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Old 26-10-2019, 07:52 PM
Hijynx (Benn)
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Tips for checking mirrors in Dobs

Hi All. I have spent some time looking at second hand dobs and have come across a few mirrors that have lead me down the path of researching mirror deterioration and maintenance.
Can anyone summarise their process/do’s and don’t for assessing mirror in second hand dobs?
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Old 26-10-2019, 08:39 PM
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Hi Benn,

Telescope mirrors, apart from the front-surface reflective coating don't really deteriorate with age provided they are cared for and occasionally cleaned carefully.

So far as assessing a mirror in a second hand telescope, you probably need to become familiar with star testing the optics and examining at high-power the intra and extra focal star images. These will reveal nearly everything you need to know about the figure on the mirror. Perform a search using star testing telescopes for a wealth of information.
The most common errors found in mass-produced mirrors are under correction (spherical aberration) turned edges and surface roughness. Star testing will easily reveal major defects. You need to carefully and correctly collimate the 'scope prior to star-testing or the results will be misleading.

An alternative that is also helpful is to make or buy a Ronchi eyepiece like this:

https://www.teleskop-express.de/shop...th-a-star.html

or

https://agenaastro.com/jsp-easyteste...ng-tester.html

that will give a very good indicator of any serious (and subtle) problems (following careful collimation).
Of course one other way is to remove the mirror and perform a Foucault (knife-edge) test. You will probably find most people selling a 'scope will be reluctant to disassemble it and remove the mirror for testing. Best to do it in situ with a star test or Ronchi eyepiece. I have owned an easy-tester for about 25 years and it is the "most borrowed" piece of equipment I own.

Best,

L.

P.S: BTW, if you enjoy being really, really unpopular with others, take your Ronchi eyepiece to a major star party, ask people if you can look through their telescope with "my own eyepiece", insert the Ronchi and then tell them bluntly and candidly what's wrong with their telescope. I haven't done this myself but have witnessed it. Nearly caused fisty-cuffs.

Sure fire way to end up with a bloody nose (actual or metaphorical) within 30 minutes.

Last edited by ngcles; 26-10-2019 at 08:51 PM.
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Old 26-10-2019, 10:18 PM
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Shine a torch down the tube and have a look at the mirror from the rear, you will see any damage to the coating like scratches etc. The coating is very thin so some light will show through even on a good mirror, but a bad coating will be quite noticeable.
Rick
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Old 26-10-2019, 10:25 PM
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hand figured

I liked what NGC-LES had to say , top information .

Before the flood of Chinese made mirrors hit the market , mirrors were hand figured & offered at longer focal ratios .

Iv'e had heaps of scopes from a 12.5 newts & 6" refractor down in aperture .

The best scope was an old Astro Optical 8" F/7 , I pushed it to the theoretical max on those few nights when the seeing close to perfect , saw the pup next to Sirius for the first time & doubles at there limit .

Cheers
Bobby
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Old 26-10-2019, 11:17 PM
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Hi Bobby,

Quote:
Originally Posted by Bobbyoutback View Post
I liked what NGC-LES had to say , top information .

The best scope was an old Astro Optical 8" F/7 , I pushed it to the theoretical max on those few nights when the seeing close to perfect , saw the pup next to Sirius for the first time & doubles at there limit .

Cheers
Bobby
Yep the AOS mirrors were very frequently excellent performers and even those that weren't, were at least very good. Many were made by Mark Suchting (Satchmo here on IIS). I have never viewed with a Suchting mirror that wasn't an excellent performer. I own one myself -- a 31cm f/5.3.

I've looked through at least a couple of dozen GSO and Skywatcher mass-produced Dobson mounted Newtonians and most have been pretty good -- some were ball-tearers so far as optical quality is concerned. They do both tend to use slightly over-sized secondary mirrors. I did a test report on a Skywatcher 12" over a decade ago for AS&T and couldn't fault the optics on that one. It wasn't a hand-picked one either. I picked the boxes myself at random off a palette-load of similar boxed tube assemblies. Many of the earlier GSOs had plate glass mirrors which is an inferior substrate to Pyrex which is a low-expansion glass. I have seen one GSO so far that had a mirror I'd describe as being poor -- but they're pretty rare so far as I can see. I have visited the Skywatcher factory in China (while I was on an eclipse tour in 2009) and thought it was a pretty professional factory set-up.

