newbie aperture question

Hi,

I'm currently using a hand me down 6" reflector (f3.5?) and when
conditions are good I can see faint banding on Jupiter. This is using
an 18mm eyepiece. (30x?) I live in an apt complex in San Diego county
with plenty of light pollution from street lamps, strip malls, etc.

My understanding is that a larger aperture telescope, say 8", would
produce a clearer picture of jupiter at the same magnification
(assuming I switch eyepieces).

But it seems as if there would be a point at which atmospheric
conditions and light pollution are such that having a higher aperture
telescope would not produce any clearer an image at a given
magnification.

So my question is if there is any way to tell whether image fuzziness
is a result of the scope or of the environment without looking through
a larger scope. In other words, how do I know if my scope's aperture
is the limiting factor?

Thanks,
Eddie.

For planets, a long focus telescope works the best, f/8 to f/15. Yours is a
short, f/3.5, built for wide field of view, sensitive to collimation. My
friend had an 8" f/4 newt, never had a good view on planets with it.
Also, larger aperture is more sensitive for seeing conditions, than a small.
All though 6"-er is not bad.
If I were you, I would buy a 6" f/8 or a 8" f/6 Dob, they perform better.

JS

"Eddie" wrote in message
om...
Hi,

I'm currently using a hand me down 6" reflector (f3.5?) and when
conditions are good I can see faint banding on Jupiter. This is using
an 18mm eyepiece. (30x?) I live in an apt complex in San Diego county
with plenty of light pollution from street lamps, strip malls, etc.

My understanding is that a larger aperture telescope, say 8", would
produce a clearer picture of jupiter at the same magnification
(assuming I switch eyepieces).

But it seems as if there would be a point at which atmospheric
conditions and light pollution are such that having a higher aperture
telescope would not produce any clearer an image at a given
magnification.

So my question is if there is any way to tell whether image fuzziness
is a result of the scope or of the environment without looking through
a larger scope. In other words, how do I know if my scope's aperture
is the limiting factor?

Thanks,
Eddie.

I hope it is NOT f/3.5. It would probably have pretty bad coma.
As to aperture vs. seeing conditions, the larger aperture will alway win, or at
least tie.
If the size of differing density pockets of air are smaller than your aperture,
it can have the effect of multiplying images and otherwise screwing you up.
But if you make a 6" aperture mask (cardboard, construction paper, whatever),
you will have a 6" when you need it, and an 8" when you can use it.
Hope the piece of paper doesn't break your budget.
One thing, before investing a bunch of money, do try to get your scope as well
collimated as you can, if you have not already done so. I kind of like the
instructions at
http://www.vvm.com/~piscescs/collimat/NoTools2.html
(maximize your screen when reading this to get the whole URL to use). There
are others, many very good.
Best wishes!
Clear, Dark, Steady Skies!
(And considerate neighbors!!!)

A 6-inch reflector is a decent scope -- it's not a world-beater but you can
see lots of good stuff with enough detail to make it worthwhile.

Rule of thumb for magnification is 50-60 x aperture in inches. Thus, your
maximum magnification will be 300X-360X. Seeing conditions will likely
limit you to around 250X and you can see just about all you ever want to see
at 170X or so.

Remember, too, that the magnification for a scope-eyepiece combination is:
focal length of scope divided by focal length of eyepiece. Assuming your
6-inch reflector has a focal length of 1200mm, your 18mm eyepiece gives you:
1200/18=67X. If you use a 2X Barlow with the 18mm, your magnification will
double, 67 x 2 = 134X

First, collimate the scope. Start here for instructions on collimation:
http://skyandtelescope.com/AuthFiles/NoCookie.asp

You will need a collimating eyepiece -- Orion makes a nice one, the Orion
Collimating Eyepice, their catalog number 3640, $39.95 -- go to their web
site and order one.

Second, get a 2X Barlow -- with the Barlow and your 18mm eyepiece, you will
have two eyepieces -- 18mm and 9mm. With the 18mm and the Barlow you can
see banding on Jupiter very clearly, the Cassini division in Saturn's rings,
and even see the shadow that Saturn casts on its rings.

Then, get an eyepiece in the 10-12mm range -- with the Barlow, that will
give you 5-6mm, fairly high magnification. Suppose you get a 10mm eyepiece.
With your scope, 1200/10 = 120X, and with the Barlow the 10mm eyepiece =
120X x 2 = 240X..

So, with 18mm and 10mm eyepieces and a Barlow, you will have magnification
of 67X, 120X, 134X, and 240X. You may want to pick up a third eyepeice in
the26-32mm range for wider views at low magnification.

Collimate the scope, get a Barlow and one or two more eyepieces, you will
amaze yourself and your friends.

--

----
Joe S.




"Eddie" wrote in message
om...
Hi,

I'm currently using a hand me down 6" reflector (f3.5?) and when
conditions are good I can see faint banding on Jupiter. This is using
an 18mm eyepiece. (30x?) I live in an apt complex in San Diego county
with plenty of light pollution from street lamps, strip malls, etc.

