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Re: [APML] sharp image with 5 mm lense
Hi Joel:
Right, you should figure on 80 to 100x for sharp images under most
conditions. (And of course, as Alan pointed out, as you go up in
magnification, your need for a good solid mount goes up.) You may do better
on a night of very good seeing, but in general ....
Barlows: Unfortunately, it's the total magnification that's limited, doesn't
depend on how you get that magnification.
What goes bad with too much magnification? You get blurry *and* dim images.
Even with large apertures and excellent optics, increasing magnification
always reduces the brightness of the image; it's a simple mathematical
relationship. The blurriness is a bit more complicated---under any given set
of sky conditions, I find that I can go to progressively higher
magnifications while retaining good sharpness up to a certain point and from
there on the quality (sharpness) of the image drops off dramatically. The
book by Dickinson & Dyer, The Backyard Astronomer's Guide, has a good short
discussion of the magnification problem.
One special aspect of image quality that can be confused with hardware
problems is seeing, especially with the moon. If you look at the moon when
the air is less than perfectly stable (ie, less than ideal "seeing"), it
will look as though it's boiling or giving off heat waves like a road
surface on a very hot day. This kind of image degradation has nothing to do
with the magnification (although it looks worse with high mag) or quality of
the optics, and everything to do with the turbulence of the air. Not much
you can do about turbulence except (1) wait for it to go away (sometimes it
does steady out later at night and early morning) or (2) go to a place known
for steady skies. Seeing does vary from day to day, depending on weather
conditions, is better at high altitudes than lower ones (in some cases), and
is definitely better in some places on earth than in others.
Regards,
Bert
Bert Katzung
katzung1@comcast.net
www.astronomy-images.com
----- Original Message -----
From: "mango" <j.kolling@chello.nl>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Monday, November 10, 2003 10:37 PM
Subject: Re: [APML] sharp image with 5 mm lense
> Hi Bert, thank you for the explanation, i did not know this.
> So my maximum magnification is 80-90x or 180-225x if assuming perfect
> optical/weather conditions.
> Does using a bigger lense (for example a 12.5 mm (= 80x mag) in
> combination with a 3x Barlow lense (result = 225x mag)
> give a better result than for example using a 5 (200x) or 4 (250x) mm
> lense when it comes to sharpness (focus) of the image?
> or does that make no difference either?
>
> Thank you,
> Joel
>
>
> Bert Katzung wrote:
>
> >Hi Mango:
> >All of the things you mention can contribute to a fuzzy image, but
probably
> >the most important is the focal length of your eyepiece. Unless you have
> >superb optics (both eyepeice and mirror), a 5 mm eyepiece is pushing the
> >magnification too high in this scope. Your 1000 mm focal length with a 5
mm
> >EP gives 200x and that's asking a lot! A reasonable rule of thumb is 20x
per
> >inch of aperture, so with your 114 mm aperture, about 80-90 x is the
maximum
> >magnification you should expect to give nice clear images. I know that
the
> >books claim a maximum of 40 to 50x per inch of aperture, but that's
assuming
> >perfect optics and perfect seeing conditions.
> >Hope this helps,
> >Bert
> >
> >Bert Katzung
> >katzung1@comcast.net
> >www.astronomy-images.com
> >
> >----- Original Message -----
> >From: "mango" <j.kolling@chello.nl>
> >To: <astro-photo@seds.org>; <videoastro@yahoogroups.com>
> >Sent: Monday, November 10, 2003 9:11 PM
> >Subject: [APML] sharp image with 5 mm lense
> >
> >
> >
> >
> >>I have a new telescope, a newtonian 114/1000
> >>and when i used it to watch the moon yesterday
> >>when i put in a 5 mm lense the image wasn't clear,
> >>could this be because of the quality of the 5 mm lense,
> >>the weather (it was not very clear and a little misty, but just a
> >>little), the mirrorsize, the focal length, or something else?
> >>
> >>
> >>
> >>
> >>
> >>
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> >
> >
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