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Premium mirrors, glass material and thermal effects.

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#1 Max T

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Posted 16 May 2015 - 07:49 AM

Hello,

Imagine owning a mass market reflector with a 8” f6 mirror of 1” thickness or a 10" f5.     The mirror is made from BK7 glass which thermally expands more than Pyrex, Suprax or Quartz materials.

Also assume that viewing is from indoors 18C taken outside to around 4C.    A big 14C or 25F drop in temperature for that mirror.     To thin the subsequent boundary layer as much as possible and have good views within 20 minutes (I can only observe for 2 hours max), a boundary fan blows a strong draft of the cold air across the mirror.     The mirror is for visual use and to enjoy everything: doubles, planets and DSO.

In this situation described, if the mirror (mine is undercorrected) was refigured up to premium quality 1/8pv wave and the spherical aberration corrected, then:

1. Would the mirror be noticeably overcorrected during the first hour as to deteriorate views, due to the sides cool faster and shrink more than the warmer core’s 18C thermal bulge?

2. Could the mirror become astigmatic ‘potato chipped’ by the strong side fan blowing cold air over the warm mirror?
(I don’t know whether this mirror actually turns astigmatic or not by the boundary fan, because my eyes are astigmatic and need new glasses!).

The reason for asking is because there would be little to gain in having the BK7 mirror refigured if, by taking it from a warm indoors, a refigured 1/8pv wave is always thermally distorted by either overcorrection or astigmatism or both down to 1/6 wave error or worse for the first half of every single viewing session.
That would seem a waste of money spent on what should be premium optics.

Instead, if the two errors are noticeable in this situation, then surely it would be better to buy a new mirror made from a lower thermal expansion material like Suprax or even Quartz (or previously Pyrex).

Or do you think these fears about thermal overcorrection and cooling astigmatism are exaggerated and irrelevant for such a relatively thin and small mirror (1” thick and 8” diameter), even if it is made from low cost Chinese BK7 glass.   And that very little distortion would be seen and it would only apply to bigger mirrors if they were taken from such a warm indoors which they’re unlikely to be?

I would really appreciate your thoughts on this.
Thank you.


Edited by Max T, 16 May 2015 - 08:22 AM.


#2 Mike Lockwood

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Posted 16 May 2015 - 09:06 AM

Based on my experience, the anneal of the BK7 glass is far more of a concern than any thermal expansion issues.  Quality of anneal determines how stable the shape of the mirror is, and strain/poor anneal itself can cause thermally-induced astigmatism.

 

From certain manufacturers, anneal/strain is a matter of the luck of the draw, and is the main criteria that should be used to decide if money should be spent refiguring it.  BK7 is also softer, like plate glass, and it is trickier to make the optical surface smooth.

 

Neither anneal nor softness is an issue with a good-quality borosilicate or quartz blank.



#3 pstarr

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Posted 16 May 2015 - 09:22 AM

You would definitely see some overcorrection in your mirror while cooling and possibly some astigmatism. Instead of taking the scope out in extremes, why not store it where the temperature differential would not be so great. I found the least distortion and the best setup is a single fan blowing on the center of the mirror. The center of the mirror is the hot spot. If you cool the sides, you will see more overcorrection for a longer time. I never had any luck with boundary layer fans. They didn't add anything to having a fan in the back mounted on Velcro and using a speed control. My 12.5" mirror is made of Supremax and is 1.5" thick. I see overcorrection while it cools. Even so, it still gives very good resolution while showing the overcorrection. It is a premium mirror. A mirror of lesser quality would show more figure errors while cooling since there would be more error to began with. The same would be true in less than perfect seeing. 1/4 wave of bad seeing would have more effect on a 1/4 wave mirror than a 1/10th wave mirror since the errors add up. Even with a quartz mirror you would still need a fan to deal with the boundary layer distortion but the figure would be very stable. An 8"x1" thick mirror shouldn't take very long to cool in any case. I would make sure it's properly mounted. You could get some distortion from the cell if it's not well designed.

