Strange effect around bright stars....

[jetstream]

7 hours ago, Peje said:

Any other thoughts would be greatly appreciated.

Nice solution to the mirror bevel scattering/glare issue- I use a non reflective paint and use a Q Tip to (carefully) apply on the bevel. This is one area that can scatter quite a bit of light IMHO. Another area of concern is the focuser drawtube and a baffle can work well here.

http://www.bbastrodesigns.com/baffle.html

edit: actually your scope looks very well baffled after looking at your first images- nice scope!

[jetstream]

5 hours ago, FLO said:

The other is to remove the clips and fix the mirror to the cell using silicone-glue (the type of glue used in the making of fish-tanks, I think). No clips - no aberration. 

I wouldn't do that- it can cause astigmatism. My OOUK VX8 mirror was siliconed to the cell for some reason and it cause some big issues. I removed the silicone, set the mirror cell up right-astig gone.

Just a thought from a newt guy.

[Peje]

8 hours ago, FLO said:

I am on holiday, rain on the roof of the caravan woke me and I couldnt resist a quick peek at SGL... I saw your thread, now I know I won't be able to go back to sleep until I have posted!  

I think you have discussed two seperate unrelated things.

Your first photo (not the one above) showed an aberration that you successfully adjusted out. 

Now you are noticing aberrations caused by your Newtonian's mirror clips. This isn't uncommon and is not related to your first concern. The three dark spikes will coincide precisely with the position of the mirror clips. They are normally only visible around bright stars and the severity depends how large the mirror clips are in relation to the mirror diameter. This is why this aberration is most often noticed on smaller diameter Newtonians - their mirror clips are larger in relation to the mirror diamater.

As far as I know there are only two solutions, one is to render them invisible by covering the remainder of the mirror edge (i.e. if the mirror clips protrude 4mm into the mirror then mask/blacken the entire mirror edge with a 4mm sup-aperture mask, as you have done). The other is to remove the clips and fix the mirror to the cell using silicone-glue (the type of glue used in the making of fish-tanks, I think). No clips - no aberration. 

Hope that helps, 

Steve

Thanks for your input Steve.

This effect is very disappointing for two reasons, 1) This is touted to be a premium imagining Newtonian. 2) My previous Europa 200, from the early 90's, did not suffer this problem; it doesn't seem that image quality should be getting worse as the decades pass.

I hope to have the ring profile completed this evening to allow for better coverage (outwards) so this will allow me to get it printed / cut from a suitable material.

[Peje]

3 hours ago, jetstream said:

I wouldn't do that- it can cause astigmatism. My OOUK VX8 mirror was siliconed to the cell for some reason and it cause some big issues. I removed the silicone, set the mirror cell up right-astig gone.

Just a thought from a newt guy.

NEVER going to do that LOL

[Peje]

Tweeked the design of the ring to allow it to go out past the edge of the mirror as this seemed like it could only be an improvement. Now just just to transfer the clip cutouts back onto the CAD and find a supplier to cut them out of thin matt black plastic without an ahesive backing

[Ricochet]

I need to do something similar to my dob to improve planetary viewing. What if instead of the mask being placed directly onto the mirror it was mounted on top of the mirror clips as a baffle? With the baffle extending beyond the edge of the mirror would it then redirect air being blown from a rear mounted fan across the front face of the mirror and scrub the boundary layer? Or would raising the mask that small amount still leave the unsilvered edge scattering light as in the first images? 

[Peje]

1 hour ago, Ricochet said:

I need to do something similar to my dob to improve planetary viewing. What if instead of the mask being placed directly onto the mirror it was mounted on top of the mirror clips as a baffle? With the baffle extending beyond the edge of the mirror would it then redirect air being blown from a rear mounted fan across the front face of the mirror and scrub the boundary layer? Or would raising the mask that small amount still leave the unsilvered edge scattering light as in the first images? 

My concern would be that light could reflect up the tube. I was considering printing something out in work using the 3D printer but it seems a plastic or vinyl ring is going to be the best solution.

Another issue with reusing the mirror clip screws is that there is a variable slope on the top of the clips as the screws are tightened, when I put my mech eng hat on I have to say it is a pretty sloppy design. The screw are designed for axial loading, this slope will be introducing shear forces.

