13 replies to this topic

[rexowner]

I was looking at Arcturus last night through this circa 1989 telescope.  If I moved

focus in and out, in one direction the image became a *slight* oval with the axis

up and down, and in the other direction the image became a slight oval side

to side.

 

Would this indicate an issue with collimation or something else?

 

[Scott in NC]

Astigmatism

[jpflood]

Call Televue and make arrangements to have them collimate the scope. Double check with a different eyepiece to verify the problem is with the scope. Not cheap but they will clean and re-flock the tube. Unless your really good, don't try this at home ...

Edited by jpflood, 15 June 2025 - 12:14 PM.

[betacygni]

Astigmatism, but could also be in your diagonal or eye. Double check these before blaming the scope.

[Polyphemos]

If the problem is more pronounced at low magnification, and less so or disappears entirely at high magnification, there’s a good chance it’s your eye.

[rexowner]

Thanks, great suggestions. 

 

I will rotate the eyepiece and swap it and the diagonals out tonight.

 

Pretty sure it's not my eye as I had an NP-101is set up identically next to it for comparison,

and did not see any oval through it.

 

 

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[areyoukiddingme]

Given that collimation is going to be fairly expensive, if you are mechanically inclined, I'd consider giving collimation a go. Either it works out or it doesn't, but you are presumably in the same spot you are in now, and you get to learn something. 

[Inkie]

Yes, I agree.  It can't be THAT hard to collimate a refractor if the cell allows for it. Lot's of videos show how to do it.  And, if you really do muck it up, what have you lost?  You're just as unhappy with the visual appearance of what the scope shows, and you still need to get expert help.  But, you might nail it, and be pumped for days afterwards. 

[areyoukiddingme]

Here's Jon's collimation thread. 

 

https://www.cloudyni...g-my-tv-np-101/

[rexowner]

I actually borrowed the Genesis from a friend as I was curious how it compared to

its newer 4" sibling.  I really only noticed this out of focus, but will pass the 

recommendations on to my friend.

[davidc135]

Is the lens cell outer flange fitted with three pairs of push/pull screws for collimation? If so, it's easy to adjust them to remove astigmatism and especially easy if you have an artificial star at, say, 25 yards.

 

David

 

PS It looks (from post 9's reference) like collimating is more difficult.

Edited by davidc135, 15 June 2025 - 06:09 PM.

[rexowner]

Is the lens cell outer flange fitted with three pairs of push/pull screws for collimation? If so, it's easy to adjust them to remove astigmatism and especially easy if you have an artificial star at, say, 25 yards.

 

David

 

PS It looks (from post 9's reference) like collimating is more difficult.

Yes, it has three screws covered with some kind of black caulk, similar to the NP-101is.

 

Not my scope, though, and I really only noticed it when defocusing.

[Jon Isaacs]

Yes, it has three screws covered with some kind of black caulk, similar to the NP-101is.

 

Not my scope, though, and I really only noticed it when defocusing.

Those three screws are not normal collimation screws, they merely attach the objective to the OTA.  If you loosen them, the objective is free to flop about.  Collimation basically means aligning the front doublet to the rear doublet.  I do recommend reading the thread I started about building a jig to collimate my NP-101.  The jig allows me to dial in the collimation using orthogonal collimation adjustments (two pairs at 90 degrees) and then tighten down those three screws without the collimation shifting.  I have used it once, 2010.  The scope has remained in collimation since then despite getting knocked over on the mount and hitting the dirt driveway objective end first.

 

I suspect my design would probably work the older scope but I don't know for sure.  Before attempting collimation, it is best to eliminate other possible sources of apparent astigmatism.  I discuss that in the first post.

 

 

https://www.cloudyni...g-my-tv-np-101/

 

Jon

[rexowner]

Thanks for all the replies, esp the link from Jon Isaacs, in which he pointed out the astigmatism may have been due to 

thermal issues.

 

I haven't seen the "ovals" out of focus when the telescope was more cooled down, so thermal issues seems like

likely to have been my problem.

 

Likely operator error on my part -- appreciate learning from others on CN.