Re: (IAAC) Planetary eyepieces
At 04:10 AM 6/17/98 -0700, you wrote:
>At 03:56 AM 6/17/98 -0400, you wrote:
>>>Since the light transmission of Naglers is a few percent less than the
>>>Orthos, and the design is not as well corrected as orthos across the field,
>>>it may seem that the Nagler is "sharper, brighter, and clearer" than a good
>>>ortho - especially at low power on a fast scope, where other aberrations
>>>will overwhelm the correction of the ortho - but in reality the perception
>>>is nothing but fantasy.
>>Al Nagler has specifically stated to me quite the opposite, and I am
>>surprised that he did a 180 degree reverse.
>Well, let's be certain we are talking about the same thing here. Al said a
>lot of good things about Naglers too, not unexpectedly since he owns the
>company. However, wrt giving brighter images, Nagler explicitly agreed with
>my comments about this at Riverside. Since there are more elements, and
>more air-glass surfaces, and there is in fact less light making it through
>the eyepiece (losses are due to absorbtion, scatter, and reflection) than
>through an ortho; thus Naglers are dimmer than the well-made orthos that
>were under discussion. However, you say:
>>In fact, he used his work in the field of television/projection
>>as a specific example of how Nagler's hold more contrast.
>There are circumstances in which Naglers might provide more contrast on
>specific scopes than an ortho would. A Nagler is optimally corrected for -
>if I recall Al's comments right - an f/5 or longer system; orthos are
>optimal at around f/7 and longer, depending on how closely one adheres to
>Abbe's design (which is the only kind of ortho I am talking about). I
>suppose that there are f/5 instruments in which a Nagler will degrade B-b/B
>less than an ortho would, due to differences in the eyepiece design.
With limited experience of a wide range of eyepieces I have a 4.8 mm Nagler
and a 4.5 mm Orthoscopic for my f4.4 20" newtonian (Dobsonian mount).
Recently I compared the view of the moon with each eyepiece. The difference
in contrast was really striking. I would say the Orthoscopic gave almost
twice the contrast of the Nagler based on a very crude estimate of the
blackness of crater shadows.
As I said in an earlier communication I could not say how much is intrinsic
to the eyepiece and how much to reflections inside the telescope and
particularily in the tubing of the focus mount. I feel these effects would
be worse with a wide angle of view eyepiece as, intuitively, a wider angular
cone of light from the exit pupil means a wider acceptance cone for the
field lens and hence more opportunity for scattered light to enter the
eyepiece. The eyepiece is very useful for deep sky and central detail in
comets but I did not really compare its performance on these with the ortho
as the wider field made it much easier to use.
BTW 4.8 mm is really too short focus and hence too high magnification for an
88" focal length Dobsonian.
Nick Martin, Bonnyton house, By Ayr, Ayrshire KA6 7EW ,Scotland, UK.
Longitude 55 24'56" Latitude 4 26' 00".
"Thou star of evening's purple dome
That lead'st the nightingale abroad,
And guid'st the pilgrim to his home."
To the Evening Star by T.Campbell