Mirror grinding Kits, what size for first time grind??

[Chris]

Right, I finally want to actually go for it and grind my own mirror! I've got as far as deciding on a mirror grinding kit from Galvoptics:

http://www.galvoptics.fsnet.co.uk/telescope1.htm

Now the only thing thats stopping me at the moment is trying to decide what size mirror kit would be best so I'm calling for an opinion poll if I may, 6" or 10"?

The things to consider are that I own a 130pds f/5 for imaging and a 200p f/6 Dob for observing. I'm therefore thinking about either grinding a 6" mirror at around f/4.8 for an imaging scope or rich field travel dob, or a 10" f/6 for an aperture upgrade for observing.

I've read that a small mirror is recommended for a first grind the reasons for which I'm not entirely sure about so if someone can fill me in on this I'd appredciate it I have also read that you don't start to break even cost wise unless your grinding a 10-12" mirror and a small mirror would be cheaper ready made.

I wouldn't say money saving is the major motivation for the project but I don't really have much money to burn. more the satisfaction and the challenge aspect of it all. My motivation stems back about a decade when I first read a book on the subject at Uni. I was thinking of doing it as part of my final year project for my Astronomy degree because my maths sucks big time and needed to find something practical to do in order to boost my overal grade. I ended up doing an Astrometric study of variable stars and Exo planet HD209458b to help qauntify the Uni's equipment with a view of professional research which I enjoyed, abeit the idea off grinding my own mirror was shelved until now.

So any advice on what size mirror and why would be great, are there any major dificulties to grinding a larger mirror over a small mirror, and any other tips?

Thanks

Chris

[Schorhr]

I can't answer any questions but I wish you good luck with grinding your own mirror :-) It's something on my to-do list though.

I'd start with a 6", as if all fails, I could still make a f/8 out of it as the strehl ratio for a 150/1200mm spherical mirror is still decent enough to use, and it would be little money wasted :-)

[JB80]

I opted for an 8", must get around to finishing that soon.

[m37]

Good on you man, I would love to do this one day. Go for 10" because if it works it would be awesome. Death or glory!!!! If it doesn't you will at least have a massive shiny ashtray, albeit a quite expensive one!!!

Seriously, hope it goes well. Keep us posted.

[Chris]

Thanks guys I think this opinion poll shows that 50% of people think a 6" mirror is the best place to start and 50% think 10" so I'm still in limbo really as there is no clear trend of opinion. I think I'll finish reading my mirror grinding book and hopefully by the end I'll have a better feel as to where to start.

Schorhr - I do think 6" is sensible and like you say f/8's need very little figuring from what I've read so it will be the easier option for sure, one day I lean towards 6" for these reasons then 10" the next because of the motivation to build a larger aperture scope than I already own, classic aperture fever

JB80 - good luck with finishing the 8", I know it can be difficult to find time sometimes

m37 - Thanks for the enthusiastic support 10" would be cool, alot of work for a newbe glass pusher though which is a worry.

Thanks again,

Chris

[Daniel-K]

10" F6 would be very nice chris

[kirkster501]

Would quite like to grind my own too Chris. But mirror is only part of the job. What about the tube, spider, diagonal, mirror cell, focuser etc? Do the math: economies of scale are working in skywatchers favour and not yours or mine on a hombrew project. How much to make a Foucault tester for that one mirror? How much to make a tube? Not being negative, just weighing the pros and cons of embarking on such a project..........

steve

[astronymonkey]

Hi Chris,

Im not sure where you are but John Nichol is doing a mirror grinding workshop in the north east in September that might be worth you considering.

http://www.nicholoptical.co.uk/seminar.htm

cheers

[Schorhr]

Good luck, whatever you decide on ;-)

Why Not grind a 6", later make a astronomy beginner Happy, when you finish your 10"?

I have no clue about mirror making except reading about it, and that it's a load of work.. Never the less many seem to get hooked, making more mirrors then they had planned...

[cassiewoofer]

I'm sure you could start with a ten and do a fine job, but I read in one book, it's quicker to make a 6" and a 10" than to just make a 10". and having done it I agree.

Schorhr makes a good point too..... my six and 8 inch mirrors or scopes will soon be donated to a worthy youngster.

You will find that figuring the 6 inch is a real challenge though due to the small amount of glass to be removed.... and the difficulty of reading the shadows when foucault testing.

I think if I were to make another six, I would make an F9 and leave it spherical!

[JB80]

JB80 - good luck with finishing the 8", I know it can be difficult to find time sometimes

Cheers, and good luck which ever way you go too.

The reason I chose the 8 was when done it would be the biggest aperture I have, why do a 6 when I already own one.

That said I'm fixing for a larger scope anyway but still 8 seems manageable and there are plenty of good travel dob designs to base it around.

I've hit a standstill atm with moving house and family etc.. plus when bevelling down the edge I managed to put a few small chips in it, nothing critical yet but it stopped me in my tracks.