Re: newbie questions for a StarBlast 4.5
- From: brian@xxxxxxx (Brian Tung)
- Date: Fri, 6 Jan 2006 10:50:52 -0800 (PST)
Liam wrote:
> I'm new to astronomy (well, interested for years, but only started
> using binoculars a few months ago and just got the telescope...) and
> haven't had any experience with equipment.
> (My wife got me the telescope within our budget after seeking a lot of
> advice online and from Sky & Telescope.)
>
> 1. Would a Barlow be useful or pointless in a scope like this? (I'm
> looking at x2 but would love to have an x3.)
> Maybe it's light pollution (live on the edge of a semi-largish town)
> but with my 15mm eyepiece I can't see M31 in any more clarity than I do
> with 7x50 partially coated binoculars. And the 6mm eyepiece is nearly
> useless. Which leads me to question...
It's not pointless, but it won't help you see dim fuzzies any better;
if anything, I would think it would make matters worse. M31, in
particular, is a very large galaxy in apparent size (and in real size,
for that matter), and it already amply fills the field of view at low
magnifications; you certainly don't need to magnify it further.
A Barlow, to first order, acts simply as a magnification amplifier. It
doesn't collect any additional light, so what little light the telescope
does capture is now spread over 4 or 9 times the area (for 2x or 3x
Barlows, respectively). That can make the galaxy more difficult to see.
What M31 and other galaxies really want is dark skies. The StarBlast is
small and portable, so if you can get out of town, and see the sky from
a dark area, do it!
Barlows are more useful on small, detail-rich objects like the Moon and
the planets. These objects emit plenty of light, so light pollution is
not an issue. The planets are small though--Jupiter at even 200x is
only as big as your thumb is at arm's length--so they take magnification
well, so long as conditions permit. In this case, the conditions don't
have anything to do with light pollution; instead, they are related to
the steadiness of the atmosphere. You've probably seen the distortion
caused when you look across the length of a hot asphalt road. Even the
atmosphere straight up in the sky entails some distortion like that. It
is much less, of course, but then you're magnifying it an awful lot, and
that can make for a lot of image distortion. Barlows will only make that
distortion (called "bad seeing") even more obvious.
> 2. Colimating. Using the instructions in the book, I THINK I have it
> colimated OK. It LOOKS all centered, although, a change in my head
> position a cm will alter the column position, so I really don't know if
> I have it right... what effects will slightly off colimating have?
> Would it make the 6mm eyepiece hard to focus on anything which is what
> I'm finding? I just can't seem to make the 6mm sharpen.
It could be the collimation, but more likely, it is the seeing or your
expectations (or both). The 6 mm eyepiece yields a magnification of
what, 150x? That much power is enough to see about any imperfection in
the image, whether it is caused by the seeing, the collimation, the
optics, or the wave nature of light (which, as you might guess, is
unavoidable).
Bad collimation can introduce coma and astigmatism into the image. If
you center a star in the eyepiece, coma is visible as a sort of cometary
shape to the star's image, so that it looks asymmetrical. Astigmatism
is visible as a crossed shape to the star at best focus. Unless these
aberrations are severe, however, they are difficult to discern in focus.
Try racking the star *very slightly* in and out of focus. Coma is then
visible as a decentering of the blurred image from the focused image;
astigmatism is visible as a slightly oval shape to the blurred image,
which changes orientation by 90 degrees from one side of focus to the
other.
The negative effects of coma and astigmatism are most evident on the
planets, since such high magnification is normally in play. Even if you
can't see the effect directly on stars, they are still there, putting
light from the Great Red Spot (say) where it shouldn't belong, making
it harder to see. (And it's already a challenge for beginners.)
> 3. Can StarryNight be a good judge of what I can expect from Barlows
> and once the scope is well colimated, etc?
> When I plug in the eyepiece specs of the telescope, and add 3x Barlow
> into the equation, StarryNight indicates I'll be able to make out the
> Saturn rings, appearantly the great storm on Jupiter, and possibly the
> four big Jovian moons.
> StarryNight has done a good job predicting what will be in my binocular
> field of view quite well. Any comments?
I find that computer displays are not really a good indicator of what
you'll see at the eyepiece through a telescope. They're much better at
predicting the view through binoculars because the magnification is so
low. Other than getting the field of view and the orientation right,
the view at the eyepiece will usually be significantly different from
what you see at the computer display in your planetarium program. The
display is usually too bright, and either too large and too detailed,
or the right size and not detailed enough, or *something*. (You may
be able to get a reasonable impression by standing back a few feet from
the monitor and squinting your eyes.)
Your first thought at the eyepiece will probably be, "Dang, that's
small." However, even at 36x, I think Saturn's rings will be visible,
and they should be obvious at 150x (although likely somewhat blurry).
You'll probably want at least 80x or so for the Great Red Spot, but it
won't be obvious; the GRS has been much paler in the last decade or so
than it had been previously. And it isn't always visible: It rotates
out of view half the time, and even when it is on the Earth-facing
side of Jupiter, it is often too close to the limb (edge) to be easily
seen. As a rule, it is well-placed for viewing for about two hours out
of each ten-hour Jovian day. Magazines like Sky and Telescope often
give the times when the GRS transits the center of Jupiter; look for it
for about an hour on either side of those times.
The Galilean satellites--Io, Europa, Ganymede, and Callisto--can be seen
even in binoculars, so they should be a piece of cake in the telescope
at 36x, unless they are (a) in front of Jupiter, where low contrast
makes them hard to see; (b) behind Jupiter; or (c) in Jupiter's shadow.
Note that (b) and (c) are not the same. At certain times, particularly
when Jupiter is high in the sky either around sunrise or sunset, you
can see one of the satellites wink out over a matter of minutes as it
enters Jupiter's shadow, even though it is clearly not behind the
planet. Sky and Telescope also gives times for these events, which are
eclipses of the Galilean satellites.
See MOPFAQ (link in my .sig, below) for a Q&A on what to expect in your
telescope on first light. Please feel free to ask any other questions.
--
Brian Tung <brian@xxxxxxx>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
.
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