Mirror life of Skywatcher.

[rory]

I'm going to look at and hopefully buy a second hand sw 10" dob over the weekend .

It's aparantly in very good condition and been used a handful of times.

My question is how long generally mirrors coatings tend to last with your average newt ?

It's about 8 years old ( 2005/06 )

Is there tell tale signs when checking the mirror ?

[Moonshane]

if it looks in good condition it probably is and will work well. coatings life depends on the life history and what it has been exposed to. if you shine a bright torch up from the bottom and see pinholes then it will need a re-coat at some point. that said, my 12" f4 is like this and I have no intention of recoating yet. the views are still excellent.

[Digger1895]

It's aparantly in very good condition and been used a handful of times.

It's funny that, 99.999% of second hand telescopes on Gumtree here in Australia have only been used a hanful of times.

Good tips in previous post, especially shining light from the bottom.

[John]

Having owned a few Skywatchers of varying ages I think there is a good chance that the coatings on the mirrors will be in excellent condition, even if has been used quite a lot.

If it was a GSO scope such as a Meade Lightbridge or a Revelation then I'd examine things much more critically as these have had problems with poor or even non-existent mirror overcoating in the past.

[rory]

Ok , I'll take a good torch . Cheers Shane .

I'm going to chat with the guy before traveling , but it all seems above board . Fingers crossed for a bargain . Can't wait to be back under the stars with a dob .

[tarotrocola]

Ask the guy about mirror maintenance, if it's been cleaned, how frequently and how. Also take a blower with you to clean some dust of the mirror and check it properly. Point the torch in different angles to look for scratches. Shine a torch up from the bottom as Moonshane said, is a good test too, but I think most of the mirrors will show some pinholes, so also check for some big spots with weak coating.

[moriniboy]

Belive it or not, this mirror worked really quite well considering it had been left uncovered for a number of years........

[tarotrocola]

   Incredible! Lucky for us the mirror acts like a whole, maybe this mirror had some amount of light lost, but was still fully functional. More than one time I've been observing through the bars of my balcony, and the planets did still show fine.. maybe not perfect, but a decent view after all.

[Paul73]

When you try the torch thing on my mirror. It is rather pretty. A bit like looking at one of those backlit planetariums.

How do you know when the pin pricks become an issue??

Paul

PS. Clearing out the spiders webs in the tube should help things a bit. I rarely look at the mirror in daylight...

[alan potts]

That picture just shows how many of us over clean things. I would never say let a mirror get as bad as that but the statement that it still works well enough is testiment.

In my humble opinion I think that one needs a clean.

Alan. 

[Alien 13]

I was allways told that proper cleaning of a mirror at reasonable intervals actually prolongs the coatings life (the muck/dust can contain abrasive particles and acids that react with dew).

Alan

[Moonshane]

That picture just shows how many of us over clean things. I would never say let a mirror get as bad as that but the statement that it still works well enough is testiment.

In my humble opinion I think that one needs a clean.

Alan. 

I think it's failed coatings Alan not dirt.

[Avocette]

I'm going to look at and hopefully buy a second hand sw 10" dob over the weekend .

It's aparantly in very good condition and been used a handful of times.

My question is how long generally mirrors coatings tend to last with your average newt ?

It's about 8 years old ( 2005/06 )

Is there tell tale signs when checking the mirror ?

Glad to read that you're about to buy a second hand scope. I bought an SW 250px Dob a year ago which I would guess was probably a similar age to the one you're going to look at. I suggest a couple of other things to check out:

The focuser may be a rack and pinion one, and in my case some of the black anodising had turned a sort of bronze/brown colour. Whilst this is not a functional defect, it may indicate that the scope has been standing for years where sunlight plays upon it through a shed window for instance.

There was also some swelling of the chipboard on one area of the Dob base. On one panel edge there was a 1mm strip of brown chipboard visible between the white panel coating and the black coating strip on the edge of the panel. I took this to be evidence that that part of the scope had had some extended exposure to condensation, damp or even rain.

The primary mirror had some dust on it, the secondary mirror looked more like spiders had made webs on it.

The mirrors cleaned up very well after some gentle care and final rinse with genuine distilled water, and I replaced the focuser (really just for aesthetics) with a second hand SW Crayford one from ABS.