 

IMG_1334-1.jpg

IMG_1335-1.jpg


Edited by pstarr, 16 May 2015 - 05:17 PM.


#4 Starman1

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Posted 16 May 2015 - 01:52 PM

Mike's answer is the most appropriate, since annealing issues can cause figure-related issues during cooling and during the night as the air cools.

Not to mention it's a nightmare for the mirror maker to deal with annealing issues during polishing.

All glass materials have thermal issues while cooling, even if the material doesn't change shape (like quartz or zerodur).

The problem caused by the boundary layer is of a larger magnitude than shape change due to differential cooling.

And once the boundary layer is gone, the mirror is probably cool enough that shape change due to temperature change is over.

Your requirements for nearly instant cooling on a mirror of that size is incompatible with reality.

So you have a couple choices:

1) store the mirror part of the scope in a place with a lower temperature differential so the fans can cool the mirror faster.

(you should use both rear and boundary layer fans)

2) use a thin mirror and place the scope outside at sunset with the fans on.  90 minutes later (or longer at high latitudes), when it is dark, you will start out the night

with the mirror at the ambient temperature.

 

I chose #2 with a 12.5" mirror 1.25" thick.  With 3 large fans blowing on the mirror for at least 2 hours (in my case) before I start observing, 

I start my session with the mirror at ambient and without a boundary layer, verified by an infrared thermometer.

 

Actually, there is a 3rd choice:

--schedule your observing so the first things you observe do not require the maximum resolution, i.e. galaxies, nebulae.

then, slowly move into star clusters and planetary nebulae as the scope cools, and end your observing session a couple hours later with double stars, planets, and Moon.



#5 Max T

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Posted 16 May 2015 - 03:32 PM

(Just got over gushing about Mike L and Don P replying- you two don’t know how much I’ve learnt from you… and you Paul as well).  
 
Reading your replies carefully, it seems I underweighted the importance of a few factors.     Annealing/ strain and its astigmatism could be a more important factor in viewing quality from that BK7 glass than overcorrection.
And the boundary layer thermal is also more of a problem than mirror figure changes.   So you've put things into better perspective.
 
Then there are solutions to these 4 problems you mention: store it somewhere cooler, put the fan in the middle, thinner mirror, put it outside to cool beforehand etc.
 
 
Which option would you go for if the scope has to be stored indoors 18C down to 4C outside, you only have 1-2 hours viewing and cannot afford more than $550 and in the UK:
1. Buy this 8” quartz mirror 1/6 pv wave (made by GSO and tested and selected using interferometer by Wolfgang Rohr (a German independent tester):
http://www.teleskop-...ertificate.html
Barely any figure change when cooling and no annealing issues, but the figure and smoothness aren’t top notch (yet it could be refigured in the future).
Or
 
2. Have the existing 8” BK7 mirror refigured to 1/8 pv wave by a premium mirror maker, assuming they will first check that it has no significant strain/ is well annealed (it could be say Nichol Optical http://www.nicholoptical.co.uk/
Might go a little astigmatised and overcorrected initially, but then the views should be better.
Thank you.

Edited by Max T, 16 May 2015 - 03:47 PM.


#6 pstarr

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Posted 16 May 2015 - 04:05 PM

Max, read this thread. It may interest you.  http://www.cloudynig...into-the-house/



#7 JimMo

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Posted 16 May 2015 - 04:09 PM

Well, I agree Mike, Paul and Don have hit the nail on the head.  I don't know where you live but I assume there is no air conditioning where you store the scope?  That's a tough one.  I don't know of any active coolers for a primary mirror /  Maybe a big bowl if ice with a room fan blowing it towards the primary?  Just thinking outside the box.  Maybe just get the mirror tested if you can and have the annealing checked.  If the anneal is not up to par get it sent back, no harm, no foul.  My two cents..... 



#8 Starman1

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Posted 16 May 2015 - 04:23 PM

Even if the mirror is quartz or Zerodur, it will still be affected by thermal issues causing a boundary layer to form in front of the mirror.

