Click here if you are having trouble logging into the forums
Privacy Policy |
Please read our Terms
of Service | Signup and
Troubleshooting FAQ | Problems? PM a Red or a Green Gu.... uh, User
Preston Smith
Military Observer
Reged: 04/24/05
Posts: 2155
Loc: Eureka, Pa
|
|
Hi Folks,
This is rather strange. I tested a 60/800mm refractor objective in an OTA and it looked very promising - great star test, no coma and no astigmatism.
I then placed the objective into another OTA that has a collimable lens cell and now I have some astigmatism. The objective was aligned with a Cheshire eyepiece and the objective does not appear to be pinched. No coma and still a good star test.
Any insight on why I am now getting astigmatism?
For those who don't know what astigmatism is:
Non-technical Answer: When you go in and out of focus, the object (such as a planet) will appear to stretch horizontally one way and vertically the other. You have two different focal points along two different planes. This is a type of optical abberation.
Technical Answer: In optics, astigmatism is when an optical system has different foci for rays that propagate in two perpendicular planes. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances.
I'm sure one of the many optics gurus here can explain this much better than I just did.
--------------------
Preston Smith, Eureka, Pa
For now we see through a glass, darkly, but then face to face. Now I know in part; but then shall I know, even as also I am known. 1 Corinthians 13:12
Stellarvue SV115T
Stellarvue NHII, SV70, F50 "Sparrow Hawk 1"
1960s Unitron 160 102mm f15
1950s Unitron 114 60mm f15
1980 Edmund "Voyager" 60mm f8
1960s Tasco 15TE 76mm f15
1960s Tasco 7TE-5 60mm f16
1950s Asahi-Pentax 50mm f12
More refractors!
|
Mark Harry
Carpal Tunnel
Reged: 09/05/05
Posts: 2505
Loc: Northeast
|
|
If it didn't have it initially, and it -DOES- now, there's obviously something wrong with the alignment in the second instance.
And I don't fully understand just what you're doing with a cheshire on a refractor??? That's a new one on me.
Regards, Mark
--------------------
Scopes in the works-
Too many for putting down here! Favorites- 8" F/6, 8" F/4.72, 4.5" F/5.4, 14" F/4.455, all completed.
|
Preston Smith
Military Observer
Reged: 04/24/05
Posts: 2155
Loc: Eureka, Pa
|
|
Quote:
If it didn't have it initially, and it -DOES- now, there's obviously something wrong with the alignment in the second instance.
And I don't fully understand just what you're doing with a cheshire on a refractor??? That's a new one on me.
Regards, Mark
Hi Mark,
I'm in full agreement that something is not aligned correctly. I'm scratching my head on this one.
When aligning a refractor, the first thing I do is use a laser to insure the focuser is in alignment with the tube. I put a piece of paper over the front of the tube and make sure that the laser is hitting the center of the paper. If not, then I shim the focuser until it's in alignment.
After the focuser is aligned, you can use a Cheshire eyepiece to initially collimate the objective - if it is in an adjustable lens cell. Just have to get those circles lined up. You can then "tweak" it with a star test if you want to go into greater detail.
Just curious if anyone has come across this before.
--------------------
Preston Smith, Eureka, Pa
For now we see through a glass, darkly, but then face to face. Now I know in part; but then shall I know, even as also I am known. 1 Corinthians 13:12
Stellarvue SV115T
Stellarvue NHII, SV70, F50 "Sparrow Hawk 1"
1960s Unitron 160 102mm f15
1950s Unitron 114 60mm f15
1980 Edmund "Voyager" 60mm f8
1960s Tasco 15TE 76mm f15
1960s Tasco 7TE-5 60mm f16
1950s Asahi-Pentax 50mm f12
More refractors!
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 6036
Loc: Bremerton Washington
|
|
Preston...the obvious question is...did you place the two elements exactly as they were from the old cell into the new cell?
Any accidental *flipping* of elements?
Do the elements fit exactly in the new cell?
Is the new cell machined square? Can you varify this?
Rob
--------------------
www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
|
Preston Smith
Military Observer
Reged: 04/24/05
Posts: 2155
Loc: Eureka, Pa
|
|
Great Questions Rob!
Hey, if nothing else this thread is an excellent exercise on how to align a refractor! 
I did put index marks on the lenses. One thing I did different this time was blacken the edges of the lenses. That made my index marks hard to see. I usually make my marks as two arrows, one on the crown and one on the flint. Both heads of the arrows point toward the front of the scope. I will go back and double check to make sure I didn't flip one accidently. Usually when I flip a lens I get other indications to include a slight halo around bright objects. That's why I initially ruled out flipping. But I'll check tonight!
The lenses fit well inside the lens cell. They slid in easily but do not slide around.
Is the lens cell machined square - another great question. Yes. The lens cell is a vintage one made by Astro Optical in the 1960s and I used the matching OTA tube which is threaded. Very good quality. The original objective that was in this lens cell and tube did not have any collimation or aberration issues.
This is great! It's always good to bring things to the collective mind!
