Building a Plossl eyepiece

[Chriske]

Just finished building an eyepiece and I was surprised it worked, and it worked rather good as a matter of fact. It was a quick test using a refactor with a 1 1/4 focuser, handhelding that eyepiece. So I could not see the entire FOV.

From what I've read the acromats should be placed as near as possible, front lenses pointing to each other, so I did.

I made a holder in which I could change the spacing between the two. Observed a large post at about 300m distance : in both occasions, lenses very near and also at about 60mm apart the result is just the same.

How is that possible...?

Lenses 50mm diameter(was a binocular) F=180mm

 

Thanks...

[Louis D]

Probably because greater separation just means losing more edge rays (I think they're diverging between groups), unless a wider eye lens group is used to capture them, and so a narrower apparent field of view results.  Give it a try to see if the AFOV narrows with increasing separation.

[John]

There are some calculators on this web page which might help. They are designed for those making their own eyepieces:

Astronomy Boy: Eyepiece Focal Length Calculators

[Peter Drew]

I've made several Plossl type eyepieces in the past.  I found that minimum separation provided by a spacer wide enough to prevent the inner convex surfaces from touching gives the best performance.  The resultant focal length is roughly half the focus of a single achromat.  The images are nice and bright with little distortion and good colour correction.  Downsides are medium APOV and excessive eye relief.     🙂

[Ratlet]

There is a guy called red who makes double plossls out of old broken binoculars. 

Apparently they work very well: 

https://youtu.be/iEUEvv4s3N4?si=Lf-bsySyMsem-CsE