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Rookie question. How dark is MY sky?



 
 
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  #1  
Old July 30th 03, 07:07 PM
justbeats
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Default Rookie question. How dark is MY sky?

A question – but first some background by way of introduction...

I've gone the typical route of a lifelong "journal interest" in
astronomy that started (and ended in practical terms) with a dept
store refractor. A recent bonus (some 30 years later) made it possible
to indulge a long-term lust for a Meade GOTO scope. So about 4 weeks
ago I bought a used 10" F/10 and have since spent nearly as much again
on a micro focuser, dew control, eyepieces, filters (on back order)
and books. CCD will have to be funded with the next bonus :-)

I've followed a (disturbingly) typical learning curve on this scope
too (only 5 sessions so far):

1. Digest MAPUG, absorb loads of theory and opinion, buy an LX200
2. Setup, look – ETX90 (other new scope - another story) is better!
3. Decide cool-down time must be critical (didn't believe). Next time…
4. Setup, cool down, look – improvement, but still not good. So…
5. decide collimation is MUST do too, not just "should do". Next time…
6. Setup, cool down, (deep breath) collimate, look – ecstatic!!!!!
7. discover looking is a skill which improves with practice too!
8. Next time, clarity and seeing better – now understand the effects
9. Friday night – best views yet including (possibly) difficult
objects

Although I "knew" all these things from reading, it was just theory
and I got problems from lack of practical experience. Which leads me
to my question as that's related to lack of experience too and I hope
you seasoned observers can answer it for me. I have read much about
the joy of "dark skies" and intend to travel to find some. But what I
saw in my last session makes me wonder if I already have them over my
house (5 miles S of Buckingham in the UK)! I have no prior experience
to compare against – so can someone confirm if the following couple of
observations sound like they're from a great site (or not)?

26/7/03 @ 0200BST. Using 10" F/10 LX200 (non-UHTC) + 12mm Nagler. The
air was not stable enough for more mag than this (211X)). The Milky
Way was prominent to the naked eye; almost horizon to horizon. The
dark lanes were clear and the nebulous clumps in Cygnus looked
semi-solid. No moon and clear sky, but faint haze and light pollution
below about 35 deg to the South. Andromeda visible to the naked eye.

With the scope, I got acres of black between both components of the
double-double. I also saw 3 diffraction rings around the stars (in
focus) during a moment of particularly stable air. Is that a sign of
good skies as well as good collimation (the rings were a bit broken
due to turbulence, but there was clearly 3 concentric "layers" there)?

M27 – wasn't a dumbbell! It was vaguely rectangular with faint
semi-circular extensions to each side (as in the astrophoto in
Burnhams – but I didn't think you could "see" images like that).
Central star was a faint blue spark in direct vision, but vivid blue
with averted vision. Like the central star of M57, I expected this to
be too faint to see. Mag 13.9 isn't it?

The question is, can I improve this (very pleasing) performance by
travelling elsewhere or should I save time and petrol? At 77 metres I
know getting higher will improve seeing (less air), and I don't
discount the social aspects of star parties etc. But in general terms,
am I wasting time looking for darker skies? If I will only achieve
marginal improvement over what I get at home, then I would consider it
a pointless exercise.

What reference objects do you use (and how) to empirically compare
clarity and darkness of skies between one place and another?

Cheers
Beats
  #2  
Old July 30th 03, 08:18 PM
guid0
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Default Rookie question. How dark is MY sky?

On 30 Jul 2003 11:07:56 -0700, (justbeats)
stepped up to the plate and batted:


and how) to empirically compare
clarity and darkness of skies between one place and another?


I always use Attila Dankos sky clock pages

Check out the sky clock closest to you from
http://www.cleardarksky.com/

Once there, click the light pollution map link for a clor coded map of
light pollution levels.

General light pollution levels can be defined using the Bortle scale,
a description of which you'll find he
http://novac.com/light/def.htm

Hope this helps.

G../0
  #3  
Old July 31st 03, 02:42 AM
Tony Flanders
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Default Rookie question. How dark is MY sky?

(justbeats) wrote in message om...

I have read much about
the joy of "dark skies" and intend to travel to find some. But what I
saw in my last session makes me wonder if I already have them over my
house (5 miles S of Buckingham in the UK)! ... The Milky
Way was prominent to the naked eye; almost horizon to horizon. The
dark lanes were clear and the nebulous clumps in Cygnus looked
semi-solid. No moon and clear sky, but faint haze and light pollution
below about 35 deg to the South. Andromeda visible to the naked eye.


Sounds pretty dark to me -- darker than I would have thought possible
so close to London. Far from pristine, but you're not going to find
pristine skies anywhere near you; England is just too densely
populated.

With the scope, I got acres of black between both components of the
double-double. I also saw 3 diffraction rings around the stars (in
focus) during a moment of particularly stable air. Is that a sign of
good skies ...


Yes, three rings in a 10" scope means very stable air. But that
is completely unrelated to how dark the skies are -- good seeing,
as opposed to good transparency.

M27 ? wasn't a dumbbell! It was vaguely rectangular with faint
semi-circular extensions to each side ...


Yes, that's exactly right. You need at least half-decent skies
to see those extensions without a filter.

At 77 metres I
know getting higher will improve seeing (less air) ...


Nah, the difference between 77 meters and the highest point in
the U.K. isn't all that significant -- certainly not worth
travelling for. Besides, lugging a 10" LX 200 to the top
of Ben Nevis isn't my idea of fun. I can't think of any
roads in the U.K. that go very high at all.

Now if you were talking about the Alps, that might be a
different matter ...

- Tony Flanders
  #5  
Old August 3rd 03, 12:08 PM
Tony Flanders
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Default Rookie question. How dark is MY sky?

Shankar Bhattacharyya wrote in message . ..

Ben Nevis is a molehill.

My father, who walked and bicycled over a good bit of the UK in his
student days, says it has a certain dramatic, spare beauty and is well
worth a visit. Still, it is a molehill.


Your father is right; Ben Nevis is a very attractive, and altogether
respectable summit. Tiny in terms of height above sea level, but not
tiny in terms of height from bottom to top, nor in terms of weather.
There are plenty of 6000 meter summits in the Himalaya that are
smaller from top to bottom than Ben Nevis.

Unfortunately, in northern Europe, as in my own part of the world
(eastern U.S.), altitude is actually counterproductive for astronomy,
because the proportion of cloudy skies increases rapidly with altitude.
There *are* times when the summits will be in clear air over an
undercast, but the opposite is much more common.

- Tony Flanders
 




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