Telescope Reviews: High power--wide field vs. small FOV

With magnification held at say around 250X, in smaller apertures, I find the light levels in the ep image to be rather marginal when using simple narrow AFOV ep's. But I have found a much brighter image obtained while using 5 element wide field ep's at barlowed higher magnifications (4.5mm/~50X/inch) and--surprisingly-- that their delivery of image detail is *very* close--if not sometimes equal-- to that obtained with the dimmer, narrower FOV ep's of about the same f/l, of theoretically better design and further, unburdoned by a barlow lens.

This seems inexplicable! The abbe orthos theoretically should be much better than a barlowed widefield ep--or any widefield ep for that matter at equivalent magnifications -- yet the images produced by the more complex wide field EPs are are consistently just as good or very nearly so.

I have a hypothisis to explain this, as it applies to the Moon only-- it is as follows; on the lunar surface the entire fov is most often fully illuminated at higher powers (much different than the situation in planetary observing.) Given this unique field illumination situation, a wider fov ep provides a view of higher overall illumination than a narrower fov ep (like an abbe otherscopic) magnification constrained. Correllate this fact with the effect upon the eye's acuity that higher illuminations affords (increased participation of the fovea centralis in image formation & communication) and I wonder if this illumination increase is what accounts for the impressive perrfomance of ep's that shouldn't by rights be impressive at all. I have found this to be consistently the case comparing a 65AFOV 12mm ep barlowed to 6mm as compared to a 6mm abbe ortho used alone as well. In other words, no doubt the absolute image is better through the abbe orthos, *but* it is too dim to be fully taken advantage of by the eye when compared to the availability of a much brighter image from a wider FOV eyepiece. One may get all that the widefield has to offer in definition but only a part of what the better corrected, but dimmer, abbe offers.

The field illumination gained *is* impressive...A 5mm with a 45 degreeAFOV in my 1180mmf/l scope, thus yields a fov of 0.19degrees of the lunar surface, where a 9mm barlowed X2 with a 65degree AFOV gives a 0.24FOV; an increase over the former by nearly 60% in effective field illumination: certainly a significantly brighter image--but at this magnification (and aperture) this is also useful brightness to get I feel. Even adding a few percent for the simpler design's increased throughput thanks to fewer elements/air/glass surfaces, still, the gain from a wider FOV on image illumination remains very significant.

So in the case of smaller apertures 4-6inches or so, I wonder: may it be beneficial to use wider FOV ep's at high powers/inch as a matter of course when lunar observing and forego the smaller FOV orthos etc altogether--dispite the lower intrinsic performance of more complex ep's? The added illumination of wider field optics provides further advantage in the utility of lightrobbing filtration at higher powers as well.

Just wondered what others thoughts were on this...

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Mardi

4" achromat, ETX-70.
Whitepeak Lunar Observatory Website

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