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Collimation Reflections

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#1 WDavidson

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Posted 23 May 2024 - 04:25 PM

I have a c8(XLT) and believe I have noticed a loss of sharpness after installing a feathertouch focuser and bob’s knobs. The stars also seem to have a haze/glow around them, instead of a pinpoint in a black canvas. I cleaned the corrector plate to ensure no dust or film was diffusing the light, but it didn’t resolve the issue. I assume something in my optical train or collimation is off.

 

When collimating using an artificial star, there are multiple duplicate reflections of the defocused star and I’m wondering if this is normal? I’m at 500x magnification, using a 10mm uwa with a 2.5x Barlow. These pictures are taken(iPhone against EP) on both sides of focus. Also, how good is this collimation to the more experienced eye?

 

 

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#2 jmillsbss

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Posted 23 May 2024 - 05:23 PM

are you certain you reassembled the corrector in the right orientation, say on a clock, or forward/backward?



#3 sevenofnine

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Posted 23 May 2024 - 05:45 PM

There are several C/N threads on Bob's Knobs and SCT's. Bottom line is that all screws need to be snug on final adjustment or collimation will  move on you. Check this and see if it helps. Good Luck! borg.gif

 

https://www.cloudyni...a-wish-i-hadnt/



#4 WDavidson

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Posted 23 May 2024 - 05:47 PM

I didn’t remove the corrector plate, I just cleaned the exterior surface, the interior looks clean.


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#5 WDavidson

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Posted 23 May 2024 - 06:03 PM

There are several C/N threads on Bob's Knobs and SCT's. Bottom line is that all screws need to be snug on final adjustment or collimation will  move on you. Check this and see if it helps. Good Luck! borg.gif

 

https://www.cloudyni...a-wish-i-hadnt/

I’ve looked into that as well and it makes sense. My knobs are snug now so that shouldn’t be an issue. I’ve just never noticed all of the star reflections before, but I’ve also never tried collimating it at 500x so I’m wondering if that is a normal occurrence.



#6 EGregerson

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Posted 23 May 2024 - 07:48 PM

I'd try it on a real star; like polaris (it doesn't move on u); and leave the barlow in the bag.  If the image is still off center, I'd try bottoming all 3 bob's knobs, then backing them out a fraction of a turn at a time, til they're all out 2 - 3 turns. then adjust from there.  Then try a triBahtinov mask


Edited by EGregerson, 23 May 2024 - 07:51 PM.


#7 WDavidson

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Posted 23 May 2024 - 07:58 PM

I'd try it on a real star; like polaris (it doesn't move on u); and leave the barlow in the bag.  If the image is still off center, I'd try bottoming all 3 bob's knobs, then backing them out a fraction of a turn at a time, til they're all out 2 - 3 turns. then adjust from there.  Then try a triBahtinov mask

I should have stated that the “star” remained center in the EP, it was me trying to hold the phone camera up to the EP that makes it appear off centered in the pictures. You think the reflections could be due to light scatter from the Barlow?


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#8 Asbytec

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Posted 23 May 2024 - 09:35 PM

I should have stated that the “star” remained center in the EP, it was me trying to hold the phone camera up to the EP that makes it appear off centered in the pictures. You think the reflections could be due to light scatter from the Barlow?

 

Very likely. 

 

Okay, so a yellow haze around stars and multiple reflections of the defocused star. These does not sound like anything involving Bob's knobs, the Feather Touch focuser, or collimation. Rather the yellow haze might be an artifact of the apparently very bright artificial star itself, or possibly the camera, and reflections are just that. There is some light bouncing around from glass surfaces somewhere. Possibly between the Barlow and eyepiece lenses. You see the reflection because the light is bright enough to show them. We sometimes see reflections between imaging filters and the protective glass covering the chip.

 

As for collimation, the first image looks pretty good for rough defocused collimation, but latter images not as good. That may be the camera is slightly off axis. It's not always easy to center the lens on the exit pupil. As for the yellow haze, are you talking about the brighter edge of the images posted above, or a yellow haze around a focused artificial star? Do you see it visually or in the camera? There can be any number of reasons, including the artificial source itself, the brightness of the source, and uh...thinking of some more. The brighter defocused image, the artificial source appears to be slightly yellow which is fine. 


Edited by Asbytec, 23 May 2024 - 09:45 PM.


#9 WDavidson

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Posted 23 May 2024 - 09:52 PM

Very likely. 

 

Okay, so a yellow haze around stars and multiple reflections of the defocused star. These does not sound like anything involving Bob's knobs, the Feather Touch focuser, or collimation. Rather the yellow haze might be an artifact of the apparently very bright artificial star itself, or possibly the camera, and reflections are just that. There is some light bouncing around from glass surfaces somewhere. Possibly between the Barlow and eyepiece lenses. You see the reflection because the light is bright enough to show them. We sometimes see reflections between imaging filters and the protective glass covering the chip.

