Help with a reflecting telescope please?

Question

I got a reflecting telescope in apparently good shape @ a garage sale

What are you supposed to see? I look through the secondary mirror to the primary mirror in the bottom of the tube. Is the entire mirror supposed to be in my view with the cross bearing the secondary mirror smack in the center or should the view be centered on one quadrent of the mirror

The telescope is currently more like the latter ith only 30% or so of the primary mirror visible

I have found some web sites about collimating the morrors but none of them make clear what the desired final result is

Answer 1

Everything should be centered when you are done. The secondary mirror should be in the center of the primary.

Answer 2

Hi. When you get into your car the first thing you do is adjust the mirror so that you see correctly out the rear window. This is a very simple "collimation". In a reflecting 'scope there are two mirrors to adjust. The primary at the bottom of the tube and the secondary at the eyepiece. Start by adjusting the secondary until you see the reflection of the primary centered. The support struts may or may not be perfectly even, so ignore them. Once you get this far put in a lens. Low power (high number of mm, say 20 to 30) and look at a star. In focus it should be a tiny point but will probably look like a small comet. On the primary end of the 'scope there should be 3 screws that adjust the tilt of the mirror. Turn one of them while watching the star (a good friend is helpful). Try to move the screw that makes the head of the comet move toward the tail. A bit hard to explain but you will see what I mean. The object is to repeat this until the star is a fine a point as possible remembering that no 'scope is perfect. Have fun and good luck!

Answer 3

Everything should appear centered. Roughly, the collimation process is, adjust the secondary mirror until you see the primary centered in the secondary, then adjust the primary so that the center of the primary appears centered. Use the three screws on each mirror mount that let you adjust the mirror tilt. There are tools (e.g., laser collimators and cheshire eyepieces) available to help you do this, but you can do a fair job if you mark the center of the primary and make yourself a collimation cap.

The star test process that Cirric describes is good for a final tweak. What he forgot to mention was to be sure to center the star before adjusting. Off-center stars will show coma (comet-shape) in a Newtonian. The effect is less noticeable at longer focal ratios - say, over f/6.

Answer 4

If you can only see 30% of the primary mirror, something is quite a long way out of allignment. I would remove any lenses and remove the end cover. Take a look down there and look for anything that looks broken or out of allignment.

Assuming that this is reasonably priced telescope, the secondary mirror is going to be supported on a plastic structure that fits into the end of the tube and is secured by screws. Check that the whole structure is in place correctly, it is always possible that someone has removed it and not put it in straight. Check that none of the plastic looks cracked or broken.

The actual secondary mirror mount is going to be what looks like a bit of plastic tube with a 45 degree cut on the lower end. This is normally screwed to the support structure with a large central screw and three small adjustment screws. Just check that the 45 degree cut looks as though it is alligned correctly. I always take a look for scratch marks around screws that might indicate that someone has playing.

Have a look down to the bottom of the tube at the primary mirror. It is usually held in place with clamps, each normally held by two screws. Shine a flashlight down there and make sure that all looks well. The primary mirror is usually fixed to a plastic structure that fits into the end of the tube and is held in place by three screws. Make sure that all this looks OK and is not broken or incorrectly inserted.

At the bottom end, you will probably find that there are a total of six screws, three to hold the mirror in place and three to adjust it. Look for any tampering. You would be surprised how many people try to fix a problem by tightening all the screws without finding out what they are for!

If all looks well, take it outside during the day and point it at a nice bit of open sky. With no lenses in, try adjusting the secondary mirror, using the three adjusting screws, until you can see the whole of the secondary mirror.. Even if the primary mirror is out of adjustment you should be able to do this.

Now put a lens in, go for a low magnification. Point it horizontally at a distant object. You want something that has well defined edges so the top of a pole or the top corner of a flat roofed building will do nicely. Focus the image using the eye-piece as well as you can. Take a good look at the image. If it is blurred you are going to have to adjust the primary mirror with the adjustment screws to get it as sharp a possible.

Now, you are not going to be able to do a very good job with an earthboud object in daylight, but you will get somewhere close and you will get familiar with what differences you can make with the screws and how far you have to move them. It is easier to learn this in daylight rather than in the dark.

Assuming that all this works. Take it out at night, use the same low magnification eye-piece, focus on a bright star and carry on adjusting the primary mirror. The object of the exercise is to see a single point of light that is not smeared or elongated. If viewing conditions are poor, you will find it difficult to do a good job. You may have to wait for a good night with minimum atmospheric disturbance to be sure of a really good job.

The whole point of the last stage is that the distant star can be considered a point source of light and should therefore appear as a point source. If it does not, then you have an alignment problem.

Do check with differnt eye-pieces. It is possible, though not probable, that you have one that is damaged in someway.

Have patience and always be prepared to go back and start again if you have to make any really big changes.

When you are happy, use a small blob of nail varnish on the edge of each screw. If the vibration cause din transporting the telescope causes things to move in the future, you will have an indication of what has happened.

Answer 5

Gross : in our giant telescope in M.t Palamar the reflecting type
is used for brining planet up closer than it looks far away by the
nakid eyes so what we do is take an image get it in line just right gama rays reflect light like a prism showing multi colored in
glass going through to the other side all directions called bent reflections

Answer 6

The best website regarding collimating a reflector is:

http://www.andysshotglass.com/Collimating.html

Which has a movie/animation of the whole process. Can't be beat!

Answer 7

http://www.madpc.net/~firmament/images/collimation/coll_outoffocus_02.jpg&imgrefurl=http://www.madpc.net/~firmament/astro/collimation.html&h=143&w=208&sz=3&hl=en&start=8&tbnid=ABpV8NoAP6qzQM:&tbnh=72&tbnw=105&prev=/images%3Fq%3Dtelescope%2Bmirror%2Bcollimation%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN
This site has pictures of what you should see when you collimate properly and fairly easy instructions.