in there. There may be a small area very deep on the visual back not covered but that's about it. You should be able to see this if it's present with your eye pressed up to the opening in the visual back.

You might also take a look at the inside of the baffle tube in daylight and notice the "flocking" used at the far end of it. On my C6 it appears to be a thin adhesive material applied down about 1" or so of the baffle tube. The rest of my baffle tube is flat black but this area at the top of it is indeed darker. On my scope the ends of the adhesive don't come together, there is about a 1/16" gap which believe it or not shows up in star tests as a slight bump on the outer Fresnel rings.

#9 charles genovese

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Posted 13 September 2009 - 03:18 PM

It is a reflection from the inside of the diagonal or eyepiece or Barlow. Inside, in daylight, point the scope at a window while your in a relatively dark room and the reflection will be obvious. Flock it

#10 mclewis1

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Posted 13 September 2009 - 03:39 PM

Considering Matt has changed eyepieces, didn't use a barlow and removed the diagonal from the scope I don't think that it's anyone of those things.

It's generally accepted that flocking a modern SCT brings very limited if any value.

#11 shams42

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Posted 13 September 2009 - 03:57 PM

Here are two pictures I took. It looks like there is a shiny area on the tube that the primary mirror rides on. I think it is shiny due to smeared grease. I would think that this would be well within the shadow of the secondary mirror though. Also, the edge of this tube (closest to the secondary) has a glossy ring around the edge -- I'm not sure why.

I really, really hope it's the 2" visual back or the 2" to 1.25" adapter.

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#12 shams42

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Posted 13 September 2009 - 03:58 PM

Second pic:

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#13 mclewis1

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Posted 13 September 2009 - 04:11 PM

Matt, in your second shot you can see the "flocking" inside the baffle tube I was talking about in my earlier post. With the flash it actually appears a bit lighter than the rest of the inside of the baffle tube (this change is just one of the problems with the use of a flash inside a scope).

That brighter ring at the front edge of the baffle tube concerns me a bit. I'm going to take a peak at my C6 tonight and compare it to your images.

#14 shams42

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Posted 13 September 2009 - 04:16 PM

Thanks Mark. Please do let me know if your C6 also has the "glossy ring."

In the meantime, we're forecast to have excellent sky conditions tonight. I've installed the old visual back. *crosses fingers*

#15 shams42

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Posted 13 September 2009 - 09:39 PM

Okay, I'm just back in from observing Jupiter. The reflection is definitely coming from inside the tube. With all accessories removed, looking through the aperture in the rear cell, I could plainly see a crescent-shaped bright reflection. It was impossible to tell where exactly it was coming from, but it looked like it might have been coming from the opening of the tube.

Where do I go from here? How do I fix this?

#16 Robo-bob

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Posted 13 September 2009 - 10:06 PM

I am thinking this is the reflection of something bright, in this case Jupiter, off or your eyeball and back into the eyepiece. Try rotating your head while keeping your pupil inside the exit pupil of the eyepiece. I see the same effect and have isolated it using this technique. It works well and may be a simple solution to your problem.

Good luck - Rob

#17 meade8r

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Posted 14 September 2009 - 10:21 AM

This problem has been cited a number of times in the past. At least, you seem to have received more suggestions this time.

Having the same problem myself, I tested my C6 in all the configurations, including the use of various eyepieces with and without diagonals. I even tried bypassing the visual back using a diagonal adapter. I finally sent it back to Celestron. It was returned in 4 weeks. Celestron cleaned the exterior of the corrector, collimated it, and replaced the visual back.

None of this helped. I have a feeling that those of us with these scopes just "live with it" since it only seems to happen on bright objects.

If you can solve this, you will be a hero.


Charlie

#18 EdKiefer

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Posted 14 September 2009 - 02:00 PM

you sure its not the tip of baffle tube not being flocked to its tip, from the pic it looks real shinny at very tip where Celestron has flocked but seems its not to edge very good (its hard to say with flash as it exaggerates lighting).

#19 shams42

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Posted 14 September 2009 - 02:23 PM

Yes, that tip (particularly the inside) is very shiny. It isn't just an artifact of the flash. I might try applying a small ring of flocking paper there and see what happens.

