Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Minimum 16", 18" mirror thick

  • Please log in to reply
25 replies to this topic

#1 AlbertoJ

AlbertoJ

    Vostok 1

  • *****
  • topic starter
  • Posts: 149
  • Joined: 20 Feb 2008
  • Loc: Madrid (España)

Posted 17 September 2019 - 07:14 PM

Hello, what's the minimum recommendable 16", 18" mirror thick to haven't any problem (astigmatism, collimation loss)? Using it in the best availlable mirror cells.



#2 starzonesteve

starzonesteve

    Viking 1

  • *****
  • Posts: 610
  • Joined: 17 May 2014
  • Loc: Central Alabama

Posted 17 September 2019 - 07:25 PM

My 14.7” mirror is .8” thick. My understanding is that these very thin mirrors become difficult to work. You need a first rate optician. I don’t know if you could get much thinner than that. Also requires a top notch mirror cell so as not to induce error after the fact.

 

What is your goal? Cooling? Weight?

 

I have heard good things about some sandwich type mirrors, although these are not without fabrication difficulties either.



#3 AlbertoJ

AlbertoJ

    Vostok 1

  • *****
  • topic starter
  • Posts: 149
  • Joined: 20 Feb 2008
  • Loc: Madrid (España)

Posted 18 September 2019 - 05:38 AM

Hello starzonesteve,

 

my goal is mirror cooling and keep a stable image without mirror cooling turbulence all night long.

I'm thinking about mirror cells like Jp astrocraft or Aurora precision.

And mirrors, if possible, different of 2" thick. Something between 1" and 1.5", depending on its performance in the cell.

I don't know if it's more suitable a 1", 1.25" or 1.5" mirror, in 16", 18".

 

Your 14.7” mirror is .8” thick, how its performance is?



#4 starzonesteve

starzonesteve

    Viking 1

  • *****
  • Posts: 610
  • Joined: 17 May 2014
  • Loc: Central Alabama

Posted 18 September 2019 - 07:09 AM

Performance is great. It is in an Aurora cell.

 

Two other considerations would be the substrate - essentially quartz vs Pyrex, and the use of a cooling fan.



#5 Roragi

Roragi

    Viking 1

  • -----
  • Posts: 654
  • Joined: 24 Oct 2013
  • Loc: Orion arm

Posted 18 September 2019 - 07:15 AM

I think I remember that my 16" mirror was 46mm.



#6 macdonjh

macdonjh

    Skylab

  • *****
  • Posts: 4369
  • Joined: 06 Mar 2006

Posted 18 September 2019 - 08:59 AM

What I've read in various ATM forums echos what starzonesteve says: mirror "thinness" is limited by how well the glass can be supported during grinding/ figuring/ polishing and how well you can design and fabricate a support cell.

 

I think you need two grains of salt for what I said, though, I have not made any mirrors, only read about it.


  • Jon Isaacs likes this

#7 Mike Wiles

Mike Wiles

    Apollo

  • -----
  • Posts: 1001
  • Joined: 04 Feb 2009
  • Loc: Phoenix, AZ

Posted 18 September 2019 - 09:43 AM

My 20" mirror is 1.25" thick and lives in an Aurora Precision mirror cell.  It cools quickly, and works perfectly.  Mike Lockwood discusses mirror thickness at length on his own website.  His website is also a great resource for finding someone who can make a really thin, really fast, really excellent mirror.  

 

http://www.loptics.com/faq.html

 

Mirror thickness vs aperture is the 9th question down on the page.  

 

Mike


  • havasman likes this

#8 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1602
  • Joined: 01 Oct 2007
  • Loc: Usually in my optical shop

Posted 18 September 2019 - 10:10 AM

Hello, what's the minimum recommendable 16", 18" mirror thick to haven't any problem (astigmatism, collimation loss)? Using it in the best availlable mirror cells.

For the best available cells with the type of edge support that I use, the minimum that I recommend taking both fabrication and cell into account, is 1.0" and 1.15".

 

I have made 20"s at that aspect ratio (16:1) and those work well in good cells with whiffletree and roller edge supports.  However, going this thin makes the fabrication cost a bit higher.

