Stars shapes: collimation, tilt, focusing, all or none

[barbulo]

I’ve just removed the primary mirror clips and the first light taken showed this:

Anyone recognizes this pattern (more evident in the TR corner)? What could be the source? Guiding was quite acceptable (~0.4ish). 
Thanks in advance. 

[malc-c]

Looking at the bright star in the top right I would say the collimation is way off.  I was experiencing strange diffraction spikes with my 200P, which turned out to be an issue with the secondary having unmirrored edges on the minor axis.  This was resolved when I installed the same size mirror fitted to the PDS version, so you shouldn't be getting the same problem as you already have a larger mirror in your PDS. 

When I was collimating the scope in order to set things up through out the year long post, twists in the secondary support veins, not having the secondary centred in the tube, and not having it collimated correctly to the focuser all gave odd shaped stars with strange diffraction spikes.

[alacant]

1 hour ago, barbulo said:

What could be the source

Hi

It's astigmatism and typical of that introduced by the Baader cc. You're also tilted vertically.

The bright star. If the pattern persists with e.g. a gpu cc and this 130 is fully modified/blackened, you'll need to 'combination substitute' secondary and primary with examples which are known to be good. Or just accept it:)

Saludos

 

Edited April 20, 2022 by alacant

[vlaiv]

+1 astigmatism.

Not sure where it is coming from - but probably CC as @alacant pointed out.

[barbulo]

Thanks @malc-c @alacant & @vlaiv 

It's the first time I see this aberration. I've had elongated stars at corners before, but never with this awful shape. I've never had a perfect flat field with the Baader MPCC MkIII neither with the 200PDS nor the 130PDS, with the 600D or the 294MM. But this is kind of catastrophic. Should I play with the spacing?

It's the first picture after removing the clips and glued the primary mirror to its cell using silicone. Could something have gone wrong?

I will try to completely re-collimate the telescope and check the next clear night.

[vlaiv]

5 minutes ago, barbulo said:

Should I play with the spacing?

I'm not sure if it will help.

Fact that you have little crosses means that there are two types of astigmatism working at the same time - sagittal and tangential

Tangential astigmatism elongates star in towards center of the field, while sagittal - perpendicular to that.

Fact that you have it combined - means that some there is optical element that produces both effects at the same time and I'm not sure if changing distance could solve this (but you could try).

Here is diagram from wiki page on astigmatism:

That is simple type - where focus point changes between two types - but notice that between two types - there is point where both are present. Having cross means that you have point of best focus, field is "flat" - but there is residual astigmatism of both types.

[alacant]

28 minutes ago, barbulo said:

Could something have gone wrong

Yes. The mirror is distorted. You probably pushed the mirror against the cell when you replaced it. It must be allowed to float on the silicone and not be glued by it.

You need just three small but generous blobs of sealant coinciding with the cork. The mirror is then replaced and allowed to fall into the cell under gravity. Leave 24 hours on a level surface before replacing.

[alacant]

33 minutes ago, barbulo said:

the first time I see this aberration.

Look carefully at your frames before. It is rare to find a Baader cc free of astigmatism.

Fix the tilt though and it should be minimal.

 

[vlaiv]

Maybe astigmatism is coming from the mirror?

If there is load on the mirror that is making it "squeezed" in one axis - that can produce astigmatism.

 

[barbulo]

19 minutes ago, alacant said:

Yes. The mirror is distorted. You probably pushed the mirror against the cell when you replaced it. It must be allowed to float on the silicone and not be glued by it.

15 minutes ago, vlaiv said:

Maybe astigmatism is coming from the mirror?

If there is load on the mirror that is making it "squeezed" in one axis - that can produce astigmatism.

It make sense: pinched optics/mirror. I'm afraid I pressed the mirror. Clumsy me!

Maybe if I remove it and glue it again more carefully...

Thanks gents!

25 minutes ago, alacant said:

You need just three small but generous blobs of sealant coinciding with the cork.

My 130PDS had the 3 squares of cork glued to a disk of cardboard (loose), not to the cell, like my 200PDS.

[rickwayne]

Probably not the same thing, but I saw this cross/"winged" pattern when the primary on my RC was way tilted. But mine was across the field. Aligning the primary fixed it.

[alacant]

22 minutes ago, barbulo said:

3 squares of cork glued to a disk of cardboard

¡Vaya! But yes. I shouldn't be so surprised. Some of ours had that too. 

If you want to keep the high tech card, then 6 blobs. One on the cork side, the other on the cardboard side. 

But NOT glue, rather non-rigid silicone sealant. And strictly gravity and very gentle pressure only. Aim for, say, no more than 2mm of silicone.

Suerte

Edited April 20, 2022 by alacant

[pipnina]

Your example looks pretty much exactly like what my 130P-DS produces with the Baader MPCC 3. This was before I removed any mirror clips.

I have decided to replace it for a TS GPU coma corrector in hopes it gives me a better image overall. (No idea if this works with the 130P-DS but I kinda broke mine by cutting the focuser tube too short so I am using this as a convenient excuse to upgrade to an 8" f4)

I did try a variety of spacings with my baader using shims (I think i ended up with too much in theory) and it didn't change a great deal.

[alacant]

2 minutes ago, pipnina said:

replace it for a TS GPU coma corrector

+1.

It may be more expensive, but it solves so many issues with low end reflectors. 

[barbulo]

Hands on, now I wonder how could I un-stick the primary from its cell. I managed to work a thin razor around. But the base is so tight that I can’t work it out.

Using any kind of dissolver so close to the coated surface just give me the creeps. 
Any suggestion?

Edited April 21, 2022 by barbulo
Solved

[barbulo]

Well, yesterday I had the fist chance to test the telescope after removing the silicone and replacing the clips. It seems the astigmatism remains, though not that strong. 

I wouldn't blame the MPCC for two reasons: never had astigmatism before the clips removal (some residual and uneven coma probably due to tilt or the CC itself) and because I rotated the imaging train 180º and the most affected areas rotated as well. 

Then my guess is that the problem resides in the telescope. Could it happen that the primary mirror is permanently deformed after the silicone-adventure? I'm sure the clips are loose enough not to pinch the mirror.

It doesn't seem to me to be way off but, could a bad collimation provoke that astigmatism?

It's frustrating spending the scarce clear nights trying to get decent stars across the field instead of imaging.

[alacant]

3 hours ago, barbulo said:

I'm sure the clips are loose enough

Thinking: mirror not evenly seated. Did you replace the cardboard/cork support?

 

 

[barbulo]

3 minutes ago, alacant said:

Thinking: mirror not evenly seated. Did you replace the cardboard/cork support?

No. Just removed the silicone and replaced the carboard with the cork. Looks it seats steadily.