[Prev][Next][Index][Thread]

Re: (IAAC) Markarian 205 - 8inch F/10

>> a long focal eyepiece (Lichtenecker 100mm!!! a huge piece of rock)
>
>Good lord, Yann - I've never even heard of a 100mm epc! "Lichtenecker"
>("light
>corners") is an apt name for such a doorstop! :)  What is the eye relief
>on an
>ocular like that??
>

Lichtenecker is a german business in telescope making (eyepieces,
telescopes, etc...). I haven't noted the exact eye relief of the 100mm, but
I can remember that it was large !  But even with the 24.4inch F/15 system
(giving 90x and exit pupil of 6.9mm), it performed like the first
generation of the Nagler series giving what we call in France (I don't
remember the proper term in english) "flying shadows" problems, and a
bright background sky.

when speaking of that, I want to know about the impression of those who
have observed in big instruments. I have often read about deceptions of
fellows amateurs looking through big amateur of professionnal telescopes
(say between 20 and 40inch) and finding the view disappointing. For
professional instruments, as I had some experience with the Cassegrain
described above, my "theory" was that proper baffling for visual
observations was not widespread: after all, these instruments are designed
for photography, CCD imaging or photometric studies and these technices
have their own specifications and baffling designs.

Rrecently, after taking long looks through amateur dobsonians of 22, 25 and
30", I have another explanation that is based on an old experience: when I
switch back from my (now) usual 17.5inch Dob to my first 4.25" Newt, I
found the sky much darker at 70x in the latter (a medium power for the
4.25) than at 70x in the previous (a low power in the 17.5). Of course, it
is natural and everybody in this newslist should be aware of that fact and
its explanation (objects and sky growing brighter with aperture increase).
To reduce the sky brightness in the 17.5, I just need to increase power,
say to about 300x. In order to achieve this in the 30" I tried some weeks
ago, it would need more than 500x.

So now I believed that the deceived observers must have been using too low
powers on the "light buckets" resulting in a lack of contrast and a
"bright" background sky. And if they have achieved the proper image
amplification, maybe bad seeing have degraded the image sharpness.
I must add that it is unfare to compare a 16" and a 40" on big and bright
objects like M31, M51 or else; observers would have a better comparison (I
think) with a mag.15 galaxy or mag.11 spiral. Big and bright objects are
nice at low power but change completely with high power.

I would like to know if netastro observers have experienced big instruments
(specially professional ones) and would like to share their thoughts.

clear skies, Yann.



Yann Pothier	tel: 01 43 41 43 29
11 impasse Canart, 75012 PARIS, FRANCE
Email: ypothier@abi.snv.jussieu.fr
Site : http://pegase.unice.fr/~skylink/publi/cielextreme
Follow-Ups: References: