Building a 20' F/3.3 Renegade Telescope: Part 1

Documenting the telescope build, including mirror grinding and figuring, and construction of the Renegade telescope body. Part 1 includes prepping the mirror for grinding, spherometry on the molded blank, fabricating a steel/plaster grinding tool, and setting up the fixed-post grinding machine for rough grinding operations.

[verdatum]

Gordon! I'm overjoyed to see you publishing vids again. You are pretty much unparalleled in the realm of telescope mirror making on YouTube. I'm amazed there isn't more competition for that spot. Keep up the fantastic work!

[timcorso]

What happened to part 2 etc? Was looking forward to a complete job, prior to install :-(

[everythingquads]

Thanks for publishing your videos, i'm looking forward to the next edition in this series. Great work.

[kippostar]

Do my eyes deceive me?! A series? I can't wait. Good to see you uploading again Gordon!

[KMegahertz]

Question for you Gordon (others feel free to comment as well).

I've recently started hogging out a 12.5" blank by hand. I've made a similar 6 inch tool as to the one used in this video and I am just using TOT/COC strokes and rotating mirror and tool every so often. I'm not allowing much on overhang but I am sure some is happening.

Progress is going well, getting about 0.2mm of depth in the center for about every hour or so of grinding. However I am getting a bit of chipout on the edge mainly on the bevel (~1/8" bevel) itself. They're rather small and none that protrude inward on the facial surface of the mirror more than 1mm or so.

I don't feel they're catastrophic and could probably be ground out and compensated for later on with rough grinding with a ceramic tile tool when the time comes and a bit of rebeveling, but I did want to ask if this is normal/expected?

Despite your efforts to reduce the chipout using the methods/tips in this video, do you still get some here and there?

Any feedback would be greatly appreciated.

Gerald

[tonisee]

Thank you for those extremely educative and information-rich videos! They made me seriously thinking about finishing my roughly grinded mirror from ca 25 years ago...
When I watched this video, few questions arised:
- are there any important differences between dental plasters or would more or less any of those fit well for grinding tool?
- do you make the tool somehow waterproof (it looked like in an another video the same tool is somehow.. more shiny)?
- the turf surprised me quite a bit - isn't there the danger that larger particles get stuck in it and may find their way out of it? Maybe it's a general question: what general procedures you follow to keep the machine and tools reasonably clean (I found one of your answers about changing grits and that made perfectly sense!)?
- where to learn about using those fixed post grinding machines and theory of using them?

[rschiwal]

Could you please show where to get the glass?

[giulianofischetto]

Idolo! Legend! love these tutorials, enjoy the grinding!

[morningstarx]

It might be worth your time to look into marble and granite fabrication tools such as diamond polishing pads and diamond grinder cups. The pads run on a pneumatic polisher, or just an angle grinder, and come in 50, 100, 200, 400, 800, 1500 and 3000 grit. A #50 pad would work really well for that beveling, and the polishers they run on have a water connection so both hands can be focused on the work. If you wanted you could run the entire set on the bevels and bring the bevel up to a nice polish. I've polished a ton of glass using those pads. I have a few sets as I program CNCs in the stone industry, as well as do fabrication.
Clearly you know your craft much better than I do, and I'm only mentioning it because in my mind it could help. The stone industry in general shares many similarities in regards to cutting and polishing tools with the glass industry.

[newton]

Hi. Very interesting video. But I have a question, how do you determine the f value of the mirror? Thanks in advance.

[attilamadai]

Hi,

I just made my first grinding tool out of dental plaster based on the video I found here (thank you very much!!!).


But, unfortunately, the tool warped significantly after pouring, during or after solidification.
The center of the tool rests in the middle of the mirror, but the edge is curled up, so it wobbles around the center (more than half a millimeter on the edge!).
If I drop it on the mirror like this, only the center works, therefore it deepens the glass disc too much, so the focus will be shortened.

Has anyone else experienced this warping or distortion phenomenon in the case of grinding tools made of dental plaster?
I suspect that the change in volume of the plaster (even if it is minimal) becomes uneven due to the steel or ceramic pieces inserted in it, and this drags its shape: the lower part cannot contract as much due to the metal pieces as the upper, homogeneous part, and this pull up the edge of the disc.

How thick should a plaster tool be made so that it does not warp like this?

Thanks a lot in advance,
Attila

[CrossoverManiac]

How long does the rough grinding take for your 20" f/3.3 mirror blank?

[carlosmero]

Best regards, all your videos are very good, please can you tell me how I can octener q products you use, to polish and finish the crystals, l'm from Ecuador ( Manabi-Manta).
Beforehand l am very grateful.

[panther]

Nuts vs Ceramic Tiles vs Plain Pitchlap. I'm getting confused which method is best....

[RandomsFandom]

You should look up how they groove mill stones for grinding grain. The pattern is important to keep equilateral in every direction to avoid a triangular grinding action. Much like a poorly sharpened drill bit. The inconsistencies in grinding plane create a wobble in the ground surface

[poly_hexamethyl]

12:30 Just wondering....how can you grind something to within a few nanometers of accuracy when it's sitting on something soft that compresses by millimeters?

[billlacasse]

How much should one expect to pay for a glass this size?

[travismiller]

i grind a lot of borosilicate myself... taking the edge off with the dremel seemed sketchy... but gets the job done. after having used resin bonded diamond grinding blades and bits, i hope never to use sintered tools again. i wonder if stained glass grinder drums would be more affordable for you though- seems dremel tools are kinda pricey. you can “sharpen” the drums with dressing stones for longer service life, and in addition to more tool surface at a lower cost, i bet the diamonds run deeper. check out Radiac’s superabrasives catalogue for a daunting variety of badass grinding tools, in sintered or resin.

[twistedhairball]

Hi Gordon, I was wondering why you would use the dental plaster rather than a copper plate, like you would in a machinists shop. I imagine it's due to cost and you being able to make them bespoke for each size you need, but are there any other reasons.

[joseplem]

Hola Gordon, gracias por compartir tus experiencias. Te hago una pregunta, cuál es la relación que tiene que tener el diámetro de la herramienta de yeso con el diámetro del espejo primario para que quede esférico?