 

As for collimation, the first image looks pretty good for rough defocused collimation, but latter images not as good. That may be the camera is slightly off axis. It's not always easy to center the lens on the exit pupil. As for the yellow haze, are you talking about the brighter edge of the images posted above, or a yellow haze around a focused artificial star? Do you see it visually or in the camera? There can be any number of reasons, including the artificial source itself, its brightness, and uh...thinking of some more. The brighter defocused image, the artificial source appears to be slightly yellow which is fine. The haze is probably normal scatter seen due to the brightness of the source. 

The explanation for the reflections makes sense in this case, so that would solve that question. Could you point out what’s off with the collimation in the subsequent images? As far as I can tell, the rings look concentric so what am I missing?

 

The haze I mentioned seems to be there on most bright objects in the sky when viewing at higher magnifications. I would best relate it to that of the moon when the sky atmosphere isn’t as clear. Maybe it is viewing conditions and particulates in the air, but it’s been so frequent that I doubted it was the atmosphere when it looked like a clear night to the naked eye.



#10 Asbytec

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Posted 23 May 2024 - 10:10 PM

Could you point out what’s off with the collimation in the subsequent images? As far as I can tell, the rings look concentric so what am I missing?

 

Maybe it is viewing conditions and particulates in the air, but it’s been so frequent that I doubted it was the atmosphere when it looked like a clear night to the naked eye.

Okay, unlike the top two images, the bottom two images the rings are not concentric. They are compressed on one side near the bottom due to either the camera or star being off axis or due to miscollimation. I presume they are the same image after collimation, so it's probably the former off axis cause. Remember, though, final collimation is best done in focus whenever possible. 

 

Haze definitely could be the sky. Humidity, particulates, etc., even if the sky looks transparent to our eye. The glow around the moon is a good analogy. Do you see this haze with an artificial star down indoors? If so, try a smaller pinhole of dimmer source to see if it clears up. Optics can scatter small amounts of light made visible by a very bright source. 


Edited by Asbytec, 23 May 2024 - 10:16 PM.


#11 Cpk133

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Posted 23 May 2024 - 10:26 PM

I used to think haze around bright objects was just poor polish, obstruction, or something in the optical chain until i observed the same in a number of high end refractors.  It’s mostly forward scatter.  No one on here is going to be able to judge collimation better than you can looking through the eyepiece (unless you use metaguide).  Slowly rackk into focus and watch the diffraction pattern collapse, everything should come together in symmetry around the poisson spot.  If you have good enough seeing, collimate on the first diffraction ring in focus.


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#12 whizbang

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Posted 23 May 2024 - 11:21 PM

500x is a bit much.  200x or better is good enough.


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#13 Eddgie

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Posted 24 May 2024 - 08:00 AM

I am sorry but I am not sure what you are talking about.

 

If you are talking about the small circles, that looks like dust on the sensor window.

 

pattern.jpeg

 

Rotate the camera and see if it moves with the angle of the sensor. Check your sensor window as well.

 

I can't imagine that this is any kind of issue caused by putting the knobs on. 



#14 WDavidson

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Posted 24 May 2024 - 09:51 AM

I am sorry but I am not sure what you are talking about.

 

If you are talking about the small circles, that looks like dust on the sensor window.

 

attachicon.gif pattern.jpeg

 

Rotate the camera and see if it moves with the angle of the sensor. Check your sensor window as well.

 

I can't imagine that this is any kind of issue caused by putting the knobs on. 

Yes, the circles are exactly what I’m talking about. They’re also there when just viewing through the EP, so it wasn’t camera dependent. I don’t think it has anything to do with installing knobs other than me not getting the collimation as good as

factory, but am unsure if it’s possible to mess anything up with the primary when replacing the stock focuser?

 

Edited to add: I’ll try rotating the EP and Barlow independently to see if that’s where the problem lies.


Edited by WDavidson, 24 May 2024 - 10:17 AM.


#15 Asbytec

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Posted 24 May 2024 - 10:17 AM

I thought you were talking about the reflections in this image. 

 

post-307716-0-44663700-1716499493.jpeg



#16 WDavidson

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Posted 24 May 2024 - 01:52 PM

I just wanted to update that I troubleshooted everything and discovered it was the reflections were coming from the Barlow, which resolved after cleaning the lens. And after some patience and more attention to detail, I was able to achieve near perfect collimation. I also made some observations with my refractor this AM and it yielded a similar “haze” around bright stars at higher mag, so I’m going to chalk that up to seeing conditions and it being particularly dry and dusty here this year. Thanks everyone for the help.


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#17 jmillsbss

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Posted 24 May 2024 - 07:19 PM

I just wanted to update that I troubleshooted everything and discovered it was the reflections were coming from the Barlow, which resolved after cleaning the lens. And after some patience and more attention to detail, I was able to achieve near perfect collimation. I also made some observations with my refractor this AM and it yielded a similar “haze” around bright stars at higher mag, so I’m going to chalk that up to seeing conditions and it being particularly dry and dusty here this year. Thanks everyone for the help.


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