[FLO]

19 minutes ago, Peje said:

Another issue with reusing the mirror clip screws is that there is a variable slope on the top of the clips as the screws are tightened, when I put my mech eng hat on I have to say it is a pretty sloppy design. The screw are designed for axial loading, this slope will be introducing shear forces.

You are misunderstanding how mirror clips work, they don't clamp the mirror into place. Indeed they hardly grip at all. It is normal to have a small gap between the clip and the mirror surface, the width of a playing card is about right. 

HTH, 

Steve

[Peje]

3 minutes ago, FLO said:

You are misunderstanding how mirror clips work, they don't clamp the mirror into place. Indeed they hardly grip at all. It is normal to have a small gap between the clip and the mirror surface, the width of a playing card is about right. 

HTH, 

Steve

It's need to be an impressively thin card to fit under that lol

[FLO]

13 minutes ago, Peje said:

It's need to be an impressively thin card to fit under that lol

Seriously. Don't use the mirror clips to clamp the mirror. If you do it can cause problems with star shapes. 

HTH, 

steve 

[FLO]

17 hours ago, jetstream said:

I wouldn't do that- it can cause astigmatism. My OOUK VX8 mirror was siliconed to the cell for some reason and it cause some big issues. I removed the silicone, set the mirror cell up right-astig gone.

Just a thought from a newt guy.

Scratching my head here, I cannot see how using silicone glue can cause astigmatism. Intriguing. 

[jetstream]

51 minutes ago, FLO said:

Scratching my head here, I cannot see how using silicone glue can cause astigmatism. Intriguing. 

Is it possible that gluing the mirror with dabs of silicone restrains the mirror? I think the mirror is supposed to be able to "slide" or move a tiny bit on the cell pads or points. OOUK has a nice cell with nylons tips for support points.

[dyfiastro]

First of all I would like to thank you for taking the time to get to the bottom of this.
I have been trying to deal with the exact same aberrations on my 130PDS which become more noticeable after flocking.
Having got to the bottom of a number of other issues (IR reflections and collimation issues) this is one that has had me stumped until now.
As you will see from the image below the aberration are pretty much identical.

Please keep this updated with how you get on as I have a strong feeling that I will be doing something along the same lines myself.
 

Thanks again

Mark

[Ricochet]

7 hours ago, dyfiastro said:

I have been trying to deal with the exact same aberrations on my 130PDS which become more noticeable after flocking.

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?

[Peje]

9 hours ago, FLO said:

Seriously. Don't use the mirror clips to clamp the mirror. If you do it can cause problems with star shapes. 

HTH, 

steve 

I will follow your advice, so you are saying just leave the clips tight enough to hold the ring in place? They were pretty tight from the manufacturer.

[barkis]

I know the scope is primarily an imaging unit, and designed as such. 
However, since these radial  aberrations are depicted quite clearly from a camera produced Image,
but have you observed them visually too, using different eyepieces?

Just out of interest.  This may have been asked by someone earlier in the thread,
but I don't remember seeing it.

[Peje]

11 hours ago, FLO said:

Seriously. Don't use the mirror clips to clamp the mirror. If you do it can cause problems with star shapes. 

HTH, 

steve 

OK, been reading online and lots of recomendations to have a small gap. My only question is, will the mirror being able to move not affect my collimation as the scope rotates / flips?

[CraigT82]

2 hours ago, Peje said:

OK, been reading online and lots of recomendations to have a small gap. My only question is, will the mirror being able to move not affect my collimation as the scope rotates / flips?

The only purpose if the clips really is to stop the mirror falling out if you turn the tube upside down! They are usually tightened at the factory to prevent the mirror clunking round during transit. It really should say in the instructions that you need to loosen them off but there you go.

In my 200p, which has the same mirror cell as yours I believe, I loosened the clips off and placed small paper spacers between the edge if the mirror and the cell to stop the mirror moving laterally. 

I still got some mirror clunk though when moving from one part of the sky to another (in a way which involved rotation of the tube). I just learned to live with this and checked collimation each time I slewewd a certain amount, takes seconds to do with a good laser. Though I appreciate that if you have a camera and CC attached that won't be easy or convenient to do. 

