Interesting experiment

[Andrew*]

From time to time I experiment with the true field of view through my eyepieces, and I always learn something new! It's a simple matter of hanging a tape measure at one end of the garden and viewing from the other!

The other evening I put all my eyepieces to the test in my various scopes and read the tape measure at the field stops, comparing it to the theoretical values. In general, the eyepieces were all within 3% of their stated field, which can probably be accounted for by rectilinear distortion in the eyepiece design and small errors. However, the results with barlows in the train differed considerably.

As far as I know, barlows do not necessarily stick to their specification. They will be closest when used straight before the eyepiece, but even then it's not guaranteed as different eyepieces have their field stop at different distances from the barrel stop. If it's closer to the eye lens, the barlow will magnify more. So, for example, my Antares 1.6x barlow in my 40mm Aero gave 1.7x, but in my ES100°s, 1.5x. When I put it in front of the diagonal in the optical train, it increased to 2.6x magnification.

My Klee 2.8x barlow gave 3.2x magnification with a Meade 24mm SWA. Didn't test it with any other eyepieces.

But there was an eyepiece surprise as well! My 3.2mm TMB Planetary II eyepiece I worked out to actually be about 2.55mm! 25% more magnification than you're expecting. I think this might have been a case of mislabelling the body of a 2.5mm, rather than incorrect specification, but who knows?!

Test your collection and see if you get any surprises!

[John]

Very interesting Andrew - I'll give it a go sometime with my lot and see what I get

I know that the Antares 1.6x barlow used with Ethos eyepieces gives a variety of amplifications and none of them are 1.6x !. Someone on Cloudynights forum worked out all the combinations - I'll try and find it for interest.

[MatthewMarkWray]

wow never fort of the tape measure ahah nice one will give this ago ! thanks

[stevepeverall]

I'm willing to bet that you're a scientist by profession. I saw a documentary a few months ago on Isaac Newton's life and when he was a boy he traced the suns shadow across his bedroom wall; made marks on his bedroom wall and in subsequent years could apparently tell the date. If you're not a scientist then you should be!

[Schorhr]

Wow, great idea!

About the Planetary-Eyepiece, I switched around the front Barlow elements of the the I have with mixed results. Perhaps they did mix it up... or does something prevent it from being screwed in all the way?

I certainly will check mine when having the chance