Expert: Tom Whiting - 7/30/2008

Question
QUESTION: Hi there, I am not that old. I am only 17 years old, but I find space very interesting. I have this Telescope wich is getting a bit old, i was wondering if you could explain the basics about scopes. I don´t understand things like  why use a small mm lense or a big mm. And stuff like the D=60 F=900 is that good or is it crap i have no idea ? Thanks

ANSWER: Hi Anders,
Odds are pretty good that it is a nearly useless scope;

The D stands for diameter of your mirror or front lens, probably the latter, in millimeters. (You can achieve nearly the same with a lot less hassles by purchasing a pair of say Bushnell Binoculars at K-mart or Walmart, 10 x 50 binoculars, for about $40 and have the convenience of a very wide field of view (telescopes inherently have a very small field of view), the image is right in front of your nose (most telescopes you are looking at a right angle to the direction of view), easy to carry and easy to use (telescopes require set up and alignment time), so make your "first Scope" a pair of binoculars.

Oh, the F stands for a 900 mm focal length, which means your 2 inch  telescope has an f-ratio of 900/60 = f15.  Only good for bright planets and the moon, and that's about it. (not for deep sky objects which typically like f4.5 - 6.0 focal ratio's.  The eyepieces rated in mm like 30, 10, 5 mm etc. give you the magnification....a 30 mm eyepiece with a 900 mm focal length gives you 900/30 = 30 power magnification, a 10 mm eyepiece gives you 900/10 = 90 power, and so on. Your maximum effective magnification
for a 2 inch scope is 50 power per inch of aperture x 2 = 100 power, so the 5 mm eyepiece is useless because it's giving you what we call empty magnification at 180 power, way above your highest limit, so you're magnifying all the defects just as much as the image, so the
image is just a big fuzzy blob.

IF you REALLY ARE  interested in Astronomy, please go to our club
website to learn how to enter the hobby the correct and proper
way; punch on Tom Whiting's Sound Advice for the Novice Observer, at
http://www.velocity.net/~bwhiting

There I outline the proper steps to take, and gives you a short
blurb on whether or not you are cut out for observational astronomy
in the first place. So put the scope away for a while, because you
have some initial learning to do first. In our great hobby, equipment
comes last, knowledge comes first.
Hope this helps,
Clear Skies,
Tom Whiting
Erie, PA

---------- FOLLOW-UP ----------

QUESTION: So you are basically telling me to start out by buying 10 X 50 binoculars. What is it possible for me to look at with them? And is it the bigger numbers the better binoculars? example: 45 x 80 is better than 20 x 100 ?

Answer
Hi Anders,
No, actually you don't need anything except your eyes and a good
star chart (map of the sky- just like you used a map to learn
the USA), in fact, you might just even have, or your parents might
have, and old pair of common 7 x 35 binoculars knocking around in
a coat closet or drawer somewhere. You can even start with those.

Binoculars do a surprisingly good job on the night sky. The large
open clusters (Pleiades, double open in Perseus, IC 4665 in Ophiuchus,  are more easily visible with bino's, and some of the
wide double stars (like Nu Draconis, Alpha Capricorni, the first
split of Epsilon Lyrae, etc etc.)....
(If you have no idea of these objects, or what I'm discussing,  all common objects, then you definitely aren't ready for a scope!) You have to first learn them. So take your star chart outside at night, and start learning and memorizing your constellations (areas) of the sky.
As far as store bought binoculars, the first number is the power.
Anything higher than about 10 power, you have to tripod mount
them because a human cannot hold them steady enough.
(You don't need that hassle while learning the night sky).
The last number is the diameter of the front lens, for astronomy
the bigger the better, to bring in more light. (IN fact that's why
we all upgrade to larger diameter scopes, bring in MORE light.)
Also get a red flashlight to read your sky atlas or chart, so you
don't ruin your night vision while referring to the chart.
Start with the easy ones, Ursa Major including the 7 star asterism
Big Dipper, Ursa Minor with Polaris, Cassiopeia (the Big "W")
and go from there. It just takes a few weeks to learn the 40 or
so constellation areas.  That's how you get started.
Hey, it's clear out...time to go out observing.
Clear skies,
Tom