There is lots on info on the net about mirror shift in newts, a lot of folks (on CN mostly) use three blobs of silicone adhesive beneath the mirror to stop it moving around. Its flexible enough to allow the glass to expand and contract but firm enough to keep hold of the mirror. 

Premium imaging newts use high quality well designed CNC mirror cells to prevent this issue, but those cells can cost more than an entire SW quattro! 

At the end of the day this kind of thing is why budget newts aren't usually recommended as introductory  imaging scopes... There's a lot of faffing to get the best from them. 

[Peje]

1 minute ago, CraigT82 said:

The only purpose if the clips really is to stop the mirror falling out if you turn the tube upside down! They are usually tightened at the factory to prevent the mirror clunking round during transit. It really should say in the instructions that you need to loosen them off but there you go.

In my 200p, which has the same mirror cell as yours I believe, I loosened the clips off and placed small paper spacers between the edge if the mirror and the cell to stop the mirror moving laterally. 

I still got some mirror clunk though when moving from one part of the sky to another (in a way which involved rotation of the tube). I just learned to live with this and checked collimation each time I slewewd a certain amount, takes seconds to do with a good laser. Though I appreciate that if you have a camera and CC attached that won't be easy or convenient to do. 

There is lots on info on the net about mirror shift in newts, a lot of folks (on CN mostly) use three blobs of silicone adhesive beneath the mirror to stop it moving around. Its flexible enough to allow the glass to expand and contract but firm enough to keep hold of the mirror. 

Premium imaging newts use high quality well designed CNC mirror cells to prevent this issue, but those cells can cost more than an entire SW quattro! 

At the end of the day this kind of thing is why budget newts aren't usually recommended as introductory  imaging scopes... There's a lot of faffing to get the best from them. 

Thanks for that info, I think my first approach will be to tighten the screws down just enough to hold the mirror in place. I will be relying on the flexibility of the rubber to allow the mirror to flex.

I tend to have my scope imaging overnight and during these sessions it will perform meridian flips & target changes which would undoubtedly [removed word] up my collimation if I was to leave the gap under the clip, I guess I am in a 'lesser of two evils' kind of situation.

[jetstream]

The miror must be able to move a bit- any form of restraint can cause issues ie astig and other strange things as the mirror distorts. My 15" never has astigmatism- one night it did- the mirror froze to the cell pads distorting the mirror. I lifted the mirror up, freeing it and the astigmatism was gone.

A small gap on the clips won't affect collimation.

Do you have a picture of the Quattros mirror cell (no mirror) by chance? I might be interested in one of these scopes.

[jetstream]

14 hours ago, Peje said:

It's need to be an impressively thin card to fit under that lol

FLO is right, the clips must not touch the mirror- this set up as in the picture will cause astig and who knows what else.. Lower the mirror in the cell if possible- I've had to do this before but not on a SW. It can be pretty fine adjusting.

[Peje]

34 minutes ago, jetstream said:

The miror must be able to move a bit- any form of restraint can cause issues ie astig and other strange things as the mirror distorts. My 15" never has astigmatism- one night it did- the mirror froze to the cell pads distorting the mirror. I lifted the mirror up, freeing it and the astigmatism was gone.

A small gap on the clips won't affect collimation.

Do you have a picture of the Quattros mirror cell (no mirror) by chance? I might be interested in one of these scopes.

I'm afraid I don't have a photo with the mirror removed.

[ollypenrice]

Back on who might make your ring, how about the Starsharp people who make B-masks?

Olly

[jetstream]

Here is a couple of pictures- the silicone caused major issues through restraint and a couple of nylon support points were "jamming" the triangles adding further restraint to the flotation system of the cell. This VX8 f3.8 cell uses a piece felt under the clip to allow for small-but needed- movement of the mirror. All this from a "premium" telescope manufacturer.

By setting the mirror cell up properly it went from about 15X aperture (on the moon) to over 39x aperture, very sharp....

When you use your "ring" the clips must still not be tight and restrain your mirror,IMHO.

 

[Starlight 1]

On a lot of scopes you fit the front cover in the back of the scope only take 5 sec to find out. just see I was only on page one.