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Why would I have a diffraction spike on a C11?

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#1 Phil Hosey

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Posted 16 March 2013 - 06:41 AM

I was going to try some longer focal length imaging with my C11 and I noticed that I'm getting this really strong diffraction spike. It appears to be coming from the scope as I have pretty much ruled out everything else:

1. If I rotate the camera, the spike rotates as well.

2. It happens with my other camera as well.

3. It happens with or without the focal reducer in place

4. I replaced the T-mount with the standard visual back, same result.

5. I looked through the rear port without anything inserted and could see nothing protruding into the light path.

:confused:

#2 John Boudreau

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Posted 16 March 2013 - 07:20 AM

Something as fine as a silk line from a spider or other insect in the baffle tube have been known to create such diffraction. Check up the baffle with a flashlight just in case.

#3 Eddgie

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Posted 16 March 2013 - 08:05 AM

Ah, I thought you ment the spike rotated with the camera, but the spike is not rotating. It is fixed against the background.

To say that it rotated with the camera would mean that it rotated with the background stars, and it stays in the same posiition in the field.

As mentioned, a spider web in the baffle or tube can cause this. Also a screw projecting into the light path can cause this.

Re-inspect. If you have a green or red laser, use it to sweep the baffle and the corrector. A bright LED flashlight might work too. Look for anything that projects into the light path.

You can also pit an eyepeice in the visual back and position the scope about a foot from the wall. Shine your laser (or LED) into the eyepeice and this will project the apeture on the wall. Look for any hair-line on the projected image. It might show. Maybe not.

This is a diffraction spike though if it is not turning with the camera (which is your case because it stays fixed on the background stars).

I know it sounds silly, but make sure that there were no power lines overhead. Sometimes we forget they are there. I have power lines in my back yard and when a bright target is perfectly aligned, I get spikes and am reminded of that fact.

#4 Goodchild

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Posted 16 March 2013 - 11:42 AM

I had this happen to me once with a short f/l refractor. It turned out to be the diagonal. There was a hint of a crease on the mirror, like they had put two pieces together and then silvered the top surface. Only once in awhile at certain angles could I look at the diagonal and see this seam. Replaced the diagonal and fixed the problem.

#5 Eddgie

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Posted 16 March 2013 - 11:47 AM

Yes, but the OP is not using a diagonal. He is going straight though with a camera I think.

#6 Phil Hosey

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Posted 16 March 2013 - 12:08 PM

Thanks for the suggestions guys, I'll look it over again and see if I can see anything.

#7 zawijava

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Posted 16 March 2013 - 01:42 PM

What cameras are you using and are they stock or modified? Possibly something in the light path within the camera(s)? -Tim

I was going to try some longer focal length imaging with my C11 and I noticed that I'm getting this really strong diffraction spike. It appears to be coming from the scope as I have pretty much ruled out everything else:

1. If I rotate the camera, the spike rotates as well.

2. It happens with my other camera as well.

3. It happens with or without the focal reducer in place

4. I replaced the T-mount with the standard visual back, same result.

5. I looked through the rear port without anything inserted and could see nothing protruding into the light path.

:confused:



#8 TG

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Posted 16 March 2013 - 05:50 PM

As Eddgie says -- and I agree -- the spike is not rotating with the camera but seems fixed to the OTA. Here's an easy way to find out if there's a diffraction causing obstruction at or near the aperture: focus on a bright planet such as Jupiter or the Moon and then take the eyepiece (or camera) out and look at the pupil. If necessary, defocus to get the pupil evenly illuminated. Any diffraction causing obstruction will then be starkly visible as a shadow in the illumination.

Tanveer.

#9 Rodeorat

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Posted 19 March 2013 - 08:54 AM

If you're imaging at long FLs, I would assume you're at low mag. Have you tried higher mag to look at that spike?
When my C11 wasn't thoroughly cooled down, the heat plume looked like a spike or a piller. I was under high mag as I was attempting to collimate. Just a thought.

