[AZ-Observing] Re: Image Question
- From: "Stanley A. Gorodenski" <stan_gorodenski@xxxxxxxxxxxxx>
- To: az-observing@xxxxxxxxxxxxx
- Date: Thu, 04 Sep 2003 12:18:28 -0800
Bill, Matt, and Joe,
My D-K is about 25 years old. It is a 12.5 incher from Coulter Optics when they
were making some better mirrors. The phenomena I am seeing could be collimation.
If I get a clear night tonight I will see if the more contrasty image is
localized to one spot of the circular edge of the EP. The EPs I am using are a
set I bought from Meade probably about 20 years ago. I noticed that the 10mm EP
did not have as good an image as I thought it should. I contacted Meade and the
owner (can't remember his name now) said that particular focal length was not as
good. I assume from his response that the others were of good quality.
When I said the markings are darker and there is significantly more contrast
near the edge of the field, I meant at the very most edge of the edge, to the
point where Mars almost vanishes from the field. At first I thought I might be
observing some lint at the edge of the glass, but when I rotated the eyepiece
the markings on Mars did not change. It almost seems like what I am seeing could
be elevated in status to something called an "Edge Effect". Could there be any
wave interference effects here? This is just wild speculation. I will check the
collimation when I get clear night.
Stan
William R Wood wrote:
> If you are talking about a symmetrical halo around bright objects, that is
> common to every scope I have looked through to some degree. This light
> scatter is probably due to several factors none of which I have any
> definitive clue about :o) I do notice that light scatter is virtually
> non-existant if my EPs are clean and seeing is very good.
>
> As to the image looking better at the edge of the field I agree with Joe
> that your collimation could be slightly off so that the sweet spot is not at
> the center of the mirror. D-K Cassegrains definitely suffer from a small
> diffraction limited field but if your scope is a Tak I can assure you that
> the entire field is very sharp. However collimation is sufficiently
> critical that the sweet spot can be moved around the field and should be
> centered which is pretty easy if you can get a night of very good seeing and
> collimate by making sure that the first diffraction ring is symmetrical
> around the Airy disc of about a mag 3 star centered in your EP at high mag,
> say 400x.
>
> However, I also agree with Matt that your EP could be the cause. What ortho
> EPs do you use? I use Tak LEs and have seen dirt on the lens and other odd
> effects cause the image to appear better off center too. This usually goes
> away after I clean the lens or get a good night's sleep :o) I know my eyes
> play tricks on me too because I can move my eye around and the image will
> get better or worse depending on where I place my eye. Wierd.
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