Cherokee #2 Pronto and Lyra

Cherokee wrote:
24 mm Konig and 24mm Konig + Ultima barlow. Do I have the wrong
eyepieces for the "double-double"?

Probably, unless you have exceptionally acute eyes. With the Barlow,
you're looking at about 40x. For most people, that's a fraction of
the magnification you need to split the Double Double. You're better
off at around 80x or so.

The stars are arranged something like this:

.. .
.. *
..
..
..
..
..
..
..
..
..
..
..
..
.. **

(Use a fixed-width font. The periods at the left there are only to
keep Google from collapsing the lines. I hope it works.)

The northern pair of stars is a fifth-magnitude primary topped by a
sixth-magnitude secondary. Both of the southern pair are of the
fifth magnitude.

Both pairs are separated by about two-and-a-half arcseconds (that's
about 1/1,500 of a degree--that's why you need higher power!). The
southern pair is actually a little tighter than the northern pair,
but because the stars are of roughly equal brightness, they are a
bit easier to split than the northern pair, where the dimmer star has
a tendency to get lost in the glare of the brighter one.

--
Brian Tung
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.txt

Ed,

How did you get the magnification numbers? I'd like to calculate those
for my Pronto.

Ed, Brian,

What eyepiece should I buy to be able to split these 2? If I need
higher magnifications for certain objects, I best buy it right away so
I don't have to skip viewing them.

peace,
Cherokee

"Cherokee" wrote in message
ps.com...
Ed,

How did you get the magnification numbers? I'd like to calculate those
for my Pronto.

Ed, Brian,

What eyepiece should I buy to be able to split these 2? If I need
higher magnifications for certain objects, I best buy it right away so
I don't have to skip viewing them.

peace,
Cherokee


Magnification is calculated by dividing the focal length of your telescope
by the focal length of the eyepiece used. In your case, 480/24=20x. Here's
a website you might enjoy:
http://www.scopesim.com/

If you don't wear glasses, a University Optics ortho is a good inexpensive
performer but it has a narrow field of view. With an alt/az mount wider
fields are nice, especially at higher powers. With a Pronto, I'd guess a
7-9mm barlowed would bring you to your maximum useful magnification.

Ed T.

Ed,

A very useful website. I spent around 30 minutes playing around with
the simulator. Can you take a moment and explain to me what FOV is all
about? I liked the FOV setting at it's widest. What FOV does my 24mm
Konig have?

Also, if I need 100x to split Epsilon-Lyra, then I need a 4.8mm
eyepiece. OK, which eyepiece should I get and from where? I'm eager
to order what I need so I can split it. I hate to leave loose
ends......

peace,
Cherokee

"Cherokee" wrote in message
ups.com...
Ed,

A very useful website. I spent around 30 minutes playing around with
the simulator. Can you take a moment and explain to me what FOV is all
about? I liked the FOV setting at it's widest. What FOV does my 24mm
Konig have?



When considering FOV, you have two types to consider.

The Apparent Field of View (AFOV) is determined by the eyepiece design.
Your Konig has a 60 degree AFOV. An unrestricted AFOV is 360 degrees, but
when looking through an eyepiece your view is limited to ~30-90 degrees by
the eyepiece. You can look through an eyepiece without a telescope and see
the limits of the field and estimate the AFOV.

The True Field of View (TFOV) is determined by the eyepiece/telescope
combination and describes how much of the night sky fits into the field of
view. For example if the full moon fills the field of view of an eyepiece
it has a TFOV of ~1/2 degree.

True Field of View = Apparent Field of View / Magnification. There are
some eyepiece distortions that make this equation less than accurate but for
now don't worry about that.

Lets take our moon example again. If you are using your 60 degree AFOV
Konig at 20x, you'll see about 3 degrees of sky (~6 lunar diameters).

Ed T.

Cherokee wrote:

Ed,

How did you get the magnification numbers? I'd like to calculate those
for my Pronto.

Ed, Brian,

What eyepiece should I buy to be able to split these 2? If I need
higher magnifications for certain objects, I best buy it right away so
I don't have to skip viewing them.


The usual rule of thumb says you can make use of 2X per millimetre of
aperture under excellent conditions. So for the maximum useful
magnification you want an EP that can deliver something like 120X -
150X -- depending on how optimistic you are. Dividing that into the
focal length of your objective, which Ed said is 480 mm, gives a
range of 3.2 mm (somewhat optimistic) - 4 mm (more conservative), but
considering that you already have a 2X Barlow you can combine that
with an EP of twice the FL for the same result. For example a 7.5-mm
eyepiece will give you 480/7.5 = 64X alone, or 128X with the Barlow.

--
Odysseus

Odysseus,

Since I have a 24mm Konig and an Ultima Barlow, this gives me a 24mm
eyepiece and a 12mm eyepiece, right? Per my other post, it appears I
would next get a 4.8mm eyepiece for 100x????

peace,
Cherokee

Using the barlow, you could get a 9mm eyepiece. That would put you a little
over the 100x, but that's okay too.


"Cherokee" wrote in message
oups.com...
Odysseus,

Since I have a 24mm Konig and an Ultima Barlow, this gives me a 24mm
eyepiece and a 12mm eyepiece, right? Per my other post, it appears I
would next get a 4.8mm eyepiece for 100x????

peace,
Cherokee

Cherokee wrote:

Since I have a 24mm Konig and an Ultima Barlow, this gives me a 24mm
eyepiece and a 12mm eyepiece, right? Per my other post, it appears I
would next get a 4.8mm eyepiece for 100x????


You certainly could (or something close to it), but a 9.6-mm you
could use with the Barlow for 100X would also give you the option of
50X when used alone, giving you in all a choice of 20X, 40X, 50X, &
100X. There wouldn't be any point to using a 4.8-mm EP with the
Barlow, because at 200X you'd be 'overdriving' your aperture.

OTOH with some 'scope-EP combinations a Barlow can create
difficulties in focusing: if a given EP requires the focuser to be
racked nearly all the way in, with a Barlow it'll likely 'bottom out'
before the view gets sharp. Since your current EP works with your
Barlow, if you can find a(n approximately) 9.6mm that's parfocal with
the 24mm you can be sure it'll be similarly compatible.

--
Odysseus

Since I have a 24mm Konig and an Ultima Barlow, this gives me a 24mm
eyepiece and a 12mm eyepiece, right? Per my other post, it appears I
would next get a 4.8mm eyepiece for 100x????

I own the Ranger, which has identical optics to the Pronto
but uses a 1.25-inch focuser instead of a 2-inch focuster.
I also have an 8-24mm zoom eyepiece which, in combination
with a Barlow, lets me "dial in" pretty much any magnification
I want.

Your 24mm eyepiece should be just great for general low-
magnification browsing. Eventually, you may want to supplement
it with a 2-inch eyepiece to give you a *really* low power
and correspondingly vast field of view. When all is said
and done, that is the Pronto's greatest strength; few other
scopes on the market can touch it for low-power work.

For closeup views of deep-sky objects, I usually like to
use a magnification of around 50X - 60X on my Ranger,
meaning an eyepiece in the 8mm - 9.6mm range. I betcha
you would see M57 as a ring at 60X, where it might be
much harder at 40X.

For planets, I usually like at least 100X, and often higher.
Tight double stars and the Moon often profit from 150X,
although the marginal gain above 120X is fairly small.
Anyway, all these magnifications are best achieved with
the aid of a Barlow, because the eye relief of a standard,
moderately priced 3mm - 5mm eyepiece is painfully short.
In other words, to see the entire field, you nearly have
to press your eyeball against the glass.

- Tony Flanders