mirror making

[monoxide]

hi guys,

since its the international year of astronomy coming up and i'm on holidays until March, i thought i'd look into grinding my own mirror.
it sounds like something i'd really enjoy doing and i'd rather spend a few hours a day grinding a mirror than playing pc games

my main question is, what am i up against as a complete beginner and whats the best way to start off?

[Geoff45 (Geoff)]

Start with a 6". It may well be rat****, but you'll be pleasantly disappointed. Your second 6" will be really good after you've analyzed and acted on your mistakes. From then on, there will be no holding you--8", 12 " no problem.
Geoff
Note added: I see that IIS automatically filters "naughty" words. My original had no asterisks

[monoxide]

thanks Geoff,
i'm pretty much counting on the first one or two being failures but its all part of the game i suppose

i guess ill do what every beginner gets away with once.. just getting in there, attacking it then coming running for help when it goes pear shaped
from what i've read, getting the mirror spherical should be fairly straight forward so i'll try for that then ask where to go from there

any recommended supplier for blanks/grit?

[theodog (Jeff)]

Sounds like a great project.
Try to source thick plate glass, say 1/2" from a glass supplier. They could cut it to size.
I had a go with some 4" blanks years ago -great fun.
Best part was building a knife-edge tester & seeing paterns that described the mirror surface, fascinating!

I recommend starting with 4", dont aim for a fixed focal lenth -leave that for the next trial & go from there. The whole process is really simple yet exacting.

Look on the net for mirror grinding kit suppliers. They are not to expensive.

Good Luck and have fun.

[monoxide]

i found a place to get everything needed

i think ill start with a 6" since i have a spare OTA and cell for a 6" mirror, will save me building another scope just to test with.

guess i need to start looking at how to build a ronchi tester or would it just be easier to buy one, $60 on the site where i was looking at blanks

i guess it just amuses me how accurate you can make something with minimal tools and home made testing gear

[Rod]

If your tube is long enough, consider making it about F10. Then you won't have to parabolise - just look for straight lines with a ronchi screen and a good edge. Your tester can then be very simple - just a light behind the ronchi screen. No measurements are needed.

I would always make my own tester. There are heaps of plans on the net and they are simple to build. Just purchase a ronchi screen - glass ones are better but the film ones are OK too.

This site may be useful re testing:

http://www.users.bigpond.com/PJIFL/ronchi_index.html

Good Luck with your project and Merry Christmas!

Rod.

[Astro78]

Good on ya Monoxide for going for it

Must be immensely rewarding to view through ones own mirror.

Hope to follow suit one day

Best of luck - please post your struggles and successes

[MrB (Simon)]

Mate, go for it.
I ground a 140mm plate glass(19mm) mirror some time back, had a pile of fun doing it too.
Never finished it, tho it's all polished out(no streaks or pits) but not yet parabolised or coated, just spherical.
Still had a first-light with it, of sorts... on the moon, the only target that was bright enough for the polished glass reflection to be visible, worked a treat.

[DaveGee (Dave Gault)]

Originally Posted by monoxide

i found a place to get everything needed

I only just found this thread. Have you got everything OK?

guess i need to start looking at how to build a ronchi tester or would it just be easier to buy one, $60 on the site where i was looking at blanks

No don't do that! Home brewed is best.

i guess it just amuses me how accurate you can make something with minimal tools and home made testing gear

Yep, It's kinda cool to think, now I'll try and shave off a few atoms of glass, here and here, figure out the stroke you need to achieve that and do it.

[koputai (Jason)]

Originally Posted by monoxide

i found a place to get everything needed

Where's that then? In Oz or O/S?

Cheers,
Jason.

[monoxide]

thanks for the support guys,

i'll be sending an email off tomorrow to order everything needed.
i thought about making my own ronchi tester but since im a complete beginner i thought i might aswell just buy one and know that its going to work properly first time.

http://www.telescopes-astronomy.com....escopes036.htm
this guy is in Adelaide and the prices seem pretty good.

i do have 1 question though, would i be better off using a glass tool or should i make a tile tool for my first go?

[DaveGee (Dave Gault)]

Originally Posted by monoxide

i do have 1 question though, would i be better off using a glass tool or should i make a tile tool for my first go?

I used a thick glass tool for my first mirror, because Texereau said so, but it's such a waste IMHO. After the tool sat in a cupboard for about 5 years, I ended up giving it to George... I think his surname starts with a V.... anyway...

Better to make a POP & tile tool AFTER you hog to depth. This might seem overkill for a 6" but you might want to make a 12" or 16" one day and this skill is a must-have for a glass that big, so you may as well learn now.

For hogging, just use a piece of 3mm thick mild steel cut into an octagon (or more sides) that is about 60-70% of the mirror size. Then glue that to some 3/4" Ply.

