Anyone who has tried to point a telescope at a particular place in the sky has very quickly learned the value of a finderscope. Trying to look down a round tube and determining exactly where the scope in aimed is a very quick lesson in futility.
There are several types of finders on the market. The most popular is a small refractor with a cross hair eyepiece which will allow the user to accurately determine the place in the sky where the telescope is pointed. The smallest of these refractors are 5 X 24 and 6 X 30 systems. That means they are 5 or 6 times magnification and have a 24 or 30 mm objective lens. These small finders are only useful for finding bright objects. However, if the only objects you are looking to find are the Moon, planets, and bright stars, then a small refractor will suffice.
Moving up to an 8 X 50 or 11 X 80 finder will start to show brighter deep sky objects and will allow the user to star hop using much fainter stars. These two sizes of finder are very popular with deep sky observers and I have an 11 X 80 myself. Besides being able to see dimmer stars and objects, there is another advantage. They make great RFT telescopes! The view of the Sword of Orion in the 11 X 80 with a UHC or Oxygen III filter is beautiful. The entire region is nebulous, with M42 showing dark markings and a large loop of nebulosity which leads to Iota Orionis. The Rosette, Lagoon and North America nebulae are also glorious in a small telescope that can show some of the area around the main object.
The last step up is a small Newtonian telescope as a finder for a larger instrument. I have used a 4.25" f/4 as the finder on my old 17.5" on a few outings when I was looking for something very faint. The problem here is that even a small Newtonian only has a field of about 2 degrees. That can be a problem unless you have a small finder on the large finder.
The one thing that must be done before a finderscope can be used to its' full potential is to align it to the main scope. The two telescope must point at the same spot so that when the cross hairs are on a location in the finder, the main scope point precisely at the same location. The most common way to do this is a set of rings which support the finder and allow the user to align the finder by adjusting the set screws on the rings. I have found a problem with this system. Using the larger 11 X 80 finder, the rings will move with the increased weight of the finder as I swing the telescope around. I have considered going to a platform mount, like the one offered by Lumicon, but I have not given it a try yet.
Now for one of my best tips on finderscopes. Do the
alignment during twilight. I know this means that you must arrive at your
observing site before sunset. It is also easier to set up in daylight.
Use a distant object such as a hill, telephone pole or (dare I say it)
streetlight. Using stars to align the finder generally makes trouble for
me because I will bump the scope and align slightly off center. Also,
it is dark and I am anxious to start observing, so I don't do as good
a job as I should.
Well, you didn't think I was going to do an article on finderscopes and not mention the Telrad. This zero power finder has made finding your way around the heavens much easier for many people. A small bull's eye pattern is projected on a piece of glass which is at a 45 degree angle. The pattern is focused for infinity, so it appears projected on the stars. It allows me to make certain that I am starting at the star I think I am starting from. It is the best $40 I have ever spent on my telescope.
There are only two drawbacks to the Telrad. One is dew. People who live in parts of the country where dew is a problem can wipe the glass often or try and rig up a small heater. I am sorry, we in Arizona don't have dewing problems. The other problem is being careful with the device. Make certain you return it to its' case after every trip. Otherwise, you will eventually break the glass. Because the only current draw on the batteries is a small LED to light the pattern, batteries last a long time. I change mine once a year or so, just because I am guilty by then.
So, I use a Telrad in conjunction with an 11 X 80 finderscope on a 13" f/5.6 Newtonian. I find that the Telrad lets me make certain where I am starting and the large finder lets me see dimmer stars to use for star hopping. This works very well when I am using the Uranometria 2000 star charts. Their limiting magnitude is easily seen in the 11 X 80 and only glimpsed in an 8 X 50 finder. I feel much more confident finding my way around with this system than any other combination I have used in the past. I have the Telrad and 11 X 80 on opposite sides of the focuser. That way neither of them gets too far under or on top of the tube as I rotate the tube for most comfortable eyepiece position.
The other tip that I can pass on concerns Amici prism diagonals. This set up on a finderscope will correct the field of view so that the orientation on the star charts is the same as the view in the finder. It makes your finder into a true monocular (half a binocular). The advantage is that the user does not have to try and re-orient the star pattern from the star chart. I found it very difficult to flip the field in my mind, the Amici prism alleviates that problem. The disadvantage is that there are some light losses in the prism, so it works better with the larger finders.
Well, that about wraps it up. As in any new piece of equipment, you might find out some more by going to an astronomy club star party in your area and looking over the equipment other amateurs are using. That way, you can pick and choose what fits your scope.