Expert: Tom Whiting - 9/10/2008

Question
Mr. Whiting,

I'm looking to buy a first telescope, likely an 8 or 10 inch Dobsonian to use potentially part of the time from a balcony (inside the building with open doors to the outside) when possible.  I'm aware that temperature differences/air currents inside and outside the building will substantially degrade the quality of the image, but am hoping to get an idea of what magnification I might be able to get away with or what level of detail I'll be able to see before this becomes a factor.  Currently I can faintly see Jupiter's moons with a 7X40 monocular from this balcony.  Since I'm in lousy light pollution I'll probably stick to planets most of the time (planets and a few of the brightest stars are about all you can see with the naked eye here).  I have an awesome shot of the ecliptic from this vantage point.

Appreciate the opportunity to ask this question!

Thanks,

Josh

Answer
Hi Josh,
In addition to your mentioned air current problem (plus a 'warm'
mirror much greater than the ambient air which also can cause
'shimmering' of your image), have you also thought of building
and balcony vibrations too?  Those can be a major problem, and
we haven't even talked about much higher winds (especially in
cities where the wind can funnel between buildings) the higher
up you are.
So with so many variables, it's very difficult to predict what
level of detail you'll see. Plus, level of detail is more dependent
on the state of the upper atmosphere. A little hazy, steady air
generally makes for higher resolution on planetary and splitting
close double stars, whereas cool, super clear, transparent nights
(where the DSO's - Deep Sky Objects, are at their best) generally
gives you poorer resolution with unstable air.

For the most part, 8-10 inch Dobsonians, (f4.5 -f6) are DSO telescopes.  For your purpose, the brighter double/multiple stars
and lunar/planetary detail, that calls for a 3-5 inch diameter,
long focal length refractor, or a DOB Newtonian Reflector mirror
that is f8 or higher.
That is what they are made for, whereas your suggested DOB is like
sending a fighter to do a bomber's job. All scopes are like
airplanes, all fly but you don't send a bomber to do a fighter's
job.  So there are...different scopes for different folks, depending
on your main interest and specialization, otherwise why don't we
all have the same type of scope? That's why...we don't.

In our great hobby, equipment (scopes) come LAST, not first. Knowledge of the naked eye night sky comes FIRST.  So, how's your
knowlege level. Can you currently point your finger to Jupiter,
Venus? How about constellations of Sagittarius and say, Leo, Hercules, and Ophiuchus?
If you go out to dark skies on occasion, how about the naked eye objects the Andromeda Galaxy, the Double Open in Perseus, the Pleiades, the Beehive star cluster? (ALL naked eye objects in relatively dark skies.  Can you point your finger at them? Eh?
If not, I humbly suggest you aren't ready for a telescope as yet.
But if so, have at it.
But I have a feeling you've a ways to go, because you're engrossed
with magnification, as are all new-comers. You have not learned as yet that most of us astronomers use our lowest, or medium power eyepiece 90% of the time.
Why? Because you lose 3 things (and you should already know this)
as you go up in magnification, light, detail, and field of view.
Most of the time we want to maximize the 3, and sacrifice just one,
magnification. (Sorry, laws of optics states you can't have it all).

So, don't be in a big hurry here; join a local astronomy club, go out
observing with the others, do some more reading on the subject.
You'll know when you are ready. It might also help to punch up our
club website and read "Tom Whiting's Sound Advice for the Novice" at
http://www.velocity.net/~bwhiting

And that will tell you where you stand right now, and where you want
to go, to some extent.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA