Meade 5000 Series Super wide angle, why mostly 2" eyepi

I have been looking for some good eyepieces for a while that wont cause me to sell my liver on ebay to fund them, so when I read the review of the Meade 5000 Series Super wide angle eyepieces in October's "Astronomy" i got excited, as i could get a full set of eyepieces that would do me for many many years.

My scope takes 1.25" eyepieces, and although the 16mm,20mm and 24mm pieces come in 1.25" size, the 28mm, 34mm and 40mm are all 2" eyepieces.

I would like to know, is there a reason why the longer focal length pieces are 2" only ( such as a long focal length like that would add nothing extra to a scope that takes 1.25" pieces), or is it just a Meade decision to do this?

High power EPs are designed for best correction on-axis while low power wide-field EPs are optimised for off-axis and on-axis viewing.

I'm not an expert in eyepiece design, but I would imagine that the shorter focal length eyepieces have a significantly smaller "sweet spot" of best correction than the longer FL EPs. The reason may be that the shorter FLs are usually used for close examination of an object under high(er) power that the observer has first found and centred under low power.

Longer FL EPs are used for low-mag wide field viewing where you want an ultra sharp image right out to the edge of the field to get the "wow" factor. Perfect correction out to the edge of the FOV takes lots of glass in exotic shapes and formes... both of which are expensive!

Just my 2c worth.

Phil.

I always thought I knew the answer to this, but on thinking about it, I cant give a definite reason why ??

I suppose the best way to look at it is imagine 2 widefield eyepieces with exactly the same specs except one is a 2" and the other is a 1.25", what would be the difference, I am guessing that the 1.25" eyepiece would suffer worse vignetting.

Fintan,

I think its a case that ultra wide field eyepieces need the whole of the light cone coming from a scope to work best. Scopes like fast newtonians have a very wide light cone and as such need the wider eyepieces to fit it all in. This can only be provided in a 2" format. By cutting off the illumination by using a 1.25" format you effectively negate the whole selling point of these ulta-wide angle eyepieces.

I always thought I knew the answer to this, but on thinking about it, I cant give a definite reason why ??

Dave I am with you in this, in that I too thought I knew the answer to this one, but being cursed with engineer's skepticism, I opened a spread sheet to calculate the field stop diameter required by different eyepiece/telescope combinations. I was surprised to see that the aperture or focal ratio have no effect on the field stop diameter :shock: (However Dave G is correct in that a faster 'scope, with its wider cone of light will require the eyepiece and focus tube to clear the edges of the light cone, but that is another calculation)

My spreadsheet, if it is correct, gives me maximum focal lengths for 1 1/4" eyepieces of 32mm for 52 deg, 22 for 70 degree and 19 mm for 82 degree apparent FOV eyepieces. For 2" eyepieces I get 38mm for 70 degree, 33mm for 82 degree and 27mm for those new 100 degree TeleVues.

I hope this helps, but most of all I hope my sums are right!

Michael --

You maths appear good to me.

There are a few higher-order design constraints which change things a little. For instance, a design which has some rectilinear distortion (either pincushion or barrel) will result in a slightly larger or smaller true field of view than predicted by the fieldstop (or, from the point of view of this thread, will have a slightly smaller or larger fieldstop than predicted by the focal length and apparent field of view).

The other one that often bites you is that a lot of the widefields use a positive/negative design -- that is, they spread out the beam before they re-focus it. Generally speaking, the spreading out lenses are inside the barrel itself, which therefore has to be larger than the fieldstop size (or it will vignette the spread-out beam). The 17mm T4 Nagler is a good example.

In some designs, the positive lenses are so far down the barrel that the field stop is actually between the positive and negative elements. I think you'd pretty much have to raytrace one of these to get any useful predictions -- other than that the barrel probably needs to be larger than the fieldstop. The 31mm T5 Nagler is an example here.

-- Jeff.