After a trip to mitzpe ramon and seeing saturn’s rings, moons of jupiter and andromeda in a 30cm telescope I got onto a telescope binge.

This article is interesting as it gets past conventional mirror/lens tech. But back in the world of imaging optics I was pipe-dreaming about feasibility of a large mylar mirror with multiple servos stretching it to some approximation of a parabola, e.g. with a servo every 5 or 10 cm behind a sheet of lets say 2m diameter.

Realistic at all?

Good telescope surface quality is apparently 1/20 wavelength which for 0.6micron is .03micron. 1/4 wavelength or raleigh limit is also some sort of standard.

Anyway to get 0.03micron movements out of a 200steps/rev stepper with a shaft diameter of 3mm (or 0.05mm/step at shaft) I’d need to gear down by a factor of >1000 which already is sounding rather unlikely . Maybe , if harmonic gears can be 3d printed and not bought.

Alternatively a servo , e.g. with feedback keyed on a calibration step? I did see an estimate of 0.05 degree for both servo and stepper.

Anyway the density of servos/steppers is another limiting factor - assuming the sheet is flat between servo points, i’ll lose the parabola shape maximally halfway bet. servos or every 5cm for 10cm spaced servos .

Assuming f/10 which is apparently good for small field-of-view stuff like planet viewing ,

focal length / mirror diam = 10 so focal length would be 20m . Lets put aside the practical matter of dealing with a 20m long focus for the time being ;

The parabola formula for that focal length would be y=1/(4f) x^2 = 1/80 x^2 in meters

The worst error is (i think) near the bottom of the parabola , if the first servo is at origin and next is 10cm away it should fix a height of

y(0.1m) =1/80 * 0.1^2 m = 0.12mm and halfway there should be at

y(0.05m) =1/80 * 0.05^2 m = 0.031mm

but will actually be at 0.06mm (half the 0.12mm height fixed at 10cm), which is already an error 1000 times the 0.03 micron desired.

So it seems servo density is a major limiter here, unless the mylar stretches naturally more towards a squared curve than straight lines between points. I suppose this depends on tension of the mylar membrane, with less tension tending towards more curvy.

Hive mind, what say you ??