Sky-Watcher secondary mirror defect?

[Mycota]

Hello there

I am not sure if this is the right section to make this post, but anyway, here it goes.

For the first time I decided to disassemble my newtonian's mirrors for cleaning and collimating.

It was then, by inspecting the secondary mirror, when I found a discontinuity of the silvery reflective coating on the glass itself (see picture).

Now, what I couldn't find was the reference of this as normal, so I'm afraid this is abnormal and thus a defect possibly affecting my imaging/observations. Can any of you confirm or clear up this for me, please?

The all scope is now all assembled and collimated, For now I have kept observing and imaging but I in case of defect confirmation, I need to contact the seller asap so they ship me a new mirror. Hopefully that discontinuity is just a normal detail of optical engineering out of my gasp.

Thanks in advance

[Daniel-K]

theres no picture ?

[Mycota]

ups, sorry

Uploaded with ImageShack.us

What do you think?

Cheers

[anthony]

Don't know the exact percentages but most of the light will be heading towards the central portion of the secondary. I doubt that any of light that hits the eye or the camera is reflected from that area at all. I wouldn't worry too much.

[JamesF]

It would be worth Malc having a look at that image. I wonder how it relates to the problems he had with his 200P. I'll PM him.

James

[JamesF]

I've PMed malc-c, but it doesn't look like he's been online for a couple of weeks so I don't know when he might answer. The thread regarding his problem which I believe may be similar to yours is here:

http://stargazerslounge.com/topic/159748-200p-colimation-or-poor-optics/

Get yourself a large drink and a comfortable chair before starting to read it. It's quite a big thread

James

[smoothound54]

my 200p secondary is the exactly same - with that tiny fillet of missing mirror surface

i just ignored it

[John]

I would not have thought that would be a problem unless the light cone from the primary mirror, at the point where it intersects the secondary, was exactly as large as the area as the secondary mirror, which I doubt that it is. I suspect your secondary will be slightly oversize for the light cone.

[JamesF]

Might be useful to know what sort of scope it is, too.

James

[Steve Ward]

The secondaries on both my 250 flextube and the 200PDS are like that . . .

They are both "oversize" and the light cone falls well inside the "defect" making no difference to the view.

Steve.

[malc-c]

I've PMed malc-c, but it doesn't look like he's been online for a couple of weeks so I don't know when he might answer. The thread regarding his problem which I believe may be similar to yours is here:

http://stargazerslou...or-poor-optics/

Get yourself a large drink and a comfortable chair before starting to read it. It's quite a big thread

James

Sorry, been a tad busy of late with family matters -

That image seem remarkably familiar, only the current mirror OVL sent me to replace the original has that mine has it's twin down the opposite side

Mycota, I would be interested to see an image of a bright star taken with a camera directly coupled to the scope.

[moriniboy]

No worries you only use a percentage of the secondary i.e. the view you see through the ep does not go right to the edge.

John and Steve Ward nailed it in their posts.

I have a 300px and mine is the same, never noticed it

[Steve Ward]

If the light cone went right to the edge of the secondary there would be no room on this forum for anything but Collimation questions . . .

Most people seem to struggle even with oversize secondaries , gawd help us if they need to get aligned right to the edge . . . .

Steve

[NGC 1502]

I really wouldn't worry about the small area of missing coating.

Have a look through the focuser without an eyepiece in place. Assuming the scope is collimated, you can probably see the primary reflected within the secondary with a bit of spare around the edge, if you can, then the light cone from the primary is smaller than the secondary at that point.

If you can do that, then all of the secondary surface is not being used, so that missing piece is of no consequence. Even if all of the secondary surface is being used, then it would be of no more consequence than the primary mirror clips hiding a tiny bit of the mirror.

I'd relax and enjoy the view myself.

Regards, Ed.

[Peter Drew]

It's of no practical consequence, just cosmetic. It probably arises from the way in which the mirror is supported in the vacuum chamber during aluminising. This appearance is quite common on large primarys, my 30" has two strips.

[JamesF]

I can't tell for certain from the posted image (though it does look like it might be the case) and I'm not going to take the secondary out of my dob to check right this minute, but for the sake of argument let's say that light can pass straight through the secondary from back to front because of the missing coating from the front. Won't that result in diffraction effects appearing in the image that aren't due to the spider? There is a nice straight edge there, after all.

If there can be diffraction effects, might it be possible that they're sufficiently faint that they'd not be easily visible to the eye, but would be picked up when imaging with the kind of exposure times used for DSOs?

James

[malc-c]

I can't tell for certain from the posted image (though it does look like it might be the case) and I'm not going to take the secondary out of my dob to check right this minute, but for the sake of argument let's say that light can pass straight through the secondary from back to front because of the missing coating from the front. Won't that result in diffraction effects appearing in the image that aren't due to the spider? There is a nice straight edge there, after all.

If there can be diffraction effects, might it be possible that they're sufficiently faint that they'd not be easily visible to the eye, but would be picked up when imaging with the kind of exposure times used for DSOs?