I did initially worry about the chipboard of the base but it has not moved in the last year. I went ahead and modified it anyway with six teflon furniture pads and a setting circle, and it behaves very well. I had planned to get hold of a piece of formica laminate to use as the mount bearing surface on top of the teflon pads, but the azimuth axis works pretty well with the original white coating.   

I did find that the primary mirror centre ring was about 3mm off-centre. When replacing it I found that the mirror silvering was slightly elliptical measuring 254mm minimum and 256mm maximum diameters. I presently use it with a permanently fitted 250mm circular baffle ring on the basis that if there is any 'turned edge' it's no longer in use.

[rory]

Some good advice Avocette, I'll be sure to examine well.

[Avocette]

 I'll take a good torch . 

I just tried the test with the torch light on the back of the mirror - very reassuring! Using a very bright 9 white LED torch, only a very few tiny points of light (holes) are visible in the silvering.   

[Avocette]

A couple of other points about the SW 10" Dob of 2005/6 vintage:

If the present owner still has them, you should find that it was supplied with the slightly better performing SW Super Plossl 10mm and 25mm rather than the Super MA versions.

The primary mirror cradle / tube end casting are more substantial than other, more recently manufactured scopes, and are fitted with chrome plated collimation and locking knobs with springs (rather than countersunk bolts, Allen screws and neoprene O rings). The knobs are quite deeply recessed in the tube end casting so there is no risk of fouling them if you stand the tube vertically on carpet while you're moving the mount outside.  

[rory]

thanks .  I hope its in good nick.

 so I can point a torch at mirror from the base and look through the open aperture end or from behind ?  sorry  if im sounding daft  !

my old 8" sw dob had the o ring and alen key combo, not great I must say.so if its old school Avocette ,then great.

[Charic]

Copied Avocette I  just put a high powered LED light on the base of the Dob, opened the Dust cover on the telescope and had a look. Apart from a slight glow around my mirror, all looks very good and no stray light or pinholes!

Remember, if you've  read my other posts, I have just recently  cleaned my mirror with ( ### N O T  A D V I S E D ### )  4 Action System Windowlene, for an experiment?

I've read about 5-10 Years is good expectancy for the mirror coating, but I bet there are older scopes out there with no issues. All depends on where the mirrors are used and what their exposed to?

Using the telescope is the best test, If you buy at night and test the telescope under good conditions, and your happy with the fees, then go ahead. I'm sure you'll see if anything is really bad. 

Best of luck

[rory]

ive a gut feeling its going to be fine ,but its good to have input from more experienced folk ,if nothing else it makes you look like you know what your looking at.

 ive some experience and can look at general wear and tear issues, but the mirror was my concern. I was unsure if they had a realistic shelf life.

  ill report back with the outcome.

[just_ed]

I recently bought a 30 year old Tasco  4 ½ inch reflector, which had been poorly stored and was quite grubby. The mirrors cleaned up nicely, so mirrors are probably not as delicate as one might fear.
 

[dph1nm]

My Newtonian is over 40 years old and has been recoated twice in that time.

NigelM

[Beulah]

Inspired by this discussion, with a help of an assistant, we backlit the mirror of a Skywatcher 200p, bought in 2009. A bit shocked to see quite a few pinholes with a few larger ones flashing like supernovas as my helper did a sweep with the LED torch.

Feeling quite nostalgic seeing the 'stars' lit from the other end...made me miss the real ones I haven't see for ages!!

However, I did take a sharp image of Luna and it's X last year so am reassured I have a few years. I am thinking about 3-5 years and it'll be sent it off for the Hi Lux treatment.

[rory]

Out of interest then , how much is a re coating of a primary . Obviously I'm assuming size = more money.

[ScottS]

I was allways told that proper cleaning of a mirror at reasonable intervals actually prolongs the coatings life (the muck/dust can contain abrasive particles and acids that react with dew).

Alan

This is my thinking also. I wouldn't want to strip it down and clean it just to remove a few spots or a light sprinkling of dust, but I wouldn't let it sit for years with grime that could be etching the surface.

I suppose it's just a case of striking a balance with careful cleaning and common sense (of which I confess I have little).

[Dave In Vermont]

After 15 years, my 200mm F/4 Newt's mirror had lost reflectivity. I remove both the primary and the secondary and carefully packed (box-in-box) and shipped them off for re-coating. Came back a week later - beautiful! Cost was $115.00. The shipping was about $15.

Reassembled & collimated the Newt. Working perfectly.

Clear & Clean Skies,

Dave