A hand-made, individually manufactured mirror is more likely to be a top performer than a mass-produced one, but it likely to cost somewhat more as well. The Chinese made Newtonians, despite some shortcomings (particularly in the mounts and the bearings) are generally very good value for money.

Doppler wrote "Shine a torch down the tube and have a look at the mirror from the rear, you will see any damage to the coating like scratches etc. The coating is very thin so some light will show through even on a good mirror, but a bad coating will be quite noticeable."

Great advice. A few pin-holes are pretty normal but if you see a lot of them, or that the whole surface is gently glowing (a thin coating) the mirror may need a re-coat.

Best,

L.
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Old 27-10-2019, 12:09 AM
Hijynx (Benn)
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Great advice guys. Thanks very much. I have done the “shining a torch down the barrel” but will need to read up on the other techniques. Most of my inspections have been done during the day. I am basically looking to avoid having to re-coat something.
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Old 27-10-2019, 08:35 AM
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Shining the torch at the reflective surface and looking down the barrel will only show dust and even a good mirror will look bad. You need to look "through" the mirror with the light source behind it. This can be hard if a fan is fitted on the rear cell.
Rick
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Old 27-10-2019, 02:28 PM
Hijynx (Benn)
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I generally put a light down the barrel and look from the pack for scratches and too many pin holes. I read about this in a few places. I also look for wear at the edges on the reflective side, like pitting or flaking. Or a poorly finished edge in manufacturing. All things I have read about. I didn’t know if anything else was worth while or if it made a difference at the eyepiece. Hand made mirrors should be more than ok I would have thought, just not sure about mass produced glass such as Meade or skywatcher etc.
I have read that the surface of a mirror is generally good for 10 years, then it should be resurfaced. I would have thought that this is dependent on amount of use and storage of the scope. Any thoughts on that?
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Old 27-10-2019, 02:47 PM
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I have mirrors that are still in good condition after 20 years, I think cleaning mirrors too often would reduce their lifespan. Storing telescopes in garages where household chemicals are also stored is not a good idea either, pool chemicals being a big no no.
Rick
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Old 31-10-2019, 12:06 AM
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Quote:
Originally Posted by ngcles View Post
Hi Bobby,



Yep the AOS mirrors were very frequently excellent performers and even those that weren't, were at least very good. Many were made by Mark Suchting (Satchmo here on IIS). I have never viewed with a Suchting mirror that wasn't an excellent performer. I own one myself -- a 31cm f/5.3.


L.
Hi Les , Thanks for that feedback about Astro optical .

I have one of those good examples , an early AOS full thickness 10" pyrex F6 , could do with a recoat , it's the scope in my avatar , never gets out of the shed to see any light for years now , someone should rescue it , I'd help with the sale price as it will be a bargain !

Bobby.
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Old 31-10-2019, 12:46 PM
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Hi Bobby,

Quote:
Originally Posted by Bobbyoutback View Post
...I have one of those good examples , an early AOS full thickness 10" pyrex F6 , could do with a recoat , it's the scope in my avatar , never gets out of the shed to see any light for years now , someone should rescue it , I'd help with the sale price as it will be a bargain !

Bobby.
Yep, before my 31cm f/5.3, I had an AOS 10" -- mine was about f/6.1-ish and full-thickness. The substrate was unusual -- not the normal clear pyrex but it had a distinctly light greenish tint. I later discovered the glass was the Schott equivalent to Pyrex (a Corning T.M product) called Duran 50. Functionally and chemically identical to "real" pyrex apparently -- just looks a tad different.

About 2,700 of my individual observing notes in my log were made with that 10 f/6.1". The mirror was excellent. Then I added about 800 notes with the 12" f/5.3 before the 18" f/4.9 arrived -- with which I added about 3,000 more observing notes. Since the 25" f/5 I've added about 600 new notes in 14 months. Currently a bit over 7,000 individual observing notes -- nearly 6,000 differnt objects observed.

Still haven't tired of visual observing!

Best,

L.
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Old 31-10-2019, 06:36 PM
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Hi Bobby,



Yep, before my 31cm f/5.3, I had an AOS 10" -- mine was about f/6.1-ish and full-thickness. The substrate was unusual -- not the normal clear pyrex but it had a distinctly light greenish tint. I later discovered the glass was the Schott equivalent to Pyrex (a Corning T.M product) called Duran 50. Functionally and chemically identical to "real" pyrex apparently -- just looks a tad different.

