Is my primary mirror beyond saving?

[TomShirley]

Hi all,
First time posting. I have a 12" dob where the primary is looking rather worse for wear.

After reading up around the web on how to clean one of these things, and additionally weighing up the numerous warnings of doing more harm then good, I decided to not shy away from giving it a go.

Check out the results in this image showing before and after 2 separate rounds of cleaning:

http://imgur.com/x3Lf4kJ

If you're wondering if there's an image compression issue playing with the clarity of the picture, alas, the mirror really is in that bad shape!

Admittedly the mirror is in really poor condition partly because of my half-hearted attempts to keep it stored away dust free when not in use and also when using the scope I need to be more careful in general.

But I wanted to get opinions if this mirror needs a re-coat or a replacement. Apart from staining that won't come off as it's most likely been there for some time and I would imagine has eaten into the coating, I think there's a bigger issue here...

My dob is a Meade Lightbridge purchased ~10 years ago and my amateur eye is telling me that the coating has deteriorated over time. Other posters have mentioned that GSO mirrors back in the day had this exact issue, but would be handy to hear what you think.

Thanks for reading,
Tom

[Shiraz]

welcome Tom

looks OK to me - unless the piccys do not show the problem properly - what leads you to think that the surface has deteriorated?. there are a couple of spots/bit of dirt, but there still looks to be a good metal covering overall. Provided it still gives good views, leave as is - unless it is going to niggle at you that it isn't quite perfect.

It is a bit misleading to shine a light onto a mirror - you can easily see the light scattered from all the small defects, whereas the vast majority bounces off as normal and you have to avoid looking at the main beam because it is so bright.

[astroron]

Hi Tom,have you done a star test or observed anything with it since you cleaned it:question:
Going by the images it doesn't look too bad, I don't think there is any great damage to the mirror it self.
I would put it back in the scope and give it a try,you will be surprised how
much you can see with it.
Sure there are some wear on the edges and maybe some spots but that won't effect the viewing too much if any.
You can get it re coated for a few hundred dollars.
True story
A 72" scope in America had six bullet holes in it but was used quite successfully for many years.
Cheers:thumbsup:

[Shiraz]

this one https://astroanecdotes.com/2015/03/26/the-mcdonald-gun-shooting-incident/

It is still listed as a research telescope and in use.

[Stonius]

What are you using to clean it?

[PhilTas]

I have an old 12" GSO dob which has quite extensive fungus damage (I bought it second hand in this condition, it was stored in a shipping container in the top end). I reckon it's probably >5 times worse than your mirror.
It gives nice views still. I'm sure that if I compared it to a new scope there would be a drop off in contrast, but still very useable.
Cheers Phil

[TomShirley]

The cleaning process I followed was gleaned from this video: http://m.youtube.com/watch?v=8TObET2F-EA

5 min soak in water with a tiny bit of detergent. Then I used cotton wool soaked in water+detergent and ran a new piece each time across the surface under its own weight.

Then finished off with a rinse of demineralized water.

[TomShirley]

I haven't performed a star test (will need to look up what's involved) however I'm planning to get out to the countryside for a viewing this weekend to see how the image quality is after cleaning.

[TomShirley]

Hi, thanks for your insight.

When I look at the mirror, apart from the spots that are dirt related, there's an irregular pattern of tiny spots that cover the whole mirror surface.

Here's the current state of the primary in a single image:https://imgur.com/GZNDk9W

[Satchmo]

My usual sermon on mirror cleaning :

Mirrors in Australia tend to get a pretty tenacious deposit of Eucalypyus oil and air pollution . There is no problem with starting off with a soak of methylated spirits on some paper towel layed over the surface and then a strong detergent solution with some pressure from a cotton wool bunch to cut through the grease.

If you breath on the mirror when it is dry and the breathe pattern looks all blotchy then you you still have grime and grease on the mirror .

Overcoated mirrors are tough - it is no different from cleaning your windscreen on your car- you wouldn't clean your windscreen would you if you were afraid of scratching it. ( I know that sounds sacrilegious but it is tru )

In my view people let their mirrors get into an aweful state and put up with low contrast views with loads of scatter around stars, and loss of planetary detail because they are afraid to touch the mirror surface .

So long as you wash off any loose grit that may have fallen on it there is no danger of scratching the actual mirror substrate in cleaning and little danger of putting scratches in the SiO or Quartz overcoat ( whichever it is ) . Coatings wont last forever anyway and neither will your eyesight :) so why spoil your views with contrast destroying scatter due to dirt dust and grime .

If you can shine a light through the back of your mirror and see hundreds of obvious pinholes then you need a recoat .

[Satchmo]

Shine a light through the back to see if you have pinholing ....

If its a GSO or the like it is not uncommon to get only a few years out of the coating .

[el_draco]

If its just the coating, then replacement is not even worth considering. You can get it realuminised or even try silvering it yourself. I agree that the images show coating in usable condition.

[TomShirley]

I will give the methylated spirits a go and apply a bit more pressure to see if the stains lift off.



My opinion doesn't carry much weight because I'm so new to this, but I tend to agree. Obviously people shouldn't clean mirrors regularly, but if it's needed then it should be done.

Lots of newcomers who've purchased a basic DOB would inevitably hit this issue over time and guidance online to help decide when to clean would be useful. From my perspective a sample gallery of mirrors in various conditions and some commentary from people who know what they're talking about would help someone in making a decision on weather to clean/recoat/replace or just leave it be.

[TomShirley]

Well... mystery solved. The coating is the issue here. Thanks for your advice everyone.

http://imgur.com/CiwC8m9

The entire mirror has these small holes in the coating, not just the area you can see in the image i've taken.

[GeoffW1]

Hi,

As others have said, that doesn't look terrible to me, especially as shining lights on the mirror tends to exaggerate flaws.

You will get an improvement with recoating, but not twice the performance. Unless the requirement is critical, I would not bother.

Good luck

[TomShirley]

Good to hear your thoughts on the matter. Thanks.

[Satchmo]

Shining a light through the back is the `acid test '. Your original coating is definitely by its use by date. The fact that some owners consider the views they have with their neglected mirrors acceptable is irrelevant in the grand scheme of things when you what your available telescope can potentially show you if you maintain it properly .

A Newtonian mirror has a great advance on refractive lenses in that it by nature will have have a high transmission of light and be completely free of false color aberrations without the $20,00 price tag ( for a meager 6" of Japanese retractor aperture ) . Coma can be corrected with a $150 add on lens train .

I was quietly appalled at the recent Macquarie astronomy night at the number of substandard views I had with low cost Chinese dobsonians through telescopes with original mirror coatings in poor condition clearly badly affected by light scatter from optics that had by admission never been cleaned since purchase . An excuse I heard a few times was `I'm into imaging - I only drag this scope out for these public astronomy nights . Shame !

Embrace the incredible cost advantage of the Newtonian reflector optics and keep your mirror in great condition with a re coat when needed . The telescope is actually worth way more than you paid for it if you treat it with respect . Don't respect any attitudes of mediocrity related to its price in relation to the care of your optics .

Just my 2 cents !

[TomShirley]

Here's a video showing the extent of the pinholes in the coating: https://vimeo.com/169077042

[astro_nutt]

Hi Tom.
Those tiny holes in area wouldn't add up to the centre spot ring, or even the shadow from the secondary mirror. Look at a full Moon or bright Nebulae and see if any bits are missing. If not, then it's ok.
Cheers!

[TomShirley]

Thanks for the feedback. It's been raining all weekend in melb but once there's clear skies I will give that a go and see where I stand.