If the fans blow the boundary layer away and the mirror isn't changing shape, then a zero-coefficient of expansion material would be useful nearly immediately.

Alas, for highest-power, high-resolution viewing, the vibration of the fans may be visible, as may the air movement in front of the mirror.

So you still have the issue of cooling to contend with, even with quartz.

 

Ergo, the simplest route would be to have your mirror checked to see about the anneal (and you'd have to strip the mirror and recoat it to test it like that) and to refigure it if it needs it.

But if the optical test shows the mirror is OK, then the 3 issues at hand to solve are:

--collimation.  Make sure you have the right tools and do it well.  

--cooling.  Make sure you have fans and try to let it sit outside for a while to get closer to ambient before you start observing.

--conditions.  Observe often and you will eventually run into better seeing conditions.  At most sites, seeing is best from midnight until dawn, which may prove difficult for you.

And always maximize your possibilities by never observing below 30 degrees (too much "soup" of air below 30 degrees) or directly above a rooftop (heat from the roof all night).

For high power, a windless hazy night may prove a lot better than a superlatively clear one with a breeze aloft.

Read:

http://www.cloudynig...observing-r1396

http://www.cloudynig...udy-night-r1413

http://www.cloudynig...observing-r1436



#9 Brian Carter

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Posted 16 May 2015 - 04:41 PM

If I have a stretch of good weather, I will keep the dob outside and a nice greenhouse effect keeps the mirror warm. I get home from work and turn on the fans. If I want to hurry it, I will put an ice pack behind one of the fans, that usually helps.

My mirror is quite over corrected when I first set it up, enough that I was dismayed the first time I looked through it. It's an old mirror, 1990, and just under 2" thick. Consequently, cooling is a major issue and even after a few hours I can see a bit of correction issues. The fans stay running, info turn them off the boundary layer stays for hours. Eventually, the mirror cools and shows itself to be very nice. Takes a while though.

Of the two issues with cooling on my mirror, that boundary layer is much worse than any change in figure. Whatever astigmatism a fan may cause for a little while, it is dwarfed by the benefit I get from removing that layer of air. I think that whatever plans you have for your scope, that should be the #1 modification.

As far as how to spend your money. I suppose you could. Get your mirror tested and maybe re figured. Depends on how bad your mirror is, you didn't really say much about that. How do you know your current mirror is bad? Keep in mind that the difference between a decent mirror and an awesome mirror is subtle, the money spent on a premium optic might benefit you more on modifications and accessories that made a bigger difference. For example, good proper air circulation and mirror cell, a dual speed focuser, and improved rocker box, or a good observing chair.

I only speak from experience. I have replaced not-awful mirrors with custom ones and concluded that the money would have better be spent in other ways. Not saying that Mike's mirrors aren't awesome, but they are best used in a scope that already does the other things right. And if you stick a warm lockwood mirror in a scope on a cold night without fans, it's probably go My to give ugly views too.

#10 Mike Lockwood

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Posted 16 May 2015 - 05:19 PM

My estimate of the typical/approximate heirarchy of bad things goes like this, from largest possible effect to the smaller.  The amounts are based on what I have seen in telescopes and in my shop, but there are always examples that are outside of the typical amounts.

 

1)  Pinched optics, very warm optics, ~1 - 2 waves

2)  Moderate boudary layer/warm optics, bad edge support, ~0.5 - 1.0 waves

3)  Gross optical errors / bad strain effects / poor edge support, ~0.5 waves

4)  Moderate optical errors / mild strain effects, ~0.25 - 0.5 waves

5)  Small optical errors / small strain effects / minor cell issues, 0.1 - 0.25 waves

 

All of these issues are fixable without destroying the mirror EXCEPT for mild and severe strain effects.  Strain can cause the mirror to change shape over time, and it can cause the optical work to be frustrating.  To fix it requires re-annealing, which then means the mirror must be re-ground, polished, and figured.


Edited by Mike Lockwood, 16 May 2015 - 08:35 PM.


