What is a C9.25 with stain on mirror worth?

[Chris-h]

 

 

 

Edited February 5, 2023 by Chris-h

[PeterCPC]

Why not try cleaning it? It's fairly straightforward to remove the corrector plate - just make sure you mark it so that it goes back in the exact same place.

[Cosmic Geoff]

Guide price for used SCT = half the price of a new one. 

As PeterCPC says, it would be worth trying cautiously to clean it.  The brown stain may turn out to be muck rather than corrosion of the reflective surface.  It would definitely be easier to sell without the stain.

[Alkaid]

Hmm...I know people do it but I’m personally against cleaning mirrors, so, so easy to scratch the thin coating.  You could have it done professionally and then re-coated? 

Or, you could offer it up at suitable price with a return policy, whereby a buyer pays for it and then has the option of returning it if not happy.  If the in-focus view is fine, this could work.  If you did get it returned you could then go down the other routes for mirror cleaning afterwards.

Annoying position...but you can get around it.

[Cosmic Geoff]

I don't think that professional dismantling and cleaning, though good for the scope, would be cost-effective for the executor.  Better to sell it off cheaply to someone willing to take a punt on being able to sort it or use as-is.

[Alkaid]

Strip and re-coat a 9” mirror is not that bad really...

https://www.galvoptics.co.uk/optical-components/telescope-products-andamp-services/telescope-mirror-coating---primary/

I was thinking that the current condition would knock a lot off the resale price.  Paying to have it done would of course be an expense, but would increase resale value back up to where it would normally sit at, more than what it is currently worth.

 

[Peter Drew]

The mirror will have enhanced coatings so a recoat (Hilux?) would need to be of a similar standard to retain the performance.  I would have thought that slightly less than the going rate for a 9.25" of that vintage as well as less the cost of a recoat might tempt a purchaser, specially as the issue will have little affect on the performance.    🙂 

[Chris-h]

Right, some new info. I asked him if he was willing to recoat it, and he said it had already been done by Orion Optics (yes, Hilux coatings)! I guess that explains why the mirror looks so smooth then. It also means the stain must have etched into the glass?

He is not looking for money in that sense, he just wants a fair price for it. Pretty much sold as is. I would have taken it, but I live on the windiest street on the planet. Something this big would be a no go for me, unless I build an observatory! 🤣

Edited January 8, 2023 by Chris-h

[Alkaid]

Ah so it’s already been re-coated then...I see.  At least that’s been tried then...

[Cosmic Geoff]

2 hours ago, Chris-h said:

It also means the stain must have etched into the glass?

No it doesn't. Surely it was re-coated before the stain occurred. I should have thought that stripping and re-coating would have got rid of the stain if it was there, or somebody would be wanting their money back.  It seems more likely that dirty water has got into the scope and dried on the glass, on top of the coating, leaving a deposit of muck. Clearly an attempt should be made to check or remove the stain before any talk of re-coating. If you feel confident about removing the corrector plate, try cleaning a small area of stain with a cotton bud stick and see how that goes.

If you don't want to, my advice to the executor would be to sell it as-is.

(If you imagine that rainwater is clean, you should check the crud that collects in the bottom of a rainwater barrel after a year or two.)

Edited January 8, 2023 by Cosmic Geoff

[Chris-h]

He had no idea. I just checked the OO site:

"Some mirrors we receive have been kept is wet conditions or conditions where moisture occasionally condenses on the optical surface and stays for hours, sometime days. This is one of the biggest problems which cause degradation of the reflecting surface. The moisture gradually seeks out microscopic pin holes in the top protective layer of a mirror’s coatings and begins, ever so slowly, to attack the aluminium underneath this layer. Over the years, this action can also attack the glass but to a far lesser extent. However, glass needs very little attack to loose a little of it’s polished finish.

Unfortunately, there are one or two cases a year when there is very little sign of any attack on the mirror’s surface and we consider that it will coat to a virtually perfect reflecting mirror. The problem is that when coated, we come across areas which are slightly duller which, were impossible to detect in it’s original condition or when the old coating was stripped off. Let me stress, this is a rare occurrence but, it does happen, even after we have given the mirror the most stringent visual tests prior to coating. The dulling of the surface in these rare instances is usually very limited and detracts only from the mirror’s appearance, it has little or no effect on the mirror’s overall performance. The only way to remove this attack on the mirror’s surface if it is very noticeable and which may detract from it’s performance, is to re-polish and re-figure the mirror. This is usually very expensive and in some instances not possible."

So maybe this is the case. It is a dull stain

Edited January 8, 2023 by Chris-h

[Cosmic Geoff]

5 minutes ago, Chris-h said:

"Some mirrors we receive have been kept is wet conditions or conditions where moisture occasionally condenses on the optical surface and stays for hours, sometime days. This is one of the biggest problems which cause degradation of the reflecting surface. The moisture gradually seeks out microscopic pin holes in the top protective layer of a mirror’s coatings and begins, ever so slowly, to attack the aluminium underneath this layer. Over the years, this action can also attack the glass but to a far lesser extent. However, glass needs very little attack to loose a little of it’s polished finish.

Unfortunately, there are one or two cases a year when there is very little sign of any attack on the mirror’s surface and we consider that it will coat to a virtually perfect reflecting mirror. The problem is that when coated, we come across areas which are slightly duller which, were impossible to detect in it’s original condition or when the old coating was stripped off. Let me stress, this is a rare occurrence but, it does happen, even after we have given the mirror the most stringent visual tests prior to coating. The dulling of the surface in these rare instances is usually very limited and detracts only from the mirror’s appearance, it has little or no effect on the mirror’s overall performance. The only way to remove this attack on the mirror’s surface if it is very noticeable and which may detract from it’s performance, is to re-polish and re-figure the mirror. This is usually very expensive and in some instances not possible."

Seems a further reason for selling it as-is and letting this be somebody else's problem.  Unless the muck comes off when carefully cleaned.

Edited January 8, 2023 by Cosmic Geoff

[Chris-h]

I'll see if I can't do a quick image test or two, just to make sure everything is working properly.