Finder Scopes
Published on Apr 18, 2006 at 6:50 pm.
10 Comments.
Filed under amateur astronomy, telescopes.
Last night was the first decent clear night to set up telescopes with the students in several weeks. We have not had rain, but lots of clouds, haze, etc. So, we set up the telescopes. By this point in the semester, I don’t help them all that much. They should KNOW how to do this by now. Well, we get going, and a couple of them are having a terrible time finding anything, so I go over to help them. Well, I center the telescope on the object that they were supposed to look for, look in the eyepiece, and …. it isn’t there. So, then I check through the finder scope, and the cross hairs are dead on. So, I ask them if they aligned the finder scope before starting. No, of course they didn’t. They said, “We did that last time that we used the telescope a few weeks ago.” Yeah, right, like no one has touched it since they last did. Like they didn’t bump it putting things away. I remind them that aligning the finder scope is always the first thing that needs to be done.
So, for those of you new to the world of astronomy, what am I talking about?
Most telescopes have enough magnification, and small enough of a field of view that they only see a tiny portion of the sky. That makes them really tough to use to find anything. So, coaxial with the telescope is a smaller telescope that we call the finder scope. This finder scope is low magnification and large field of view, making finding things easier. Good finder scopes have a set of cross hairs visible when you look through them. You center the cross hairs on the object of interest, and then when you look through the main telescope, you can see what you want. This only works, though, if the finder scope is properly aligned with the telescope. The finder scope has a set of set screws that can be adjusted to properly align it with the main instrument. If you do this very carefully, and if the finder is very securely attached to the telescope, and if you don’t jar it when you put away the telescope, then the finder should stay pretty well aligned. However, with the student telescopes, this is never the case. They don’t treat the equipment as I do mine. My personal telescope rarely needs adjusting because I am always careful. So, I teach the students to always check the finder scope alignment first thing.
Well, this brings up the point about what sort of finder is best. There are lots of opinions here. Often you find that the smaller telescopes, or the telescopes that you buy at discount stores, have very inexpensive (we can call them cheap) small finder scopes. The general opinion is that they are next to worthless. Why? Well, this gets back to how telescopes work. They gather light and they magnify. Really, the gathering light is more important than magnification. The bigger the aperture, then the more light that is gathered. So, you want to have as large an aperture as is reasonable given price, portability, ease of use, etc. This same goes for finder scopes. The typical finder scope is too small and too much magnification for its size. The problem is that the more you magnify, the more you spread out the light, and the dimmer things will be. Usually, you are looking for something too dim to see with the naked eye. So, making it dimmer is bad. The bigger the finder, then brigher it will be, unless you have too much magnification. The typical finder with a small telescope has an aperture not much bigger than your eye’s pupil, so any magnification makes things look dimmer than they would with the naked eye!!!! So, I always recommend replacing the small finder with something else. There are a couple of options. One is to replace it with a bigger finder scope. This works if the telescope itself is large enough to support the weight of a larger finder. I normally recommend a finder with no more than 10 times magnification. Any more, and you wind up reproducing the problem of the telescope itself — seeing too small a part of the sky. Magnification lower than about 5 times magnification, though, and you are not zooming in much better than you see with the naked eye, so why bother magnifying? Also, the larger the aperture of the finder, the dimmer things that you can see through it. If the finder is large enough, and not too high magnification, then it will show things that you normally could not see with the naked eye, making setting the telescope on these objects far easier than just guessing where they are. Binoculars are good for this, and among the best binoculars for astronomical work are 10×50 binoculars. So, I usually suggest a 50mm aperture finder scope. For most amateur telescopes, then, something like a 9×50 finder seen here is quite good. For many of the telescopes that we use at the college, we replaced the stock finder with a 50mm finder scope similar to this. The more that you can see with the finder scope, then the easier it is to use it to point the telescope. Even if the target is still too dim to see through the finder, you can look on a star chart and see, for example, that Object X is about a third of the way between two stars that you can see in the finder scope. Put the cross hairs about a third of the way between, and you should be pointing pretty darn close to the target — at least close enough to find it with the main telescope. But, the finder needs to be able to see the reference stars.