About 2,700 of my individual observing notes in my log were made with that 10 f/6.1". The mirror was excellent. Then I added about 800 notes with the 12" f/5.3 before the 18" f/4.9 arrived -- with which I added about 3,000 more observing notes. Since the 25" f/5 I've added about 600 new notes in 14 months. Currently a bit over 7,000 individual observing notes -- nearly 6,000 differnt objects observed.

Still haven't tired of visual observing!

Best,

L.
Hello Les ,
I've never heard about that Duran 50 but sounds like the 10" f/6.1 was a great scope ~ 2700 visual notes taken - Wow !

And then the apertures started to go up & up
What a fantastic effort to notch over 7,000 individual & close to 6,000 different objects observed .

Bet you don't need a go to system nor star atlas !

I have a 12" f/5 & have never looked through a big scope
If you get the chance can you describe the visual difference between your 25" & what I can see through my 12 " please ?

Cheers & Beers
Bobby .
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Old 31-10-2019, 07:26 PM
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Hi Bobby,

Quote:
Originally Posted by Bobbyoutback View Post
Hello Les ,

I have a 12" f/5 & have never looked through a big scope
If you get the chance can you describe the visual difference between your 25" & what I can see through my 12 " please ?

Cheers & Beers
Bobby .
Here's one very recent instance where I have revisited an object with the 25" that was previously observed with the 12": NGC 7814 an edge-on galaxy in Pegasus near Gamma Pegasi.

31cm x186: This is a fairly bright galaxy found to the NE from a couple of magnitude 9.5 stars that are aligned NNE-SSW and about 3' apart. Elongated in PA 150, it is a fat lens shaped halo. The boundaries of the halo are about 3' x 1.75', within which is a large round and slightly higher S.B core that seems about 1' diameter and does not rise further to the centre. Due S by 11' is IC 5381.
63.5cm x347: This is a moderately bright to bright, good SB oval galaxy elongated in about PA 150. Large oval or lens-shaped halo seeming 4' x 1.25', the outer halo is tenuous and of very LSB and no tips, a squashed oval with a slightly brighter, large, roughly round core about 1.25' diameter that grows within itself broadly and slightly to centre. There is a very narrow dark lane, almost a line that bisects the core-zone in the same PA as the galaxy halo. That dark lane isn't immediately visible at first glance and better seen at the higher magnification. There is a tiny weakly brighter spot-type nucleus dead centre adjacent NE to the centre of the dark lane. It is hardly brighter than the surrounding core. I attach an amateur image of this galaxy Image credit Tim Schott, Pegasus - 10" Third Planet Optics RC Truss - Nikon d810a - Losmandy g11 - Easton, Pennsylvania USA - October 27, 2019. N is left and W is up.

Here's another from this time last year: ESO 540- AKA Zwicky's Ring or the Cartwheel galaxy:

PGC 2248 (ESO 350-40) Gx Sculptor
31cm x186: Extremely faint – this is the famous Zwicky’s Ring galaxy also known as the Cartwheel Galaxy. Found nearly due S of the W most member of a very long scalene triangle of magnitude 10 & 11 stars which is just out of the field to the N. Very faint galaxy, a diaphanous patch of gossamer only 40" diameter. A magnitude 15 star is to the NW 2'.
63.5cm x260: Astounded and stupefied. The main galaxy of the three is actually seen as a ring! The ring is about 1.25' diameter and of very low SB but definitely seen as a closed loop with a slightly darker interior. The ring is slightly ovoid in PA 120. A magnitude 16.5-17 star is seen embedded on the W edge of the ring and below this trailing around for about 90 degrees to S are four very, very faint star-like spots occasionally visible. There is another star-like spot on the NE edge of the ring near where the other two companion galaxies are. PGC 2249 & PGC 2252 are <1' to the NE.
PGC 2249 Gx Sculptor
63.5cm x126: Both these galaxies that are companions to Zwicky’s ring are of similar appearance -- about 15" diameter spots of gossamer with no structure. PGC 2252 is probably slightly the fainter.
PGC 2252 Gx Sculptor
63.5cm x260: Both these galaxies that are companions to Zwicky’s ring are of similar appearance -- about 15" diameter spots of gossamer with no structure. PGC 2252 is probably slightly the fainter.

