Mirror making

[Ian]

is anyone interested in trying their hand at mirror making? There is an advert in the back of AN this month for free blanks and grit etc available to a society.

Is it worth further investigation?

[Whitters]

Sounds interesting

[Whitters]

Ian, which edition of An is that?

[Ian]

current one I picked up on Saturday.

[malcolm]

strikes me it's a hell of a lot of very accurate, sometimes messy, sometimes medically clean hard work. Unless a large mirror is contemplated it may not be worth the effort.

[Rick]

Is telescope-making something the Society could do with a talk about? I know there are some folks who've made mirrors etc.

[Ian]

I've read a great deal about mirror making but never done it. It seems to me, if you want a mirror, buy one. If you want the satisfaction of having made one, well then it's worth the effort. I would dearly like to have a go, just for the crack. It is possible to make mirrors by hand that exceed the standard produced by commercial operators, bearing in mind when a person makes a mirror for fun, they stop when they think it is good enough, a commercial operation will stop when the mirror is just good enough.
The reason I brought the ad to the attention of the society is it comprises of the right ingredients to practice on for free. We could easily have several dobs available for anyone to use at obs evenings and such, or perhaps to loan out. Having attended several obs evenings, it seems that there are a relatively small number of nutters who are sufficiently motivated to get their kit out, and it might be good to be able to put up a couple of dobs for use by peeps who would rather not cart their expensive kit around, or haven't succumbed yet, to partake in the relatively dark sky at Tripes Farm.

[Ian]

oh, and little mirrors are much easier to practice on, as they take much less time. Those 20" binos will have to wait...

[Delphine]

Every now and again I think I would like to try and make my own telescope.  

[Greg]

Just as a aside. I have ground and polished a 14.25 inch f5 mirror for the original Roloscope (that red ball thing I've got). Mirror making is NOT that difficult. Let's face it if I can - anyone can. Having said that, I never made the secondary. Aparently that's more difficult than the mirror itself!!

[Delphine]

Is it possible for the task of grinding and polishing a mirror to be shared by several people?  If a few people were involved it wouldn't be quite so onerous.  

[Mike]

Im surprised that making the secondary is more difficult when it is a flat mirror !!

[Ian]

it does seem counter intuitive, but the tolerances on a mirror are defined mostly be the nature of light and it's habit of diffracting. I don't really understand the details but depending on what you read, due to diffraction there is little measurable difference between a mirror whos surface is within 0.125 times a chosen wavelength of light away from ideal to one that is very much tighter. With the flat secondary however, there are measurable effects if the surface is greater that 0.1 times the chosen wavelength. I suspect that a poor secondary may prevent a scope from focussing at all, whereas a poor primary may just cause poor contrast (although it depends why it's poor)

[Ian]

interestingly, the focal length of a 6" primary that is indistinguishable from an optical flat is in the region of 13 miles. Anyone fancy working out the fov and mag using a 25mm plossl on that scope?

[Whitters]

You would get 0.13 arcseconds per mm on a detector at the prime focus on such a leviathon compared to 101.51 arcseconds per mm from a 8" f10 scope.
Don't know what thte field of view would be with ian's 'Nagler' beast.

[Rick]

Aligning the beast might be a bit tricky...

[Ian]

collimating the scope would need a 26 mile walk for every adjustment. Great if you're in training for the marathon