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FINDERSCOPES AND REFLEX SIGHTING DEVICES
Some of my finderscopes and reflex sighting devices.
Without the advantage of an expensive GO-TO mount to locate objects in the sky, you have to find objects the old fashioned way- by looking for them using your eyes. Even at lower powers, the average telescope doesn't show a very large portion of the sky.Trying to locate an object by looking through the eyepiece alone can be difficult, especially in areas of the sky without brighter discernable objects to help guide you. Depending on the telescope and setup you have, the image can also be inverted on one or more planes which makes it even more confusing. A sighting or "targetting" device makes things a lot easier. Once properly lined up to be aimed at the same spot as the main tube they really help center objects.
There are two basic types of sighting devices for telescopes. One is a finderscope, which is really a small, low-power telescope in itself with a large field of view. The other is a "reflex" sighting device, which has no lenses and doesn't magnify. Instead it projects a dot, crosshair, bulls eye, etc. onto a clear plate overlapping the real sky view. Each type has it's pros and cons.
Left:6X30 finderscope. Right: 9X50 finderscope.
Most telescopes usually come with a finderscope. They are mounted on a bracket near the viewing end. Every finderscope I have seen has a standard cross-hair in it: two lines that cross each other at right angles in the center of the field of view. They have a low magnification, usually between 6x and 10x, but higher powered finderscopes are available. The numbers represent the magnification and aperture of the finderscope: for example, a 9X50 finderscope has a magnification of 9X, and an aperture (main lens diameter) of 50mm.
Due to the nature of the lenses, the image in finderscopes like those shown above is upside down and mirrored hoizontally. This can be confusing to some people, as if you are looking for a small, dim point of light (dimmer planet, asteroid, star) amongst others with the use of a map, it's hard to find your way around when they are all just points of light that have been flipped on two planes. For this reason they also sell finderscopes that give a properly oriented image. They use an erecting prism, much like the ones you can buy for a refractor telescope to make the image correct. NOTE: A mirror diagonal is not the same thing as an erecting diagonal. A mirror diagonal only corrects the image on one plane, but leaves it reversed on the other. This is why if you view objects on land with a mirror diagonal in a refractor things appear right-side-up, but are still inverted left to right. Finderscopes that show a correctly oriented image are more expensive than a standard one, so it's up to the individual to decide if the need is enough to warrant the cost.
Finderscopes are useful for finding objects that are bright enough to be distinguished from the background stars when looking through them. If a galaxy, nebula, etc. is bright enough to be seen in the finderscope, it doesn't really matter if it is upside-down for the purpose of aiming the main telescope. You just center it in the crosshairs and it should be in the main viewfinder.
REFLEX SIGHTING DEVICES
Left to right:Telrad, Rigel quick finder, Antares red dot finder.
Reflex finders have no lenses or mirrors, and don't magnify the image. They consist of an LED light that projects a small circle, dot, or crosshair onto a clear plate. You line up an object by looking into the plate and aiming the telescope until the object is centered on the dot or crosshair. In the case of a Telrad (my favorite reflex sighting device) the diameter of the largest circle in the bull's eye is 2°, no matter how close your eye is when you look at it. Reflex sighting devices can be used to line up the Moon or one of the brighter planets, but they are better than a finderscope for locating an object too dim to show up in a finderscope. Using a reflex finder, you not only see the portion of the sky that is in the clear plate, but all of the surrounding sky as well. Furthermore, since you are not looking through the lense, everything you are seeing is correctly oriented. All you need is a star chart or map showing where the object is, and you can aim the telescope in the right spot.
AIMING A SIGHTING DEVICE
In order for a sighting device to work, it has to be collimated with the main telescope so that they are both pointing in the same spot. Both types of sighting devices are collimated the same way, and it is best done during the day the first time. After ensuring that the sighting device is installed correctly and secure, pick a stationary target a long way off and center it in the eyepiece's field of view (it doesn't matter if it is upside down or sideways). Choose something that has a point on it that can be easily distinguished to tell if it's centered in the finderscope or other sighting device (the end of a pole, top of a tree, or if you're lucky a circular sign). Start off with a low power. This will give you a larger field of view to work with. In the field this is what you would use to find an object, then you just keep re-centering it as the power climbs. Once centered in the eyepiece,lock down the telescope if it has clutches or lockscrews. If it doesn't, be careful not to bump or move the tube.
Now look through the sighting device. The device must be aimed so that the object you're looking at is centered in the crosshair, dot, bull's eye, or whatever the device uses to center objects. There are small screws or knobs on the mounting bracket or device itself to facilitate this adjustment. On older model finderscopes, you will have to loosen one or more screws slightly before you can tighten another. Newer finderscopes have two screws and one spring loaded support where the 3rd screw would be, and you can turn the 2 screws freely to adjust the devices aim. It doesn't take long to figure out which way to turn which screw to center the object in the sighting device.
Whether you are using a finderscope or reflex sighting device, it's important to remember that the higher the magnification you use, the smaller the actual portion of the sky you will see in the eyepiece is. While it might be easy to locate an object at 25X in your telescope, that same object might be nowhere in the eyepiece's field of view if you line up your sighting device when your magnifivation is 300X. Start with a lower power, then re-center the object each time as you gradually increase the magnification.