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Old 18-05-2015, 09:52 PM
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alocky (Andrew lockwood)
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Mirror substrates Pyrex vs exotics.

Hi all,
I'm doing some due diligence before buying another scope, and am trying to decide what benefit ultra low expansion substrates offer in practice. Ignoring considerations about stability during figuring, since that won't be my problem, at least, what sort of conditions 'require' the use of better substrates than pyrex? I'm looking at about 2800mm focal length, so I expect the main limiting factors will be thermal equilibrium and local seeing.
I'd welcome any informed opinion on the topic!
cheers,
Andrew.
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Old 18-05-2015, 10:12 PM
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Shiraz (Ray)
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http://www.rfroyce.com/pyrex.htm

http://www.skyandtelescope.com/astro...lescope-myths/

probably worth a read Andrew, if you haven't already come across them.

also note that the Riccardi Honders design is based on crown glass optics (eg BK7) and seems to be successful.

Last edited by Shiraz; 18-05-2015 at 10:33 PM.
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Old 19-05-2015, 06:36 AM
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I have heard that the closer the thermal expansion is to zero the easier it is to polish the mirror -
there is less cool down time between tests.


cheers
Allan
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Old 19-05-2015, 07:17 AM
glend (Glen)
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That Sky and Telescope - Four Myths article is a good read and debunks some of the fanatical rants we see on here from time to time. Pyrex vs BK7 is a good example, and of only real value in marginal improvements in cool-down times for the people making the mirrors - not significant to end users. Much of the noise about mirror substrate is nothing more than marketing efforts to differentiate a product to put a premium price on it. Also Pyrex is a brand itself and thus attracts a premium because of that. I have heard, but can't say it is proven, that Taiwan and Chinese made mirrors can not access Pyrex stock because of trade restrictions and /or price premiums put on by owners of thd Pyrex brand and thus favour BK7. Taiwan maker GSO turns out a very nice mirror for the price and sell these for scope making. Royce, Zambuto, etc are premium brands that differentiate themselves on design (Royce conicals for example) and/or substrate brand, figure and finish (Zambuto). You choice needs to take into account your end use, for example an imaging scope can perform great with a mirror the visual guys would shun - horses for courses.
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Old 19-05-2015, 10:25 AM
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Thanks Ray, I had seen those before. I guess more specifically, I'm considering a 16" f7 Dall-Kirkham for imaging, and was wondering at what sort of temperature gradients the additional thermal stability of the exotics might make the rather significant investment worthwhile. Perth has a fairly moderate climate, so I think I've answered my own question to some degree. Certainly when you look at the results from Damien Peach, Anthony Wesley etc, there's no mention of exotic substrates in their OTAs. The primary advantage would seem to be reducing the cooldown time, but a small airconditioner in the observatory will be significantly cheaper than zerodur!
interested to hear from anyone who's had the chance to compare the two side by side!
regards,
Andrew.

Quote:
Originally Posted by Shiraz View Post
http://www.rfroyce.com/pyrex.htm

http://www.skyandtelescope.com/astro...lescope-myths/

probably worth a read Andrew, if you haven't already come across them.

also note that the Riccardi Honders design is based on crown glass optics (eg BK7) and seems to be successful.
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Old 19-05-2015, 02:19 PM
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sn1987a (Barry)
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I've got a square kilometre of pyrex aching for some photons
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Old 19-05-2015, 02:49 PM
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Quote:
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I've got a square kilometre of pyrex aching for some photons
Ha ha - my Pyrex monster is feeling a bit deprived as well. Bloody weather!
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Old 19-05-2015, 04:23 PM
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Andrew,
assuming the substrate is (reasonably) stress free,
the impact on the resolving power (of a 16" DK) for a temperature differential between the primary mirror and the air will be dominated by thermal convection for any substrate that you are likely to encounter.
Focus stability is a separate issue that can be addressed through automation and the appropriate equipment.

I think it is more cost effective to address the issues associated with temperature gradients at a fundamental level. Remove them (through active cooling) rather than compensating for one (minor) symptom using exotic materials.

c
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Old 19-05-2015, 04:31 PM
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My RC has Zerodur mirrors made of "lithium aluminosilicate glass-ceramic", gaud knows what that is, but it sounds fantastic!!.

I focus once every 2 weeks or so which is convienient but not really a critical feature. I wouldnt mind having to focus every night, its not hard.
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Old 19-05-2015, 04:37 PM
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Thanks Clive - it may be confirmation bias, but that is precisely what I was thinking too. My current imaging system is far more prone to atmospheric distortion a than figure changes at f4.4, and for the sake of 5 minutes every hour the SGPro focus routine easily keeps ahead of it.
Still - there must be some rational arguments for getting the ceramic stuff, or they wouldn't sell it, right?
Cheers,
Andrew.

Quote:
Originally Posted by clive milne View Post
Andrew,
assuming the substrate is (reasonably) stress free,
the impact on the resolving power (of a 16" DK) for a temperature differential between the primary mirror and the air will be dominated by thermal convection for any substrate that you are likely to encounter.
Focus stability is a separate issue that can be addressed through automation and the appropriate equipment.