 

If absolute minimum thickness is not critical, then I prefer about 1.25" for a 16" and 1.35" for an 18".  (Edit - just saw Mike's post above, he has one of the 20"s I was referring to.)


Edited by Mike Lockwood, 18 September 2019 - 10:11 AM.


#9 areyoukiddingme

areyoukiddingme

    Skylab

  • *****
  • Posts: 4373
  • Joined: 18 Nov 2012

Posted 18 September 2019 - 04:09 PM

Hello starzonesteve,

 

my goal is mirror cooling and keep a stable image without mirror cooling turbulence all night long.

I'm thinking about mirror cells like Jp astrocraft or Aurora precision.

And mirrors, if possible, different of 2" thick. Something between 1" and 1.5", depending on its performance in the cell.

I don't know if it's more suitable a 1", 1.25" or 1.5" mirror, in 16", 18".

 

Your 14.7” mirror is .8” thick, how its performance is?

 

My inclination would be to go for a quartz mirror. My experience is limited to an 8" 20mm thickness, but it is up and ready to go in a few minutes. It's in an aluminium tube with an aurora precision cell.


  • stargazer193857 likes this

#10 AlbertoJ

AlbertoJ

    Vostok 1

  • *****
  • topic starter
  • Posts: 149
  • Joined: 20 Feb 2008
  • Loc: Madrid (España)

Posted 18 September 2019 - 07:46 PM

Thank you all for your answers.

 

Mike Wiles, I read your past problems with your 20" mirror. Now, in the suitable and awesome support for this very thin mirror, you say it cools quickly, and works perfectly.

That proves that very thin mirrors aren't ineffective for bending like a pizza, as it used to be said some years ago, when the main recommendation was 2" mirror. The necessary condition is get the best primary mirror cells available.

 

Mike Lockwood, it is an honor and a luxury to have one of the world best mirror manufacturers answering the questions and doubts of this forum users. Your answer solves almost all my doubts.

One more question, is there a big cooling difference between the standard mirror (pyrex, supremax) and the quartz one?


Edited by AlbertoJ, 18 September 2019 - 08:25 PM.


#11 starzonesteve

starzonesteve

    Viking 1

  • *****
  • Posts: 610
  • Joined: 17 May 2014
  • Loc: Central Alabama

Posted 18 September 2019 - 11:36 PM

Certainly Mike Lockwood can answer this with more precision than I if he happens onto this thread again. In the meantime, my understanding is that the thermal expansion of pyrex is approximately six times greater than quartz. This is what can be measured in the lab. The difference in what you would see in the field depends on how and where you are using your telescope. If you bring it out for immediate use under circumstances of large temperature change then the quartz would provide an advantage as the mirror acclimates to ambient. Also, if the temperatures drop rapidly or severely where you are viewing then quartz may be less affected. On the other hand, use of a cooling fan and having the scope out for enough time before observing may mitigate any differences between substrates one might see at the eyepiece. Therefore, when deciding on whether to pay more for quartz it is worth understanding what conditions you expect to be observing under.



#12 Mike Wiles

Mike Wiles

    Apollo

  • -----
  • Posts: 1001
  • Joined: 04 Feb 2009
  • Loc: Phoenix, AZ

Posted 18 September 2019 - 11:48 PM

Thank you all for your answers.

 

Mike Wiles, I read your past problems with your 20" mirror. Now, in the suitable and awesome support for this very thin mirror, you say it cools quickly, and works perfectly.

That proves that very thin mirrors aren't ineffective for bending like a pizza, as it used to be said some years ago, when the main recommendation was 2" mirror. The necessary condition is get the best primary mirror cells available.

To be clear....none of my previous problems had anything to do with the mirror.  My previous experience had everything to do with the structure....which leads me to one more thing.  

 

I'm also of the opinion that the structure is every bit as important as the optics. 


