3 inch Eyepiece Project

A large refractor is a great telescope in that it has the potential to do pretty much everything well in one scope. However, when you begin to get towards apertures large enough to do excellent planetary and deep-sky work, the focal length tends to become too long for effective wide field viewing. For example an 8” f15 refractor is a stunning instrument but using the best wide-angle commonly available in 2” size, the 31 Nagler, the true field of view is less than one degree.

This is a pity because there’s nothing inherent in the optics of a big refractor that prevents it achieving a much wider view, in fact just the opposite. A big refractor has the capacity to produce stunning coma free wide views. The problem lies with the eyepieces.

Physics dictates that the widest available field of view is limited by the field stop of the eyepiece which in turn is limited by its physical diameter. The upshot of this is that if you have a 3” eyepiece then you can achieve a much wider view than you can with a 2” eyepiece.

The good news is that many modern refractors have really big focusers such as the Starlight Instruments Feathertouch 3.5”. Starlight Instruments produce a 3” adaptor that screws on the back of this focuser for about $109, enabling you to use a 3” eyepiece.

I recently built an 8” f9 refractor and began experimenting with 3” eyepieces so that I could achieve wide field views. I designed and built a prototype 65mm 70° that gives me a field of view of about 2.5°. After a bit of messing about, I managed to achieve a really nice flat field of view, good eye relief and low aberrations, making this one of my favourite eyepieces. It’s not quite as well corrected as a Nagler but it’s really close – and way, way better than those generic Chinese 80° eyepieces that in my experience have mushy edges. I don’t really see the point in an 80° field of view if only the middle 45° is sharp. You may as well buy a Plossl!

I’ve recently secured a commission to build a large refractor that will require another of these 3” eyepieces so I’ll be making a couple more. Now it occurs to me that there are quite a few folks out there that own big refractors so perhaps some of them may be interested in having such an eyepiece too.

So here’s my plan. I’m going to have a few made using professional CNC machined and anodised barrels (rather than the delrin and brass the prototype is made of.) Two of these will go with the new refractor commission and I’ll keep one myself. I’ll send the last one off to an independent and impartial reviewer on Cloudy Nights or Astromart that has lots of experience and a suitable telescope. Not sure who yet but I’d welcome suggestions. If his/her review is positive then I’ll be offering some for sale, if the review says it’s a jam jar bottom and Tim's eyes must be screwy, then I guess I’ll just shrug and drop the idea of selling them.

Optical design

Naturally I'm not going to publish the exact details of my design, but surfice to say it's different to conventional wide angle eyepieces. Not an Erfle, wide Orthoscopic or knock-off of a Nagler or other commercially available wide angle design. It uses 9 elements in 6 groups with all optics antireflection coated.

What telescope will you need?

Well any big refractor (6” up) of f7 or longer will work superbly with this eyepiece. Unfortunately a regular Newtonian won’t because the secondary will be designed for 2” accessories only. If you have a monster Newtonian with a 3.5” or bigger secondary, then you might be in business if the f ratio isn’t too short (I know this design works well at f7 but haven’t been able to test it on anything shorter). Likewise a Cassegrain with a 3.5” centre hole in the primary will work superbly too.

Potentially interested in having one?

If you think you may potentially be interested in having one of these, send me an email and I’ll keep you up to date on the progress of the project. (There’s no obligation at all, just looking for rough numbers at this point.) I would estimate that if the commercial side of this project goes ahead the eyepieces may be available by the end of 2011. I’m not sure of the cost yet because I don’t know the number of orders or how easily I can source the components in quantity but it will be somewhere in the few hundred dollars range. I have also made myself a 3" diagonal and may consider making these commercially too if there's enough demand.

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