ATM - mirror making and testing

Hi All,

Just thought I'd give you a peek into how the 14" Folded Stevick-Paul scope is coming along. I am about half way through polishing out the 14" f/15 primary and am fine-tuning the radius of curvature (10,500mm) by polishing only. I fine ground and smoothed the curve to 10,400mm and then switched to polishing with the mirror underneath the tool so as to ensure a perfect edge (no TDE) as the edge gets polished out first in this position.



I am also using a fine nylon gauze/mesh when cold pressing to micro-facet the tool. This simple technique makes a startling difference to the speed and quality of the polishing process.


Above: Micro-faceted pitch tool


Above: After several hours of polishing the facets spread out and threaten to touch each other and need to be trimmed with a chisel

Anyone who has made a mirror for themselves will know the difficulty of seeing, let alone interpreting, the shadows using a traditional pinhole Foucault tester. I am instead using a slitless tester with an ultra-bright green LED for the light source. This produces a much brighter image and clearer shadows. I am also using a video camera on the Foucault tester stage to display the image on the TV. I can zoom in and fill the screen with the image for a very clear view. I can also make a large Couder screen and stick it to the TV screen using static cling! This way there are no diffraction effects to confuse the shadows and null-zone under test – especially at the edge, where the zones are very narrow.


Above: My video/TV Foucault tester lash-up. I use an old laser pointer on a swing arm to align the tester and mirror.


Above: The video/TV/Foucault combo produce a very clear and sharp image. Here it shows the 52% (approx) zone nulled, and also that the centre of the mirror is still not polished out (the dark splodge at the very centre – the LHS of which should be fully illuminated.) Actually, this Foucaultgram shows that there is a whacking great hole (50% of the diameter) which needs to be reduced by further polishing to bring it to the same centre of curvature as the outer 50%. Lots more sweat and grunt-work to go then!

If anyone’s interested, I’ll post up more pictures of the project as it progresses.

Dark skies,

Phil.

Thanks for sharing that Phil - very interesting. I'd like to see updates if your arm hasn't dropped off

Cheers

Dave McD

Hi Phil,

Looks very nice, how long do you think it will take to finish? (the scope that is)

Later,

Jed.

Hey man that looks cool
Please post more pictures as you progress as it would be very helpfull to me as I am going to be making my own mirror,I ordered the glass yesterday so I will be starting when I get it.

That's really cool! Very impressive!

Very interesting.
Would you be able to describe this "folded" scope to us, what is the light path?