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Optical quality in mirrors of 16 "F4.5 of GSO

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#1 pastorgalactico

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Posted 22 November 2017 - 03:47 PM

Hello;
I'm building a 16 "Dobson and it's time to look for the 16" F4.5 optics and I do not want to ruin myself by buying very good optics that are worth a lot of money and more than I can afford.
That is why I ask those who have the optics of 16 "F4.5 GSO.
   How is the optical quality of these GSO mirrors?
Greetings and I hope you help me choose well.

P1040525.jpg

P1040522.jpg

J.Tapioles


Edited by pastorgalactico, 22 November 2017 - 05:17 PM.

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#2 Codbear

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Posted 22 November 2017 - 05:47 PM

While I have heard that the overall quality of GSO mirrors are improving, the quality and consistency still isn't nearly what you would get with any given Zambuto, Lightholder, Lockwood, etc. mirror...for a lot more money of course as you alluded to.

 

However, I can tell that the gems are definitely out there. My 16" F4.5 Teeter Dob has a GSO primary and Rob Teeter told me that, incredibly enough, the mirror had been mistaken for a Zambuto by experienced amateurs (I bought the dob used from Rob - it was used for outreach).

 

Rob is a man of his word...the views are absolutely stunning. Since I also have a Teeter 11" Zambuto dob, I can also attest that the 16" GSO is just a smidgen off the Zambuto. 

 

It's a really sharp-looking Dob you're building and would certainly be deserved of high quality optics. Perhaps if you could hold off on first light (as hard as that would be!bawling.gif ) until you've saved the funds, you could pull it off.

 

Good luck!



#3 pastorgalactico

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Posted 22 November 2017 - 05:59 PM

Hello;
Thanks for answering.
I know that the GSO mirrors are coming out good lately, since those mirrors are mounted by both GSO and Meade telescopes and some more.
Currently I can choose to buy them in Spain Brand kepler / GSO or buy it at https://www.teleskop.../shop/index.php and have two GSO mirrors analyzed and the analyzer tell me which of the two It is better for an extra price of € 220.
But if they say more fans here than the standard quality is from good to very good I would not doubt it that much anymore.
If I had more than enough pasta I would go to a superior one of zambuto jejjeej and whoever doubts it, but that's just a dream.
Greetings and I will wait for more amateur tests that have optic of 16 "F4.5 of GSO.
J.Tapioles



#4 sixela

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Posted 22 November 2017 - 06:14 PM

I’ve seen some bad Lightbridges, so I don’t subscribe to your “since”. Personally, I think even now the mirrors are quite rough, and the figure is sometimes not that good (especially if the blank’s stresses interact badly with figuring —the really ugly ducklings have astigmatism, esp. in the larger sizes).

If the Pyrex-like borosilicate and fused silica mirrors that Teleskop Service also sells also come from GSO (thry do come in very similar sizes and thicknesses), then these do tend to be better (probably because the blanks are better than the normal BK7 ones and polish to a better finish), plus Teleskop Service offers to have them tested by Wolfgang Rohr for a small fee. Even if they’re not GSOs, they’re worth looking into.

Edited by sixela, 22 November 2017 - 06:17 PM.


#5 sixela

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Posted 22 November 2017 - 06:16 PM

BTW, pastorgalactico, are you in Europe or in the US?

#6 pastorgalactico

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Posted 22 November 2017 - 06:26 PM

Hello Hubble;

I am in Europe - Spain and especially what I am interested in knowing is how the GSO optics are really seen by the fans that use them and I do not want this to be treated as knowing which ones are better if the GSO or the zambuto or some handmade mirrors and I suppose that the zambuto and many of the handicrafts would win.
I do not want to pay a fortune for a zambuto, but I have offered handicrafts both in Spain and in England at € 2100 that would be about 700 € more than the GSO and I do not know if that would be worth it and that difference would be seen in the optical aspect and visual of the optics.
I am using the google translator so I apologize if any word is not understood as I would like.
Regards;



#7 sixela

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Posted 23 November 2017 - 02:45 AM

In Europe, you should give John Nichol a look; the last three 16” mirrors I’ve seen from him were a lot better than typical GSOs (though the sample size I have is limited). If you want my opinion about Oldham Optical or Orion Optics: not good (I can point you to some personal anecdotal evidence for both).