Iconic image by HST of Zwicky's Ring attached.

I looked at the Horsehead Nebula (IC 434 & B33) several times with the 12". It is still a difficult and indistinct object in that smaller 'scope. With the 25" it is visible using direct vision, no filter and "pretty obvious. You can even see the snout of the head distinct from the main body opf the dark nebula.

If I get a chance I'll find a few others.

Simple arithmetic tells you the 25" brings about 410% more light to the focal plane than a 12". That makes a big difference on most things faint & fuzzy.

Best,

L.
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Old 31-10-2019, 08:49 PM
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Thanks

Les ' thanks for those extremely detailed descriptions & for posting the photos

That Zwickys ring looks mind boggling !
Fancy be able to see the horse head with direct vision through your 25" I've never seen it in my eyepiece .

And as you said 410% more light then a 12" makes the magic happen

Regards
Bobby .
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Old 31-10-2019, 11:19 PM
ausastronomer (John Bambury)
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Originally Posted by ngcles View Post
Hi Bobby,



Yep the AOS mirrors were very frequently excellent performers and even those that weren't, were at least very good. Many were made by Mark Suchting (Satchmo here on IIS). I have never viewed with a Suchting mirror that wasn't an excellent performer. I own one myself -- a 31cm f/5.3.

I've looked through at least a couple of dozen GSO and Skywatcher mass-produced Dobson mounted Newtonians and most have been pretty good -- some were ball-tearers so far as optical quality is concerned. They do both tend to use slightly over-sized secondary mirrors. I did a test report on a Skywatcher 12" over a decade ago for AS&T and couldn't fault the optics on that one. It wasn't a hand-picked one either. I picked the boxes myself at random off a palette-load of similar boxed tube assemblies. Many of the earlier GSOs had plate glass mirrors which is an inferior substrate to Pyrex which is a low-expansion glass. I have seen one GSO so far that had a mirror I'd describe as being poor -- but they're pretty rare so far as I can see. I have visited the Skywatcher factory in China (while I was on an eclipse tour in 2009) and thought it was a pretty professional factory set-up.

A hand-made, individually manufactured mirror is more likely to be a top performer than a mass-produced one, but it likely to cost somewhat more as well. The Chinese made Newtonians, despite some shortcomings (particularly in the mounts and the bearings) are generally very good value for money.

Doppler wrote "Shine a torch down the tube and have a look at the mirror from the rear, you will see any damage to the coating like scratches etc. The coating is very thin so some light will show through even on a good mirror, but a bad coating will be quite noticeable."

Great advice. A few pin-holes are pretty normal but if you see a lot of them, or that the whole surface is gently glowing (a thin coating) the mirror may need a re-coat.

Best,

L.
Les has given you some great advice. If you can find a Suchting mirror buy it, regardless of the coating. Just use the coating quality as a price bargaining point. Mark's mirrors are all exceptional and well worth getting re coated. The mass produced Taiwanese / Chinese mirrors from GSO and Synta are generally pretty decent. I have looked through well over 50 of these scopes over the years and I would assess them as follows. Of the 50 plus scopes I have looked through I have seen 2 ashtrays that I wouldn't waste my time looking through. About 1 out of 10 I would rate as ordinary, but it's going to have to do. About 6 out of 10 are pretty decent and happy to use them, 2 out of 10 are very good and 1 out of 10 are exceptionally good. I have a 10" F5 GSO dob that I have had for about 18 years that I leave at a friends rural property so I can use it when I go there without having to transport a scope. I also have a premium 10"/F5.3 SDM with Suching mirror, which is as good as it gets for a 10" scope. I got very lucky with the 10" GSO. It has an exceptional mirror for a mass produced scope. David Collis Bird bench tested it for me many years ago and his only comment was "I don't know how they can make a mirror this good for that price". Is it as good as my Suchting mirror ? Not quite, it's not quite as smooth so contrast under really good skies is slightly down, it has a touch more scatter and it is thicker and takes longer to cool. However, it takes good conditions with dark steady skies, good eyepieces and an experienced observer to pick a difference. The defining factor however is that 1 in 10 mass produced mirrors are this good and all of Mark's perform at the same level.