#11 pstarr

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Posted 16 May 2015 - 05:25 PM

 

 

Ergo, the simplest route would be to have your mirror checked to see about the anneal (and you'd have to strip the mirror and recoat it to test it like that) and to refigure it if it needs it.

But if the optical test shows the mirror is OK, then the 3 issues at hand to solve are:

--collimation.  Make sure you have the right tools and do it well.  

--cooling.  Make sure you have fans and try to let it sit outside for a while to get closer to ambient before you start observing.

--conditions.  Observe often and you will eventually run into better seeing conditions.  At most sites, seeing is best from midnight until dawn, which may prove difficult for you.

Agreed, even though BK7 isn't the best material, there sure seems to be allot of it out there now that people are happy with. It seems to be the material of choice coming out of China. If these are working well in so many scopes, anneal must be acceptable. Of course each example is different.



#12 Pinbout

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Posted 16 May 2015 - 09:25 PM

 

The mirror is made from BK7 glass which thermally expands more than Pyrex, Suprax or Quartz materials.

 

Imported mirrors are not BK7, they are faker BK7, BK7 like.

 

meaning they probably use the same content but not the same annealing process.

 

the material to make BK7 is relatively cheap compared to the cost of processing it correctly, so they can pay for the licensing fee, to actually say they're using real BK7.


Edited by Pinbout, 17 May 2015 - 06:49 PM.


#13 eyepiecedropper

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Posted 17 May 2015 - 12:43 AM

Max, it seems you´re from Europe. You have other options than TS.

 

https://www.fernrohr...04b68be5f2fe6df

 

It´s a GSO glass ceramic mirror. Zero Expansion.

 

BTW: I do offer a used one incl. ronchigram from TS for 100,- Bucks + shipping. Coatings o.k.

 

also interesting:

https://www.astrosel...io.php?Id=50578

 

Zen is premium, and will fit in your GSO cell. Have the very one in my GSO.

 

 



#14 Max T

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Posted 17 May 2015 - 07:06 AM

Max, read this thread. It may interest you. http://www.cloudynig...into-the-house/

Thanks Paul. Yes a 8” quartz mirror at ¾” thickness on a 9 point cell sounds gorgeous.

My Skywatcher has a 3 point cell which seems ok based on Plop software, but the mirror would need to be full thickness 1” and this adds to the boundary layer.

 

Alas, for highest-power, high-resolution viewing, the vibration of the fans may be visible

Fan vibrations have been a struggle.
It seems to help if long steel bolts are taped to the fan outer body, as this serves to increase mass and inertia resistance to vibration.
Also decoupling the reduced vibrations by inserting the fan into some 10mm foam that holds it very loosely (so the fan can wiggle easily) and connecting the foam to the OTA with two layers of Velcro hooks and loops does well.     There’s no elongated airy disc at x240 at least.
 

--collimation. Make sure you have the right tools and do it well.
--cooling. Make sure you have fans and try to let it sit outside for a while to get closer to ambient before you start observing.
--conditions. Observe often…

It’s thanks to you Don saying the above that my f6 scope is autocollimated using a little hand mirror to stack the vanished triangles and has lots of night air blown into the tube.

On better nights, all 4 Jupiter’s moons appear as crisp little discs, thin blue festoons arch across and a little darker area in the GRS can be seen… from a cheap Synta scope!

 

My estimate of the typical/approximate heirarchy of bad things goes like this…
1) Pinched optics, very warm optics, ~1 - 2 waves
2) Moderate boundary layer/warm optics, bad edge support, ~0.5 - 1.0 waves
3) Gross optical errors / bad strain effects / poor edge support, ~0.5 waves
4) Moderate optical errors / mild strain effects, ~0.25 - 0.5 waves
5) Small optical errors / small strain effects / minor cell issues, 0.1 - 0.25 waves

Whatever astigmatism a fan may cause for a little while, it is dwarfed by the benefit I get from removing that boundary layer of air.