If you have more money, also look at nauris.de .

If you are very rich, there’s Mirro-sphere in France.

In any case, if you’re worried about a GSO’s quality, ask Teleskop Service how much it would be to get it tested independently. Also ask if the “Pyrex” (that’s a misnomer, since Pyrex is a Corning trademark) or fused silica TS Optics mirrors are available in 16” format (the only ones on the site are the conical ones for the moment, but those require different cells) and what it would cost to test these.

Edited by sixela, 23 November 2017 - 02:47 AM.

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#8 CHASLX200

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Posted 23 November 2017 - 06:28 AM

It's a crap shoot. You could luck out and get a winner or could get a bomb.  Pay more from a top mirror maker and your chances are much better.



#9 pastorgalactico

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Posted 23 November 2017 - 07:01 AM

Hello;
Hubble the way you tell me things it seems you're an optics salesman or optics tester, is that true? or I just get confused.
I know Nikol optis for a friend from Madrid who has a mirror of his and I also met Joan Lopez Vila for experience with my 12 "that is his.
But the problem is that either of those two artisans go to 2100 € or something else.
Regarding the other manufacturers of mirrors I do not know them although I suppose that their prices will already be close to those prices.
The GSO mirrors of TS are from BK7 and the TS mirrors are in pyrex but they are cones and those mirrors do not get along with my cell.
I know that TS can send the mirrors that I buy to analyze and I have these reports of GSO mirrors;
http://r2.astro-fore...gel-16-inch-im- vergleich
I think I would have to pay an extra € 220 to choose 1 among 2 GSO mirrors that would analyze them.
It was said by TS directly to me that in 2017 of 70 GSO mirrors sold only one returned or they returned it, because I asked it.
Hubble that experiences you have with GSO mirrors of 16 "F4.5.
Have you tried them on any observation?
Have you analyzed them meticulously?
You tell me why you heard it?
I would like more details if possible.
As I have not yet decided I have time to investigate qualities and prices.

Greetings and thanks for the information.

J.Tapioles



#10 pastorgalactico

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Posted 23 November 2017 - 07:08 AM

Hello CHASLX200;
For what you comment it seems that you have had some GSO mirror.
  if it is not asin and you have observed by some telescope that it had optical GSO of 16 "F 4.5 and that the telescope had some other cause like its collimation etc I would say that this opinion can not be considered valid since I would prefer that those reports were of someone who has in his possession a telescope of those characteristics and then already the opinion of that amateur would be more accurate.
You can explain to me in more detail your opinion about that 16 "F 4.5 optics
It would be a pleasure for you to explain it to me.
Sincerely;

J.Tapioles



#11 bvillebob

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Posted 23 November 2017 - 09:35 AM

There's sure been a lot of discussion about these lately.

 

All I can offer is a sample of one, my Meade 16" f/4.5 was about 8 years old, so it's not current production.  It worked fine for DSO but poor for planetary.  A lot of the problem for me was trying to cool it, as well as optical issues.

 

I eventually replaced it with a Hubble Optics sandwich style mirror, which is far better optically and much easier to cool.  I purchased from them off Ebay, the price was very little more than the GSO mirror and it's much better.

 

Also remember a lot of people have seen problems with the larger Meade secondary mirrors, it's not just the primary you need to be concerned about.


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#12 pastorgalactico

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Posted 23 November 2017 - 09:59 AM

Hello;
The problem of acclimatization would not be much problem since the telescope will always be in the observatory and will keep more or less the same ambient temperature and also the tube already has a built-in fan in the 120 mm cell and also 2 fans in the walls of the tube to sweep the boundary layer of the primary.
I can go to the hubbay optis ebay website where mirrors come out, why I visited it and no primary mirrors come out.
My telescope I want it to perform well in both deep and planetary sky and that is why I have minimized the secondary to 80 mm of minor axis.
I know that the secondary mirror has to be in tune with the quality of the primary.

regards



#13 txmnjim

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Posted 23 November 2017 - 10:35 AM

just get what you can afford now and have it re-figured later

if it doesn't prove optimal. good luck!



