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Oleg, I calculate the maximum overhang and offset with those formulae. Sometimes I deviate from those numbers, depending on the actual figure of the mirror; its a matter of excperience. Then I start at offset 0 (or sometimes 5 or 10) and then decide how many offset positions I want to use. Most of the time this will be 5 or 6, sometimes 7 in the beginning of parabolizing and sometimes 4 at the end stages, when less parabolization is needed. I also decide beforehand about the number of overhang positions. Most of the time I do 5 or 6, sometimes 7-8 at the early parabolization stages, sometimes 3-5 at the end stages. Most of the time I diminish the overhang 3 mm at a time, sometimes, closer to the end 4 mm or, when I only use a few overhang/offset positions in the end stage, even 5 mm. I hope this helps.

@@janvangastel6763 Thanks for the detailed answer. I'll try to apply it to my mirror with an optical diameter of 406 mm. For the edge zone we have a 240 mm tool. According to calculations, the maximum offset is 68 mm and the overhead is 72 mm. Using your tips, we will choose 7 times at the beginning of parabolization. Will have Offset: 0, 10, 19, 29, 39, 49, 58, 68 mm and OVH 72, 69, 66, 63, 60, 57, 54, 51 mm. Time spent in each overhang position similar to video 10, 5.5, 4, 3.5, 3, 3, 3, 3 minutes. In your opinion, is this the correct calculation and time for the initial stage of figuration? Thanks!

@@oleggerashchenko2201 Yes, I think this is correct. But it is always important to check after a session, if the result is as it is meant to be. Sometimes, especially in the beginning, the edge zone will lag behind. And if that's the case it will be necessary to adapt the progression accordingly.

Not true, what's polished in can be polished out.I did this by at least 30 mirrors.

No, stroke length is 2 cm. This is not critical at all. Might also be 3 cm or something like that.

@@janvangastel6763 Thanks! What speed do you usually use for the table and eccentric during polishing? A good sphere is made.

A good place to buy glass to make mirrors is here: www.stathis-firstlight.de/spiegelschleifen/material.htm

No, I do not make mirrors commercially.

I use a concentration of about 1:100 when parabolizing. When polishing about 1:50.

@@janvangastel6763 Thanks for the answer! In other words, for 100 ml of water you should take about 1 gram of cerox. Did I understand correctly? Does this affect smoothness???

@@oleggerashchenko2201 It does affect smoothness a little.

Maybe this is not necessary, but it does not cost much time and I do it to be as sure as possible that mirror and tool do not get into resonance.

makes sense - So there is absolutely no possibility of there being any affinity between the two systems I guess. You are decoupling them by rotating every so often, "just in case". @@janvangastel6763

Very good book is: David A. Harbour: Understandig Foucault. The book covers Foucault testing and different ways of parabolizing. Other very good book, a classic: Jean Texereau: How to make a telescope.

@@janvangastel6763 I appreciate your recommendations👍. Grinding/polishing telescope mirrors is fascinating, I hope I can figure it out and make my own one day

Yes, after polishing the mirror needs to be parabolized. I have also a video about how I do that.

Thank you.

Yes, that's is right.

Je kunt de rit op Strava downloaden en een poging wagen Klaas-Jan.

I think so, but I am not sure.

If a reading is higher then ideal - above the 100% in the M-L plot in figure xp - then that zone is overcorrected, so too deep.

@@janvangastel6763 thank you sir for the reply. 🙏

I test with the Bath interferometer, see this video: th-cam.com/video/YSZZrUhg2DU/w-d-xo.html

I never correct by hand during polishing. The only thing I do by hand is removing astigmatism if necessary. I also have a central hole sometimes after polishing, but I don't care, as long as it is not deeper then it will be after parabolizing. In the early stages of parabolizing, the hole vanishes. Only for an f/7 mirror of longer it can be a problem and should be removed before parabolizing. I do that - on the machine - with a 75% tool, center over center stroke, with 15% overhang.

@@janvangastel6763 When removing the central hole your way, will the curve change? notice you do it with 75% tool and 15% overhang, does the magic also work here that will keep the curve not changed? I'm asking because my goal is to make a optic spherical mirror. BTW, thank you very much for the quick reply, it is very helpful answer!

No, I do nothing by hand. Here is a video about how I hog out a flat blank: th-cam.com/video/AdCW64TZFaw/w-d-xo.html

@@janvangastel6763 Thank you for your quick response! it is very informative and helpful.

The glass thicknes of this blank was 40 millimeters. I have also done blanks of 32 millimeters and 25 millimeter thickness. Largest was an 18 inch diameter blank of 25 millimeter thickness. No problem at all.

@@janvangastel6763 thanks. Also want to know what max f ratio I can go for incase I am making a 16 in mirror with 25mm blank?

@@satyajitbhadraroy7723 The 25 mm thick 18 incher I made was f/3.9 and was no problem at all. I am sure f/3 would be fine too. Making a good support for the back of the mirror is more important then the depth of the curve.

@@janvangastel6763 I have got it sir. You helped me a lot. 👍

The pitch I use is named 28° pitch. It is comparable to Gugolz 64, with a working temperature between ~20 and ~24 degrees C (~68 - ~75 F).

I find it easier, but maybe they might also be placed on the tool. I don't know, never tried.

I use borosilicaat glass. It's not laminated.

@@janvangastel6763 Just one more question, I'm about to buy a glass like this here where I live in Brazil but should it be tempered or not? The supplier has two types of glass.

@@Gezielthuler The glass should not be tempered

@@janvangastel6763 Thanks for the answer.

I don't know. I don't make mirrors commercially.

Yes, they did.

I clean the machine after Carborundum 80 and after carborundum 180 because of the large amount of dirt. I do not disassemble the rotating table to do that, just wipe it clean with some water. After 3 micron grit, so before I start polishing, I disassemble the rotating table do be able to do a more thorough cleaning. It's all very easy to do and does not take much time. In this video I see that I did not clean the machine after 180 grit, sometimes I don't and just take care that I do not touch the dirt before touching the mirror surface.

@@janvangastel6763 thank you so much :)

The rotating disc can easily be taken off, including the drive shaft. And also the drip pan can be easily taken off the machine. Takes only a few minutes. I clean the machine every time when I have finished the grinding phase of a mirror, to be sure there's no grit left that might scratch the mirror surface when polishing and parabolizing. I place everything I want to clean in my garden and use a hose to clean it, after having removed most of the grit-glass mud, so that I do not pollute my garden.

I have tried that, but there's not enough space around the machine to get good views from more then one side.

It took 20 hours to get to the intended sagitta.

@@janvangastel6763 woah!!!! i am building an MoM, and I hope I can hog out an 8" f/5 fast enough.

@@janvangastel6763 how many hours do you think i will require for an 8" f/5 with a steel tool?

@@astrosathya1984 I never used a steel tool, so I don't know. With a 60% (so ~120 mm diameter for an 8 incher) glass tile tool it would take me about 9 hours to get to the right sagitta with #80 carborundum.

Oblique.

@@janvangastel6763 That's really good oblique. It looks like 400x magnification which is the limit for oblique but this looks absolutely awesome for 400x mag!

@@benjamind2008 Thank you.