This is very useful in showing where efforts should be focussed to reduce wavefront error/ improve views.

Maybe the bigger the temperature difference between stored mirror and night air and the less time or patience you have in wanting high mag, high resolution views…
… Then the more fans on less thermal expansion glass is needed (plate glass and BK7 expands the most, Pyrex and Suprax and Supremax in the middle, Quartz or Zerodur the least) or thinner glass with better mirror cell support.

Thank you eyepiecedropper and others for solutions to this issue of cooling and mirror buying.
 

If [Chinese] BK7 is working well in so many scopes, anneal must be acceptable. Of course each example is different.

Imported mirrors are not BK7, they are faker BK7, BK7 like.
Meaning they probably use the same content but not the same annealing process.
The material to make BK7 is relatively cheap compared to the cost of processing it correctly so you can pay for the licensing fee to actually say you’re using real BK7.

Paul and Danny, I get the feeling that BK7 glass used for mass market scopes, just like the figuring process, needs only be good enough to do the job.
If 1/6 wave figures take more polishing time and full annealing takes more fossil fuel, then a company giving good value telescopes will reduce costs and go for “good enough acceptable.”    So presumably BK7 annealing can be as much a “lottery” hit and miss as the finished mirror figure.



With this particular mirror, there’s too big a likelihood that the anneal is poor.     Because whilst my eyes are a little astigmatic without a new prescription, the views in winter were *really* astigmatic.     For example at low power it reveals twin overlapping Jupiters in the centre of FoV!    Two for the price of one.     Stars shaped like eagles’ wings flying towards you.
To paraphrase a famous quote in Blade Runner movie “I have seen things you people can scarcely believe” :)

Whereas in the last few weeks, with a much smaller temperature difference between indoors and outside, the low power views are now far less astigmatic.
The mirror figure on such a small 8” aperture surely cannot be ‘potato chipping’ furling and contracting so much by the side fan as to cause the astigmatism. So instead the cause could well be poor BK7 annealing.

Other faults with the mirror include undercorrection and a few millimeters of turned down edge (masked off).     Considering the low cost of a new 8” mirror, a new replacement will be bought.

And with the unusual personal viewing circumstances (I have to store it indoors, often around 4C outside, short sessions, strong fans), it’ll be a lower expansion glass than BK7.  

I really appreciate all of your advice and hope that this thread might help others too.


Edited by Max T, 17 May 2015 - 07:46 AM.


#15 pstarr

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Posted 17 May 2015 - 07:57 AM

Max, reading your last post about the temp. problems with astigmatism many have found this type of thing to be a problem with how the secondary is mounted. I would make sure it's mounted correctly as it too can cause what you are seeing. A very good article to read.  http://www.loptics.c.../starshape.html


Edited by pstarr, 17 May 2015 - 08:03 AM.


#16 Max T

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Posted 17 May 2015 - 08:15 AM

Max, reading your last post about the temp. problems with astigmatism many have found this type of thing to be a problem with how the secondary is mounted. I would make sure it's mounted correctly as it too can cause what you are seeing.

You could well be right.   In winter when the astigmatism was really bad, the secondary was mounted the usual Synta way: big white blob of hard glue connecting it to a heavy block of metal as the stalk.   If either the glass or the metal shrinks at a different rate, there could be trouble.

 

Four weeks ago, it was reglued with three blobs of TGV silicone glue to a smaller diameter wood dowel (thermally inert).    Astigmatism has reduced roughly around this time too, but the outside temperature has also warmed up too.   

So I'm clueless as to what has helped reduce astigmatism!

 

It just reminds when measuring the effectiveness of modifications- change only that one thing alone and then observe over several occasions.     Thanks Paul.



#17 pstarr

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Posted 17 May 2015 - 08:42 AM

You could easily see where the astigmatism is coming from by rotating the primary 90* and see if the astigmatism also rotates.



#18 Max T

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Posted 17 May 2015 - 09:16 AM

A very good article to read.  http://www.loptics.c.../starshape.html

Read it several times!   Excellent article.
 

