Ronchi eyepiece for pennies

[Ben the Ignorant]

I received a free piece of plastic Ronchi grating with an optics catalog years ago. After stupidly storing it in a little box and using it by simply resting it on the eyepiece holder, I finally mounted it in a convenient tube, a 24x36mm film canister; these are well known among astro do-it-yourselfers for having nearly the same diameter as a 1.25" eyepiece barrel.

I cut the film in a shape that would naturally get stuck inside the canister's cap, but for safety I glued it in place with two drops of the blackboard paint I covered the inner canister with. The viewing hole was made with an 8 or 9mm drill, and deburred with fine sandpaper. Being able to remove the cap makes repair or replacement so easy.

 

I also painted the area around the viewing hole because the canister's material is rather shiny.

 

The whole object weighs only a few grams, so there is no need to tighten the lateral screw, friction keeps it there.

 

And how sensitive and useful is it? It proved my 80mm f/7.5 achro is quite good, ever-so-slight curved lines with some violet and yellow fringing:

 

I could also see with great relief that my new 80mm f/7 quasi-apo (with 51 glass) has very, very little spherochromatism, from red to purple the difference in curvature is barely there:

 

With only 13 lines per millimeter I know it is not as sensitive as other films but it detects figuring errors in other telescopes, an unfortunate friend's astigmatic 90mm triplet, for instance, sorry, we didn't take a pic. The near-straightness in my quasi-apo's violet range stuns me. While the achro shows some purple and yellow separation when tested with a pure white artificial star, the quasi-apo shows none when tested with a whitish-purple diode that I chose because its excess short wavelengths would make spherochromatism more apparent.

It might not be as sensitive as other finer etched-glass gratings, but given the usual obvious difference in straightness from red to blue, the fact it detects almost none here makes me happy about my sharp 80.

There is no visible color separation at all, and these full-spectrum lines are nearly straight. The dent in one line is due to a scratch in the grating that happened when I stored it carelessly. Needless to say, I will never sell my Astro-Professional 80. Except for a very faint trace of purple in certain conditions with certain eyepieces, it is perfect, that's why I call it a quasi-apo. 53 glass could improve on chromatism, but I don't see how Ronchi lines could be straighter.

All this info and more (I obviously tested my other scopes, all turned out straight) with a tool that I almost got for free. I don't remember where but this and finer plastic gratings can be bought for less than 15€. I hope someone will repost the link I saw in this forum.

Very telling optical assessment for very little money, ordinary colored filters do the job for checking spherochromatism in refractors. And while defocused star-testing seems subjective to the inexperienced, especially in refractors, judging if lines are straight or not is not subjective. That is the beauty of the Ronchi grating, best of all it costs so little everybody should have one.

[JRM]

Made one of those back in 2001 with and old empty eye piece,

think someone patented my idea after that, see them for sale

nowadays, but super easy for anyone to make.

www.ronchiscreens.com  sells them, you just have to remove the screen from the film slide it

comes in.

Cheers

Rick M