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Secondary Mirror Offset (Collimation Problems... :))

Collimation
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#26 Usman

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Posted 14 August 2021 - 11:33 AM

Thanks for your reply, Don. I'm using a Celestron Cheshire eyepiece.

 

https://www.celestro...-eyepiece-125in

 

Do you think this undersized secondary mirror would significantly affect the optical performance of this telescope (since there might effectively be a loss in the total amount of light received by the secondary from the primary mirror)?



#27 Starman1

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Posted 14 August 2021 - 11:46 AM

It might be better to take a pic though that tool when the scope is pointed at a darker surface so we can see the inside of the scope better.

To adjust for the sky brightness, your camera is stopping down, making everything else appear black, with no details.

Your Tool, by the way, is a combination tool, combining sight tube (the crosshairs) with a Cheshire (the bright white circle with dark center)

I think that may be why Celestron calls it a "Collimation Eyepiece"

 

Here is the kind of image we need to see (there are residual errors in this collimation, too--centering, rotation, tilt, et.al):

Attached Thumbnails

  • Farpoint cheshire--tiny error revealed.jpg
  • Farpoint annotated.jpg

Edited by Starman1, 14 August 2021 - 11:52 AM.

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#28 Usman

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Posted 14 August 2021 - 12:20 PM

Please let me know if this is OK. I slipped a sheet of paper behind the secondary and then pointed the telescope at a darker surface.

 

DDD.jpg



#29 Usman

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Posted 14 August 2021 - 12:40 PM

And this is with the focuser fully racked out.

 

EEE.jpg

 

 



#30 Starman1

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Posted 14 August 2021 - 12:52 PM

If post #28 is with the focuser all the way in, and the collimation tool fully inserted, that puts the apex only an inch above the focuser and the focal plane must be farther in.

Or, what I suspect, the secondary is just undersized for the light cone.  The images in post 27 weren't even with the focuser all the way in and you see how much image there is outside the reflection of the primary.

You still have some secondary tilt error since the crosshairs are not centered in the white reflection of the Cheshire.  That won't help the illumination issue, though.

If you remove the tool, can you get your eye close enough to see more than the primary reflected in the secondary mirror?


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#31 Usman

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Posted 14 August 2021 - 01:05 PM

Yes, I took the photo in post 28 with the focuser fully racked in and  the collimation tool was also fully inserted. The view definitely looks very different from that in your photos.

 

When I remove the tool and just peer in, I can just about see the primary edges in the secondary but still nothing like your photos.



#32 Vic Menard

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Posted 14 August 2021 - 01:37 PM

Your image in post #28 appears to be taken with the Celestron tool's pupil close to the apex--the actual edge of the secondary mirror (green circle) is almost the same apparent size as the reflected edge of the primary mirror (larger red circle). I've also annotated the Celestron tool's cross hairs (green) and the center relative to the bottom edge of the focuser(?)/Celestron tool (blue cross hairs)--not really sure here.

 

Note that neither cross hair aligns with the primary mirror center marker, although both are well within the high magnification error tolerance (6- to 8mm for your scope). For what it's worth, the primary mirror center marker also appears to be off center relative to the primary mirror edge, but my primary mirror edge reference could easily be off a bit, and at f/6 you can tolerate a center spot that's a little more than a millimeter off center, so I think you're OK there too. 

 

An undersized secondary mirror can impact the image performance, but that impact might only be noticeable at higher magnifications (30X to 50X per inch of aperture). And (inexpensive) smaller secondary mirrors are often better than larger secondary mirrors...

 

I think it's time to "see" what your scope can really do. Try Albireo or the Blinking Planetary, and maybe Saturn if it isn't too low.

 

 

 

Attached Thumbnails

  • post-275768-0-06114200-1628961648_thumb.jpg

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#33 Usman

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Posted 14 August 2021 - 07:23 PM

Thanks very much again, Vic. I'd really like to test it out in the field (or at least on my balcony :)) but the weather isn't cooperating. :/

 

Can you guys please suggest a collimation tool that can help me see these errors while I'm collimating the scope? As far as I can tell, the Celestron collimation tool is excellent for getting me close (and within tolerance limits) but I would like to get everything as close to perfect as possible without having to take any photos with my camera. I've read that laser collimators can sometimes be unreliable since their own collimation can be off.

 

Thanks once again, Don and Vic.



#34 Vic Menard

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Posted 15 August 2021 - 08:13 AM

...Can you guys please suggest a collimation tool that can help me see these errors while I'm collimating the scope?

The image you provided with the tool you're using allowed me to "see" both axial alignments and the secondary mirror placement. If you want to see a wider field of view (like Don's images in post #27), you can use a simple collimation cap like the Rigel Aline (  https://agenaastro.c...t-eyepiece.html  ). If you want to see a wider field of view with cross hairs, with your apex so close to your focal plane, you would need to use a Catseye TeleCat XL. The noticeable gain in precision with the TeleCat would likely be from the elimination of the 2- to 1.25-inch adapter, but I'm not sure it would be significant.

 

...I would like to get everything as close to perfect as possible without having to take any photos with my camera.

Unfortunately, taking a photo with your camera is also no guarantee of "perfection" (camera decentering relative to the pupil causes the same parallax issues you can see when you move your eye relative to the pupil). Then there are errors caused by tool/adapter registration, and a variety of other less than precision mechanicals that allow mirrors to move and parts to flex... Thankfully, at f/6 "good enough" doesn't have to be perfect!

 

...I've read that laser collimators can sometimes be unreliable since their own collimation can be off.

Yeah, I've read that too. Generally speaking, those complaints are not about good lasers. I use a Glatter laser (mine's at least 25 years old...)--I think Don has had good results with a (less expensive) FarPoint laser. When I get asked to assess someone's collimation, I always bring my TeleCat and my Glatter--and when even more precision is required, my Infinity autocollimator. 

 

To see how each collimating tool can be used to assess specific alignment signatures, read this:  https://www.cloudyni...obs/?p=4651500 


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#35 Usman

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Posted 15 August 2021 - 08:29 AM

OK. Thanks once again, Vic. 


Edited by Usman, 15 August 2021 - 08:41 AM.



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