Struggling to star-test and collimate my refractor : (

[pipnina]

Just come in after about 2 hours of staring at slightly defocused stars and twiddling with the front lens cell.

I have either made things a lot worse, or have managed to change nothing! I tried only adjusting one of the 3 collimation points (loosen main screw, tighten grub screws) which was the one roughly in the direction of the offset bright dot in the middle of a defocused capella.

I could see in my 10mm EP (80x on a 130mm scope), a large outer ring, some blurriness in the middle (a bit of atmospheric turbulence tonight) and a bright spot to the right of center.

But no matter how much I adjusted the front lens cell, the bright spot would not move.

Do I need to put the lens cell in a certain orientation for it to be adjustable? Like loosen all the grubscrews and just do up all the main screws just barely, then start loosening them off to adjust tilt?

I also noticed that when I turned to jupiter, the dot appeared on the left side instead? And I could only see through a small part of the EP, but maybe that was a reflection... I dunno. It seemed very odd. It seemed like getting closer I never got to see the whole 50 degrees of afov, only a small off-center circle of it, but then, jupiter was in the field and very bright around that area too...

Are there any tips for doing this by eye? I seem to be making a mess of it : (

Considering sending it away for someone professional to work on it haha. Do @FLO offer that service maybe?

[Franklin]

You would be better off doing this in the daytime with a short cheshire collimating eyepiece. Leave the lens cap on, direct the cheshire window at a bright light and make your adjustments until the reflections are concentric. Keeping the tube horizontal helps and only very slight turns on the three push-pull screws. Once you twig which screws move the reflections in the correct direction, you'll find it easy. First job is to make sure the focuser is square on the tube though and you can check that with a laser collimator and a paper mask over the dew shield, the red dot needs to be central.

Having said all of that it all depends on the scope, if it's a triplet I'd leave well alone and pay a professional with access to an optical bench to set it up correctly. Doublets are quite easy to set up though, once you get the hang of it.

[pipnina]

27 minutes ago, Franklin said:

You would be better off doing this in the daytime with a short cheshire collimating eyepiece. Leave the lens cap on, direct the cheshire window at a bright light and make your adjustments until the reflections are concentric. Keeping the tube horizontal helps and only very slight turns on the three push-pull screws. Once you twig which screws move the reflections in the correct direction, you'll find it easy. First job is to make sure the focuser is square on the tube though and you can check that with a laser collimator and a paper mask over the dew shield, the red dot needs to be central.

Having said all of that it all depends on the scope, if it's a triplet I'd leave well alone and pay a professional with access to an optical bench to set it up correctly. Doublets are quite easy to set up though, once you get the hang of it.

There's Trouble with Triplets? Might explain my frustration then as I'm using a triplet apo (SET Optics 130 f6, can't find much about it, got it 2nd hand)

It has a rather large number of little screws around the lens cell, but I haven't touched them as I assume they adjust the three lenses individually. I have touched the screws around the edge that seem like they're for tilting the whole lens cell in one go however.

Maybe I'll have a look and see if I can get someone to look at it 😕

 

[Peter Drew]

It could be that one of the three lens elements has become decentred, if so, tilting the lens cell won't cure the issue.     🤔

[Rusted]

Slightly off topic, but:

I have often considered using a camera and monitor to help to help me collimate.
Though the Cheshire does not readily lend itself to fitting a camera at the peephole. DIY adapter?
Walking back and forth between adjustments. Climbing and descending ladders every time. 
Is more a trial of memory, agility and balance than some innate skill at collimation.
Seeing the changes I make "live" on a screen, from the objective end, would be a huge advantage.

[Stu]

I would think if you are only adjusting the three sets of screws which tilt the whole cell, then there should be no difference between a doublet and triplet in complexity. If, however, you need to adjust the individual lens elements then that’s a while other ball game and definitely needs expert help I would say.

[Franklin]

Sounds like a job for Es Reid in Cambridge.

[Franklin]

8 hours ago, pipnina said:

It has a rather large number of little screws around the lens cell, but I haven't touched them as I assume they adjust the three lenses individually.

Yes, they will be the adjustment screws for centralizing the 3 elements within the cell and, as you rightly say, the 3 push-pull pairs of screws are for adjustment of the cell tilt. Even though you haven't touched the element centralizing screws, being secondhand, a previous owner may have and messed it up a little. Indeed, that may be the reason they sold it on!

[Franklin]

3 hours ago, Rusted said:

Though the Cheshire does not readily lend itself to fitting a camera at the peephole.

The short cheshire could be used with an eyepiece projection camera adaptor maybe?