#20 PHampson

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Posted 15 September 2009 - 08:39 AM

My C6 had the same thing. I believe it is a reflection off of the inside of the primary baffle tube. Someone already mentioned the insert they put into the end of the baffle nearest the secondary mirror. It's effective in stopping reflections from the extreme end of that tube but the problem is that it's too short. That leaves a small portion just underneath it that's still 'glossy' when viewed from an angle. I repainted that small 'ring' flat black and roughed it up a bit. It didn't eliminate the problem but it did help. I now only see the reflections on extremely bright objects in the periphery of the field of view. Someday I may open the OTA up and try to eliminate it altogether but, for now, I'm OK with it. Good luck.

Paul

#21 BluewaterObserva

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Posted 17 September 2009 - 02:21 PM

I think it's the draw tube. I mean in the pic it looks shiney and the culprit to me.

#22 Eddgie

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Posted 17 September 2009 - 03:12 PM

Ok, I am following the post and now I have an idea.

Go to a craft store and get some black craft paper.

Roll it into a tube and slide it up until the end JUST goes past the tip of the baffle.

See if the reflection goes away. I bet that it does.

If it does, then you know that this is the area that is causing the problem.

I would NOT leave the paper in though because it will vignette the light cone even more than the standard baffle.

If the reflection IS coming from the inside front of the baffle, the BEST solution would be to use ultra-flat black paint on it.

Don't do ANYTHING that adds thickness at this point because it will introduce even more off axsi vignetting than the design already has.

I think you have found the problem though. Oblique rays are getting down the baffle.

Let us know how it turns out.

#23 HandyAndy

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Posted 17 September 2009 - 03:58 PM

Hi,

My friends old 12" tube showed glare on the planets even tho the plastic tube was black painted matt black inside. I showed him when we replaced the OTA how shiny the inside of the tube was at an oblique angle.

I was shown the workshop where they were diamond turning up the nested annular mirrors for the first imaging X-Ray space telescopes. At an oblique angle even X-Rays are reflected of a surface they would not be at right angles.

In his new tube I painted the inside with black matt brush paint then threw in a generous helping of fine sawdust and rolled it about till all the paint was covered. Once it was dry the loose was shaken out then a couple of cans of matt black acrylic car paint were sprayed in to get it back to black. That cured the reflections.

Even matt blacking the main tube and the baffle will still leave some residual reflection for bright objects. For a near complete cure you need some form of flocking or my treatment.

Cheers. Andrew.

#24 mclewis1

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Posted 17 September 2009 - 05:05 PM

Matt,

OK, after a bit of a delay I've had a close look and took a picture of the interior.

First my scope is stock with the exception of a Feathertouch SCT microfocuser and a set of Bob's Knobs ... it is as it came from the factory in 2006. It is a black tube model purchased as part of a C6-ASGT package. Visually through my C6 I do not see the ring you described when viewing Jupiter. I would class the optics of my scope as very good, it is an excellent example of a SCT. The views of Jupiter on nights of very good seeing rival those through my TMB115 refractor.

The picture clearly shows a few things ...

1) The factory applied flocking is an adhesive material applied to the inside of the baffle tube ... it is not sprayed on or anything like that. This is clear from the gap as well as the upper edge, that white ring is the adhesive layer.
2) The flocking of the upper inside of the baffle tube goes all the way to the outer edge ... no shiny ring on the inside of the baffle tube.
3) The flocking in my scope is not "complete", there is a gap between the edges visible as that darker line. This line shows up in my star tests as a small "bump" in the outer fresnel ring.
4) The baffle tube itself has ridges in it as well. This isn't that easy to see in the image posted here but is clear on the full size image that came off the camera.
5) The flocking shows up a bit lighter than the rest of the interior of the baffle tube with the flash.

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#25 shams42

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Posted 17 September 2009 - 09:41 PM

Alright, I got some Protostar flocking paper. I flocked the last inch or so of the tube. This was harder than I expected. Anyway, I'll test it again as soon as it clears off.

Here's a pic.

Attached Files








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