How do you collimate a Newtonian telescope with a star?

Question

When I use 50x (20mm) the image is good, but when I use 200x (5mm), there is a diffraction pattern off to one side (not symetric around the star) when it is focused, and the secondary mirror and web are not centered on the star when slightly defocused. I was playing around with the locking and focusing screws on the primary and the screws in the center of the web for the secondary to see if I could tell which way improved the collimation, but I couldn't and the star kept moving both from the pressure on the scope to turn the screws and from the adjustments made to the primary mirror. I also noticed a balance issue on both RA and DEC when I placed the Celestron X-Cell 5mm on the scope, so I increased the counter-weight when switching to the higher power. How do you know which screw to turn which direction for the diffraction and off-axis issues? What do you do about moving stars with primary mirror adjustments? How do you switch between eyepieces of different weights?

What is a "reticule eyepiece?" Is it a collimating eyepiece (I am still waiting for it to arrive)? Do you need to mark the mirrors if you have a collimating eyepiece?

I have a Celestron 5" Newtonian (127mm aperture, 1000mm focal length) and the manual wasn't very helpful when I first looked at it, but I'll take another look when I get home.

Which primary adjusting screws do you turn and which direction do you turn them to center the black secondary dot on the star?

Answer 1

Congradulations. You've discovered the joy of collimation!
Don't worry if you are thourghouly frustrated by now. Even experiences amateur astronomers spend hours on end trying to collimate sometimes. Have patience though, you;ll get it.

Before you try anything else though, do this little trick someone told me about...
Flip the optical tube so the front end is facing down and the back end is facing up like a table top.

Carefully undo the locking screws and completely loosen the adjustment screws until you can grab the locking screws and lift the mirror.

At this point you should be able to carefully lift the mirror/cell out by the captured locking screws.Only remove it though if you will not be using a laser collimator and must mark the mirror with a center spot. To do this, you will have to take the mirror out, remove it from the cell, trace around it on some paper,cut the paper circle out, fold it in quarters, and snip the tip of the triangle to make a perfect centered template to mark the mirror. Carefully lay the template over the mirror and mark it with something. Then secure the mirror back in the cell, and holding on to the locking screws, replace it in the tube. If the mirror is dirty, do not attempt to clean it before reading how to do so properly.

Now using a screw driver and holding it with the tip of your thumb and index finger, gently tighten the locking screws just enough such that they're snug. Use no force.

Then using the same method, gently tighten the adjustment screws.

The assumption here is that the telescope was made with a degree of quality such that the mirror will sit right without serious tweaking of the adjustment screws. The processes will also release any tension on the mirror caused from screws being too tight.

Check to make sure the mirror cell is secure before flipping the telescope back over and star test it again. If it still isn't collimated, go here to figure out what kind of problem you're looking at:

http://www.company7.com/books/products/testing.html

Scroll down the page to the different start test images. It may be difficult for you to tell what you're looking at but I doubt there is a problem with the mirror itself. Celestron is a very good brand.

Now onto collimating...
It's best to start with aligning the secondary mirror properly by adjusting the position of the spider vein. When this is off, it usually presents as the center dark spot on the star test being off center.

Go here:

http://www.atmsite.org/contrib/Carlin/collimation/

Go down the page about 3/4th of the way to a section called "The peephole"

Make one of these or buy a collimator. The image below this section is what you should see when you look down the tube through the peep hole. Aren't you glad about that center spot now? Do this with the telescope pointing at the daytime sky or a bright wall so you can see inside the tube. You will have to fiddle a lot with the spider vein if this is off but don't touch the screws holding the actual secondary mirror. Study the image on that website well. It's the holy grail of getting your telescope collimated. After you get the spider properly adjusted your only task is to adjust the position of the primary mirror IF need be.

As for your question about which screws to turn to get the mirror to move in the direction you want...it's an enigma. Loosen the locking screws and work with two adjustment screws until you need the third one. It's a good idea to only turn them a quarter of the way at a time.

Good luck.

Answer 2

Using a star to collimate is really doing things the hard way. Usually a star test is only used to verify collimation. Yes you should have a center dot on your primary mirror, but not the secondary.

To collimate. Look through the focuser with no eye piece. You will see a reflection of your eye in the big, primary mirror. Around your eye you will see the bottom of the focuser and then the secondary mirror around that. Turn the adjuster screws on the back of the scope till the image of the secondary is centered on the primary. If the image of the primary mirror is not centered in the secondary you will need to adjust the secondary first. Secondary mirrors don't need to be adjusted nearly as often as the primary.

No, is not used to collimate. A reticle eye piece just has cross hairs in it. It's used for guiding when taking a long exposure astrophotograph. There are tools to help collimation. A "Cheshire" eye piece is made to help and the easiest is a laser collimator. These both require a center dot on the mirror.

When center marking the mirror use one of those stick on three ring binder reinforcement things. The light colored ring is much easier to see in the dark. Don't worry about it messing up viewing. The center of the mirror in in the shadow of your secondary. You could mark it with a golf ball and still not have it effect your observing.

Answer 3

Star collimation isn't terribly difficult but it is only as precise as your powers of observation which usually isn't that great.

First, the secondary mirror must be aligned properly and this cannot be done using star collimation.

Once that is done, use a fairly high-power eyepiece and point at a bright star, and defocus it enough so that you can see the diffraction pattern and the central obstruction. Move the defocussed image (by moving the scope) to a point on the field where the central obstruction appears centered in the light disk. If you can't do it in the current field, then just get it as close as possible. Then, adjust the collimation screws on the primary mirror to move that image back to the center of the field. Repeat as many times as necessary.

Answer 4

Your best bet is to download the manual for your Celestron make and model. The manual should offer you fairly easy instructions on how to get your collimation corrected.

I own a different model Celestron and I wouldn't begin to try to talk you through without the manual and a similar scope in front of me. I wish I could be more helpful.

A reticule eyepiece is designed to project a set of double cross hairs in the same focus as the image. To make this possible, the eyepiece has a separate focus adjustment on top just for where the cross hairs will be in focus. So, after the image is in focus, you focus the cross hairs so they show up in the image. The cross hairs can be lit with an illuminator.

Answer 5

This is too complex a process to go into in detail in a forum like this. I'd recommend reading this article:
http://www.backyardastronomy.com/appendix/cleaning.pdf

Collimation is something best done in daylight with the proper tools, usually a sight tube and/or Cheshire eyepiece. As another answer indicated, you need to mark the centre of your primary mirror. If your scope is a "short tube" Newtonian, you may have extra difficulty because of the extra lens in the focuser. A final touch-up of collimation can be done on a star, usually Polaris because it doesn't move.

Switching eyepieces: a good mount has enough friction that it will stay in place while you change eyepieces, although you may have to keep a finger on the focuser while you do this.

Answer 6

It's too difficult to collimate in the dark (on a star). It is much, much better to put a black dot on the center of the main mirror, and use that to collimate.

I've done it both ways. Trust me, the dot-method is the easiest. While I can't find the reference I use (I printed it out and the URL is at home, here is a decent site I found:

http://www.oarval.org/collimatE.htm.
.

Answer 7

I think its cheating just to dump a URL, but this is such a well covered subject and why reinvent the wheel?

http://www.astro-tom.com/telescopes/newtonian.htm