Help with focus

[gordonbaldry]

Please can any one give me some advice when i try to focus in on the moon i get a ghost image around what iam looking at ..why is this and how can i stop it i have a 8 inch reflector also if i use a 25mm eye piece its ok but when i use a 9mm i can not focus it at all ?? please help

[scotastro]

gordonbaldry

The is no obvious answer to you problem without some more information.

Firstly, what model of reflector do you have. Without wishing to worry you a friend bought a scope off ebay which was so badly made it was impossible to collimate it and gave double images of everything a low magnification.

The most obvious thing to ask is have you check the collimation of the scope - assuming you know how to do this.

Are you using plossl eyepieces or some other type.

If you can provide me with the above we can eliminate the possibilities. It's most like poor collimation.

Scotastro

[The Warthog]

Here are a couple of things you might try. If your scope cap has a hole in it about 1/4 the size of the cover, try opening that hole, putting the cap on the scope, and seeing if it improves the view. If it does, I would suspect some flaw in your mirror, but see if you can beg or borrow another eyepiece or two, as a cheap eyepiece can do all sorts of interesting things to your view.

With the 9mm in place, try and catch the edge of the Moon in the field of view, and slowly focus from all the way in to all the way out. If it doesn't come to focus, but looks better when it is all the way out, slightly loosen the set screw and draw the eyepiece slowly out of the tube, and watch to see if it comes to focus then. If it does, and you can still tighten the set screw on it, you will be able to use it for higher power views.

What kind of telescope is it? Unfortunately, some ebay scopes have spherically ground mirrors, rather than parabolic ones. This limits the usefulness and total magnification of your scope. You should still be able to see a lot of things, but not at the full potential of an 8" mirror.

As to collimation, have a look down your focuser tube in daylight, while pointing at a light coloured object or wall, and you should see your eye looking back at you from the centre of the primary mirror, and you should see the primary mirror from edge to edge in the secondary (the little mirror at 45degrees to the tube.) If it's close to lined up, you should be OK for now, but if it is far off, you will have to read up on collimation and fix it. This may be all the trouble you have. It's something most people with opposable thumbs and forebrains can learn with only a little fear and trembling, so don't worry; if I can do it...

Hope this helps, eh?

[Ajohn]

If it's the scope shown in your listing - open truss tube - it looks to be oldish and substantially built.

It could be that you are having problems with stray light. Try wrapping towels or something round it and see if things improve also check the collimation. Both things may be needed.

If blocking out stray light helps then I would suggest purchasing some foam mats that walkers etc carry around to sit on and fixing these in place. Could also use varnished cardboard or many other things that may be to hand. You might also benefit by extending the covering past the end of the tube. Those foam mats are very useful for that and make excellent dew shields. It's just a matter of cutting to size and using velcro tape to join the ends.

This link should give you a good intro to collimating your telescope.

http://www.galaxypix.com/Stargazing/collimate.html

John

[gordonbaldry]

Thanks for all your comments i will give it a go and see what happens and let you know ... my telescope is the one in the small picture its a 1960 Charles frank 8inch

[Ajohn]

That sort of telescope should have a decent mirror but some did fit scopes with 1/4 wave optics then. Having said that though this may well be better than many are today.

If all else fails you might like to consider getting Oldham optics to refigure the mirrors for you. They are on the web. Not sure if they offer that service but it would be worth asking.

The ghost image you mention could well be down to seeing the whole mirror in the 2ndry due to stay light. That's the problem with open scopes - it needs to be pitch black. The other problem could be down to the same thing, poor collimation or a bad mirror/mirrors. Even a combination.

John

[John]

Thanks for all your comments i will give it a go and see what happens and let you know ... my telescope is the one in the small picture its a 1960 Charles frank 8inch

I used to use one of those scopes in the early 1970's at our school astronomy club. At the time they were well thought of scopes.

I would have thought that poor collimation was the most likely culprit for the ghost image with the 25mm eyepiece as otheres have said. The 9mm not focussing at all is harder to understand - is it because you run out of either inward or outward travel in the focusser ?. Are these modern eyepieces or are they of the same vintage as the scope ?.

John