Another tool you will probably need to make is a tool to grind the back of the mirror flat, before hogging. Post a picture of the blank when you get it. Again, get a piece of 3mm mild steel but 1.5x bigger than the mirror, cut it into and octagon and glue that to some 3/4" ply.

Paint or varnish the backs of the steel tools to keep out moisture. Use any old paint or varnish.

The bigger tool is used Mirror-On-Top (MOT) to grind the back flat and even.

The smaller tool is used Tool-On-Top (TOT), working Centre-over-Centre (CoC) to hog to depth.

If the above is not clear, just say so and I'll post photos of my grinding tools.

BTW, PM me with your snail mail addy and I'll post you enough glass tiles to cover a 6" tool.

[monoxide]

thanks Dave,

i was thinking a tile tool might be a better idea, that way if something goes horribly wrong atleast i have a second blank to have a go at.
im not sure i understand fully what you mean with the octagonal tools though.

i haven't recieved a reply to my email yet but as soon as i get everything on the way i'll send you a PM, many thanks for the help

btw i have been doing a LOT of reading and can highly recommend this website for anyone else thats thinking about giving it a go:
http://stellafane.org/misc/links.html

[MrB (Simon)]

The "ATM list archives" is pure gold, thats why it's up the top end of their links list on the Stellafane site

[monoxide]

still no reply from the guy i emailed

does anybody know anywhere else i can buy supplies from? (in aus hopefully)

[kinetic (Steve)]

Originally Posted by monoxide

still no reply from the guy i emailed

does anybody know anywhere else i can buy supplies from? (in aus hopefully)

Hi TJ,

Maybe the Adelaide contact is on holidays at this time of year?

I made my own mirror a lifetime ago, it was an 8 " F7.
When we did it, we bought the whole grinding kit and two
pyrex blanks (the old style thick ones) from Astro-Optical
in Sydney. Chances are I probably dealt with a bloke who is
a member here

I still have what is left of the grinding powders and cerium oxide.
I would offer them to you TJ, but I would be doing you a disservice
because they are contaminated by adjacent different powders.
What I mean is, the bags they originally came in started to perish
after being in storage for 20 odd years.
When I re-bagged them, I couldn't guarantee that any of a certain
powder did not end up in another powder.

When you grind a mirror you almost need a laboratory clean work area.
That's pretty hard to do in a backyard shed but something even worse
is having one speck of a lower grade grit in a higher grade grit!
You could get to say, 4th powder and have a terrible scratch (caused
by me ) , and have to go back to an earlier powder.
If you just want to have a dabble, by all means give me a pm and
you can have what's left anyhow.
I can't see myself ever using it again.I'm in Adelaide too.

If you wanted a clean batch and the contacts you mention don't pan
out, maybe try a lapidiary (gemstone cutting) supplier, they could have
a range of silicon carbide grits.


Hope this helps,

Regards,
Steve B

[monoxide]

thanks Steve, its great to see everyone pitch in to help out
no need to worry, if i cant get ahold of this guy then i wont have a blank to use any of the grit on anyway
cheers for the offer though!

i'm hoping its just that he just doesn't check his email too often, might try giving him a ring tomorrow if i get a chance.

[Satchmo]

tJ, 19mm thick plate glass will be fine for a 6". Most larger glass shops can cut this as they make table tops with it. Plate glass ( not the tempered stuff ) is usually well annealled. The glass will be so cheap that I'd recommend the luxury of a nice glass tool to grind it on.

Get a 4" angle grinder with a $10 diamond blade and cut 6mm deep channels in the glass on a 1" grid pattern, and that will give you great abrasive flow and prevent the discs sticking together on finer powders.

If you can't get your abrasives and polishing powder locally in descent time I can give you the name of a place in Sydney that will supply.

I'd suggest at this time of the year you may have to be patient with your Adelaide supplier

[DaveGee (Dave Gault)]

I couldn't guarantee that any of a certain powder did not end up in another powder

Hows this, If you grab Steve's stuff and use the coarse grits and I'll send you the fine grits (600, 1200 & 1800 aluminium oxide) A film container of each is more than enough. Now it's just the glass!

I have an 8" plate glass blank you can have too, if you want it.

What's left...
- pitch
- Cerium Oxide
- blood, sweat and tears (you can supply that, but skip the blood bit if you can)

[Rod]

I can supply some cerium oxide and some polishing pads if you want to try them. I also have an 8 inch plate glass blank (1 inch thick). It might be a good tool to go with Dave's blank. It has two clam shell chips - one on each side. I believe this would grind out in rough grinding if you used this blank as a mirror and would not be a problem as a tool.

I also have some pitch from a friend's old kit. I don't know it's quality but you are welcome to it.

Let me know if you want any of the above.

Rod.