However, even a finder scope is sometimes hard to use, since even the finder shows a small portion of the sky. I often sight along the barrel of the telescope to line it up with where I want to look. That way, when I look in the finder scope, what I want is often in the field of view, or at least near the field of view, so this makes finding objects easier. Well, if that is the case, then if you could just visually align the telescope more precisely without a magnifying finderscope, it would be much easier to use. A number of years ago, amateur astronomers used a variety of devices to get past this problem. I have seen small plastic pipes tied to the barrel of telescopes that you can look through to point them in the general direction of what you want. Some would install screws at the front and back of the tube adjusted so that lining one up with the other and with the location of the target would do the trick. This was fine for the Moon, Jupiter, certain stars, or other objects visible with the naked eye. For many other objects, you could still look at star charts and see where they are located in relation to brighter naked eye stars. You could then put your sights about a third of the way between two stars if you know that the object is about a third of the way between those two stars. This works better if you are using very low power with the main telescope, so that it sees the largest portion of the sky. Then, someone had the idea of having illuminated finding aids. This eventually took the form of the Telrad sight. The Telrad is basically a sort of heads-up display. You look through a piece of glass at the sky, and a target is projected onto the glass. You adjust the target to center on where the telescope points. Then, you move the telescope to where the target is over the object that you want to look at, and if you did it right the object will appear in the eyepiece when you look through the telescope. The only problem with the Telrad is that it is big and heavy. It is not appropriate for a small telescope. That is OK, though, since there are now several other smaller non-magnifying sights available. They, in fact, are often smaller and lighter that the tiny and nearly useless finder scopes that the smaller telescopes come with, and I think far better as well.
But, of course, it all eventually come down to having the finder, of whatever type, aligned with the telescope. Checking this alignment should always be a first priority when setting up the telescope. But do you need to do this for the goto telescopes? After all, you might think that since they go to the object by themselves, you don’t need a finderscope. Well, unless you have a GPS unit that plugs into the goto controls so that the telescope knows immediately how it is oriented, then you will still need to point the telescope at several known stars to help it orient itself. That means using the finder scope.
So, if you plan on getting a telescope, plan on spending more money to get a good finder scope. If you don’t, then you will wind up frustrated and not using the telescope much.
-Astroprof
Sandy on December 26, 2006 at 7:19 am: 1
I purchased a telescope for my son, but the view through the finder scope is upside down. The instructions that came with the telescope doesn’t talk much about the finder scope, it’s use or what the view is supposed to be. Is seeing things upside down correct? Help! We’re very confused.
Astroprof on December 26, 2006 at 8:52 am: 2
The finderscope is normally supposed to make things appear upside down. There really isn’t any up or down in space, so astronomers don’t care. Some moon maps and star maps are printed upside down just for the telescope. I have a later post at: http://astroprofspage.com/archives/88
that addresses setting up a telescope. But, perhaps I’ll write a brand new one here in a day or so for all the people who got telescopes for Christmas. Check back at http://astroprofspage.com for more postings.
Dennis Valade on July 8, 2007 at 6:16 am: 3
I have a Bushnell Sky Tour 76mm reflector telescope and my finderscope is mounted on the front end. Where do I position my head to look through the finderscope? At the mirror end and sight along the tube which puts the finderscope about 2 feet away from my eye? Or do I place my eye along the top of the tube just behind the finderscope?
Your guidance is appreciated.
Astroprof on July 8, 2007 at 10:22 am: 4
You have to look through the finderscope where it is. For a Newtonian reflector, I sometimes sight along the tube to point in the general direction, and then move my head up to look through the finderscope. You’ll have to move your head to the front end of the telescope, anyway, to use the eyepiece. The finder should be on a small stalk to stand off far enough from the tube for you to get your head in close enough for at least one eye to look through it.
Urban on August 13, 2007 at 4:25 am: 5
why do i see my own eye in my reflector telescope, is that how it should be?
Astroprof on August 13, 2007 at 7:35 am: 6
If all the mirrors are lined up right, then you should see your own eye if you look into a reflector without the eyepiece.
Marilyn on January 9, 2008 at 6:58 pm: 7
why do objects appear upside down when I look in my telescope and do I have to buy anything else to make them appear up right/ please help me
Astroprof on January 9, 2008 at 8:06 pm: 8
It takes extra optics to rectify the image, and that means dimmer objects. Most of the time in astronomy you are dealing with very dim to start with, so getting dimmer is bad. Also, things rise one way and set upside down, so there is no real up or down in the sky. Thus, astronomical telescopes don’t worry about rectifying images. So, we all just get used to things being upside down. That is where using a Telrad or something would be good, since it is not upside down.
DeeC on January 3, 2011 at 5:13 pm: 9
I’m keen to resurrect my telescope;
when lining up the viewfinder cross hairs
to an object in the distance how far away
should the object be?
Trina on February 1, 2012 at 3:08 pm: 10
I just bought a huge 4 1/2 foot Meade telescope that I think I have figured out is a reflector, it was used with no manual. I cannot find anything on it when I look at night, I am not sure if there is any pieces missing, I did see the moon for a split second but cannot keep it steady enough. What I did see was nothing special just white light. When I look through I see a spot, during the day I can see my eyeball looking through the eyepiece. I really need some advice, I thought I was getting a great find but have discovered that it isn’t as great as I thought it would be. Please any advice is great!