The Celestron red dot sight is standard equipment on all of the single arm mounted computer driven scopes such as the NexStar 80GT and NexStar 5 series. The idea behind this device is quite simple: a little red LED reflected in a small tilted window appears as a point at infinity, as though the scope had a laser dot sight (which, of course, would be unusable for pointing at anything over a football field or two away). To aim the telescope, you move it until the red dot falls on the object. It's simple, and it works at night.

Background

The problem of pointing a telescope is closely related to its focal length. A small telescope with a focal length of 400mm, when combined with a common 40mm Plossl eyepiece, have a field of view 4.6 degrees across, or 9 full moons wide. In this case, you will be able to get close to just about anything you want to see just by sighting along the barrel of the telescope. Up to around 600mm focal lengths, this still works fairly well. But when the focal length gets to 1000mm or longer, then the best field of view available drops to a degree and a half wide or narrower, and here pointing can be very difficult. If the object you want to point at is something you see in the sky, then getting to it with the telescope can be a very serious problem as it is slowly moving as you try to guide your telescope to it.

The first red dot sight I saw was something made by the Israelis to use with a rifle which was fairly large and heavy since it was all metal, and needed special lithium batteries to drive it. It seemed as though these devices had disappeared for a while before inexpensive astronomical ones showed up in large numbers in the late 1990s.

Today they are offered in several sizes, but what is true of all of them is they have the same small dovetail clamp taken from versions made for BB guns. Unfortunately, the linkage between these devices and their weapon heritage has not been completely broken, and even the nicest ones have dovetail rails made for the tops of various weapons instead of something more useful for astronomy.

Description and Usage

The Celestron Red Dot Sight is the smallest size of these finders made. The window is about half and inch (12mm) in diameter, and has a small amount of tint (probably so ones used on air guns can't be seen from the front, but it interferes with its use as a finder). The finder needs and adapter to attach it to the telescope, and Celestron has used one variant with a pair of screw holes to go on a Schmidt Cassegrain Telescope (SCT), and has a version with a stalk and a dovetail base to go on small refractors (which have a finder base built in which has a dovetail to preserve alignement from the last time it was installed).

The rail system these attach with is an upside down dovetail with a clamp mounted on the sight to take hold of it. The clamp has a simple through-bolt with two plates on either side. In practice this means it is impossible to take the sight off and put it back on and get the same alignment- the plates move around a litte differently every time, so the pointing changes. Especially in the case of the SCT, this leaves a lot to be desired since the mount is relatively susceptable to any small bump as well.

The sight has a small knob under the front end to turn it on and adjust brightness. The brightest setting is barely visible in full daylight. The side of the sight has an adjustment screw for slewing it left and right, while the back end has the screw for moving the dot vertically. To line it up, you will need to take the telescope and point it at a distant point feature. For example the light on a television antenna is a perfect alighment target. With the scope there, move the dot to merge with the light on the antenna.

The sight has a slide-out tray which holds the watch battery it uses for power. Note, you need to turn this off when you are done with it. Be gentle while turning the knob under the front or this can result in moving the mount (so much for pointing).

For the NexStar telescopes, which use computer guidance for most of their pointing, this sight makes a lot of sense because you only need it for getting a few alignment stars at the beginnning of observing. They even have a reminder to turn it off show up on the controller.

However, for use as a primary pointing device, this finder does have some flaws. First, the red dot is not a magnified point; all you have is your eyes for alignment. Especially with larger scopes such as SCTs, the result here is a lot of things are visible in the telescope which are completely invisible to the eye. If you are trying to find something a bit dim or under any level of light pollution, this can make it nearly impossible to get there with the red dot sight.

A red dot sight can work well if mounted in parallel with a finder scope, where it can serve to get the finder scope in the ballpark of where you want to be pointed. If the finder has a 90 degree diagonal on it, then this may be the best way to get in the neighborhood of where you want to look.

And that comes to the main point: this item has limited usefulness. If you can see what you are guiding to, you will get there. If you can't see it, then this item is useless unless you have some serious mojo (like me) and can dead-reckon where the object is based on what you can see [and yes, that's a joke- I have only been able too do that with Double Cluster].

Conclusion

The Celestron Red Dot Finder is the absolute minimum you can have as a pointing device for a telescope. If a computer is going to do the hard part and all you have to do is sight in on some obvious stars to initialize the scope, then this works very well. If you are going to point at bright targets like the moon and planets, it can handle this as well (but not for planets beyond Saturn). If you are going after deep sky objects, especially nebulas away from bright stars, then this finder has little to offer other than to get your actual finder scope in the neighborhood. If your scope has one slot for a finder, I recommend getting something with optics instead of one of these.

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