Getting ready for the grind

[Geoff45 (Geoff)]

It's years (in fact decades) since I last ground a mirror and the urge is getting too strong to resist. Got the grit, cerium oxide, pitch and the blanks and have almost completed the grinding stand--copied from the Stellafane website (see thumbnail). Still need to make the Foucault tester, but that's no big deal.

I've decided to get my hand in again with something easy by starting with a 150mm (6" ) at 1000mm focal length for astroimaging. I know it's a strange focal ratio (f 6.67), but I wanted something with a bit more image scale than my current 770mm focal length 110mm APO and 1000mm seems a nice round number.

The current plan is for the next scope to be a 10" at around f5 and then touch up the mirror for my 12" GSO Dob, which looks to be a bit under corrected with a slight turned edge.

For one reason and another two nice homemade 8" mirrors never made it to Oz when I migrated in the 1970's.
Geoff

[joe_smith]

Geoff did you run a course at Sydney observatory and the one out west a looong time ago on mirror grinding ???

[Geoff45 (Geoff)]

No Joe, wasn't me.

Originally Posted by joe_smith

Geoff did you run a course at Sydney observatory and the one out west a looong time ago on mirror grinding ???

[robz (Robert)]

I ground my first 6 inch mirror when I was 13.
When it came to figuring..............no idea..............had an elderly gent do it for me.

It was an F9,and was left spherical.

Still a great feeling to make your own mirror

[astro_nutt]

I'm in the same process. I was very fortunate to have mirror blanks, grit and other stuff given to me and I will hopefully finish off the 7 inch f6 mirror once I get some 12 and 9 micron grit. Ditto Robert, it is a great feeling making your own!
Cheers!

[hikerbob (Bob)]

"it is a great feeling making your own"

Will you guy's stop that, there is already enough projects on the go around my place without that line of thought creeping in.

Bob

[Rod]

Originally Posted by astro_nutt

I'm in the same process. I was very fortunate to have mirror blanks, grit and other stuff given to me and I will hopefully finish off the 7 inch f6 mirror once I get some 12 and 9 micron grit. Ditto Robert, it is a great feeling making your own!
Cheers!

You can get grit etc from the ASV's instrument making section if you are a member. I am pretty sure I have some 12 and 9 micron spare or similar size. PM me if you are interested.

Rod

[astro_nutt]

Thanks for the kind offer Rod but I will be visiting the Instrument Section in March.
Cheers!

[peeb61 (Paul)]

Ooh...those were the day's. My first and only, up to now was a 12inch mirror, it ended up at f5.7. I cannot forget the ghostly image of the full moon on the un-coated glass during the testing stages! Still haunts me to this day.

Paul

[Satchmo]

Nice project ,Geoff.

I can recommend using a Ronchi grating on a star as final check /tweak on correction. Foucault reduction can often be misleading if you are inexperienced. I say that because I tested 30 or more amateur mirrors when I used to work at AstroOptical years ago and only found one ( an 8" F7 ) that produced a decent star image on the bench

I'm sure that if these guys had used a simple grating on the a real star to check their figure , they would have ended up much better mirrors. I even remember one which the owner assured me had a 1/10 wave figure- it might have but it was in the wrong direction - an oblate spheroid instead of a paraboloid, which would have been instantly diagnosed using a grating