James

This was the conclusion Ed Reid came too when he was testing the replacement mirror in my 200P. The fun part was that using his test equipment, the diffraction pattern I was getting in the photographic images could be seen visually, confirming that the issue was no a CCD anomaly or caused by the CLS filter / camera internal optics. The only way to confirm this (other than replace the secondary) would be to remove the mirror from the central boss and rotate it in the same plane and see if the diffraction pattern moved, but as this would render the scope damaged we opted not to try that option and chose to try an over sized mirror instead...

[Mycota]

Wow people,

Thank you very much for your swift feedback. Although amazingly informative, I have to say I don't feel fully clarified.

I have read a good part of malc-c's thread suggested by JamesF and only reinforced my impression that collimation is a very hard, time consuming, expensive, and doomed to imprecision task (at least on the most common fast newtonians). I will continue reading the rest of the thread.

Your comments on the size of the light cone versus the secondary mirror surface shed some light on the non-importance of this uncoated stripe in terms of reflection into the focuser. However, JamesF mention a valid point, what about the light hitting that uncoated stripe? Significantly or not, it will be refracted onto the primary mirror, eventually reaching the eye/sensor.

As this scope was bought last May, it's probably in the warranty still, and since I got no confirmation that such thing is not a defect, I will contact the seller inquiring about the subject.

I've cleaned and collimated the scope yesterday and tonight is kind of cloudy, so I have no star test pics yet. I will post them as soon as I can.

That's it for now. Once again, a thousand thanks for the help, I'll keep you updated

[moriniboy]

I paint the edge of secondary mirrors with matt black paint.

If I notice an area like the one mentioned above I usually paint this as well to prevent interference.

The attached is a photo from my TAL 2, you can just see the far edge is black to cover the area concerned.

[crashtestdummy]

The missing bit is likely from where the mirror was held during coating or just crappy chinese quality control.i doubt it will cause any viewing problems.my 300p secondary came back with a piece missing after recoating but i doubt it has caused a problem

[JamesF]

I paint the edge of secondary mirrors with matt black paint.

If I notice an area like the one mentioned above I usually paint this as well to prevent interference.

The attached is a photo from my TAL 2, you can just see the far edge is black to cover the area concerned.

I think blocking light from the back and potentially the sides too by painting or covering them might be the easy way to resolve any problems that do occur as a result of the incomplete coating of the mirror. Looks like the TAL secondary holder obscures the entire back of the mirror from your picture so perhaps that isn't such an issue for you. The Skywatcher secondary holder clearly doesn't do that.

James

[Astro_Baby]

You could alwaya have a secondary shroud made of something like protostar flock that sits a little proud of the mirror that would kill any interference.

By the way my own 200P shows the same edge ....i never worry about it as the secondary is oversized in these things anyway. The edge of mine is painted as well with blackboard paint.

I'd be pretty confident that given the huge numbers of these scopes outs there and the huge numbers of pele using them for imaging that this problem would have been seen before now if it were critical.

[JamesF]

You could alwaya have a secondary shroud made of something like protostar flock that sits a little proud of the mirror that would kill any interference.

By the way my own 200P shows the same edge ....i never worry about it as the secondary is oversized in these things anyway. The edge of mine is painted as well with blackboard paint.

I'd be pretty confident that given the huge numbers of these scopes outs there and the huge numbers of pele using them for imaging that this problem would have been seen before now if it were critical.

Assuming that all mirrors are equally affected. It could be that only certain batches of mirrors are affected, or only certain sizes, or only those produced after some change in the manufacturing process. I guess it's even possible (though I admit it seems unlikely) that even when imaging the additional diffraction effects might be disguised by stray light reflected in the OTA unless you flock the tube as Malc did.

Initially I understood Malc's problem to be that the light cone intersected the uncoated area, perhaps misreading Malc's explanation of what the optical engineer had said. I'm persuaded now that it's nothing to do with the light cone. It does look to me far more likely that light passing through (rather than reflected by) the secondary is a more likely cause.

I would agree though that if it's an effect that only shows up on the brightest of stars most people may well never see it. I shall test my own newts when I have the opportunity. Right now I've not seen our nearest star for what seems like weeks, let alone a bright point source many light years away.

James

[MadAlbert]

Another one for the pot...I've just checked my 200p and have the same discrepancy. I've not noticed any issue with observing, in fact since 'the great flock' it's been great. I wonder what the image of bright star would reveal...

[Astro_Baby]

Well imaging is always going to be more demanding......but ai would have thought painting the sides and back of the secondary with black paint OR flocking the back would kill anything of this sort.

When I flocked my scope I also eliminated all possible reflective surfaces, the somewhat shiny spider vanes were painted flat, the secondary had its sides and back painted and the vanes screw fittings were also painted flat black. Painting the edge of the scondary was a scary moment for sure but it all worked out.

I havent read the main thread on this, and to be honest I dont have that much time but I would have thought if its stray light going THROuGH the secondary than painting or flocking its back would kill it and if its stray light bouncing off the edge than a paint job there would fix it.

You have to remember these tubes are made down to a price, I'd be scared to think about what the manufacture

d cost of one of these tubes would be at the factory gate, probably less than £50......a lot less would be my guess. In the main they perform perfectly well with hundreds of happy users but at the extreme end of use they are bound to show some issues.