Don’t be afraid of Foucault

[markse68]

like I was! well not really afraid but I wrongly thought it was a bit technical and a lot of hassle but it really isn’t and is a really useful tool- you all knew that though, right?

Heres my quick and dirty foucault tester. It took about 30 mins to make but evolved a few times during last Saturday. It’s basically a stick with 2 holes in it, an led powered by a pp3 battery and series resistor (pot optional for dimming) and a dulled craft knife blade. The led doesn’t need to be bright- it will look bright when it’s focussed back to your eyeball (or better a camera) This is called a slitless tester and is all the rage. Traditionally a pinhole light source is used and avoids linear diffraction effects but this is simpler and gives better illumination from a dim led.

I mounted the mirror to a bit of ply I had kicking around with 4 screws(bits of ply are remarkably useful!) and a machine vice keeps it upright.

I was lucky to have a couple of cross slides also kicking about which make it much easier to accurately position the knife edge but you could just move the tester around by sliding it on the bench if you don’t have and don’t want to invest. The tester is placed at the radius of curvature of the mirror (2x focal length) and needs to be manoeuvred to the exact ROC, but that’s easily done by just observing the “shadowgram” as you move the knife edge across the light cone.

Its a beautifully simple idea- for a spherical mirror like i was testing, all light from the led should reflect back to a perfect centre at the ROC. Any deviations from the sphere will reflect the light to a different position off the ROC and the KE will block them leading to those areas looking dimmer. 

It revealed the source of the problems i’ve had with my Cape Newise- the primary isn’t spherical! 😞 Here’s a vid of the shadowgram - for a sphere it should evenly dim as the KE crosses the ROC- it clearly doesn’t. No amount of collimation will fix that!

 

 

I confirmed this by testing the mirror from my Tal-1. The Tal mirrors have a reputation for being good and mine certainly seems to be- it was text book with no areas of shadow showing anywhere- it evenly dimmed to extinction

Wish I hadn’t procrastinated so much on this- it’s really a useful tool and great fun to be able to examine your mirrors surface form like this.

Anyone got a spare 8” f3 spherical mirror kicking around or being used as a paper weight/salad bowl?

Mark

 

 

Edited July 5, 2022 by markse68

[CraigT82]

Nice 👍🏼. I had a go at making one last year using a cheap eBay cross slide never got around to using it… was thinking of checking my 300p mirror to see if it was worth having it refigured. 
 

How and where did you mount your camera Mark? 

Edited July 5, 2022 by CraigT82

[markse68]

eye and camera need to be as close as possible to the KE Craig. I had the KE at the end of the bench and used a cheap tripod to position the camera. The camera has a lot of advantages- bigger image on screen and more importantly it’s locked off- head wobble is like KE wobble- hard to get a steady image. The camera was just a cheapo usb thing with a c-mount. Lens was 25mm but would have been better shorter fl- am scouring ebay now for a 20mm or wider. 

Of course testing a parabolic mirror is a bit different to a spherical mirror which is a null test. Now i need to learn to make meaningful measurements with it- so far i only know it’s “not right”

Mark

[CraigT82]

Yeah there is a mask you can print out to put over the mirror, there are segments to cut out of the mask and you move the tester back and forth to illuminate the different segments and note the displacement of the tester, those measurements you can then enter into a spreadsheet to give you quantified errors on the mirror ( I bought a cheap dial indicator with magnetic stand for this) . Again never got round to actually doing it, mainly because I needed a much longer bench to do my 12” mirror! 

[markse68]

My chances of finding a replacement mirror are close to nil I think. Any experienced glass pushers out there? Does this look like something that could be polished out or would it need major regrinding?

Mark

[markse68]

So Foucault identified there was a problem with the primary but i wasn’t quite prepared for this

Just removed the mirror from its cell and found a huge clam shell chip in the back face! No sign of what could have caused it and it doesn’t look like the mirror was removed before but this must have relieved stress in the glass causing it to go slightly pringle.

I fear that means the scope is doomed unless I can find a suitable replacement mirror somewhere…

Mark