I think it is more cost effective to address the issues associated with temperature gradients at a fundamental level. Remove them (through active cooling) rather than compensating for one (minor) symptom using exotic materials.

c
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Old 19-05-2015, 04:39 PM
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alocky (Andrew lockwood)
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Quote:
Originally Posted by Bassnut View Post
My RC has Zerodur mirrors made of "lithium aluminosilicate glass-ceramic", gaud knows what that is, but it sounds fantastic!!.

I focus once every 2 weeks or so which is convienient but not really a critical feature. I wouldnt mind having to focus every night, its not hard.
Thanks Fred - I think economy will win out! The scopes look like there should be very little expansion anyway, so it will be interesting too see how they compare in practice. That does sound very cool though :-)
Cheers,
Andrew.
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Old 19-05-2015, 05:36 PM
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Having used telescopes with a number of different substrates, I'd have to agree that just because a mirror is made from ZeroDur or Astro-Sitall that is by definition going to be a good mirror. In fact, I had to return a very expensive Sitall mirror to Star Optics for re-figuring due to a turned edge.

But the elephant in the room that Royce and Seronik don't mention is focus and figure stability. Provided the substrate is homogeneous and isotropic, bulk temperature changes only cause a focal shift. Temperature gradients will however distort the figure, and are harder to control as the mirror size increases.

Optics with other than a zero CTE can and do shift their focus as the ambient temperature ramps up and down.
If you are a planetary or visual observer focus shift matters not. But if you intend on taking 30-minute subs over several hours ....pain and frustration come to mind.

The two instruments I've owned with Astro-Sitall optics showed not even the slightest focus shift, in fact on my 36cm RCOS, focus was stable for weeks at time.

Figure changes are far more problematical. A thermal gradient across a zero CTE mirror simply makes no difference to its figure. That cannot be said for most other materials.
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Old 19-05-2015, 05:49 PM
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I'm interested to understand why focus would shift over weeks but not with night/day temperature variation?
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Old 19-05-2015, 05:58 PM
glend (Glen)
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Andrew there is an endless discussion of this topic over on Cloudy Nights, if your interested:

http://www.cloudynights.com/topic/50.../#entry6589460
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Old 19-05-2015, 06:12 PM
clive milne
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Uses 0.5kW of power with a cooling capacity of 2.5kW.
Can be controlled remotely over the internet.
Approximately $800 + installation.
I'd spend the extra $1k for solar plus what ever it takes to insulate the observatory
then just set it to keep the scope below 17C
Problem solved.

http://www.samsung.com/au/air-condit...9FSSSBWK1.html
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Old 19-05-2015, 07:27 PM
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Quote:
Originally Posted by peter_4059 View Post
I'm interested to understand why focus would shift over weeks but not with night/day temperature variation?
In truth I could leave the focus for months, hence I suspect any variation was
my fixing what wasn't broken in the first place.

Re: Temperature compensating focusers...I tend to think of them in the same way as auto-guiding.

The more stable and accurate the mount is, the less work your guider has to do.
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Old 19-05-2015, 09:57 PM
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My CDK17 has pretty stable focus also. Its carbon fibre body seems to be the most important factor there. I do see the scope won't achieve sharp focus until temp difference between ambient and mirror is 1C or less.

I also notice the AP RHA with its crown glass mirror and vented primary with fans holds focus really well also despite temp shifts. There is some but its pretty minor.

I have not noticed the CDK17 to shift focus with temperature changes.

Imaging at long focal length shows up the bad seeing better is probably the correct statement. It also makes achieving sharp focus very hard as the focus images with no change to focus will appear in focus at one instant and out of focus the next just from the seeing when its bad.

Greg.
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Old 20-05-2015, 06:15 AM
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marc4darkskies (Marcus)
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Quote:
Originally Posted by Peter Ward View Post
In truth I could leave the focus for months, hence I suspect any variation was
my fixing what wasn't broken in the first place.... .
I confess that I was also in some disbelief that the combination of sitall and carbon fibre meant I didn't have to refocus. Once I focus in good seeing now I just leave it alone. With parfocal filters, refocusing is essentially uneccessary. I do let the scope and observatory equilibrate for 30 mins or so before imaging but my mirrors rarely get to within 2C of ambient . I do tend to do a focus run occasionally but all that does is confirm that I didn't have to ... even after large ambient changes! My imaging system is F8.7 too so the large CFZ will be contributing to this stability as well.

Cheers, Marcus
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Old 20-05-2015, 08:23 PM
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Thanks guys - some really good discussion here! I'm probably going to go with pyrex, and use the cost difference to put active cooling in the observatory as per Clive's suggestion. I've settled on this OTA as a compromise between deep sky and planetary, and will be trying to do both from within the light pollution of Perth.
The OTA is a mixture of CF and aluminium, so I will definitely report back on focus stability. However, since the first floor of the house is still being built, and the observatory is on the second floor, it will be a while before anything actually happens. Don't watch this space for at least 9 months!
cheers,
Andrew.
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Old 20-05-2015, 10:59 PM
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Steffen
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Interesting, I had always assumed that the mirror warping in all kinds of ways as it cools down affects its figure and hence its performance. Is the consensus now that the boundary layer has a much worse effect?

If so then I suppose the ideal mirror material would polish up very smoothly and have a low thermal capacity, so as to not store much heat.

Which of the common substrates gives the smoothest surface given comparable effort?

On that topic, the back of my watch is made from zirconia and allegedly polished to a roughness of 4nm – how does that compare to telescope mirrors?

Cheers
Steffen.
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