  • Starman1, Jon Isaacs, Kunama and 1 other like this

#13 AlbertoJ

AlbertoJ

    Vostok 1

  • *****
  • topic starter
  • Posts: 149
  • Joined: 20 Feb 2008
  • Loc: Madrid (España)

Posted 19 September 2019 - 03:25 AM

I think, some structures can work very good with most of mirrors. But I think too, that in critical mirrors (very low focal ratio and very thin mirrors), like your 20" f/3, only can be made by the very best optician and  only work at the highest level in the very best specific structures to be able to support it.



#14 Allan Wade

Allan Wade

    Soyuz

  • *****
  • Posts: 3925
  • Joined: 27 Jan 2013
  • Loc: Newcastle, Australia

Posted 19 September 2019 - 04:19 AM

The traditional process is to pick an optician of your choosing and then be guided by them. A lot of the top opticians have builders they work closely with, so the end product is a complete high quality system. It seems typical that the optician is the first point of blame when the new scope doesn't perform up to standard. But in more cases than not, its the structure that's at fault, and particularly the mirror cell when you're talking the new generation of thin and fast mirrors.

 

In over 30 years the best mirror I've seen was a thin, quartz 22" made by Mike Lockwood. It was no surprise that the mirror was in a dob and cell made by John Pratte, who puts all the cutting edge features into his mirror cells.


  • Mike Wiles likes this

#15 Mike Lockwood

Mike Lockwood

    Vendor, Lockwood Custom Optics

  • *****
  • Vendors
  • Posts: 1602
  • Joined: 01 Oct 2007
  • Loc: Usually in my optical shop

Posted 19 September 2019 - 09:06 AM

.....my understanding is that the thermal expansion of pyrex is approximately six times greater than quartz. This is what can be measured in the lab. The difference in what you would see in the field depends on how and where you are using your telescope. If you bring it out for immediate use under circumstances of large temperature change then the quartz would provide an advantage as the mirror acclimates to ambient. Also, if the temperatures drop rapidly or severely where you are viewing then quartz may be less affected. On the other hand, use of a cooling fan and having the scope out for enough time before observing may mitigate any differences between substrates one might see at the eyepiece. Therefore, when deciding on whether to pay more for quartz it is worth understanding what conditions you expect to be observing under.

What Steve said is exactly right.  The main effect is that quartz (temporarily) changes shape less as it cools.

 

There is a small difference because quartz has a slightly lower specific heat, so it will cool very slightly faster, but I don't think it would be noticeable under normal observing conditions.

 

The claim that quartz takes on a better polish is often used for marketing, however in the real world the effect of that would usually be impossible to see.


  • Starman1 and Mike Spooner like this

#16 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42745
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 21 September 2019 - 01:03 PM

Just bear in mind that the thinner a mirror is, the more complex the mirror cell will need to be.

A 1.25" thick 16" works fine on an 18 point cell, but a 0.8" mirror has a lot more induced error in the wavefront with 18 points

and the figures look better with 27 points.  That's certainly not impossible to make, but a little thicker is easier.

At 45°, where the telescope is used often, mirror wavefront error increases, so keeping cell-induced errors to a minimum makes sense.

Mike Lockwood has done some good research into edge supports and induced errors, for example.

 

As for cooling, add boundary layer cross fans and a high CFM rear fan, start your fans at sunset, and by dark, you'll be at the ambient temperature

with a 1.25" to 1.5" thick mirror.

The issue is keeping the mirror at the ambient temperature, so figure the fans will run all night, maybe slowed down a tad to reduce vibration.

 

The big advantage to fused quartz as a mirror material is that, if the boundary layer is blown away, the mirror can still be warm and yield excellent star images

because the mirror doesn't go through a figure change as it cools.  So if you have a fused quartz mirror, you can do some high power planetary observing a bit earlier

in the evening than someone with a borosilicate glass mirror, assuming both mirrors are still above the ambient temperature.


  • Mike Spooner likes this

#17 AlbertoJ

AlbertoJ

    Vostok 1

  • *****
  • topic starter
  • Posts: 149
  • Joined: 20 Feb 2008
  • Loc: Madrid (España)

Posted 24 September 2019 - 06:02 PM

Ok, quartz changes shape less and the mirrors would suffer much less distortion throughout the cooling period. And practically nothing in the case of zerodur.