You could easily see where the astigmatism is coming from by rotating the primary 90* and see if the astigmatism also rotates.

Certainly, mind you I first need new glasses since unfortunately the little eyes are astigmatic and the prescription lenses are a bit old.

This helps anyone determine if they have eye astigmatism:
http://www.prokerala...matism-test.php

After new glasses, I'll really just buy the TS Optics 1/6 pv wave quartz mirror as a replacement:

http://www.teleskop-...ertificate.html

(if anyone would like to advise on such a mirror).     

 

It'll be nice to introduce a new set of mirror errors to pick over have a totally stable primary mirror figure that can withstand the difficult temperature changes.   And just observe through the scope rather than at the scope for a while.

Thanks again Paul.


Edited by Max T, 17 May 2015 - 09:26 AM.


#19 pstarr

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Posted 17 May 2015 - 10:58 AM

 

A very good article to read.  http://www.loptics.c.../starshape.html

Read it several times!   Excellent article.
 

You could easily see where the astigmatism is coming from by rotating the primary 90* and see if the astigmatism also rotates.

Certainly, mind you I first need new glasses since unfortunately the little eyes are astigmatic and the prescription lenses are a bit old.

This helps anyone determine if they have eye astigmatism:
http://www.prokerala...matism-test.php

After new glasses, I'll really just buy the TS Optics 1/6 pv wave quartz mirror as a replacement:

http://www.teleskop-...ertificate.html

(if anyone would like to advise on such a mirror).     

 

It'll be nice to introduce a new set of mirror errors to pick over have a totally stable primary mirror figure that can withstand the difficult temperature changes.   And just observe through the scope rather than at the scope for a while.

Thanks again Paul.

 

Sounds like a good plan to me and your welcome. What are doing for a secondary?. Updating to quartz also.


Edited by pstarr, 17 May 2015 - 11:22 AM.


#20 Max T

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Posted 17 May 2015 - 12:29 PM

What are doing for a secondary?. Updating to quartz also.

It will be an Antares secondary in pyrex 1.83" (46mm).  

 

Only the finest sellotape is used...

 

17595309718_26c205ca3f_z.jpg


Edited by Max T, 17 May 2015 - 12:36 PM.


#21 pstarr

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Posted 17 May 2015 - 03:21 PM

 

What are doing for a secondary?. Updating to quartz also.

It will be an Antares secondary in pyrex 1.83" (46mm).  

 

Only the finest sellotape is used...

 

 

 

FYI, Antares sells quartz secondary's. They don't list them. You have to email or call to inquire about them. They also don't always have them. I was able to get a quartz 2.6" 1/30th wave for my scope. They aren't cheap, about 25% more than the regular ones. Just thought I'd let you know if you may be interested. See correspondence below.

 

Paul
On 10/17/2014 8:37 AM, AntaresOptics wrote:
> Hello Paul and Linda,
> We have a dozen 2.60" quartz secondary mirrors in stock at the moment.
> We received an order for 50 mirrors from one of our customers. We
> always make approx 20% overage to cover the order.
> We didn't break any so we offer the overage to anyone who inquires
> about them.
> The quartz substrate mirrors cost 25% more than our standard
> supremax33 substrate mirrors. 1/18-1/30 wave, your choice.
> Have a great weekend.
> Best regards,
>
> Frederick Koch
> Antares Optics
> 261 David Parkway
> Ontario, NY 14519
> 585-265-4820 ph.
> 585-265-1052 fax
> www.antaresoptics.com
> info@antaresoptics.com



#22 Max T

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Posted 17 May 2015 - 05:45 PM

Thanks Paul, it's something to consider, since a quartz primary might perhaps be held back during early viewing if the secondary is still cooling and is of a higher expansion material.  

 

Mind, at 1.83" the secondary will only be 10mm thick so it should cool fast enough.  Perhaps more of an issue for bigger mirrors.



#23 mark cowan

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Posted 17 May 2015 - 06:39 PM

The secondary is really never in that situation.




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