difference in plate glass primary vs pyrex?

[Garyh]

Hi all,
Been thinking of planning my next newt astrograph.
I have been on the lookout for some cheap blanks etc around the 7-8" size.
Can anyone point me to a site or some info on the differences between plate/pyrex and other low expansion materials for a mirror this size..
Would it make much of a difference in small mirrors as they cool down.
Plate is easy to get while pyrex is rather hard to find except buying from the states. What about BK7 glass?, how does this rate?
I`m planning on a 1/20 pv mirror if all goes well to get nice crisp stars!!
cheers Gary
oh....where to get blanks over here at reasonable prices?

[Blue Skies (Jacquie)]

I've a lot of discussion with friends (other mirror makers) about this topic and the general conclusion was that in the climate of Perth, at least, there was little difference in use, and the main difference would only be seen if you actually tried to make a mirror yourself (ie when you got to the figuring stage you would have to be more careful). In an area where there is a small diurnal change in temperature, such as Perth, there was no significant advantage in using Pyrex-type glass. However if you live in an area where there is a large difference between day and night temps then pyrex would make more sense.

Other peoples milage on this subject may vary, as they say.

[Garyh]

Thanks for the reply Blue Skies, is there any particular way to figure a plate glass mirror? like what is the difference in figure from a warm mirror just figured to a cooled down one..if the mirror showed overcorrection when warm would it settle closer to correct or would it tend to be more overcorrected when cool?
The scope probally won`t go through huge changes in ambient temperature as it will be outside in a shed that will be well ventilated so should stay close to outside temps. Here I would expect a 20 degree swing from daytime top temp to nighttime low at worst..

Ok anyone know where I can get a couple of plate glass discs cut in sydney out of 20mm-25mm sheet?
cheers

[cristian abarca]

Hy Gary. Pyrex is better than plate glass. Once cooled down the differences are not noticeable. In my opinion plate glass will be a problem if you take it from a warm enviroment to a cold one or (visa versa) and you expect to start viewing straight away. If the mirror reaches ambient temperature then there is no problem at all. On the subject of figuring there is a difference. Figuring a plate glass mirror is done in the same way that a pyrex one is. The difference is in the testing. By this I mean that the mirror will look distorted if tested immediately after a figuring session. The mirror must be left to cool down before any testing is done. This can mean that you do 10 or so minutes of figuring and then leave the mirror on the stand overnight to see the result. The changes can be substantial. It is a much slower process and more patience is required that's all. On where to buy glass, again there are not too many choices. Most plate glass supplied in Australia (by Pilkington glass anyway) is 19mm thick. I tried to purchase a 10 inch piece and they gave me a couple of numbers of suppliers. I was quoted around $180 plus GST. I found this a little expensive. You should try http://www.telescopes-astronomy.com....escopes036.htm . They have blanks for $99 which is about as cheap as you can get. I looked around for a while at the beginning of the year and ended up buying from him.

Regards Cristian

[cristian abarca]

I forgot to say Gary, that if you scroll down his site you will see plate glass 3/4 inch thick $90 and 1 inch $99. I would go for the 1 inch.

Regards Cristian

[Blue Skies (Jacquie)]

Originally Posted by Garyh

Thanks for the reply Blue Skies, is there any particular way to figure a plate glass mirror? like what is the difference in figure from a warm mirror just figured to a cooled down one..if the mirror showed overcorrection when warm would it settle closer to correct or would it tend to be more overcorrected when cool?

What Cristian just said above I guess what I was trying to say is that your hands do transfer some heat into the glass when you're holding it so there is a possibility that the glass might heat up a little to much while you're figuring and then when it cools the figure is different to what you were aiming for, so the progress is slower.

As for glass - I suppose if you can afford the pyrex-type stuff you should go for it, especially since you only want to make a medium size mirror anyway, but if the plate glass is handier it's still do-able. The plate is softer so progress will be quicker, so if anyone else is reading this and tends to be impatient but wants to give mirror making a go with something in the 6" to 8" range the plate might suit your needs better.

[ballaratdragons (Ken)]

In Australia there is no real need for Pyrex. While it may be nice to have a Pyrex Primary it doesn't really matter. We don't go from 24 degrees inside the house to -20 outside.

Also, Corning Incorporated has stopped production of Pyrex and Flourite Crystals. They have moved into new materials. Once all the Pyrex and Flourite is gone, it's gone. The new material is 'ULE': ultra low expansion ULE® glass, a material that exhibits virtually no dimensional changes over extreme temperature variations and is generally considered the best in the world for astronomical optics.

[Satchmo]

Originally Posted by ballaratdragons

In Australia there is no real need for Pyrex. While it may be nice to have a Pyrex Primary it doesn't really matter. We don't go from 24 degrees inside the house to -20 outside.

Also, Corning Incorporated has stopped production of Pyrex and Flourite Crystals. They have moved into new materials. Once all the Pyrex and Flourite is gone, it's gone.

Whether you can `get away ' with plate glass instead of Pyrex depends totally on the thickness of the glass. Plate glass bends three times the amount of Pyrex for a given Delta and with Pyrex you need to get your delta to ambient with in 2 degrees to get get the figure of the mirror as the maker intended. Anyone who has set up digital thermometers will know how hard it is to keep a mirror within 2 degree of ambient when the temperature is falling.

In the case of plate glass , the owner has very little chance of maintaining the intended figure if the temperature is falling fast enough. I happen to have a 38mm Plate 14" mirror in my scope at the moment and it gives me plenty of trouble at certain times of the year. Smaller thinner mirrors in the 8 to 10" range aren't so troubled becasue of their fairly low mass. It also depends very much on how discriminating the observer is of course.

The idea that the world is running out of Pyrex is a bit of a USA forum beat up. Supplies of 2" sheet Pyrex are indeed over. Corning have a new plant in China and Schott are ramping up their production. If the industry want to cling to 2" pyrex blanks their going to have to get them moulded. Theres more Pyrex thanever in production now.