#14 Pinbout

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Posted 23 November 2017 - 11:05 AM

just get what you can afford now and have it re-figured later

if it doesn't prove optimal. good luck!

 

With the bk7- like material, not always that easy. That substrate can jump around like crazy while figuring and testing, thats why some won't touch it.


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#15 nirvanix

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Posted 23 November 2017 - 11:11 AM

If you want good planetary viewing you need good optics. No getting around that. Save up your euros and get something good. Not to say GSOs aren't good  - I've got one and it's very good. The problem is you're only buying 1 mirror so it's got to be the right one and there's no point playing the averages.



#16 pastorgalactico

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Posted 23 November 2017 - 11:29 AM

Hello;
I do not understand what you said; (Not to say that the OAB are not good, I have one and it's very good.)
Did you mean that you have a GSO and it is good?
I know I can only buy one but I can have the option to analyze 2 mirrors of 16 "F4.5 and stay with the best of them at least that option I offer the German store TS.
I also know that the BK7 glass is difficult to repulse again and also the collar would be more expensive than the greyhound.
Greetings.



#17 Pinbout

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Posted 23 November 2017 - 03:11 PM

 

Did you mean that you have a GSO and it is good?

get the gso primary and get a antares 2ndry

 

what he's saying is he's lucky he has a good one.

 

the difference between good and great is the ability to push the mag when the seeing is good. when the seeing is not so good a lot,..no one gets to do high magnification.

 

but a lot of people blame seeing, when really its their optics. so you need to be able to test the optics to know the difference.



#18 sixela

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Posted 23 November 2017 - 04:59 PM

GSOs are certainly usually usable, unless you have bad luck and get one with lots of astigmatism (and when I say "lots", the worst one I saw had 1 wave of 'stig on the wavefront with images already severely degraded at 150x). Sadly your chance of getting a comparative lemon are larger in the larger sizes. So getting the mirror tested is a _really_ good idea,  since at least you can reject anything that doesn't meet your minimum criteria, given the consumer protection laws in the EU (you might have to pay again for the testing if you decide to try a second mirror, though).

 

Whether you think they are "good" depends on your definition of "good". They're certainly usually not as good as my own 400mm mirror currently is (it started its life as an Oldham Optical which was decidedly "meh", with a colossal bulge on the centre and quite some astigmatism, but not enough for it to be fatally flawed, but is now refigured) and not as good as the three last Nichol mirrors I saw.

 

The figure can be "OK" but the RMS error of small scale roughness is frequently something which I'd guess is around 5nm on the surface (and was measured by some French -- you've been on that forum as well -- as 5-6nm as well); it's not unusable but even on a Foucault-tester you can clearly see the mirror's rough, with very characteristic striped patterns due to rapid parabolisation. You can see an example that _I_ think is a GSO (it's only opinion, or course, but there's some anecdotal experience behind it since the roughness pattern and the stress figure in the blank are _very_ recognizable) here:

http://www.loptics.c...hop/shop40.html

 

That's also a mirror with a fairly OK figure but fairly rough (and with strain in the blank that makes it harder to guarantee it can be refigured easily).

 

So there's your example: if you think that's a "good" mirror then you should go for it -- that's what you can expect if you don't get a lemon. It will also pass the independent test quite well, since the figure's fairly OK.

 

I have an issue with some of the French owners who gave Teleskop Service a hard time about that roughness (it should not matter too much for planetary imaging, which was what the user said was "impossible"; my guess is that it's the GSO mirror thickness that are causing that user a headache), but they're certainly less smooth than most Synta mirrors I see.


Edited by sixela, 23 November 2017 - 05:25 PM.


#19 turtle86

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Posted 23 November 2017 - 05:19 PM

It's a crap shoot. You could luck out and get a winner or could get a bomb.  Pay more from a top mirror maker and your chances are much better.

 

I agree, especially with the larger mirrors.  If it were me, I’d save until I could afford a premium mirror, especially if you want to observe planets at high mag when the seeing is good.  



#20 CHASLX200

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Posted 23 November 2017 - 05:44 PM

 

It's a crap shoot. You could luck out and get a winner or could get a bomb.  Pay more from a top mirror maker and your chances are much better.

