Telescope Basics 6 (of 6): Understanding telescope finders, types, and alignment

Hi there, welcome to Part six, and the last of this video series, “Telescope Basics.”

My name is David Fuller, host of the “Eyes on the Sky” weekly stargazing series

that can be found at eyes on the sky dot com. In this video, I will explain the various

types of finders used on small, inexpensive telescopes and what you need to know about

them.

There are two basic types of finders for telescopes: Magnifying, and non-magnifying finders. I"m

going to cover the basics of the lower cost versions of each of these.

Let"s start with non-magnifying – also known as zero-power finders.

The basic style of the zero-power finder is what"s known as a red-dot finder, or RDF.

It consists of an adjustable base which holds a small watch-sized battery,

a switch, a red LED, and a clear, flat lens onto which the LED"s light is projected. The

idea of a red-dot finder is that, once the finder is properly aligned

with the main telescope, you can place the red-dot onto the object you wish to see, since

what you see in the finder is what you can see with your eyes, then that object

should be visible in the eyepiece of the main telescope. In theory, this works well….

for brighter objects. The problem can be when you go to look for dimmer objects,

or when you are looking for objects in light polluted areas – as many of us are. Now,

if you only plan to look at bright objects like planets or very bright objects, or

are good at estimating where to point the red dot finder in between 2-nd and 3-rd magnitude

stars, that may not be a problem for you. Or perhaps you do live in an area

where the skies really get dark, and you can see 5-th or even 6-th magnitude stars. In

those cases, a red dot finder may be just fine.

The other type of finder, a magnified finder, does not suffer from this. These are basically

small, wide-field of view telescopes attached to the main telescope that

provide a wider field of view than what can be seen in the bigger scope. And telescopes

can show you more stars than what you can see naked eye; this is the same concept

as being able to see far more stars in your larger telescope. To understand how much more

you can see, look at the numbers on them: The first number is the magnification,

the second is the aperture of the lens in millimetss. So a 6×30 is a 30 millimeter aperture

finder that magnifies the view 6 times. One problem with these is that instead

of seeing the sky as you see it – as the zero-power finders show – the star patterns

in the sky are reversed. In other words, when you move the entire telescope assembly to

the left, the stars will move the wrong way (opposite what you would see in binoculars,

for example). This can make it challenging to find objects, when the stars move the

opposite what you"d expect. In any case, with good magnified finders, you can see enough

more stars that you can “star hop” your way to dimmer objects that you cannot see

naked eye.

Ah…. but that"s another problem with these smaller finders. Many of the ones included

with inexpensive scopes are NOT good ones. The typical “5 x 24” finderscope

that comes with lower cost instruments often are of poor quality: Singlet lens, poor eyepiece,

difficult to focus and sometimes stopped down to smaller apertures than

what they are stated to be. In cases such as this, sometimes using a red dot finder

may be easier and better – or upgrading to a quality 6×30 or larger magnified finder.

There are good quality, small finderscopes that can be purchased separately and added

to telescopes to make it easier to find things if you already have a telescope with

a poor finder – a 6×30 finder really is a good starting option. But if you are in

the market to buy a scope and are not sure which to buy, I can state these things from

firsthand experience:

Red dot finders may not show you all the stars, but they are easy to use

Bad 5×24 finders are worse than red dot finders. However, I do know that Orion Telescopes places

a good quality 6×26 magnified finder on many of their less expensive telescopes, and it

does not reverse the view, making it easier to use.

Be sure to align your finderscope with your main telescope. Do this during the day by

aiming the main telescope at a distant object, a least one quarter of a mile or half a kilometer

away. Center an object that is not moving, such

as the top of a telephone pole, tower, or tall building. Center a prominent section

of that object in your main telescope, and then place a shorter

focal length eyepiece in it, and center that object again. Lock the telescope in place,

so it does not move. Now turn the adjustment screws on the red dot finder or finderscope

until the red dot or the cross hairs in the finder are

lined up with the object in your main telescope. Look in the main telescope again to ensure

that the telescope did not move. When you have completed that, it will make

it much easier to finder objects in the night sky because the wider field of view you can

see with the red dot finder or finderscope will ease the difficulty of aiming the main

telescope at the object you wish to see.

So that is a quick look at inexpensive finders for astronomy telescopes, and the end of this

series for understanding telescopes. With the free star charts available and weekly

night sky videos available at eyes on the sky dot com, you are now prepared to find

and see more objects in the sky. Thanks for watching; keep your eyes on the sky and your

outdoor lights aimed down, so we can all see, what"s

up.

Telescope Basics 6 (of 6): Understanding telescope finders, types, and alignment