Objective Lens Misalignment

[Roy Challen]

I've been getting back into my visual astronomy since Comet Neowise graced our skies, buying new stuff and whatnot. 

I got my astro stuff out of the loft, the big Tak mount is now in a place where I can deploy it without necessitating a visit to the chiropractor, and the Tal is ready to go as well. 

I also got my Skylight out of the loft, but on inspection, I found there was a rattle from the front end. It turned out to be one, or both, of the lenses  rattling about in the cell and I could see they had moved relative to each other as the marks left by the foil spacers were in different positions.

On unscrewing the cell, I found the retainer ring almost completely undone, however, it seemed to be just a case of move the lenses so that the marks line up again - I just went to the nearest marks - tighten it back up and crack on (jeez, don't say crack near a lens, please!). So I did that, and checked the view on a tree branch about half a mile away.

It seemed ok, but at higher mags - more than x120 or so, the view didn't seem to come to focus properly - it didn't seem as sharp as the same view through the Tal. However, this scope used to provide sharper views of stars than the Tal, so I think something isn't right.

It could be that I have moved the lenses to the wrong marks, or they are correctly aligned to each other, but not with the cell. Another problem is there are no marks on the side of either lens.

I think before taking anything apart, I'll check the view with an actual star a night, just in case. But if it isn't that, I'll have to take the cell apart.

The plan would be something like this.

1. Mark the positions of the spacers according to the witness marks (luckily, both lenses have marks on them-I didn't clean them). 

2. Try each combination in order against a star, Polaris for convenience.

3. If the diffraction rings look good in any of the 3 positions, consider myself lucky.

4. If the rings look poor, mark the crown and flint, say, position one and then mark one or the other every 45°, and check each position. When the best is found, refine from there.

Obviously, this will have to be done in the dark, but it doesn't have to be done all in one night if I keep a record of what I've done.

So, what can go wrong?

[johninderby]

Have you collimated the scope?

Use a laser collimator and put a paper target over the objective. Print out a bullseye the right size. Keep checking after adjusting the objective until it is right.

Edited October 7, 2020 by johninderby

[Roy Challen]

Hi John,

 

Good point, I haven't checked the collimation, but the lens cell isn't adjustable anyway.

[NGC 1502]

50 minutes ago, Roy Challen said:Good point, I haven't checked the collimation, but the lens cell isn't adjustable anyway.

 

Sounds like something happened to the scope.  Seems very odd the lens retaining ring being very loose, hard to imagine it would get like that without a deliberate act.

 If collimation is off, it’s sometimes possible to adjust the focuser so it’s pointing accurately along the tube, depending on how the focuser is attached.  Not all refractor issues are at the front end.

Hope you sort it, Ed.

[Stu]

@Roy Challen did you make sure not to over tighten the the retaining ring? This may cause issues I think, better slightly loose than over done.

[Roy Challen]

12 minutes ago, NGC 1502 said:

 

Sounds like something happened to the scope.  Seems very odd the lens retaining ring being very loose, hard to imagine it would get like that without a deliberate act.

 If collimation is off, it’s sometimes possible to adjust the focuser so it’s pointing accurately along the tube, depending on how the focuser is attached.  Not all refractor issues are at the front end.

Hope you sort it, Ed.

Yes, it's a bit strange. I don't know how it came to be that loose.

I use a Tal diagonal, which is collimateable to some degree, so that could be of help.

[Roy Challen]

9 minutes ago, Stu said:

@Roy Challen did you make sure not to over tighten the the retaining ring? This may cause issues I think, better slightly loose than over done.

I only used hand force when tightening, just enough to stop any rattling.

[NGC 1502]

47 minutes ago, Roy Challen said:

Yes, it's a bit strange. I don't know how it came to be that loose.

I use a Tal diagonal, which is collimateable to some degree, so that could be of help.

If you have an adjustable diagonal then perhaps that could be part of the solution.  I can recall the mirror in those was spring loaded to facilitate adjustment.  Most diagonals don’t have that useful feature but sometimes it’s possible to shim the mirror with very thin plastic sheet or tape.  Could be a laborious trial and error job, but worth it.

[John]

Does the scope use foil spacers ?

If so and one of those has moved a bit, that can make an impact on image quality as I've been finding out with a 90mm F/11 achro that I've been playing around with lately.

 

 

[Roy Challen]

Hi John,

It does have foil spacers but they don't appear to have moved relative to each other. It was only the flint lens that looked like it had moved as the witness marks from the spacers were about 20-30 degrees offset. Interestingly, on Saturday morning I looked at Mars through both the Skylight and the Tal, and other than the difference in brightness, the views were very similar. I forgot to star test though🤦‍♂️

Edited October 12, 2020 by Roy Challen