Strange effect around bright stars....

[Peje]

It has occurred to me that I am being a bit unadventurous... (if that's a word)

The weather is pants so I have time to experiment. I can leave the clips slack, put the scope in the mount and collimate. Then slew about and see what happens to the collimation...adjust screws until I'm happy.

[Peje]

OK, did some fiddling tonight. Even with the clips just touching the mirror enough to hold the ring in place the collimation moves about quite a bit up at the focuser (not huge movement at the primary).

I don't think I can face leaving the clips loose as there's a good 2-3mm of free space so that would mean I would need almost constant recollimation. I'm wondering if I could apply silicon bonder between the side of the mirror and the side of the clip to restrict lateral movement? (Perhaps someone suggested this earlier as it rings a bell lol)

[Stu]

1 hour ago, Peje said:

I'm wondering if I could apply silicon bonder between the side of the mirror and the side of the clip to restrict lateral movement? (Perhaps someone suggested this earlier as it rings a bell lol)

From Gerry's comments based upon his experiences I would tend to avoid silicon. Perhaps some kind of spacer between the clip and the side of the mirror would take up the slack whilst still allowing expansion?

[Peje]

14 hours ago, Ricochet said:

Do you have any comparative images from before you flocked it? Did flocking make the effect more apparent by darkening the background or did it somehow increase the effect?

So I decided to see what options I had for silicon blobs, I removed the mirror and to my surprise there is very little space in there for lateral movement...by very little I mean less than 1mm.

This does mean my ring design is now scrapped, I'll have to look at painting around the edge of the mirror.

Am I right in assuming three blobs of silicon would be the right number? And that I want to get the base of the mirror flat against the cork bits? What would be the negative impact of running a thin bead of silicon around the ring, say 2-3inches long?

Pete

[Peje]

3 minutes ago, Stu said:

From Gerry's comments based upon his experiences I would tend to avoid silicon. Perhaps some kind of spacer between the clip and the side of the mirror would take up the slack whilst still allowing expansion?

I was thinking about this, I have lots of self adhesive flocking material that would likely do the job. Can't testify to it's compressibility though.

[dyfiastro]

I set about tonight trying to come up with a ring to block out the edges on my 130PDS.
See what you think but this may be an alternative idea to attaching it to the primary. Instead of putting it between the mirror and the clips, instead I put it between the metal and rubber sections and cut out holes for the screws. mine is in no way perfect but may give you an alternative way to try and attach something.

I cobbled my "prototype" using a template I made in photoshop and then put flocking material on both sides.

[Ricochet]

Lol, snap.  I haven't worried about flocking my prototype though. Fitting it was a bit fiddly though as the Bresser design has two washers per screw between the rubber clips and mirror cell and there is a tendency for at least one to go walkabout. I'm not sure if a metal ring is going to be the best permanent solution as a bit of flexibility when fitting is probably needed. I also used a glass marker to blacken the unaluminiumised bevel around the edge of the mirror. I'm not sure if that is what has been causing my issues or if it is the very edge of the aluminium deposit. Hopefully there will be some clear skies sometime in the near future to actually test what works best. 

 

[Ricochet]

1 hour ago, Peje said:

So I decided to see what options I had for silicon blobs, I removed the mirror and to my surprise there is very little space in there for lateral movement...by very little I mean less than 1mm.

This does mean my ring design is now scrapped, I'll have to look at painting around the edge of the mirror.

Am I right in assuming three blobs of silicon would be the right number? And that I want to get the base of the mirror flat against the cork bits? What would be the negative impact of running a thin bead of silicon around the ring, say 2-3inches long?

Pete

Are you thinking of using the silicon to stick the mirror to the cell or just a thin smear around the outside to take up the less than 1mm space? 

[Peje]

9 hours ago, Ricochet said:

Are you thinking of using the silicon to stick the mirror to the cell or just a thin smear around the outside to take up the less than 1mm space? 

I meant under the mirror. I think some self-ahesive flocking material around the perimeter of the mirror (stick to the metal wall) will also help with the lateral movement.

A question, is the movement we are trying to allow primarily the vertical expansion of the mirror? @FLO

[Skyline]

The mirror clips are only there to stop the mirror falling out of the tube, some people say you need a width of a business card for a gap in between the clip and the mirror.

