15 replies to this topic

[wargrafix]

After cleaning the corrector, and recollimating, it would seem that there is a tri spoke versus a ring , I suspect pressure on the secondary, I don't have clear weather for this evening, but would backing off the collimation screws help to easy the pinch?

 

 

Mars

 

[RedLionNJ]

A few people get those discontinuous, tri-lobed diffraction rings. Since the secondary is the only piece of the puzzle with three pressure points, it would make sense to assume that's where the problem lays.

 

Are the screws in really tightly? Can you just loosen them all by the same quarter-turn, for example?

 

 

Nice Mars, btw. Looks awfully like mine from the night of the 4th!

[wargrafix]

Honestly, not very tightly since I followed the loosen 1 then tighten the 2 others. I may have tighten a bit too much earlier this year. Have I screwed up my sct?

[RedLionNJ]

Honestly, not very tightly since I followed the loosen 1 then tighten the 2 others. I may have tighten a bit too much earlier this year. Have I screwed up my sct?

Probably not irreversibly, no.

[mr_maup]

Yes that typical aberration is referred to as "Trefoil" and there is two versions of it (Z9 and Z10). Indeed this may be print through of the mechanical mounting support/screws. Suggest to loosen up the secondary adjustment screws (stepwise) and then recollimate. Obviously they should not be too loose since that will impact your stability. Try to find a good compromise. Good luck !

btw: That PSF still looks very good to me and very much diffraction limited

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Edited by mr_maup, 07 December 2022 - 02:58 PM.

[JMP]

That kind of trefoil can also be due to thermal issues such as falling temperature. I've seen it in my scope and I've seen it go away on a better night.

[wargrafix]

I will have to a star test before I futz with anything. Does anyone know what pinched optics look like in planetary images?

[Tulloch]

I see it all the time on my C9.25" and it doesn't seem to make a huge difference on the planet...

 

Sometimes my moons look a little odd though, this is a stacked, sharpened and over-stretched Europa from a recent session. It's pretty easy to correct though with a bit of levels adjustment...

 

 

Andrew

Edited by Tulloch, 07 December 2022 - 07:24 PM.

[wargrafix]

You had a fantastic session

[wargrafix]

Tulloch, did you see thr test session ser link for the 20 inch?

[Tulloch]

You had a fantastic session

Thanks, worked out pretty well, considering it was still pretty bright when I started

 

Tulloch, did you see thr test session ser link for the 20 inch?

Hmm... not sure I did?

 

Andrew

[wargrafix]

https://www.cloudyni...20-inch-issues/

[Kokatha man]

My advice Amrit - it's fine! You ought to be able to collimate further btw, and (dare I use this word?!?) don't agonise over it too much...   

[Lopper]

I've seen that trefoil pattern with my C9.25, too. I had thought that it might be due to having the secondary screws too tight so I loosened them up a bit. I think that might have helped to some degree, but it didn't completely eliminate the trefoil all the time. On the few rare occasions that I've had really good seeing I've been able to get a mostly-unbroken first diffraction ring since I started keeping my collimation screws looser (see the Airy pattern in this post). In average or below average seeing the trefoil pattern comes back (see the Airy pattern in this post). To echo JMP in #6, I've also noticed that the trefoil is particularly prominent when the scope is experiencing a large temp drop.

[Tulloch]

I actually question whether the trefoil is really an issue anyway - we expect to see diffraction rings anyway, does it really matter if they are more concentrated in 3 locations? Maybe they are brighter than if they were more evenly distributed, but I've seen my trefoil Airy discs spin around while collimating as shown here.

https://www.cloudyni...022/?p=12224781

 

Andrew

[wargrafix]

Thank you! I need a nice imaging run. The team worked our butts off on the 20 inch last night.