Spots on My Mirror !

PCH (Paul) · #1 13-07-2009, 06:43 PM

Hi all,

I know there are various dob mirror cleaning threads on this forum, and I've read them all. But I still have a question for someone who knows the answer to this ...

I was aware from reading the threads that it isn't strictly necessary to clean the primary mirror too often, as a dirty mirror won't degrade the image too much. And so, as I've looked at my mirror every once in a while over the past few months, I've noticed it getting dustier and dustier.

When I looked at it yesterday, it looked particularly bad. Not only was there the regular dusty appearance on it, but there's also a very decent smattering of what look like oversized iron filings. Now I know they couldn't be iron filings, but that will be enough to provide a good description of what these particles look like. And so I thought I'd bite the bullet and give it it's first ever wash.

Well, the cell came out easily enough, but when I started washing it - initially using a bit of warm water and dishwashing liquid to lift any surface yuk off easily - I was surprised to see the "iron filing" marks seem to be embedded into the mirror surface. Sort of like a bit of dust that's "leaked" and the leak has spread to the surrounding area. The feel of the marks as I run a finger over the surface is such that you wouldn't know they were there unless you actually look.

The cleaned mirror does look generally much cleaner, and it's reinstalled and appears not to be a problem with images. But I'm a bit concerned about what these marks are in the first place. The scope is kept in a dedicated outside shed that's waterproof, and the scope ahs a dust cover thrown over it.

Has anyone any light to throw on this? All replies appreciated.

Cheers,

DavidU (Dave) · #2 13-07-2009, 06:50 PM

How old is the mirror? If you look from the back of the mirror looking towards a bright light can you see "pin holeing"? Can you see light through it?

PCH (Paul) · #3 13-07-2009, 07:55 PM

David,

thanks for those tips. The mirror is well less than two years old (it's on a Meade 12" LB), but as for the pin-holing, I'm afraid I didn't check that while the mirror was out. I knew I should have left it until I'd got a few replies to this question - drat

However, I did get the impression whilst washing and holding etc, that the blotches were surface imperfections rather than holes in the coating.

I may take it out again to check in the next few days to check this. I'll wait to see what other pointers get posted

Thanks again David

astronut (John) · #4 13-07-2009, 09:00 PM

Paul,
My 12"LB is just over 3 years old, and while it still gives killer views the coating has degraded quicker than I had hoped.
This appears to be a manufacturing fault: maybe not enough material used or applied incorrectly.
I'm saving up my pennies ($300) to have Chi Qin , here in Sydney re coat the mirror.
They are a premium dealer with a reputation for very good work.

John.

DavidU (Dave) · #5 13-07-2009, 09:14 PM

John, I have read about simlar problems over the last few years and what I wonder is if some of these mirrors were not SiO overcoated thus explaining the rapid Al oxidation. Also this would explain the white circular colouration of some reported "spots" on LB mirrors.
Once PCH tests his mirror opacity we will then know if a Chi Qin re coat is in order.

PCH (Paul) · #6 13-07-2009, 11:46 PM

Hi Guys,

first of all, thanks John and David for your thoughts.

Another thing I've thought of is to mention that the LB normally rests with the tube upright. So if it stays like this for some time, then I suppose it's possible that dew forms, and because of the bowl shape of the mirror, there's nowhere for the water to run off. So I suppose it must just evaporated leaving any residue in it's wake to do it's corrosive thing.

Could this be a possibility guys? I had the scope out this evening and although the cell does look very poorly, the views appear to have been in no way compromised. I'm now resting the scope in the horizontal position just in case

jjjnettie (Jeanette) · #7 13-07-2009, 11:57 PM

I'd suggest drying the mirror with a hair dryer before storing. Best not to let the condensation sit on the surface.

DavidU (Dave) · #8 13-07-2009, 11:59 PM

Quote:

Originally Posted by PCH

Hi Guys,

first of all, thanks John and David for your thoughts.

Another thing I've thought of is to mention that the LB normally rests with the tube upright. So if it stays like this for some time, then I suppose it's possible that dew forms, and because of the bowl shape of the mirror, there's nowhere for the water to run off. So I suppose it must just evaporated leaving any residue in it's wake to do it's corrosive thing.

Could this be a possibility guys? I had the scope out this evening and although the cell does look very poorly, the views appear to have been in no way compromised. I'm now resting the scope in the horizontal position just in case

Yep, very possible. don't cover the scope until it is dry. I always kept my Meade Starfinder upside down on it's mount. Not possible with a dob.
It's good the views through the scope are still good

erick (Eric) · #9 14-07-2009, 12:02 AM

Yes, always leave the OTA to dry horizontally. And if you can store it horizontal, that's a good thing to do as well.

astro_nutt · #10 14-07-2009, 10:00 AM

I found that if my 10" dob has dew forming on the primary mirror when brought indoors, I have it sitting at a 60 degree angle..(to reduce the chance of droplets forming), take the dust cover off but place a light cloth over the front so no more dust can stick to the primary mirror and by morning it'll be dry!
Cheers!

jjjnettie (Jeanette) · #11 14-07-2009, 10:12 AM

Dew + dust = mud

JimmyH155 · #12 14-07-2009, 12:42 PM

Yes, my 12 inch LB mirror - 3 years old has one place where there is a 5 mm circle with no silvering on it. Light passes straight through!!

Someone on this forum suggested that I put a dollop of black paint on it to try and stop the hole growing. That has seemed to do the trick.

Haven't noticed any drop in performance, but not too impressed with the quality. As you say, maybe Meade quality control was up the gurgler in those days.

PCH (Paul) · #13 15-07-2009, 07:33 PM

Hi Guys,

thanks David, John, Jeanette, Erick and JimmyH155. I'm going to try just storing the scope in a horozontal position for the while and see if that helps.

It does seem as if a few have a similar tale to tell regarding the quality of the earlier models. But since I keep all my astro gear in a purpose built, new, carpeted shed, - I'm hoping there won't be much dew since the temp of the scope should be permanently similar to ambient outside temp.

Thanks again all who answered. Clear skies

Stuart78 (Stuart) · #14 02-08-2009, 08:57 PM

Isn't some minor pinholing normal i have seen many mirrors with the tiny pinholing maybe 1 or two tiny holes while shining a bright light through the back???