I found the clips should just barely rest on the mirror without force. You must be able to move the mirror without any slop in the mirror cell. But not loose that is wobbles.

Sometimes I get the same effects using my 130PDS on bright stars, but quite like the effect of the mirror clip shadows.

One of my images showing the shadow of the clips using the 130PDS.

 

 

[Ricochet]

I would say there must be both horizontal and vertical expansion/contraction of both the mirror and the cell. The mirror is only about 1" thick but 8" wide so the horizontal effect will be 8x as much and so that is what you are primarily trying to allow by having the mirror clips loose. In my mind if you had a mirror cell with a central support point then a single blob of silicon there to centralise the mirror while allowing movement across the rest of the points would be fine but by fixing it around the edge you prevent those points from moving and so any expansion or contraction between those points relative to the mirror cell can only occur if the mirror distorts vertically, which you want to avoid. I would probably just try a layer of flocking and see how it goes.

[CraigT82]

15 minutes ago, Skyline said:

The mirror clips are only there to stop the mirror falling out of the tube, some people say you need a width of a business card for a gap in between the clip and the mirror.

I found the clips should just barely rest on the mirror without force. You must be able to move the mirror without any slop in the mirror cell. But not loose that is wobbles.

Sometimes I get the same effects using my 130PDS on bright stars, but quite like the effect of the mirror clip shadows.

One of my images showing the shadow of the clips using the 130PDS.

 

 

Yes that's what I tried to do too... tighten the clips just enough to hold the mirror whilst still being able to rotate the mirror in it's cell by hand

I must admit I do find the diffraction effect pleasing!  Gives a more arty, ethereal look to the image.  That's a lovely M45 by the way

[Ricochet]

Just got back in from an unexpectedly clear night and the baffle mounted on top of the mirror clips definitely works (visually at least). None of the scatter around Jupiter and the sky around the edge of the moon looked darker than I remember. 

[Starflyer]

I see the same problem, but to a much lesser extent, on my OO Newt and I should really get around to solving it.

Just a thought, has anyone tried convex mirror clips?  This is what I'd always planned to do to try and solve it, see attached diagram for an idea of what I'm on about.

 

 

[AKB]

14 minutes ago, Starflyer said:

has anyone tried convex mirror clips? 

Well, that would be easy to try with just a stick-on shape over the existing ones.  I'm wondering, though, whether the problem isn't as much from the beveled edge all the way around as from the clips themselves.  Experimentation should reveal all!

 

[Ricochet]

2 hours ago, AKB said:

Well, that would be easy to try with just a stick-on shape over the existing ones.  I'm wondering, though, whether the problem isn't as much from the beveled edge all the way around as from the clips themselves.  Experimentation should reveal all!

 

It is the edge of the mirror scattering light that is the problem. Otherwise my exposed clips would still be scattering light. Also winding the focuser into the tube would previously add a fourth shadow into the scatter pattern. 

[Peje]

Another fairly pants night here so I spent some more time slewing about, and watching the laser collimator. I've got the mirror clips tightened just enough to hold the ring in place, I will be replacing the ring with some form of paint on the edge of the mirror and following the 'piece of card theory.

I've come to the conclusion that the movement I can see in the mirror is actually play in the mirror cell's sprung bolts. To test this I torqued up the locking screws for the primary and was pleasantly surprised that the collimation varied only slightly...this is great news for me.

Currently my revised procedure for collimation will be to slew to somewhere near my target and collimate before fitting the camera, then forget about it.

[Peje]

Well I think all my work is complete, I ended up transferring a new ring design onto some adhesive backed flocking material I had left over from doing my previous scope and sticking this to the edge of the mirror.

As recommended by @FLO I left the clips loose enough to slide a piece of cardboard under them, a quick fiddle seemed to confirm that any movement in the mirror was down to the mirror cell.

Thanks for all the advice guys, really appreciated. If anyone wants the CAD drawing for the ring I'd be happy to send it over.

[AKB]

Really glad to hear it. I must do the same... would welcome the CAD: I think the mirror is actually slightly oversized beyond the nominal 200mm?

Looking forward to seeing some stunning, artefact-free, images!

AK

[dyfiastro]

Glad to see you came up with a solution and post the results up once you get a clear night.
I have not had a chance yet to try my "prototype" but lets hope everyone here can put pay to the ugly.