Jump to content

Mirror blanks

dark star

By dark star
in DIY Astronomer

Recommended Posts

dark star

dark star

Posted
Posted

I am nowhere near finishing the six inch mirror I started making a while ago, although I have made some progress on the hogging out. I have started making a foucault tester, so the grinding has been put on hold until I finish this.

I am already thinking about getting a 14 inch (350mm) mirror blank, the 6 inch mirror is just a trial run to get the hang of it!

Galvoptics sell 'Dobsonian mirror blanks', Chance Pilkington Glass. This costs £66.68 for 350mm and is presumably plate glass. It is 25mm thick. They also sell low expansion mirror blanks. They are £283.50 for a 360mm blank and are 40mm thick.

I read in one of Glasspusher's threads on this forum that plate glass of 25mm thickness should be alright up to 14 inches, so I intend to get this. It is a lot cheaper!

Is there any big advantage to low expansion glass? Apart from a shorter cool down time? I can put up with a longer cool down time, especially as the finished telescope may well be stored in my shed, if it will fit!

David

Link to comment
Share on other sites
Rustysplit

Rustysplit

Posted
Posted

I did my 14" with plate glass for galvoptics. As it is only 25mm thick the cool down is pretty quick anyhow. My thoughts were that as a first time grind for me, the outlay for low expansion was quite a risk. Having successfully finished the mirror, if I were to make another of around the same size I would still go with plate at 25mm. It works!!

Link to comment
Share on other sites
Astrobits

Astrobits

Posted
Posted

I guess one of the major advantages of low expansion glass is that it has better thermal stability as temperatures vary whilst it's in use.

James

Actually, the thermal stability of low expansion glass has no effect on it's performance once in the telescope compared to plate/float glass. This was believed to be the case many years ago before we realised that thermal equilibrium with the atmosphere was far more important for image stability. You need the mirror to follow any cooling of the atmosphere as fast as possible. Slight differences in temperature between the atmosphere and the mirror might well lead to distortion of the mirror but the thermals that arise in front of the mirror are far more destructive of the image. Theoretical work, ( published in Sky & Telescope, I haven't got the reference offhand ) showed that a 2 1/2" thick mirror could not keep cooling fast enough to keep up with the air temperature if that was falling 1 deg C per hour.

Therefore go for the thinnest glass blank and disregard the actual glass type.

There is one advantage to low expansion glass and that is during the final figuring stage of mirror making. You have to leave plate glass longer to stabilise after a bit of polishing and before testing than low expansion glass as plate glass has about twice the thermal coefficient of expansion as low expansion glasses and you can see the effects in the figure.

1" thick is fine for a 14", my 16" is also 1" thick and performs to my satisfaction.

Nigel

Link to comment
Share on other sites
JamesF

JamesF

Posted
Posted

Actually, the thermal stability of low expansion glass has no effect on it's performance once in the telescope compared to plate/float glass. This was believed to be the case many years ago before we realised that thermal equilibrium with the atmosphere was far more important for image stability.

That's very interesting. I had no idea it was the case.

Is my understanding therefore correct that it's more important for the mirror to be at the same temperature as the atmosphere than it is to minimise expansion and contraction due to change in temperature?

James

Link to comment
Share on other sites
Astrobits

Astrobits

Posted
Posted

That's very interesting. I had no idea it was the case.

Is my understanding therefore correct that it's more important for the mirror to be at the same temperature as the atmosphere than it is to minimise expansion and contraction due to change in temperature?

James

That's correct James. Hence the proliferation of fans to cool the mirror and get rid of the boundary layer in front of the mirror.

SandT

First page (link) is just an intro to the 4 points it makes which you find lower down the page as 2,3,4,5.

How true/false/correct any of it is I have no idea. Just book marked it some time back.

There was a much more comprehensive article some years ago. I will try to find it in my stack of S&T's when I have time tomorrow.

Nigel

Link to comment
Share on other sites
FraserClarke

FraserClarke

Posted
Posted

Therefore go for the thinnest glass blank and disregard the actual glass type.

