Spherical mirrors are way easier and cheaper to make. Any cheaper telescope will have spherical mirrors. But, if the f number is larger, or Numerical Aperture smaller, this really doesn't matter. It's like the difference between a Chevy and a Porsche. Depends whether you have lots of budget or just want to get from point A to B

To the best of my knowledge, you need to spin cast to yield a parabolic shape. Which, is what you want… if you are going to spend you time and money to do something, well… anyway.

My understanding so far is that it's the characteristic of the strokes and the relative size of the tool Vs blank, but since he says the tool may be as big as the blank I think that means the push and pull means that more pressure is placed on the edges of the tool than the centre in that case but perhaps the grind is unstable so that any deviation is amplified because the tool edged will be proud from the perspective of the mirror surface as it begins to curve.

In other words, the blank sags a little when pulled halfway off the tool. Mine had a handle on the blank. I just walked around. Pushed and pulled it. Wash it off, wet it, point toward Sun. You can see the focal length change.

An even simpler explanation - The tool is always grinding away at the centre, since the strokes used are always catching more than half the blank at once. The edges don't get cumulatively ground as much as the centre - that is, there is always an outer sector of the blank which is NOT in contact with the tool. The central area is always in contact on every stroke. Doing this, you are guaranteed that the centre will be deeper and will end up with a dished shape. As you go from the centre to the edge of the blank, the amount of cumulative grinding decreases smoothly. The uniformity of the dish (i.e. the variability of the height around the edge relative to the centre) depends on rotating the blank regularly and evenly.

Hah nice! What a good book!

Naturally. If you ground it like it is shown in the video it will naturally create a concave mirror and convex tool.

Because a sphere is made of a bunch of circles. For this to work, you have to be very diligent in keeping even. The more gradual the material removal, the easier that is. It won't be perfect when you do it by hand like this, to make it perfect you have to make a machine to automate the process. The point here is that you can make a good enough mirror to get started for way easier and cheaper than a super fancy telescope. You totally can automate the lapping process, but you can also imagine how much work that would be, and a beginner needs easier quicker reward for his efforts, so he's motivated to push on

Thanks!

For a good mirror, tolerances are close to quarter wavelength, unachievable by 3D printer… and for the tiles glued in it, you don’t really need any pre curve in the tool.

Thanks!

Thank Manoj!

Thanks Pinak!

Hi, thanks for watching! The tool starts out flat!

@@AustralianAstronomy does the tool get curved itself while griding? What if the mirror gets convexed 😅?

Hi John, My route would be to ask your local astronomy club esp. if they have a telescope making section, you'll likely find people who could sell you a blank, if not you could ask around on facebook or similar platform in astronomy groups. Cheers!

Thank you very much for the information Logan.

Maybe you can share this info there if anyone you know is interested.

Discovery world for the kit.

The only thing is is they have to be stress free so well annealed, for good measure I'd go through an astronomy mirror retailer. Cheers!

Yup. You send it in to a business that has a chamber that vaporizes aluminium which then deposits onto the glass making it a mirror.

@@UpcomingJedi ohhhh okay. You don't have the name and info of this business by any chance?

Cloudynights, or gotgrit

Yeah, you make the tool with regular plaster like his.