[AZ-Observing] Re: Image Question
- From: "William R Wood" <wmrwood@xxxxxxx>
- To: <az-observing@xxxxxxxxxxxxx>
- Date: Thu, 4 Sep 2003 11:19:34 -0700
I have a D-K Cassegrain too, a Tak Mewlon 250. If yours is like mine there
is a 3 arm spider holding the secondary mirror at the open end of the OTA.
The spider causes diffraction spikes around bright stars and planets. The
spikes do look like beams of light coming off Mars which is now super
bright. Maybe that is what you are referring to??
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.
Regards,
Bill Wood
Fountain Hills, AZ
> -----Original Message-----
> From: az-observing-bounce@xxxxxxxxxxxxx
> [mailto:az-observing-bounce@xxxxxxxxxxxxx]On Behalf Of Stanley A.
> Gorodenski
> Sent: Thursday, September 04, 2003 1:43 AM
> To: az-observing@xxxxxxxxxxxxx
> Subject: [AZ-Observing] Image Question
>
>
> When I look at Mars straight on I can make out dark markings. When I get
> the disk of Mars to the edge of the eyepiece field of view the dark
> markings become much more contrasty and much more prominent. I thought
> it might be that there is too much light in the straight on view, but
> when I cross polarize it does not increase the darkness of the markings
> and the contrast. What is causing this? Is it a problem with the
> orthoscopic eyepiece? What can I do so that I see the better image when
> viewing it straight on. I feel pretty certain I have my Dall-Kirkham
> (cassegrain with an open tube) well baffled from the primary mirror
> through the eyepiece tube. One other thing I have always noticed is that
> there is always a hazy area projected around the planet like a beam of
> the light of the planet. This seems to indicate a baffle problem?
> Stan
>
>
>
>
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