Going to grind first mirror ever for 6" Long Focus Reflector - Need some help...

[Jethro777]

Hi,
I am really looking forward to grinding my first mirror!
I have picked up a lovely DIY telescope made 57 years ago, and I would love to see it do another 57 years.
The mirror has an upturned edge and isn't in the best condition, so if I replace it, and put on a proper secondary (which I have simply ordered), I think it will be lovely!
I have ordered some texts to learn from (Jean Texereau's How to make a Telescope and Sam Brown's All About Telescopes), and have made some preliminary decisions - but having no practical experience, I don't know how well founded those are.
I am aiming for a 6" f/9 mirror 1/18 wave PV or better (aim for the sky, right?.. RIGHT?). I wonder if I can ever do that.
I was going to use just a glass blank (150mm (6") diameter x 25mm (1") thick), then make my own tools using plaster and tiles. But, I could get a Glass Tool 150mm (6") diameter x 19mm (3/4") thick at the same time I buy the blank - is that better?
For my grits, I was going to go Silicon Carbide 120, 240, 400, 600, Aluminium Oxide 800 then polish with Red Rouge (Yes, I know it's messy..)
I'm rather worried about the testing equipment. I think I can hang it on it's side and do a knife test, but I can't do much more, and I might need some help with figuring it at the end.
As far as grinding goes too, I don't have much to grind on - so I was thinking of trying to sit on a bench "John Dobson style".

https://youtu.be/snz7JJlSZvw?t=835

Is that okay?
I was going to follow his instructions somewhat for finding the focal length I need, as I need an f/9, but am also getting a spherometer to measure the depth accurately.
As far as the physical scope goes, I might be able to enhance it a little further. The current secondary is too large (It had a massive 1.8" diagonal :eyepop:) and is obstructing too much light (30%). I am replacing it with a 1.3" (22%).
I can add some extra blackening behind the secondary too.
I was thinking of cutting some air vents into the bottom to expose the bottom of the glass to air, and perhaps create a cooling 'door' near the mirror.

Would love any suggestions or advice on grinding or any advice you might have. If you make mirrors and are around Logan, Brisbane, I'd love an email!

[Merlin66]

Jethro,
Have you considered re-grinding the existing mirror? 90% of the work is already done...
I would add N E Howard's "Handbook for Telescope Making" to your knowledge base.
Howard and Texereau guided me through a number of mirror making sessions.

[Jethro777]

Well, I was trying to have a fallback plan in case I completely screwed it up. :)
Here is a picture of the existing mirror...

PS - Just ordered Howard’s book. :)

It just occurred to me that I should do a knife test for the existing mirror and see how it goes, right?

[Merlin66]

Yeah,
that would be worth while - the mirror may be acceptable.
You could also check with a Ronchi grating and centre of curvature - at f9 the lines should be pretty well straight.

[Jethro777]

I don't have any experience testing mirrors yet, but I have looked at some videos. I'd love to see how to (1) actually use one properly, try it myself, with a mentor next to me, or perhaps (2) get some help building it (I'm not much of a carpenter) or buying one from someone who doesn't see themselves testing mirrors anymore.
Is there anyone in Brisbane that might be able to give me a hand with test equipment?

As well as that, the mirror is coated, obviously - so it would need to be resilvered and recoated after the refiguring, right?
Does the coating need to be removed first? How is that done safely and evenly?

[Wavytone]

Jethro this path is for those who are reasonably good at making things - or at least have the space for all the mess and prepared to buy the tools and materials and spend the time to learn.

In particular to make this work you must value your time as zero because even at zero, because you’ll discover this is going to take many hundreds of hours.

If not, to be honest you are far better off buying a finished mirror if you want a working scope in finite time.

And I’m aware the mods will delete this post for daring to express a contrary opinion.

[bojan]

Totally agree...

I tried to make two mirrors in the past (long time ago). I was at high school then, living with my parents in two bedroom flat....
The noise produced by grinding of 13cm was temporary detrimental for our relationship with neighbours.. Not to mention mess and frustration when mirror stuck to tool (fortunately, the attempt to separate them by applying cold and hot water was succesful, but I almost dropped the mirror into bathtub, it could have ended with disaster...)
This mirror was silvered and used in telescoipe, build for federal competition.

The next one was supposed to be 20cm, never finished.. because at the that time ('80-ies) Coulter Optical from USA offered 10" f/5.6 for 105.00 US$.. the one I still use in my bartelised dob.

So... think about it.

[Jethro777]

We all start somewhere and I do want to learn. But I acknowledge I need some help.

>prepared to buy the tools and materials and spend the time to learn

Yes, prepared to do that, but I know enough to know I will need a mentor. Experience doesn’t have to be the best teacher. ;)

[bojan]

Yep..

BTW, there is sticky on this forum, probably you know but just in case her eit is:
http://www.iceinspace.com.au/forum/showthread.php?t=81660&page=18