Dollond ep

[markse68]

I just looked up @Robs “new” Meade MA9 ep and came across a thread on CN where someone talked about converting one into a Dollond ep- an ancient super simple, highly compromised but apparently unbelievably sharp eyepiece.

so I went rummaging and found this old “junk” plossl and will take it apart later to create a ~20mm Dolland ep which I’ll try with 2x barlow next time I’m out- quite excited about it 😉 

They talk about sharpness and contrast that is a step up from the best orthos! And completely glare/scatter free! A poor mans monocentric apparently. Obviously the severe aberrations and very narrow fov make it pretty impractical but 😳 got to be worth a try I reckon

Anyone else tried this simple hack?

Mark

Edited June 19, 2020 by markse68

[mikeDnight]

I've not tried this but I'm very interested in your results, especially regarding on axis sharpness. Who knows, you may have found yourself a profitable side line rehashing old eyepieces! ☺

[markse68]

I will let you know how it goes Mike- I’m super intrigued myself. 

Surgery is done- very simple indeed. The idea is you use one of the plano convex doublets and invert it so the convex part faces the eye. The doublets strangely were  partially edge blackened so I fully blackened the lower one to use for this- the top one  having cleaning marks from its past life. I used some small o-rings to make up for the missing lens cos I was too lazy to make a proper spacer.

Looking through it at a bright led desk light it shows none of the usual hazy flare you get with a lot of eps as you move the light across the fov and it internally reflects.  The fov doesn’t seem all that small though only the central part will be usable.

Mark

[John]

Interesting experiment.

The ultimate low scatter, high transmission eyepiece is reputed to be the "ball" eyepieces comprising a single spherical element. Miniscule AFoV and eye relief though.

Siebert produced a commercial version, the Planesphere series:

https://www.siebertoptics.com/Planesphere.html

 

[Rob]

Nice Mark.. I did read through the same CN threads. I was intrigued about the dolland conversion also. Thanks for posting this

Best Rob

[markse68]

I couldn’t wait to try so went out last night for a quick first look at Jupiter with it.  Conditions weren’t the greatest with a bit of wind and a lot of whispy cloud Interrupting viewing but the seeing was pretty ok when the clouds parted. I compared it to these 2 10mm eps- the Parks Gold GS-5 and BCO. It seems if you use half a plossl you effectively double the focal length so the 10mm plossl became a 20mm Dollond. So I used my 2x Tal barlow with it. Barlowing  makes a lot of sense actually as the field curvature of the doublet is really bad and doubling the focal length of the ota helps a bit there.

so how was it? Well I don’t want to draw any conclusions after just one session in not the best conditions but what I can say is it was no slouch when it came to sharpness and contrast, revealing just as much detail as the other 2. Was it better? Not earth shatteringly if at all lol. I couldn’t really see much difference between any of them! But that in itself is kinda amazing- that such a simple lens can compete with a well regarded ortho in terms of sharpness and contrast! Of course that was only in the centre but that was plenty to view the whole disk at 160x- the edges dropped off a cliff with the moons blurring into smears) 

 The Parks Gold is interesting too- it seems to my eye to give a lower contrast almost pastel like view yet it showed as much detail as the others with a wider more comfortable afov!
 

It was “fun” using the Dollond though- on a rickety old dob in gusting breeze with a tiny fov 🤦‍♂️ I persevered but it wasn't ideal. On a driven mount or still conditions it’d be more usable. The afov wasn’t the biggest problem though- it was the eye relief that I found most troublesome. Unlike the orthos which can be a bit too close, this had huge eye relief- too huge really. It was quite tricky to hold your head just right without suffering blackouts and I thought at times kidney beaning! So not really a comfortable experience. 

I tried it with a 3x barlow against some 6mm orthos with similar results but the fov was sharper at the edges with the longer effective ota fl. but much more of a mare tracking targets!

(I also discovered that the CZJ 6mm ortho that came with my telementor despite more flare from older coatings showed I think more detail than the BCO 6mm 🤔)

Looks like we have a return to fine weather next week if nobody buys any new Astro gear! 😉🤞 so will give it more time in hopefully much better conditions then 😊

[DaveL59]

interesting experiment Mark, not sure I'll pull any of my TAL plossl's apart tho  I've yet to try the new TAL 2x Barlow I recently acquired tho I expect it'll perform nicely when I do get the chance.

[markse68]

2 minutes ago, DaveL59 said:

interesting experiment Mark, not sure I'll pull any of my TAL plossl's apart tho  I've yet to try the new TAL 2x Barlow I recently acquired tho I expect it'll perform nicely when I do get the chance.

Hi Dave, I like my Tal barlow- it’s very compact and optically I haven’t noticed any issue. I compared it to a more modern 3 element “apo” barlow from AE and it compared well though did suffer some slight warm sepia colouration but I only really notice that when doing a direct comparison like that.