--------------------
Preston Smith, Eureka, Pa
For now we see through a glass, darkly, but then face to face. Now I know in part; but then shall I know, even as also I am known. 1 Corinthians 13:12
Stellarvue SV115T
Stellarvue NHII, SV70, F50 "Sparrow Hawk 1"
1960s Unitron 160 102mm f15
1950s Unitron 114 60mm f15
1980 Edmund "Voyager" 60mm f8
1960s Tasco 15TE 76mm f15
1960s Tasco 7TE-5 60mm f16
1950s Asahi-Pentax 50mm f12
More refractors!
|
rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 6036
Loc: Bremerton Washington
|
|
Quote:
Great Questions Rob!
Hey, if nothing else this thread is an excellent exercise on how to align a refractor!
I did put index marks on the lenses. One thing I did different this time was blacken the edges of the lenses. That made my index marks hard to see. I usually make my marks as two arrows, one on the crown and one on the flint. Both heads of the arrows point toward the front of the scope. I will go back and double check to make sure I didn't flip one accidently. Usually when I flip a lens I get other indications to include a slight halo around bright objects. That's why I initially ruled out flipping. But I'll check tonight!
The lenses fit well inside the lens cell. They slid in easily but do not slide around.
Is the lens cell machined square - another great question. Yes. The lens cell is a vintage one made by Astro Optical in the 1960s and I used the matching OTA tube which is threaded. Very good quality. The original objective that was in this lens cell and tube did not have any collimation or aberration issues.
This is great! It's always good to bring things to the collective mind!
If you conclude that you have alligned the objective as square as it can be, in the new cell, and the focuser is alligned...then it may be possible the the new internal stop distance of the adjustible cell isn't the same as the old and you may be experiencing slight optical distortions off the end of the focuser tube, which might also be vingetting do to the slight FL error(that you don't notice mechanically). Your Astig image might be an optical illusion do to items mentioned. 
Did you factor out *gremlins*? 
Rob
--------------------
www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
|
Preston Smith
Military Observer
Reged: 04/24/05
Posts: 2155
Loc: Eureka, Pa
|
|
Quote:
If you conclude that you have alligned the objective as square as it can be, in the new cell, and the focuser is alligned...then it may be possible the the new internal stop distance of the adjustible cell isn't the same as the old and you may be experiencing slight optical distortions off the end of the focuser tube, which might also be vingetting do to the slight FL error(that you don't notice mechanically). Your Astig image might be an optical illusion do to items mentioned.
Did you factor out *gremlins*?
Rob
Gremlins are always a possibility. Just ask William Shatner!  
(if you are laughing at this statement then you are just TOO old!)
Actually I was starting to think along the same lines. I need to see how the light stops are lining up in the two OTAs. I'm pretty sure I'm not vignetting within the OTA but there could be some reflected light getting through. Also the focuser is different and now I'm trying to remember if there was a stop in the focuser tube. That could be an issue. I'll have to draw out the light cone.
--------------------
Preston Smith, Eureka, Pa
For now we see through a glass, darkly, but then face to face. Now I know in part; but then shall I know, even as also I am known. 1 Corinthians 13:12
Stellarvue SV115T
Stellarvue NHII, SV70, F50 "Sparrow Hawk 1"
1960s Unitron 160 102mm f15
1950s Unitron 114 60mm f15
1980 Edmund "Voyager" 60mm f8
1960s Tasco 15TE 76mm f15
1960s Tasco 7TE-5 60mm f16
1950s Asahi-Pentax 50mm f12
More refractors!
|
Preston Smith
Military Observer
Reged: 04/24/05
Posts: 2155
Loc: Eureka, Pa
|
|
Hi Rob,
The objective lenses are not flipped. Next is the light stops and then on to the focusers! 
--------------------
Preston Smith, Eureka, Pa
For now we see through a glass, darkly, but then face to face. Now I know in part; but then shall I know, even as also I am known. 1 Corinthians 13:12
Stellarvue SV115T
Stellarvue NHII, SV70, F50 "Sparrow Hawk 1"
1960s Unitron 160 102mm f15
1950s Unitron 114 60mm f15
1980 Edmund "Voyager" 60mm f8
1960s Tasco 15TE 76mm f15
1960s Tasco 7TE-5 60mm f16
1950s Asahi-Pentax 50mm f12
More refractors!
|
mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
|
|
But how can you have a "good star test" in the presence of astigmatism? If your star images are pinpoint there's something else going on - the star test is not just "what does it look like racked in and out" it's "what does it look like closely inside and outside of focus" where astigmatism is quite apparent.
I think you might want to check your tolerances carefully... 
Best,
Mark
|
Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1963
Loc: Boechout, Belgium
|
|
Is there a diagonal involved...? Maybe you should check on that one...?
--------------------
Chris
|
ken scharf
sage
Reged: 02/09/08
Posts: 393
|
|
Quote:
Gremlins are always a possibility. Just ask William Shatner!
(if you are laughing at this statement then you are just TOO old!)
At first I thought you meant 'tribbles' then I remembered the twilight zone. Actually Shatner was in at least one other TZ episode. (And you don't have to be TOO old. The SciFi channel runs a 48 hr TZ 'thon at least twice a year.)
|
|
13 registered and 2 anonymous users are browsing this forum.
Moderator: Don W, Joe Cipriano
Print Thread
|
Forum Permissions
You cannot start new topics
You cannot reply to topics
HTML is disabled
UBBCode is enabled
|
Thread views: 256
|
|
|
|
|
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
[Re: 