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18-05-2015, 10:52 PM
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PI popular people's front
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Join Date: Aug 2010
Location: perth australia
Posts: 1,291
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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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19-05-2015, 07:36 AM
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Registered User
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Join Date: Jan 2012
Location: Melbourne
Posts: 3,689
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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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19-05-2015, 08:17 AM
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Registered User
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Join Date: Jun 2013
Location: Lake Macquarie
Posts: 7,096
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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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19-05-2015, 11:25 AM
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PI popular people's front
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Join Date: Aug 2010
Location: perth australia
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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
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19-05-2015, 03:19 PM
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Registered User
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Join Date: Dec 2007
Location: Rockingham WA Australia
Posts: 725
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I've got a square kilometre of pyrex aching for some photons
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19-05-2015, 03:49 PM
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PI popular people's front
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Join Date: Aug 2010
Location: perth australia
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Quote:
Originally Posted by sn1987a
I've got a square kilometre of pyrex aching for some photons
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Ha ha - my Pyrex monster is feeling a bit deprived as well. Bloody weather!
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19-05-2015, 05:23 PM
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Registered User
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Join Date: May 2011
Location: Freo WA
Posts: 1,443
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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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19-05-2015, 05:31 PM
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Narrowfield rules!
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Join Date: Nov 2006
Location: Torquay
Posts: 5,065
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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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19-05-2015, 05:37 PM
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PI popular people's front
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Join Date: Aug 2010
Location: perth australia
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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
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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19-05-2015, 05:39 PM
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PI popular people's front
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Join Date: Aug 2010
Location: perth australia
Posts: 1,291
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Quote:
Originally Posted by Bassnut
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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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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19-05-2015, 06:36 PM
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Galaxy hitchhiking guide
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Join Date: Dec 2007
Location: The Shire
Posts: 8,276
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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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19-05-2015, 06:49 PM
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Big Scopes are Cool
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Join Date: Jun 2007
Location: SE Tasmania
Posts: 4,558
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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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19-05-2015, 07:12 PM
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Registered User
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Join Date: May 2011
Location: Freo WA
Posts: 1,443
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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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19-05-2015, 08:27 PM
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Galaxy hitchhiking guide
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Join Date: Dec 2007
Location: The Shire
Posts: 8,276
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Quote:
Originally Posted by peter_4059
I'm interested to understand why focus would shift over weeks but not with night/day temperature variation?
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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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19-05-2015, 10:57 PM
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Registered User
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Join Date: Feb 2006
Location: Sydney
Posts: 17,998
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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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20-05-2015, 07:15 AM
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Billions and Billions ...
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Join Date: Mar 2007
Location: Quialigo, NSW
Posts: 3,142
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Quote:
Originally Posted by Peter Ward
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.... .
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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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20-05-2015, 09:23 PM
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PI popular people's front
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Join Date: Aug 2010
Location: perth australia
Posts: 1,291
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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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20-05-2015, 11:59 PM
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Ebotec Alpeht Sicamb
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Join Date: Feb 2010
Location: Toongabbie, NSW
Posts: 1,968
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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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