SW120ED collimation advice

[jetstream]

Basically - any ideas?

I put the cheshire in and its off a bit, smaller gap on the bottom than top.... yeah, advice appreciated.

Gerry

[R26 oldtimer]

Check focuser tube sag, then the whole focuser to tube connection (3 screws) before messing with the lens cell.

[wookie1965]

If you have a laser collimator put a piece of paper over the front, now check where the dot lands without the diagonal spin 180°  check again then do the same with the diagonal you will see if they are out. With my Tal obviously different to a ED I loosened the retaining ring then slapped hard on the cell all the way round that seemed to get the crown and flint aligned whether you can do that I am not sure.

Paul

[jetstream]

1 hour ago, R26 oldtimer said:

Check focuser tube sag, then the whole focuser to tube connection (3 screws) before messing with the lens cell.

I have a big moonlight for it with the adapter and it did the same thing. I just checked with the focuser racked all the way in. I had tweaked the focuser to work better previously ie smoother, hold weight better.It actually works pretty good right now.

The collimation was out before tweaking the focuser.

[John]

Unfortunately there is no collimation adjustment on the ED120 objective cell. The trick of loosening the lens retaining ring (scope pointing upwards) and gently slap around the cell to settle the elements, as suggested by @wookie1965 is worth a try. If the objective as a whole seems to be tilted then you could check that the cell is not cross-threaded where it screws onto the scope tube. 

 

[jetstream]

On 21/03/2023 at 16:15, John said:

Unfortunately there is no collimation adjustment on the ED120 objective cell. The trick of loosening the lens retaining ring (scope pointing upwards) and gently slap around the cell to settle the elements, as suggested by @wookie1965 is worth a try. If the objective as a whole seems to be tilted then you could check that the cell is not cross-threaded where it screws onto the scope tube. 

 

Thanks John, well its nice when both the cheshire and laser say the same thing... as disappointing as it is. There is zero focuser sag, and the laser dot (collimated Glatter) stays in the same spot racked in or out. I did put a scopestuff short 2" diag holder on the scope but the issue was there from the beginning. Obviously there laser can "move around" a bit before and when tightening, but the thing looks like the image every time. Years ago I confirmed all this on a star and artificial star- then put the scope away and quit lol.

 

Edited March 27, 2023 by jetstream

[John]

That looks like the optical axis of the focuser is not aligned with that of the objective. Moonlite focusers have collimation adjustment I seem to recall so you should be able to get that laser spot central. It might take some trial and error though.

 

[jetstream]

24 minutes ago, John said:

That looks like the optical axis of the focuser is not aligned with that of the objective. Moonlite focusers have collimation adjustment I seem to recall so you should be able to get that laser spot central. It might take some trial and error though.

 

What would miscollimation look like? the reflection in the cheshire showed the difference too but there was only 1 ring surrounding the cheshire reflection itself- so I guess the lenses are aligned? There were not multiple reflections.

Can the focuser misalignment cause degraded views?

[John]

This is what I look for in the cheshire:

My (probably limited) understanding is that you should check for focuser tilt first (using the laser) and address any that is there. Once you have the laser exiting the centre of the objective, re-test with the cheshire. If you still get something like the right hand image above, the objective has some tilt that needs to be addressed.

When I've star tested refractors where there was either focuser tilt or objective tilt (or both !) the diffraction rings are not concentric around the airy disk but offset to one side or another. A bit like this:

 

 

 

 

[jetstream]

Ok, I get the top left image in the cheshire, as in no multiple reflections like the right, however there is a gap difference. When I star tested as in the bottom image I got between good and worse, I'll have to re check that after focuser alignment. Now to figure that out...

The Moonlight is back on but the laser is still not centered...

Thanks John.  I might have 2 problems ....

[jetstream]

I cant find my Suiter star test book - anyone tell me the distance to put an artificial star for a 120mm f7.5?

[jetstream]

Ok the Moonlight was easy to adjust and I'm now about 20 thousands off -needs to go to the right .02" roughly,. .02" would bring it to the exact center with regard to how I'm testing. Better than the 6mm+ off to begin with anyway.

How fussy is the focuser alignment?

I'll re check the cheshire now. Its sure too bad they are not making these focusers anymore. It a CFL 2.5" large format.

Edited March 27, 2023 by jetstream

[John]

That is looking much better to me 🙂

On the distance to put an artificial star at, it's difficult to find any precision in this. I've seen figures that vary between 40 and 100 metres. I guess the further away it is, the more the artificial star will appear like a real one.

Edited March 22, 2023 by John

[jetstream]

1 hour ago, John said:

That is looking much better to me 🙂

On the distance to put an artificial star at, it's difficult to find any precision in this. I've seen figures that vary between 40 and 100 metres. I guess the further away it is, the more the artificial star will appear like a real one.

Thanks John, just did the cheshire and it was even all the way round now. Nice when the tools agree with each other. I used a small Christmas ornament ball last time glinting off the sun, seemed to work but I'll test again.

Id hate to get the distance to the artificial star wrong - you never know it might add spherical aberration or even the much talked about and hated spherochromatism  😲

Edited March 22, 2023 by jetstream

[Stu]

4 hours ago, jetstream said:

Thanks John, just did the cheshire and it was even all the way round now. Nice when the tools agree with each other. I used a small Christmas ornament ball last time glinting off the sun, seemed to work but I'll test again.

Id hate to get the distance to the artificial star wrong - you never know it might add spherical aberration or even the much talked about and hated spherochromatism  😲

Some info in here Gerry suggesting a distance in metres which is 1/3 of the aperture, so for you it would be 40m.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10781_TS-Optics-artificial-Star-for-Telescope-Tests-and-Collimation.html
 

I’ve also seen 20 to 25 x the focal length quoted which would be less, 18 to 22.5m but best to go on the long side I reckon.

[jetstream]

16 hours ago, wookie1965 said:

If you have a laser collimator put a piece of paper over the front,

Thanks for the help Paul! Great idea and easy in application.

[jetstream]

3 hours ago, Stu said:

Some info in here Gerry suggesting a distance in metres which is 1/3 of the aperture, so for you it would be 40m.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10781_TS-Optics-artificial-Star-for-Telescope-Tests-and-Collimation.html
 

I’ve also seen 20 to 25 x the focal length quoted which would be less, 18 to 22.5m but best to go on the long side I reckon.

Thank you Stu!

I just got up to another what looks like 8" of snow lol Once I get that sorted I might try the artificial star today but I'm not holding my breath as I dont want to wade through the snow to get the ornament positioned to the sun .

I wonder if anybody has other ideas for the artificial star other than what I was thinking?

 

[jetstream]

I'm curious if anyone knows what the effects of refractor focuser misalignment are with respect to the views?

[Peter Drew]

If the misalignment is gross I would expect the extrafocal rings to be skewed to one side.  To be honest, I've corrected alignments where the centres had been 1cm out and not noticed a significant improvement to the performance.  Some objective designs are more sensitive in this respect.   🙂

[jetstream]

13 minutes ago, Peter Drew said:

If the misalignment is gross I would expect the extrafocal rings to be skewed to one side.  To be honest, I've corrected alignments where the centres had been 1cm out and not noticed a significant improvement to the performance.  Some objective designs are more sensitive in this respect.   🙂

Thanks Peter, I'm hoping this ups the performance of the scope but I guess 300x on the moon isnt too bad anyway.