cheap mirrors?

[Jarrod]

hi, i am thinking about grinding my own mirror for the first time and im wondering if there are any cheap alternatives to the proper mirror blanks.
i am considering porthole glass (if i can find some!), but are there many other half decent alternatives? i have also thought about metal mirrors, although this wouldent count as "grinding".

thanks, jarrod.

[cristian abarca]

Hi Jarrod. There are a few alternatives to Pyrex glass but it all depends on what size mirror you want to make. It also depends where you are. If you are in the US then Pyrex isn't that expensive and compared to the trouble you might have with other type of glass I think that it would be unwise to try an alternative. If you live in Australia then you might try an alternative like plate glass. I have heard of people making mirrors out of porthole glass but not here in Australia. Might be able to give you better advise if I knew where you are and what size you are contemplating.

Regards Cristian

[Jarrod]

hi cristian, well for a start i live in victoria, australia. porthole glass appealed to me because i live near the coast and there are a few places around that i might find some porthole glass. i am thinking of mabye making around a 10" mirror, proberly around f:6.

jarrod

[cristian abarca]

The first mirror I made was a 10 inch F7 and I made it out of Pyrex. I got the Pyrex from my Astro society for about $150. I don't know what the price for this glass is normally nor do I know the price of porthole glass but it definitely should be much cheaper. By all means try porthole glass specially if you can get it. I looked around for it but could only find it in the US. The availability of glass will be you biggest problem. You will also need powders and pitch and plaster and tiles etc. You might consider buying a kit that comes with everything. You should inquire in Geelong about an astronomical society (I think they have one). If there is an astronomical society near you then they will probably be able to guide you in the right direction. I would personally use Pyrex glass at my first attempt at mirror making. It will make your life easier specially if you've never made one before. Plate glass is an alternative, it is cheaper but 1 inch seems to be the thickest that you can get around here. Have a look here http://www.newportglass.com/ they are pretty good but they are in the US. They provide kits as well. I have found them to be the most reasonably priced. Also look here http://www.galileotelescope.com/ These guys deal in plate glass and they are in India. I have emailed the guy in charge a couple of times and he seems pretty keen on doing business. You can always try here http://www.telescopes-astronomy.com.au/telescopes034.htm They are in South Australia. Have a read of this website http://www.bbastrodesigns.com/ and of this one http://bobmay.astronomy.net/ I started making my own mirrors after reading Bob's website. You can also have a look at my unfinished site which is not quite to these guys standards but will give you an idea of what is involved. I'll finish it one day. http://cmabar.googlepages.com/ Hope this helps and if you have any more questions feel free to ask.

Regards Cristian

[Jarrod]

thanks for all the helpful info cristian. there isan astro society in geelong, i'll pay them a visit. i've asked a few friends regarding materials and the only thing i can't track down is the suitible glass. i got a friend who works with boats in geelong to ask around regarding porthole glass. hopefully i'll start making some progress.

thanks, jarrod.

[Sharnbrook]

I'm guessing here, but I would think in this day and age, modern porthole glass would probably be built up and laminated. Maybe 4 x 6mm pieces, or 2 x 12 for a 25mm overall thickness. Has anyone tried using laminated glass for a mirror? Or would the lamination break down in the aluminising process?
I have read that it is quicker to make a 6" mirror and then a 10" mirror, than to make a 10" mirror as a first effort. The learning curve on a 6" mirror is quite gentle, but on a 10" it is very steep. I have never tried either.

As the depth of the curve on a 250mm mirror is only about 2.6mm (I think), there would be plenty of meat left in the front layer of glass. However, I only offer this suggestion for consideration and comment by others, and would not recommend it unless there is a positive reaction.

Regards,

Mike

[Jarrod]

i asked a friend about porthole glass, he wrote back in an email... so if someone was to use porthole glass it would have to come from an old boat, at least 1970's i'd think. coating laminated glass? does the coating process require heat, because that would proberly melt the layers between the glass.

jarrod

[tjquoll]

I've occasionally seen portholes on ebay being flogged as collectibles but they usually want $100's for them.

If you're adventurous there is an example at http://lerch.no-ip.com/atm/default.new.htm on how to cut blanks from 3/4 inch plate glass. Just got to find a source of cheap thick glass.

I'd hazard a guess that an issue with glass laminates might be whether they'd retains the mirror's figure over a range of temperatures. If materials had different expansion coefficients it may distort the mirror. Also couldn't out-gassing from the glues cause problems when coating the mirror?

[Jarrod]

well the portholes on ebay ussually have the metal frames still attached. would it be possible to have laminated glass cut into circles by one of those places that cut glass to fit window frames? if so, would it cost much?

jarrod

[cristian abarca]

I asked around for 19mm plate glass and I was quoted $180 for a 250mm piece. Not all glass shops cut glass 19mm thick but if you ring around you may find some that do. One of the problems with cutting glass is that you may release strains on the glass and that may not allow you to get a good figure. Have a look at this site for plate glass http://www.galvoptics.fsnet.co.uk/telescope3.htm . I came up into a site in the US who were selling porthole glass but I can't remember the name. I'll keep looking and if i find it I'll let you know.

Regards Cristian

[cristian abarca]

I found the site Jarrod http://www.commercialsalesinc.com/inv3.htm They have porthole glass for $20USD plus delivery.

Regards Cristian

[Jarrod]

thanks cristian, but i've got a friend working down at corio bay this weekend who's gunna ask around regarding portholes. i'll see what he comes up with and take things from there. i really don't want to order glass over the net unless i have too.

thanks, jarrod

[Blue Skies]

Just to make it clear, you can't use laminated or tempered glass to make a mirror. The first wont hold it's figure and the latter will probably shatter at some point into many tiny little pieces. I've also been asked about using the glass from a tv screen, which is tempered glass too, so that's a no-no, but I've also been told they have a significant amount of lead in them. So that why you need plate glass.

Then you will find that thick plate glass is hard to get hold of these days as it's not made very thick anymore - if you need thick glass you get a laminated pane for safety reasons, so that's one reason why most people turn to pyrex type glass. If you contact Schott-Garsco in Sydney you will find they have ready made mirror blanks in various sizes and if you want to try a bigger mirror you can do what a friend of mine did and buy a sheet of thicker stuff, get it water-cut to the diameter you want (he got two from the sheet, sold the other blank to a another grinder).

When I tried to find portholes in Perth I just got laughed at, so eventually we (there was three of us searching for suitable material) we pieced together the glass situation story and ended up getting a shipment of blanks from Schott in Sydney. We kept going and also found that that sometimes you can get a scrap or offcut of the thicker plate glass - if you're lucky and you know the right people - but the glass companies didn't want to know you if you didn't have a size in mind. At one point I went halves getting a piece of plate water-cut into different size blanks and the result was not as good as I had hoped. They managed to loose a couple of blanks in their tank and the others were a bit rough around the edges.

[Satchmo]

Jarrod, I don't know what your budget is but I have a couple of full thickness Pyrex blanks here allready with F6 curve. PM me if yo're interested.

Mark