A telescope's finderscope is a pointing device that rides on top or the side of the main telescope tube. A telescope's finder scope is used to help point the main telescope at the specific area of the sky you want to observe. Knowing how to use your finder scope is an important part of learning how to locate objects in the night sky with your telescope.
Why do we need a finder scope?
Finderscopes are generally very low in magnification- between 6x and 9x the naked eye- while some have no magnification at all. Without the finder scope, locating objects simply by looking in the main telescope would be very difficult. Even at a telescope's lowest magnification, it is still far too much magnification for locating objects easily. Basically, a finder scope is a tool for bringing objects into your telescope's field of view.
Aligning your finder scope with your telescope
Used properly, a finderscope will see exactly what your main telescope sees. In order to do this, they need to be properly aligned. Finderscopes are not delivered attached to your telescope, so everyone will have to do this at least once, and frequently if you remove the finder scope for storage.
Aligning your finder scope to your main telescope is easy. Your finder scope will usually have some sort of aiming device, such as a red dot, or a cross-hair. You need to get the finder scope to point exactly to what the main telescope points to. Here's how:
- Start by attaching your finder scope to your main telescope during the day.
- Using your lowest magnification eyepiece, begin by pointing your main telescope at something distant, like a street light. Center the top of the street light in your telescope eyepiece.
- Without moving the telescope, center the finder scope on the same distant object, getting the dot or crosshair as centered as possible.
- To test your alignment, start by moving the telescope. Using your finderscope, center a different distant object in the dot or crosshair. Now look into your main telescope. You should be looking at that exact position.
- Aligning your telescope during the day gets the finder scope and telescope roughly aligned. For final alignment, repeat this process at night with a star. This will perfectly align your finder scope with your telescope.
Different Types of Finder Scopes
There are generally two different types of finder scopes- optical tube finder scopes or reflex sight finder scopes.
Optical Tube Finder Scopes
An optical tube finder scope is a simple refractor telescope usually between 25mm and 50mm in aperture with magnification between 6x and 9x. These feature a crosshair in the field of view and in some cases even have illuminated crosshairs. Another design element to consider is that some optical tube finder scopes offer straight-through viewing, while others are equipped with a 45-degree viewing eyepiece.
Reflex Sight Finder Scopes
The other type of finder scope is the reflex sight style. These are often called "red dot finders." They usually have no magnification power and simply project a red illuminated dot or circle for aligning. These finder scopes are very easy to use and align.
The TELRAD Reflex Sight
One style of finder scope is so popular it almost needs to be called out on its on. Many TELRAD finder scope owners consider this to be one of the most valuable accessories in their astronomy arsenal. The TELRAD is by far the best selling finder scope. Most TELRAD owners replace the finder that came with their telescope because of its ease of use.
TELRAD finderscopes project three red rings spaced at 4 degrees, 2 degrees and 1/2 degree in diameter. These rings can be used to "star hop" from object to object. For example, if a galaxy is 10 degrees south of a star, you simply count two 4-degree jumps and one 2-degree jump, and you're there! If you're a Dobsonian telescope owner, a TELRAD is a absolute must-have accessory.
But I have a GoTo Telescope--I don't need a finderscope...right?
Wrong! The GoTo Telescope still needs to go through its alignment procedure. Here is where a finderscope is the most critical. When aligning your GoTo telescope, you need to align and center two or more stars. Without a properly aligned finder scope, you will have substantial difficulty getting your telescope's GoTo feature to function correctly.