[SAS] Fwd: Finding faint things with star charts
- From: cave8@xxxxxxxxxxx
- To: sas@xxxxxxxxxxxxx
- Date: Wed, 4 Mar 2015 17:58:50 +0000 (UTC)
I thought some of you might be able to use this info
----- Original Message -----
Dear Fellow Astronomer,
I've been hearing from people who've had trouble using star charts to find
Comet Lovejoy with binoculars. (By the way, Lovejoy is still in good view .)
This gets to the problem of how does anyone learn to find faint things in the
sky using charts with binoculars? Or a telescope for that matter?
Some figure it out by trial and error, but that's the hard way. A few tricks
make for huge shortcuts in mastering this. 1) First: know how big your view is.
To compare what you see in binoculars (or the finderscope on the side of a
telescope) with what's printed on your chart, you have to know how much sky -
how much of the chart - your binoculars are showing you. In other words: what's
the binocular's true field of view? For 7-power binoculars, the view is
probably about 7° wide. For 10-power binoculars, it's probably about 5° wide.
You don't have to know exactly, but you should have a general idea. Now get out
your charts. I'll assume you have the Pocket Sky Atlas . because it's
super-popular, handy and fairly cheap, and just about ideal for binoculars and
small telescopes because it shows stars to as faint as magnitude 7.6 - about
the limit of typical binoculars and good finderscopes. Look on the sides of the
charts. There's a degree scale running down the sides. This shows declination
on the map (like latitude on Earth), but it also shows you how big your 5°
field of view appears on the map! On the Pocket Sky Atlas , 5° is 1 inch wide.
That's probably a lot smaller than you imagined your binoculars showed. It's
much smaller than most constellations. That's why you see so little of a
constellation at a time! Remember this size. Maybe draw it on the inside of the
front cover so you'll always have it handy. The star patterns on the charts
that fit into that size circle are the star patterns you'll see in your field
of view.
2) Which way is up? On star charts, celestial north is up. On the sky,
celestial north is always the direction toward Polaris, the North Star. No
matter at what angle that is from wherever you're looking. You do need to know
how to spot Polaris. Once you do, you'll know which way is celestial north from
wherever you're looking. Turn your chart around so its top matches that
direction.
3) Start from a naked-eye landmark near the object you're hunting for. You need
to know at least a few constellations. (There's an excellent naked-eye
constellation map in the center of each month's Sky & Telescope The
constellations provide starting points for zeroing in on the exact point where
a faint thing on your chart should be. On the chart, working from a starting
point you know to your faint target, look for star patterns - triangles, kite
shapes, whatever - marking the path. In choosing these patterns, use the
brightest available stars that will fit in your field of view. Now, with the
binoculars (or your telescope's finder), work your way along this trail,
matching the chart with what you see, step by step. This is called
"star-hopping." If you get lost, start over. You'll get the hang of it. And
then you'll know how to do it for the rest of your life. You'll discover that
when you know the exact point in your field of view to examine, it's amazing
how much deeper you can see. And how much more confident you'll be that you've
indeed found what you were looking for.
I hope you'll use these tips to help you enjoy the Comet Lovejoy, while you
still can!
Alan MacRobert
Senior Editor, Sky & Telescope
P.S. One reason binoculars are nice is they show a view that's right-side up
and also is not mirror-imaged. Some telescopes and finderscopes show views that
are upside-down or tilted, and/or are mirror-imaged.
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