My understanding is that a larger aperture telescope, say 8", would
produce a clearer picture of jupiter at the same magnification
(assuming I switch eyepieces).

But it seems as if there would be a point at which atmospheric
conditions and light pollution are such that having a higher aperture
telescope would not produce any clearer an image at a given
magnification.

So my question is if there is any way to tell whether image fuzziness
is a result of the scope or of the environment without looking through
a larger scope. In other words, how do I know if my scope's aperture
is the limiting factor?

Thanks,
Eddie.

Hi Eddie,
as others have suggested, make sure it's collimated.
next, get a higher power eyepiece. at 30x, i can't see more than 2 bands
either. 300x should be attainable with a 6 in.
sounds like you have an interesting scope. do you know it's history?

peace,
jon


"Eddie" wrote in message
om...
Hi,

I'm currently using a hand me down 6" reflector (f3.5?) and when
conditions are good I can see faint banding on Jupiter. This is using
an 18mm eyepiece. (30x?) I live in an apt complex in San Diego county
with plenty of light pollution from street lamps, strip malls, etc.

My understanding is that a larger aperture telescope, say 8", would
produce a clearer picture of jupiter at the same magnification
(assuming I switch eyepieces).

But it seems as if there would be a point at which atmospheric
conditions and light pollution are such that having a higher aperture
telescope would not produce any clearer an image at a given
magnification.

So my question is if there is any way to tell whether image fuzziness
is a result of the scope or of the environment without looking through
a larger scope. In other words, how do I know if my scope's aperture
is the limiting factor?

Thanks,
Eddie.

So my question is if there is any way to tell whether image fuzziness
is a result of the scope or of the environment without looking through
a larger scope. In other words, how do I know if my scope's aperture
is the limiting factor?

Looking through other scopes is a good way to see if how yours compares.

But there are many variables as others have pointed out, collimation, cool
down, atmospheric stability (seeing) quality of the mount, quality of the
eyepieces...

And of course it depends on when you are viewing Jupiter. Right now Jupiter is
low in the early evening, so you are looking though lots of atmosphere and
muck. Got to wait until it gets at least 30 degrees from the horizon.

I see you are a fellow San Diegian. What part? I am in Clairemont and could
likely help you get your scope straightened out.

Best wishes, dark skies

jon

At 30x with a 18mm EP that means that the focal length is 540mm with a 150mm
aperture or about F3.5. This is a scope which is more usable for the larger
deep sky things in the sky than as a planetary scope.
For a planetary scope, you really want a long focal length so that you can
easily get magnifications of 200x or more with that EP.
Things will be somewhat fuzzy with any scope to some degree or another with
about 50x per inch of aperture being approximately the highest magnification
usable with any aperture.
FWIW, I have a telescope of 4.25" aperture and is 200" focal length. That
scope works wonderfully on the planets, producing excellent images that are
hard to compare to other scopes. At a recent telescope meet, I was seeing a
crescent with sharp tips on Mercury when all the other scopes could only see
that it was elongated. Then again, looking at any large deep sky object was
difficult to do as the scope really can't see that much of the sky. Getting
M81 and 82 in the same view was almost impossible to do.

--
Bob May
Losing weight is easy! If you ever want to lose weight, eat and drink less.
Works every time it is tried!

Hey Jon,

The scope is an orange Celetron tube. It's from at least 1991, but that's
all I know about it.

Later,
Eddie.

Hey Jon,

The scope is an orange Celetron tube. It's from at least 1991, but that's
all I know about it.

Later,
Eddie.

Celestron made a Comet Catcher or some such thing which is a 5.5 inch F3.64
Schmitt Newtonian with a focuser that moves the secondary mirror and eyepiece
up and down the optical tube rather than moving the eyepiece in and out.

These are best for Deep Sky viewing and widefield photography rather than for
viewing the planets.

If you want to give the planets a try you will need some shorter focal length
eyepieces and probably a barlow or two.

Jon

(Eddie) wrote in message . com...
So my question is if there is any way to tell whether image fuzziness
is a result of the scope or of the environment without looking through
a larger scope.

Yes - look at the same thing, with the same scope, every available
night for for months and months.

I know that's a trite answer, but it's the truth - and I learned it by
doing it.

The experts will give you loads of valuable and relevant advice
regarding collimation, seeing, cooling (of the scope), eyepieces,
learning to see, gettting experience, etc. etc. etc. Don't disregard
this, it's right - but it won't give you a satisfying answer. Just do
it!

Until you experience it yourself, you'll always be doubtful. Trust me
- after you've looked long enough, you'll be blown away by what your
scope can deliver - and 99% of the time it *IS* limited by seeing.

(To the experts; sorry if I've overstepped the bounds of giving advice
- you all know I'm a newbie, but this aspect is something I've
experienced over the last few months - not a distant memory as it is
for you. Evangelism of the converted and all that... :-)

Cheers
Beats