Finder Scopes
The narrower the field of view, the less area you can view.With a narrow field of view (FOV) it can be difficult to locate deep sky objects in the night sky.
To overcome this, a finder scope is often mounted on the telescope to make it easier to locate objects. A finder scope has a field of view much wider than a telescope. Finder scopes make it easier to locate objects, and are useful when aligning the scope to the night sky and are especially useful with computerized “go-to” telescopes.
What follows is a brief explanation of finder scopes and what I use.
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Field of View Comparisons
William Optics Zenithstar 61 wide field view telescopeShown above is my smallest telescope (widest field of view) mount on my Celestron CGX Equatorial Mount.
Below is my largest telescope on the same mount. This has a much smaller field of view than the
Celestron C8 Simulation
Using the calculator on the Astronomy Tools Website we can see what the Hercules Star Cluster will look like using a 25mm eyepiece with the Celestron C8 whose field of view is 0.8 degrees (see below).
Now let’s compare it to the William Optics Zenithstar 61 using the same eyepiece, which provides a 4.53 degree field of view. The Hecules Cluster now looks like a single bright star (see below):
In other words, the Zenithstar’s field of view is more than 5.5 times as wide as the Celestron telescope.
The Hercules cluster is a very bright object. When looking for even smaller objects, such as a single bright star it becomes very difficult indeed to find it with the wider FOV of the Zenithstar, and nearly impossible with the narrow Celestron C8. So what we do is month a very wide FOV finder scope on top of the telescope.
Finder scopes do need to be aligned with the telescope. When a finder is first installed on a scope and an object is found, the telescope may not be centered on the object, but it should be close enough to at least see the desired object.
Now the telescope is adjusted so the object is centered in the eyepiece, meaning the object will no longer be centered in the finder scope. Next the finder scope is adjusted so the object is also centered in the finder. After this adjustment, slewing the telescope and using the finder scope to center the object, will also center it in the main telescope.
Types of Finder Scopes
There are two basic kinds of finder scopes:
Red-Dot Finders
“Red-dot” finders use a laser beam to point to an object in the sky. Usually these have red lasers, thus “red-dot” finders. However some use different color beams. There is no magnification in these finders.
My NexStar 8se came with an inexpensive red-dot finder, which I never did like and I soon upgraded to Achromatic finder scope.
Celestron StarPointer Red-Dot Finderscope
Achromatic Finder Scopes
Achromatic finder scopes are miniature telescopes. They have lenses in them and also magnify the view to help locate objects. Low cost finders will show a reversed image, sometimes making it more difficult to adjust the telescope mount. More expensive achromatic scopes use a prism to keep the object oriented and some come with a diagonal (90 degree elbow) to make viewing easier.
Years ago I replaced my Red-Dot Finder with an Orion brand 9×50 Right-Angle Correct-Image Finder. I still use it and have been completely satisfied with its operation. Last year Orion went out of business so the finder scope is no longer available.
Most folks mount their finder on the top of the telescope. I found it more convenient to mount mine near the bottom, which made it easier to look through and the eyepiece was close to the level of the telescope’s eyepiece. Plus it was easier and more convenient to mount my DSLR camera on the top section of the telescope.
Astromania 9×50 Right Angle Upright Corrected Image Finder Scope
Which Type is Best?
Best for one person can be the worst for someone else and visa versa. There are two camps when it comes to amateur astronomers — the red-dot camp and the achromatic camp.
I like the achromatic because it does magnify the view and has a “cross-hair” in the viewer. The red-dot makes it easier to find things in the night sky if one is moving from object to object and the telescope is not a computerized go-to scope.
The Best of Both Worlds
For a time I mounted both the red-dot and the Orion achromatic finder on my telescope.
I removed the red-dot when I found myself no longer using it.