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Re: (IAAC) Markarian 205 - 8inch F/10
Yann,
I agree with you completely. It has been my experience that large optics really start to pay off
and perform well for visual use when used at high magnifications. When I first started observing
with my clubs 30" dob a little less than 2 years ago I set up my 13" dob next to it on several
occasions to compare images. At low powers, less than about 125x, the views were only very slightly
better in the 30". But I was amazed when I increased the magnification. At 300x the images in the
13" were becomming extremely dim and little detail could be seen. But not so in the 30"! The
magnigication could be increased to whatever power the turbulent Missouri skies allowed, sometimes
600x or more, and the image stayed nice and bright.
One exception could be the use of filters for observing nebulae. I have little experience with
filters on scopes other than the 30" but have to believe that, even at low powers, a large scope
will outperform a small scope when using filters such as the H-Beta or O-III. Have the more
experienced observers on the list found this to be the case?
> 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.
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