Telescope Tips: Finder Scopes vs. Red Dot (Reflex) Finders
Aiming a telescope is tricky: It sees only a tiny portion of the sky, it might not be in focus for what you are looking at, and slight bumps can throw off your aim. That is why most telescopes come equipped with some kind of aiming device to help you find objects in the night sky. In the past this was almost exclusively with a Finder Scope – a little, low poweder telescope on top of the main telescope’s optical tube with a cross-hair that was used to aim.
The design of these little telescopes would vary from 20mm to 50mm in diameter, and the power was from 5x to 9x, typically. The holding bracket originally
would be two metal O-Rings with three thumbscrews each to adjust the aim of the finder scope. More recently that design would be replaced with a single ring with two thumbscrews and a spring bracket. Those were the good ones, anyway. Cheaper telescopes would often have a plastic 5x finder scope with a single holding bracket with
three plastic thumbscrews that would often frustrate new telescope owners as this mount is clunk and hard to control and the optics in the finder scope were poor.
This frustration would lead to a buig change in smaller telescopes around 10-15 years ago as they switched from cheap finder scopes to using reflex finders.
Reflex finders, or red dot finders involve no magnifying optics. Instead the reflect finder has a window that you look through and a red dot is projected to show where the telescope is aimed. Adjustment is made by two knobs. This was much easier to deal with fot new astronomers as the main frustrations with cheap f
inder scopes were mostly alleviated by using a red dot finder. But unfortunately they were replaced with new issues. The first being that all too often the new astronomer would leave the Red Dot Finder on after the viewing session was over, which would drain the battery. Long term storage would also be an issue as many would forget to remove the battery and they might leak acid onto the electronics.
The final issue was that once the astronomer gains some experience they will not be able to use the Red Dot Finder for a technique that is ued by more advanced astronomers to find objects: Star Hopping. This is where the viewer jumps from star to star in the field-of-view of the finder scope to get towards an object they are seeking such as a small nebula or globular cluster. The technique involves having one bright known star that is near another known star (not as bright) such that they can both be in the field of view of the finder scope. That 2nd start is then centered in the finder scope and a 3rd star that is near the edge of the field of view is found and so on. It is a tricky technique to learn and unfortunately you can’t do it with a Red Dot Finder.
SO which to choose? Well, some do not:
More determined astronomers will actually have both designs on their telescope. A red dot finder to easy aiming along with a larger finder scope for close work and star hopping. This may not be an option for those using smaller telescopes as they have limited space for such extravengance.
Here is a summary of the main points along with some other advntages and disadvantages:
- Have actual magnification
- Can be used to star hop
- Magnification gives a better sense of where you are viewing
- Can be purchased as ‘right angle’ which makes using them on Reflecting telescopes easier
- Harder to use for new astronomers
- Trickier to align properly with the optical tube
- Cheap ones extremely hard to aim
- Need to keep clean
RED DOT FINDERS
- Easy to use, especailly for new astronomers
- Much easier to align with the telescope
- Batteries can be drained if you forget to swithc off
- Batteries can leak in long term storage
- No magnification means no star hopping
Have fun viewing whichever you use!