#10 Patrick

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Posted 19 March 2013 - 01:53 PM

I also suspect it's not a diffraction spike at all, but rather a heat plume. Check to see if you still have it after you've given your scope plenty of time to cool down. One way to check is to see if the spike is pointing straight up in relation to the ground.

I've seen these plumes in my scope quite clearly when it's still warm.

Patrick

#11 Phil Hosey

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Posted 19 March 2013 - 03:22 PM

I did the bright pupil test and what I saw as I was approaching focus is what looked like very faint, evenly spaced vertical lines running through the defocused image, but you could only see them just barely and only at a small range of focus. It almost looked as if there was a B-mask on the scope, only there wasn't. I have used a B-mask that was close to the corrector before, I wonder if it had touched the corrector if it left a mark behind, leaving its signature on the corrector (or part of it) ?

#12 rmollise

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Posted 19 March 2013 - 05:02 PM

I did the bright pupil test and what I saw as I was approaching focus is what looked like very faint, evenly spaced vertical lines running through the defocused image, but you could only see them just barely and only at a small range of focus. It almost looked as if there was a B-mask on the scope, only there wasn't. I have used a B-mask that was close to the corrector before, I wonder if it had touched the corrector if it left a mark behind, leaving its signature on the corrector (or part of it) ?


Even if it did, it would NOT cause the effect you are seeing.

#13 Phil Hosey

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Posted 19 March 2013 - 05:37 PM

I did the bright pupil test and what I saw as I was approaching focus is what looked like very faint, evenly spaced vertical lines running through the defocused image, but you could only see them just barely and only at a small range of focus. It almost looked as if there was a B-mask on the scope, only there wasn't. I have used a B-mask that was close to the corrector before, I wonder if it had touched the corrector if it left a mark behind, leaving its signature on the corrector (or part of it) ?


Even if it did, it would NOT cause the effect you are seeing.


Then I have no idea what it could be. I saw nothing obvious doing the pupil test, I can't see anything in the light path. It happens no matter which camera or wether or not I use the focal reducer. The spike is thin, so maybe I'm just not seeing what it is. I would have thought that anything that coudl cause a diffraction spike would show up in the de-focused star image if I racked it far enough out, but I don't see anything.

#14 David Knisely

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Posted 19 March 2013 - 05:40 PM

If it is due to the scope, diffraction spikes can also result from a thin film of foreign material on the corrector plate (or even some other part of the SCT optical train). Try as they might, not all of the optics can be cleaned perfectly (or can stay perfectly clean), so this can sometimes result in a pair of diffraction spikes that show up on exposures of brighter stars. A famous example of this happening was the so-called "Saturn-like Object" that amateur Chuck Shramek imaged next to Comet Hale-Bopp. It was just a background star that his SCT had introduced a diffraction spike on for the reasons stated above. Clear skies to you.

#15 Da Bear

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Posted 20 March 2013 - 03:01 PM

Tape two thin stings in a cross-hair pattern over the end of the scope. You should see diffraction spikes in a photo at infinity. Do you also see your odd difraction spike as well? Rotate the the "cross -hairs" until they parallel or cover up the odd spike. Now you know the location in the scope of the cause.

You can also try a really long temporoary dew shield (24") made of cardboard to block any stray light. Still see your odd spike? If so, that will give you a clue where offending blockage is interecting the scopes light path.

You can try blocking 1/8 of the incoming light using cardboard wedges at differernt rotations around the axis of the scope.

Flock the interior would be logical next step after evaluating these tests.

Also there may be a micro chip or micro flaw on one of the mirrors coatings.

Good luck.

Da Bear

#16 rmollise

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Posted 20 March 2013 - 06:45 PM

If it is due to the scope, diffraction spikes can also result from a thin film of foreign material on the corrector plate


Did you look at his picture? The only "thin film" on a corrector that would cause a diffraction spike like that would be immediately evident, I'd guess. ;)

#17 JoseBorrero

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Posted 20 March 2013 - 08:40 PM

I noted you have the hyperstar, can you check the secondary visually by removing it? can be scratched.

