Weird Diffraction spikes

[MikeWilson]

Just a few cameraphone pictures from inside my OTA for comparison...

[pook]

OK.. that all looks the same as mine. I was wondering if they'd changed anything (SW) in order to cut costs. With my scope being newer and all.

I think before I start taking things apart, I'll do two things:

A) Get a Bahtinov mask made at work ( I have access to a laser cutter ) to ensure better focus.

Image something from a darker sight with no off axis light sources.

I'll report back when I've done this.

Thanks for all your help so far with ideas etc.

[Carbon Brush]

Very interesting. Last night, using my SW200 I got a few pics with dffraction spikes from M45. Until this thread I gave them little thought.

I will give the images a closer inspection.

[sixela]

What's making my diffraction pikes look strange? Not the spikes themselves... but the fan shapes in between.

Two effects are visible:

a) your vanes aren't actually in two parallel pairs (which is why you have one split spike).

Mirror clips add the other things (as others have said).

The mirror is relatively small and the clips protrude quite a bit. On my large scope, I have another system (with some covered bolt shafts as clips) and the clips don't produce sectors like this but faint spikes (less prominent because the relative area is a lot smaller), but on my Starblast what you see is what I see.

[sixela]

I didn't use any filters at all.

I'll paint the draw tube first, as that seems a likely candidate.

It's a diffraction effect, so that won't help.

One thing you can do is use a ring baffle just over the mirror and occluding the clips (and slightly reducing your aperture). It can even serve as a good baffle to direct airflow in a fan system for mirror cooling.

You could also, if the mirror is rather thick for its diameter, glue the mirror to the cell supports and do away with the clips, but if the mirror's too thin that'll cause astigmatism.

[pook]

Not had chance to image anything yet!

Someone else posted up a flame neb image in here this forum though, and it had exactly the same pattern, and that was a SW150P.

I'm just starting to think this is what you get when you pay £260 for a 6" newt and a GEM. SOmething got to give Unfortunately, it's the optics it would seem.

[pook]

Did some imaging from a fairly dark site tonight, and had the same problem, so it's not off axis light. Something in this tube is causing this pattern.

I'm rapidly going off this scope. I don't think it deserves the reputation it has to be honest. I wish I'd waited a little longer and got the OO VX6.

[rfdesigner]

odd effect..

one thought... if you centre a bright star right on the centre of the FOV then the light from the primary mirror really ought to miss all the clips. My scope USED to have a secondary that was a tad small for the primary and actually set the apparent aperature. I assume this isn't the case on your scope but it's worth checking. Place your eye at the focal plane in the middle of the FOV without an eyepiece, a cheshire 'eyepiece' would be ideal, you should be able to see the whole primary mirror unobstructed.

Also.. if the secondary clips are flat to the secondary, then they could be just reflecting light off their surface, depending on how the clips are manufactured they could disperse light substantially in just one direction.

So are the clips blackened metal (a bit shiny), or are they painted matt black?

Personally I'd get rid of the clips entirely and fix the secondary to the spider with some othe method (bathroom/silicon sealant is excellent for glueing mirrors.. and removable when necessary, my secondary's been in position for >10 years with this method)

Just throwing up ideas.. I hope they're of some use.

Derek

EDIT: rereading.. Doh! looks like it's your primary that has the clips.. yes all stars will be effected.. I'd try some matt black paint on the clips.. or try fixing the mirror without the clips. can you rig up a temporary arrangement with the clips removed?

[pook]

Hmm.. remove the clips. Sounds interesting. I was going to mask them off, but didn't want to lose any aperture on such a small scope. This sounds much better.

What adhesive did you actually use? Any particular brand?

[rfdesigner]

probably B&Qs finest... and that was 10 years ago, the rest of the tube has long since gone off and is now in landfill somewhere.

To allow later removal ensure the sealant creates a pad at least a few mm thick.. i.e. >2mm, otherwise you can't get a knife in to cut it out.

Derek

[MikeWilson]

I plan to put a little bit of flocking material and/or a little bit of dark black paint applied with a very small brush onto the clips.

[pook]

Ahh.. I just re-read rfdesigner's post.. He's referring to the secondary mirror.

The secondary is bonded to the spider already. No clips. I thought he was referring to bonding the primary.

Oh well. Maybe flat black paint on the primary clips will work.

