Limits of optical designs (e.g. Delos)

[Chipela]

While planning a perfect eyepiece collection (), I started to wonder what (in general) are the limiting issues in not offering longer or shorter focal lengths with given optical design. (I assume here that e.g. Delos line of eyepieces share a common optical design and only the dimensions & lens parameters are varied to provide different focal lengths.)

For example, a lot has been speculated about Delos eyepieces with focal lengths > 17.3mm but they never appeared on the market. One theory I read was that the field stop had reached its limits for 1 1/4" barrel size. For me this doesn't sound too big a limit since why couldn't they move to 2" and continue providing longer focal lengths. So there might be some other reasons.

The situation is a bit unfortunate since the mechanics, eye relief and comfort are so good in Delos line that I'd be very happy to eventually get still one more, longer focal length, Delos. Now moving up would mean to move to Panoptic and Nagler line of eyepieces if Televue is still preferred.

[John]

Actually many eyepiece ranges don't share a common design through their focal lengths. Some do, like the Nagler T6's, but many don't. Those that do are referred to as "scaled designs".

Tele Vue won't produce eyepieces that push the design paramaters to the extent that they won't measure up to their exacting standards of sharpness etc. Thats why you won't see things like a Tele Vue 25mm Ethos even though you could in principle squeeze the field stop into the 2" barrel format.

It's worth reading the articles on their website - whether you like their products or not Al Nagler does know what he is on about I feel:

http://televue.com/engine/TV3b_page.asp?id=154

 

 

[andrew s]

Eyepieces are much harder to design than any telescope. I would recommend Telescope Optics by Rutten & van Venrooij for an excellent discussion on eyepieces.

Regards Andrew

[Chipela]

3 hours ago, John said:

Actually many eyepiece ranges don't share a common design through their focal lengths. Some do, like the Nagler T6's, but many don't. Those that do are referred to as "scaled designs".

 

Thanks for answers. I see that my question was over simplistic, I assumed that scaled designs would be more common.

The references you provided look interesting, thanks for those as well!

[Northern Soul man]

Hi Guys, great advice here, but another thing to think of away from direct lens placement/design - just have a look at previous designs/focal lengths - when a "new" design hits the market - think of all the previous designs that  came before - surely X company in their advertising blurb aren't going to say that this design is better than their previous designs of the same focal length - they just ain't going to sell the well tried and tested designs of yester year down the line.

Same scenario if you can compare washing powder to EP's !! - you've seen the adverts "our best formula yet!! - if you believe this - not that us Astronomers are going to use detergent to clean our mirrors!! - I think the real clever part of the advertisements for new products is to word it in such a way that they can keep plugging their older designs - and keep sales up - whilst bringing newer lines in - obviously, the new designs are going to show a surge in sales, because they are "new" and the "best yet" - but also keep a beady eye on sales of previous lines to keep these still "current".

 

Paul.