The other thing I should add is that generally what you observe as coating defects either by day, or under torchlight, don't have as noticeable an effect on optical performance under the stars as you would think they might have. My 14" Zambuto has plenty of pinholes in the coating and it still performs flawlessly. My 18" Obsession has essentially no coating left on the outside 1 cm of its entire circumference to go with its multitude of pinholes and it still performs fine. Its probably 5% to 8% or so down in its limiting magnitude threshhold and in its object brightness presentation, but because it deteriorates very slowly over time you don't notice it. In addition to that, laboratory tests have proven that the human eye cannot detect brightness changes <=5%

Cheers,
John B
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Old 31-10-2019, 11:42 PM
ausastronomer (John Bambury)
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Quote:
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Fancy be able to see the horse head with direct vision through your 25" I've never seen it in my eyepiece .

And as you said 410% more light then a 12" makes the magic happen

Regards
Bobby .
The Horsehead is a bit of an enigmatic target. The Horsehead image is created by a dark nebula (Barnard 33) sitting in front of an emission nebula which is a massive HII region (IC434) which actually illuminates it. HII regions emit a lot in the upper red end of the spectrum. Different people have different sensitivities to red light under dark adaption. Those whose vision doesn't extend as far into the red will have a lot more trouble observing the Horsehead than people who can see deep into the red. This was demonstrated at the IIS Astro Camp at Lostock in 2007. Andrew Murrell had his 25" Obsession set up beside my 18" Obsession and we were running sky tours with a group of about 15 to 20 people sharing the views in both scopes. Someone asked to see the Horsehead in my 18" Scope, so I put it in the FOV. It was unfiltered, but conditions were excellent and I had no trouble seeing it. Our friend Gary was helping us with the sky tours and went to the eyepiece for a look. He said to me, "I can't see anything". I was quite surprised at this as Gary is a very skilled observer and I thought he would see it easily. I called out to Andrew to put "the Horsehead in the 25 inch" which he did. I then went to the eyepiece to compare the view between the 2 scopes. Unfiltered in the 25" it stood out to me like the proverbial. Gary went to the eyepiece and couldn't see anything. I was perplexed at this knowing his observing skill and experience. About 6 months later I read about different human sensitivities to red light and immediately realised that this was the reason Gary couldn't see the Horsehead, but Andrew and I could. That particular night about 30% to 40% of the people around the scopes could not see the Horsehead.

Cheers,
John B
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Old 01-11-2019, 12:18 PM
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Les has given you some great advice. If you can find a Suchting mirror buy it, regardless of the coating. Just use the coating quality as a price bargaining point. Mark's mirrors are all exceptional and well worth getting re coated. The mass produced Taiwanese / Chinese mirrors from GSO and Synta are generally pretty decent. I have looked through well over 50 of these scopes over the years and I would assess them as follows. Of the 50 plus scopes I have looked through I have seen 2 ashtrays that I wouldn't waste my time looking through. About 1 out of 10 I would rate as ordinary, but it's going to have to do. About 6 out of 10 are pretty decent and happy to use them, 2 out of 10 are very good and 1 out of 10 are exceptionally good. I have a 10" F5 GSO dob that I have had for about 18 years that I leave at a friends rural property so I can use it when I go there without having to transport a scope. I also have a premium 10"/F5.3 SDM with Suching mirror, which is as good as it gets for a 10" scope. I got very lucky with the 10" GSO. It has an exceptional mirror for a mass produced scope. David Collis Bird bench tested it for me many years ago and his only comment was "I don't know how they can make a mirror this good for that price". Is it as good as my Suchting mirror ? Not quite, it's not quite as smooth so contrast under really good skies is slightly down, it has a touch more scatter and it is thicker and takes longer to cool. However, it takes good conditions with dark steady skies, good eyepieces and an experienced observer to pick a difference. The defining factor however is that 1 in 10 mass produced mirrors are this good and all of Mark's perform at the same level.


Cheers,
John B

Thanks John for your valuable input .

I already have a AOS 10" in an old truss dob that probably is a Suching mirror , it's performance was outstanding from what I remember many years ago .
I'll pull the mirror out & maybe sell it with the cell and other parts .
It deserves to see again !

Very interesting about the Horse head & individual red light sensitivities .
No wonder I could never find it back in the 70s with my 8" f/7

Have Fun
Bobby .
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