#18 David Knisely

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Posted 21 March 2013 - 12:20 AM

If it is due to the scope, diffraction spikes can also result from a thin film of foreign material on the corrector plate


Did you look at his picture? The only "thin film" on a corrector that would cause a diffraction spike like that would be immediately evident, I'd guess. ;)


Not really Ron. For a bright enough star, a diffraction spike can be caused by something on the surface of *any* of the SCT's optical surfaces that might not be easily visible on casual visual inspection of that surface (you are forgetting Chuck Shramek's little fiasco with his C8 and Comet Hale-Bopp). It can sometimes be not easily seen visually, but if the star is bright enough or the image exposure is long enough, even a minor residual film of stuff that was not quite cleaned off when the scope was put together can introduce just enough of an effect to show the spikes. I have seen this on a few SCTs, so it does happen, although not a lot. Clear skies to you.

#19 Phil Hosey

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Posted 21 March 2013 - 06:05 AM

I noted you have the hyperstar, can you check the secondary visually by removing it? can be scratched.

No issue with the secondary

#20 Phil Hosey

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Posted 21 March 2013 - 06:09 AM

The previous owner of the scope pointed out a micro scratch on the primary about 1" long radially positioned, which I thought could be the cause, but the diffraction spike is somthing like 90 degrees off from the direction of the scratch. The scratch itself is only visible under very bright flashlight at a certain angle.

#21 freestar8n

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Posted 21 March 2013 - 06:39 AM

That sounds like the problem because the diffraction pattern of a line will be perpendicular to it. This whole thing was puzzling to me if nothing was evident on the corrector - but if there is something on the primary that would explain it better. It may help just to mask it off with a thin strip of black paper just to try. You may need to put another on the mirror 90 degrees away to help balance it. Masking will introduce its own diffraction spike - but it may not be as bad, particularly if balanced with another one at 90 to it.

You might even be able to draw over the scratch with a black marker - but that would be harder to undo than a simple paper cover.

Frank

#22 Eddgie

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Posted 21 March 2013 - 08:27 AM

I have my doubt that a small scratch would cause a diffraction spike. Maybe.

To the OP..

A star test will amost always show sources of diffraction.

Have you use aimed it at a bright star and racked it way in and way out of focus?

If the scratch were bad enough to cause some kind of diffraction artifact, I would think it would be glaring in the star test.

You want to be out 20 or 30 waves in each direction.

#23 Phil Hosey

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Posted 21 March 2013 - 10:24 AM

I have my doubt that a small scratch would cause a diffraction spike. Maybe.

To the OP..

A star test will amost always show sources of diffraction.

Have you use aimed it at a bright star and racked it way in and way out of focus?

If the scratch were bad enough to cause some kind of diffraction artifact, I would think it would be glaring in the star test.

You want to be out 20 or 30 waves in each direction.


Eddgie, I have had it way out of focus on both sides but never noticed anything, but then again I wasn't looking for anything either. I'll try this again and this time I'll be specifically looking for the issue.

#24 freestar8n

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Posted 21 March 2013 - 10:47 AM

What your images indicate is a very faint streak that only appears next to bright stars. There is no reason this would show strongly in a visual star test near focus since it appears to be faint - and its faintness, plus its perpedicularity to the scratch, for me points to the scratch as the cause.

Unless there is something else going on inside - it is the only lead to this unusual effect I have heard that is at least plausible.

Frank

#25 TG

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Posted 21 March 2013 - 04:56 PM

I have my doubt that a small scratch would cause a diffraction spike. Maybe.


My MN-66 has a 1" scratch in the corrector it does cause a faint diffraction spike on the brighter planets. Can't say I have noticed any spiking on the stars.

I can actually see two faint spikes and was initially puzzled since I had bought the scope knowing only about the scratch. The illuminated pupil test showed two apparent scratches but the other scratch was nowhere to be seen on the corrector, primary or the secondary. Turned out it was a stria in the corrector and very hard to see directly but easy to spot in the illuminated pupil. So I believe that if there's anything on the corrector or primary, the OP should've been able to spot it easily.

This test, BTW, will also easily spot any oil leaking onto the surface of an oiled objective.

Tanveer.






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