[F15Rules]

I have a suggestion...buy a refractor:)??

Only joking!

I must admit those diamond effect spikes do look quite attractive as in the Capella image, remind me of the astro books I read as a boy.

I can imagine it must drive you nuts not to know what the cause of the other shadows/flares are, but they do seem to be perfectly spaced apart at 120 degrees, suggesting something on your main mirror, eg the clips?

Hope you find peace and a solution soon:rolleyes:

good luck

Dave

[rfdesigner]

Ahh.. I just re-read rfdesigner's post.. He's referring to the secondary mirror.

The secondary is bonded to the spider already. No clips. I thought he was referring to bonding the primary.

Oh well. Maybe flat black paint on the primary clips will work.

Yes I was.. until I realised it was the primary.. but we all seem to be comming to the same conclusion.. it's the clips and it doesn't really matter if they are on the primary or the secondary, they're not helping.

For the record, I just remembered my 5.5" scope has the primary bonded to it's cell with the sealant as well, and that's lasted over 10 years without issue.

Derek

[pook]

Hmm.. looking at it again, it's as if the clips are just shadowing, or blocking out something being reflected from the edge of the mirror, so if I removed the clips somehow, then instead of getting rid of the problem, it would instead just produce a complete hazy ring around the stars instead. It's almost as if there's something about the edge of my mirror that's wrong, and the clips are just hiding this defect, not the cause of it.... if that makes sense.

How likely is it that my mirror is faulty, or badly figured at the edge?

[JKB]

I had same prob with my 150p

Was the spider vanes being twisted slightly! Hard to spot. Also the paint is not great on the sharp edges of vanes.

I took mine out, rounded them off slightly (Fine Wet/dry) and resprayed with Matt black.

Not had the same problem since. HTH

[FraserClarke]

Hmm.. looking at it again, it's as if the clips are just shadowing, or blocking out something being reflected from the edge of the mirror, so if I removed the clips somehow, then instead of getting rid of the problem, it would instead just produce a complete hazy ring around the stars instead. It's almost as if there's something about the edge of my mirror that's wrong, and the clips are just hiding this defect, not the cause of it.... if that makes sense.

How likely is it that my mirror is faulty, or badly figured at the edge?

You'll get that pattern from the diffraction from the clips anyway I think.

However, you could be right, and it could be that the clips are masking a bit of turned down edge, which is otherwise spraying light out from core of the image.

Try a little cardboard mask around the edge of the primary It's a quick and reversible experiment -- much easier than removing the clips and siliconing the mirror. Will at least help identify if this is the problem area (be it the clips, or the mirror edge).

[pook]

I hope it's not a turned down edge, as that's a quality killer! Is there a test to see if this is the case? If that mirror is has a bad edge I'm damned well returning the scope for a refund!

Others have the same scope, and don't have this problem, which has made me think it's not the clips. If it was, everyone with a SW150P would be having the same issues as I am.

[peter shah]

This type of diffraction is caused by your primary mirror clips, they need to either rounded to reduce them or reduced as far as you dare and a baffle ring put around the primary to hide them....i did a lot of work with orion optics on this very problem

[pook]

So what exactly did OO do to solve this? Baffle around the edge? I'm sure I've seen a OO scope mirror, and I don't recall seeing a baffle.

[peter shah]

They do offer baffles around the mirrors....but they have also reduced the size of their clips too.

[pook]

How's the mirror held into the cell of your AG12?

[pook]

I've almost decided to remove the mirror clips entirely, but am worried about the security of just using silicone to hold the primary. Would it be possible to use brackets like this?

That's a side on view.

[peter shah]

If you only use silicone on the primary you will get flex and if you use too much it might pinch the mirror, the mirror wont fall out as long as you dont face the OTA face down. It will be fine for visual but not for imaging. I would not remove the clips just grind them down and round the edges then fit a baffle, Orion Optics will make you a baffle. The baffle needs to go to a knife edge to reduce diffraction. Or you could make one out of thin black card. The AG12 has rounded clips and a baffle, they only just show, but the later ones the clips are hidden, a mod that i will do another time.

[pook]

Is there any chance you could photograph your mirror for me? I'm intrigued now. What do you mean by hidden?

I'd just like to see how it's been done to the OO AG so I don't make a complete mess of mine.

Thanks