But convection from the warm surfaces causes air currents that distort the viewing until the mirror has cooled. I don't know if these currents stop when the  outside surface of the mirror is cool, or the whole mirror must be cool.



#18 mark cowan

mark cowan

    Vendor (Veritas Optics)

  • *****
  • Vendors
  • Posts: 9447
  • Joined: 03 Jun 2005
  • Loc: salem, OR

Posted 24 September 2019 - 06:34 PM

The surface temp has to be tracking the ambient air temp closely for your last sentence to be true.  

And the surface temp over time depends on the distribution of heat in the bulk substrate, the ambient air, and how the heat moves inside the mirror to the surface.

 

So thinner mirrors having less heat capacity at the same starting temp have less heat to transport, which makes them faster to acclimate and follow the ambient.



#19 Pinbout

Pinbout

    ISS

  • *****
  • Posts: 22926
  • Joined: 22 Feb 2010

Posted 24 September 2019 - 08:51 PM

Slumping the glass let’s you go thinner


  • stargazer193857 likes this

#20 MitchAlsup

MitchAlsup

    Soyuz

  • -----
  • Posts: 3904
  • Joined: 31 Aug 2009

Posted 25 September 2019 - 10:27 AM

Hello, what's the minimum recommendable 16", 18" mirror thick to haven't any problem (astigmatism, collimation loss)? Using it in the best availlable mirror cells.

I have a 13" F/3 that is 1.1" thick and a 20" F/3 that is 1.3" thick.



#21 AlbertoJ

AlbertoJ

    Vostok 1

  • *****
  • topic starter
  • Posts: 149
  • Joined: 20 Feb 2008
  • Loc: Madrid (España)

Posted 26 September 2019 - 03:36 AM

I have a 13" F/3 that is 1.1" thick and a 20" F/3 that is 1.3" thick.

Hello MitchAlsup, what mirror cell do you use for each one of them?

how their performance is?



#22 MitchAlsup

MitchAlsup

    Soyuz

  • -----
  • Posts: 3904
  • Joined: 31 Aug 2009

Posted 26 September 2019 - 10:21 AM

The 13" F/3 rests on a std 6-point cell.

The 20" F/3 will rest on a 12-point cell.

 

Both of which I construct.

 

20cell01.JPG


Edited by MitchAlsup, 26 September 2019 - 10:24 AM.


#23 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42745
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted 26 September 2019 - 11:53 AM

Mitch,

I know you have a lot of experience with scopes, but I see a 12 point support on that 20" results in 50+nm of wavefront error (about 1/10 wave) in deformation and that is pointed up.

It's worse at 45°.

Even assuming 1/4 wave optics, that's a lot to add to the wavefront error.

I see 12.7nm of error with 18 points, which is about the minimum number of points I think might be desirable to use.

27 points reduces that to 5.9nm of error.

That mirror is really thin for the diameter.

I'm sure you figured that out too, so why 12 points?



#24 AlbertoJ

AlbertoJ

    Vostok 1

  • *****
  • topic starter
  • Posts: 149
  • Joined: 20 Feb 2008
  • Loc: Madrid (España)

Posted Yesterday, 05:31 AM

These 2 mirrors have similar price :

 

Zambuto 16" , 29mm thick Quartz.

Zambuto 18",  41mm thick Supremax.      

 

Which of them would you choose by images they give?



#25 Starman1

Starman1

    Vendor (EyepiecesEtc.com)

  • *****
  • Vendors
  • Posts: 42745
  • Joined: 23 Jun 2003
  • Loc: Los Angeles

Posted Yesterday, 09:00 AM

If you have adequate fans, the 18" will be fine and has 26.6% more light grasp.

But, the mirror would be a lot heavier, as would the scope be, so it depends on the weight you will tolerate.

 

Also, it depends on your viewing style.  With boundary layer fans, the fused quartz mirror will be as sharp as it can get right at the moment you start observing,

whereas the borosilicate mirror might take an hour from the start of fans to achieve final equilibrium.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics