Re: What magnification do I want for deep sky?

On or about 23 Feb 2006 12:46:44 -0800 did "Don't Be Evil"
<g626700-gg@xxxxxxxxx> dribble thusly:

I have an 8" f/6 dob. I presently have a 40mm eyepiece for wide field
observing giving me 1.6 deg TFOV, 30X mag, and 6.7mm exit pupil.
Unfortunately, I live in seriously light-polluted suburbs, so the sky
is just too bright (mag 4 on a good night). Often, I can't even see
M31. I'm thinking of getting a wide angle eyepiece in the 26-32mm
range. Should I do this, get a filter, or something else?

I used to live in mag 4.5 skies myself (now about 6.0), so I
understand where you're coming from. I could still see M31 in
binoculars, but only just.

As magnification goes up, brightness goes down. That's true for any
extended object in the field, which includes DSO's and the background
sky itself.

However, depending on the object, you might reach a point where the
sky is darkened enough to let contrast increase visibility, despite
the object being dimmed as well. Some objects are so small that
you'll need to magnify them to appreciable size anyway, despite the
dimming effect, so getting a higher-mag EP with a wide field will be
useful regardless of how well it works on M31 (which is quite large).

A light pollution filter might help quite a bit, since it has the
effect of darkening the sky without equally darkening the object
(since it's designed to block the wavelengths put out by normal
streetlights). There are also nebula filters for preferentially
letting through those wavelengths put out by emission nebula. I
haven't tried either, however.
--
- Mike

Ignore the Python in me to send e-mail.
.

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