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Re: Star Hopping



Michael Edmiston wrote:

John Denker mentioned being able to find celestial objects by knowing where to look relative to easily observed stars. Bernard
Cleyet correctly referred to this as star hopping and mentioned how difficult this can be when you live in an area with "poor sky."

There are various limiting cases.
1) At one extreme, I've been in situations (high up in the
mountains) where I could see a dozen Messier objects
with the naked eye. No star hopping required; just
point the scope at what you want to look at.
2a) At the other extreme, if the sky is sufficiently undark
there are some objects that you won't be able to see,
no matter how much technology you bring to bear; they're
dimmer than the skylight.
2b) Even under dark-sky conditions there are some objects that
you won't be able to see with eyes looking into telescopes,
because they are too dim; these are accessible by photograpy
only.

Situation (2) applies to nebulae, and not to star clusters. A
telescope can't increase the surface brightness of an object; it
can only bring the object closer. Stars have plenty of surface
brightness; many nebulae are so intrinsically dim that even if
you were standing inside them they wouldn't be very bright.

good rigid contact lenses

Wonderful things.

No amount of "see that bright star there... now move up and to the left about two hand
widths..." seems to get them to see things that even I see.

Two possible problems. Possibly they just can't see it,
in which case you're stuck. Another problem that is more
challenging than I would have expected is communicating my
notion of "that star yonder" to the students. A trick I've used
to advantage is to have a really bright flashlight (about a
hundred times brighter than what you'd use for reading charts)
which I use to point at stars. The backscattered light (from
dust and moisture in the atmosphere) gives me a pointer that
is much longer than my arm and much more effective, but even
that doesn't work unless I'm standing shoulder-to-shoulder
with the student. Somewhat better is to use _two_ flashlights,
held at arm's length on both sides (to give a long "baseline"
between the beams) and pointing them both toward the same
object. The beams converge at infinity, indicating my
intended object with rather more specificity than a single
beam would. But even that doesn't work unless the students
are quite nearby.

their coats were chosen more for
style than warmth. (For subsequent labs this eventually improved.)

Well, at least they learned _something_ in class :-)

< What it boils down to is, if you can't see
it in the finder scope, or there is a nearby star you can see in the finder and the desired object is close enough that you don't
have to move more than one finder field of view... you're not going to find it.

... which puts a premium on having a big field of view.
I've got an eyepiece with a long focal length (low mag)
and a wide apparent field of view, so I can get > 1.5
degrees field of view even with a 10" f/10.

I have a finder scope but I can't remember ever using it.
I can get close enough using the Telrad (gain-of-one optical
sighting device). More than close enough.

We have so much water vapor and so much light
pollution that Andromeda Galaxy is not usually visible with the naked eye, and also typically not visible in a one-inch finder
scope.

... in which case I wouldn't expect to see much of it in
the main scope, either. Liouville's theorem and all that.
Outside the core, it doesn't have much surface brightness,
unless you are planning to resolve it into stars (which
ain't gonna happen with 10-inch scopes).

At some point you have to give up on dim objects and settle
for planets and star clusters.

... use the setting circles to find something.
... A neat thing we did this way was find Venus in the daytime sky

Cool.

This posting is the position of the writer, not that of Berenice,
Hercules, or Leo.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.