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Adam J    687
Posted (edited)

Today I removed my primary mirror for cleaning. On removing it and prior to cleaning I could immediately see that the coating was thin in places around the edge with light penetrating from the rear. This is the first time I have had it out and am quite disappointed in the quality of the mirror coating. 

Has anyone else seen this on SW mirrors? 

Cleaning went well and I replaced the mirror. 

Am wondering if it's worth having it recoated in a couple of years time when it dusts up again. Or many it's just as well to buy a whole new scope what with the 130pds being so cheap...problem is I could just end up with another poorly coated mirror. Does anyone know if SW put any other coatings on or is it just plain aluminium? 

Edited by Adam J

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Lockie    4,219

I'm guessing that's not right. I've owned plenty of Synta mirrors and never noticed translucent coatings. Synta (Skywatcher/Celestron) mirrors are supposed to be overcoated with something like silicon dioxide for longevity.

Do you think the thin coating will photograph? It would be interesting to see.

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Adam J    687
2 hours ago, Lockie said:

I'm guessing that's not right. I've owned plenty of Synta mirrors and never noticed translucent coatings. Synta (Skywatcher/Celestron) mirrors are supposed to be overcoated with something like silicon dioxide for longevity.

Do you think the thin coating will photograph? It would be interesting to see.

Mirror is back in now could never see it untill I removed it. But I'll have a go. 

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Peter Drew    6,243

Provided that the mirror looks clean and reflective when in the telescope I wouldn't worry too much about it. Too "thick" a coating may not copy the accurate figure polished on the mirror before coating.   :icon_biggrin:

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Adam J    687
3 hours ago, Peter Drew said:

Provided that the mirror looks clean and reflective when in the telescope I wouldn't worry too much about it. Too "thick" a coating may not copy the accurate figure polished on the mirror before coating.   :icon_biggrin:

You can only see when you shine a bright torch on it down the tube.  Otherwise it looks perfect. 

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Peter Drew    6,243

Shining a light on to optics is second only to looking at the Sun without protected optics on the list of things not to do with a telescope.   :D

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Adam J    687
1 hour ago, Peter Drew said:

Shining a light on to optics is second only to looking at the Sun without protected optics on the list of things not to do with a telescope.   :D

Perhaps I am being dumb or missing the sarcasm, but I was under the impression that only highly parallel light would be focused by a telescope mirror.

At any rate here are some pictures (I removed it again just for you chaps).

From above you can see the purplish areas of thin coating.

From off axis that are largely none visible.

image.thumb.png.acdf49ceca74771b6003f1b9902d8212.pngimage.thumb.png.084d13c4c5e66a41b5c65f36479cdd43.png

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pipnina    538
3 minutes ago, Adam J said:

Perhaps I am being dumb or missing the sarcasm, but I was under the impression that only highly parallel light would be focused by a telescope mirror.

I believe the nature of parabolic curves is that, irrespective of distance, light from a point source which is on-axis (center frame) will always come to focus. I could be mistaken however.

The purple spots do seem strange, but I've not seen anything like it before so I'm afraid I cannot provide any insight :(

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Astrobits    563
Posted (edited)

It certainly looks a bit on the thin side but as Peter Drew implies shining a light onto the mirror reveals stuff that doesn't affect the operation of the telescope for normal use. To re-coat the 5" primary only VCSM charge £32 +£6 p&p. and you will not see any change in performance.

Leave it alone.

Nigel

Edited by Astrobits
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RobertI    1,167
Posted (edited)

I sympathise. I had a similar experience when I shone a torch on the secondary of my Heritage 130P to see if it had dewed up - it looked tinted red and blotchy and not good at all. On taking it indoors later it looked  even worse. I was reassured that this is normal (and told not to do it again :)). All this happened after I had posted several reviews saying how great my observing results were and how good the optics are, so in theory I should not have worried, but of course now I cannot shake the feeling that it is slightly sub standard despite the good results I get! Mind you, it only cost me £80!

Edited by RobertI
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Lockie    4,219

Thanks for taking the mirror back out and taking some pics. I can see what you mean, when the torch is shone on axis it does look rather blotchy, then off axis you would never know. 

Nigel has given you the costings for re coating the mirror and that doesn't seem too expensive. I guess it depends how much it bothers you verses the money needed to sort it out? They are probably right in saying that the difference wouldn't be that noticeable to your eyes, but I totally get how it could bug you just knowing the mirror is a bit mottled. 

From what Rob has just said it wouldn't surprise me if lots of mirrors are like this and we just don't notice! 

Ignorance is bliss. 

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Adam J    687
Posted (edited)
47 minutes ago, Lockie said:

Thanks for taking the mirror back out and taking some pics. I can see what you mean, when the torch is shone on axis it does look rather blotchy, then off axis you would never know. 

Nigel has given you the costings for re coating the mirror and that doesn't seem too expensive. I guess it depends how much it bothers you verses the money needed to sort it out? They are probably right in saying that the difference wouldn't be that noticeable to your eyes, but I totally get how it could bug you just knowing the mirror is a bit mottled. 

From what Rob has just said it wouldn't surprise me if lots of mirrors are like this and we just don't notice! 

Ignorance is bliss. 

Well I have had the scope for three years and the mirror is probably a bit older than that, its nether bothered me in use as such. This is the first time I have removed it, mostly because I left the cover off the tube for two weeks in the obsy without noticing and when i looked some larger flecks of something had settled onto the mirror surface and I figured it was time to give it a clean. 

I probably wont touch it again for at least another two or three years, ill consider a re-coating at that point assuming I have not replaced it with the GSO 6 inch F4 carbon fiber scope that I have always wanted....or a ES Comet Hunter max-newt. 

Edited by Adam J
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Lockie    4,219
58 minutes ago, Adam J said:

Assuming I have not replaced it with the GSO 6 inch F4 carbon fiber scope that I have always wanted....or a ES Comet Hunter max-newt. 

That ES comet hunter looks like a cracking scope for a small obsy.

 

1 hour ago, Adam J said:

I probably wont touch it again for at least another two or three years, ill consider a re-coating at that point

That's probably best I reckon... or at that point use it as an excuse to buy comet hunter ;) 

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