[pipnina]

39 minutes ago, Franklin said:

Yes, they will be the adjustment screws for centralizing the 3 elements within the cell and, as you rightly say, the 3 push-pull pairs of screws are for adjustment of the cell tilt. Even though you haven't touched the element centralizing screws, being secondhand, a previous owner may have and messed it up a little. Indeed, that may be the reason they sold it on!

Wouldn't be the first time something unexpected was wrong with the scope!

I only just got the focuser issues sorted, as the 2" crayford that came with it was absolute rubbish (would not sit straight, and slipped out slightly every time the auto focus motor moved!). Took a lot of stress and expense (and bribing of the machinist at work) to replace it with a suitable TS 3" R&P.

So it would not surprise me in the least to learn something else complicated is wrong!

48 minutes ago, Franklin said:

Sounds like a job for Es Reid in Cambridge.

I'll have a Google for the guy! Thanks for the name!

 

9 hours ago, Peter Drew said:

It could be that one of the three lens elements has become decentred, if so, tilting the lens cell won't cure the issue.     🤔

I really hope not! But in the case that it's so, does it show up in a certain way in star tests? While I have noticed symptoms of miscolimation in images and at the eyepiece, so far I think it only looks like (to my newtonian-familiar eyes) primary tilt as described by images online, but does decentralised individual elements also have a specific set of symptoms? Tests showed the scope does only produce about 800mm of focal length despite the previous owner saying it should be f6.6, which is closer to 850mm, although the distortion model was immaculate in pixinsight and there is no issue with colour as the autofocus routine is finding all 3 of my RGB filters to be parfocal.

 

Cheers

[Franklin]

On 16/12/2022 at 08:55, pipnina said:

I'll have a Google for the guy! Thanks for the name!

I see you're in Devon, the guys/gals at FLO in Exeter will help you contact Mr Reid I would think.

Edited December 18, 2022 by Franklin
can't spell

[Peter Drew]

Triplet lens elements are very sensitive to lateral displacement and can cause all manner of optical effects.  The decentring of the central spot despite adjusting the tilt of the cell was what made me suspect the issue.     🙂 

[Peter Drew]

Just noticed my post count!     🥳

[JeremyS]

39 minutes ago, Peter Drew said:

Just noticed my post count!     🥳

Congratulations Peter.

Looks like you maxed out now tho 🤔🤣

[pipnina]

I've just gotten home from work and decided to take a look at it in daylight. Having aired my brain for a while.

I sadly don't have a Cheshire any more (sold it with the old newt I had, figured the newbie needed it more than I did), but I might have to go and get another if this it's going to be useful for fracs too!

I do however have a TS 2" concenter and a Baader laser.

I tried to see if I could see any reflections using the concenter, but sadly with no way to illuminate the lens while it's in place (maybe I could rig something, torch inside the tube maybe?) I only saw a foggy objective... I did notice however that the concenter was NOT showing the edge of the objective to be concentric to the rings in the etched acrylic...

I got the Baader laser out and my printed target paper, sure enough my focuser is out having already been through this once. I suppose tightening the screws that keep the focuser on the rotation bearing must have moved it. Maybe if I double-triple check that the focuser is square after fiddling with it again, I could also set the lens cell back to being flat against its own tilt adjustment... And then work from there in whatever method I can use to get it proper...

Ai ai ai this is not what I hoped for haha.

 

[Franklin]

For refractors a short cheshire works best, but get a good one. Some of those cheapy made in China ones are not accurate and also the budget lasers are naff. I used to own a number of achro refractors that had collimatable cells and I almost became obsessed with trying to get everything lined up, I didn't mind playing around with them because they were budget scopes. What I found was that, the longer the focal length, the more any focuser misalignment showed. Cheapy cheshires rattle around in the eyepiece holder, essentially making the test useless. With a self-centering eyepiece holder the results were better but then I discovered that the laser was out of collimation itself. After buying a better cheshire from TS and a Baader laser, I did manage to get those scopes from showing "figure of eight" reflections to a nice "bulls-eye".

But what a palaver! These days, with my Vixen ED's the doublet is in a fixed cell set by them, which screws on to the tube and the focuser screws on to the other end, so everything is square from the factory. Any misaligned can only be from play in the drawtube which can be tweaked if necessary.

If I were you I'd get the focuser aligned first and then test the optics for any astigmatism, to see if any of the three elements need recentering or the whole cell re-tilting.

You have a very nice scope which would warrant the expense of getting it on an optics bench and set up by a professional if needed.

[scotty38]

The thing is you cut the back of the scope off so are you absolutely sure that's not part of any issue you're seeing? If that's not been machined 100% in line with the rest of the optics I guess you're on a hiding to nothing.