 

I agree, especially with the larger mirrors.  If it were me, I’d save until I could afford a premium mirror, especially if you want to observe planets at high mag when the seeing is good.  

 

only way to fly.



#21 pastorgalactico

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Posted 23 November 2017 - 05:50 PM

Hello;
Good explanation.
Well, reading everything I have said so far and what I have been told in other forums, what I'm going to do is try to wait a little longer to buy a decent mirror and if it can not be that I'll at least pay a bit more in TS for that I choose the best mirror between 2 GSO and at least I'll have an assurance that at least the mirror is not bad and that means 220 € more.
If I save a little more, maybe I'll go to Joan Lopez Vila or Nikol Optis, which would be ideal.

Greetings and many thanks.

J.Tapioles



#22 Starman1

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Posted 23 November 2017 - 05:52 PM

Hello;
I'm building a 16 "Dobson and it's time to look for the 16" F4.5 optics and I do not want to ruin myself by buying very good optics that are worth a lot of money and more than I can afford.
That is why I ask those who have the optics of 16 "F4.5 GSO.
   How is the optical quality of these GSO mirrors?
Greetings and I hope you help me choose well.

P1040525.jpg

P1040522.jpg

J.Tapioles

Most I've tested have been average--not horrible, just not great.

1 Meade 16" (GSO) was the closest to perfect I've seen in a production scope. Simply stunning and no errors of any size in the intra- and extra-focal star images.

It was easily better than 1/8 wave on the wavefront.

That's NOT usual.

The mirrors are thick, slow to cool, and made with lower-end BK glass.

 

So:

--ask if you can return a mirror that isn't good enough for you to keep.

--design the scope so you have boundary layer fan(s) and fans behind the mirror.  Don't test until two hours outside with fans running.

Even then, if you see thermals in the images, wait to test later.

--the scope in the picture, with one small fan, will not be adequate for cooling.


Edited by Starman1, 23 November 2017 - 05:53 PM.


#23 CHASLX200

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Posted 23 November 2017 - 06:12 PM

 

Hello;
I'm building a 16 "Dobson and it's time to look for the 16" F4.5 optics and I do not want to ruin myself by buying very good optics that are worth a lot of money and more than I can afford.
That is why I ask those who have the optics of 16 "F4.5 GSO.
   How is the optical quality of these GSO mirrors?
Greetings and I hope you help me choose well.

P1040525.jpg

P1040522.jpg

J.Tapioles

Most I've tested have been average--not horrible, just not great.

1 Meade 16" (GSO) was the closest to perfect I've seen in a production scope. Simply stunning and no errors of any size in the intra- and extra-focal star images.

It was easily better than 1/8 wave on the wavefront.

That's NOT usual.

The mirrors are thick, slow to cool, and made with lower-end BK glass.

 

So:

--ask if you can return a mirror that isn't good enough for you to keep.

--design the scope so you have boundary layer fan(s) and fans behind the mirror.  Don't test until two hours outside with fans running.

Even then, if you see thermals in the images, wait to test later.

--the scope in the picture, with one small fan, will not be adequate for cooling.

 

Being closed off in the back is not good...



#24 pastorgalactico

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Posted 23 November 2017 - 06:36 PM

Hello;
Thank you starman1 for all the information that you have passed me.
I have also observed by a meade 16 "F4.5 and it was quite good and comparing it with another 16" F4.5 side that mounted a much more expensive optical pegasus and that theoretically would have to have more optical quality.
To which CHASLX200 comments that being a closed tube would harm its performance or that it would be good ...
I am observing inside a 3.3m dome and having the tube closed I will avoid the internal turbulence of the dome that I could not control without the optics inside a tube.
I usually observe with the back fan working and producing a laminar updraft inside the tube sweeping all the turbulences.
This I have very verified with my current 12 "dobson also in tube.

Greetings.



#25 sixela

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Posted 24 November 2017 - 03:35 AM

Being closed off in the back is not good...

 

 

Did you miss the huge fan?

 

If you have a fan being closed off in the back is GOOD.


Edited by sixela, 24 November 2017 - 03:36 AM.

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