I agree with that, but of course the thinner the blank the greater the mechanical challenges of supporting it in the correct shape, both during polishing and in the telescope. Not insurmountable, but something to consider.

Link to comment
Share on other sites
Glasspusher

Glasspusher

Posted
Posted

When figuring a plate glass mirror you have to wait longer for the mirror to reach equilibrium with the air in the test chamber than would be the case with a low expansion glass. For an ATM time is not an issue, so the use of plate glass is fine you just have to be patient during testing. Some good points from Fraser and Nigel.

John

Link to comment
Share on other sites
dark star

dark star

Posted
  • Author
Posted

Thanks for all the useful information. I will most probably not buy the 14 inch mirror blank before finishing the 6 inch mirror, I don't want to try to run before I can walk! I hope to make a lot of progress on the Foucault tester this weekend.

Can anyone tell me the weight of a 14 inch plate glass mirror blank? This will help me to start planning and hopefully in the next few months start building the mount. I am probably going to make a truss dobsonian after seeing the amazing ones built on this site!

David

Link to comment
Share on other sites
Astrobits

Astrobits

Posted
Posted

I, and John, have made the point that you have to wait some time for a plate glass mirror to stabilise after a bit of polishing and before testing. I mentioned that you could see the change but forgot to mention that you will only see the change with a null test. The Foucault test might measure it but you will not see the difference after half an hour's cooling. It is quite interesting near the last stages of figuring to see that the figure appears worse than before if the mirror is examined immediately after polishing. Go away for half an hour and have a cup of tea and when you get back the figure will be much better.

Nigel

Link to comment
Share on other sites
fwm891

fwm891

Posted
Posted

Thanks for all the useful information. I will most probably not buy the 14 inch mirror blank before finishing the 6 inch mirror, I don't want to try to run before I can walk! I hope to make a lot of progress on the Foucault tester this weekend.

Can anyone tell me the weight of a 14 inch plate glass mirror blank? This will help me to start planning and hopefully in the next few months start building the mount. I am probably going to make a truss dobsonian after seeing the amazing ones built on this site!

David

David, Its fine knowing the weight of the optics its the final focal length in some ways thats more important. The length of truss tubes and the size/weight of the secondary cage will have a big effect on where your OTA balance point is. The last thing you want to end up doing is adding a great weight behind the primary to balance out the OTA because things didn't go quite to plan...

Link to comment
Share on other sites
FraserClarke

FraserClarke

Posted
Posted

Can anyone tell me the weight of a 14 inch plate glass mirror blank?

Density is about 2.4 g/cm^3 (depends on the glass, but that's a reasonable median). So I think that makes a 14"x1" blank about 6kg.

Francis is right about focal length though. If you build the mount first, you'll need to control the focal length during grinding/polishing.

Link to comment
Share on other sites
Rustysplit

Rustysplit

Posted
Posted

David, I need to take my 14" mirror out for a rinse thanks to a sticky "blob" that appeared during use last night. I will weigh it for you when I do.

If you know in advance roughly what focal length you are going to make, and the weight of the mirror, this should give you enough info to make a fairly balanced design. So long as you do not cut your trusses to length until you fit your mirrors, you will be fine. Keep an eye on the desired saggita when making the mirror, then all you have to do is fine tune the tube lengths on completion. I can't see a major problem building the mount first. My advice would be to try to keep the weight of the UTA to an absolute minimum, as it is far easier to balance a bottom heavy 'scope than a top heavy one.

Link to comment
Share on other sites
dark star

dark star

Posted
  • Author
Posted

I think it will take me quite a while figuring out exactly which design of truss dobsonian I want to make. There are so many different designs!

I will concentrate on making the Foucault Tester, I have cut out most of the pieces of wood for it now. Then I need to make a mirror stand and progress with my 6 inch mirror.

Thanks for all the advice, I now have a better idea how to start calculating the sizes of everything. I have enough wood and will hopefully make a start with the mount in a few weeks. I plan to make it a f/5 mirror. I expect to finish it sometime next year. I tend to progress in stops and starts! At least I can now use a router, although I need more practise!

David

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

Go to topic listing

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.