Collimation Question

cjmarsh81 · #1 21-01-2006, 04:53 PM

This may sound a bit odd, but what happens to the image when your telescope is out of collimation. Everywhere I look people say make sure the scope is collimated and they are repeatedly doing it to their own. No-one that I have seen has explained the effects of a poorly collimated scope.

I have not yet collimated my 10" Dob. It is as I got it from the shop. I can see crystal clear images of Saturn and I can see amazing images of the Moon.

Does this mean my scope is already correctly collimated? I am a little wary of attempting a collimation until I see someone else do it. I think if I start adjusting mine I may make it worse for no reason.

Can someone please give me a basic explanation of what happens to the image if the telescope is out of collimation. Is it blurry and unfocusable? Is the image off centre? Can you see no image at all? Is it only important at higher magnifications?

Sorry if this sounds like an obvious question, but I am a little confused.

davidpretorius · #2 21-01-2006, 05:03 PM

http://legault.club.fr/collim.html

the images at the top are an inidcation. you notice it most whilst imaging at large magnification.

with my 5x powermate and my toucam acting as a 6mm eyepiece, i get 1044x, which you would never bother trying to look through. you are hoping to have your collimation spot on so that at this magnification, you can extract the most detail out of your movie you capture.

also, i find focussing better when the collimcation is good. you start with a defocussed image which looks like a CD rom and then you start to slowly focus it. you want to retain that circular shape all the way down to the tight focussed point for a star. if it starts get out of whack and your star grows hair, then the collimation is out

cjmarsh81 · #3 21-01-2006, 05:06 PM

I think my collimation is pretty good at the moment. I read somewhere if you focus on a star, then move it out of focus until it gets much larger, if the image retains the shape of a circle then it is fine. I tried this last night and it appeared to stay round.

davidpretorius · #4 21-01-2006, 05:09 PM

start at when the circle is really big, then the shadow of the secondary should be right in the middle.

it is when you are nearly focussed then you want it to be circular etc. ie from very big circles all the way down to pinpoint

cjmarsh81 · #5 21-01-2006, 05:13 PM

I have just looked at the link you provided and especially at the two pictures of Saturn at the top.

I am going to take another look at Saturn tonight. I think what I saw last night was nearly as good as the left correctly collimated picture, but I want to check it again tonight.

cjmarsh81 · #6 21-01-2006, 05:24 PM

Asimov. That is exactly the way I see it. If it works don't play with it. I was just trying to establish if I am missing on some quality or detail as I haven't looked through any other telescopes. I think I will not touch the collimation at this point. I will put it off until I know a bit more about it and need to (ie when I get higher magnification eyepieces).

davidpretorius · #7 21-01-2006, 05:39 PM

at normal high magnification ie around 200x, you would not notice too much, at 1000x when imaging, it is noticable, the imperfections in the system ie seeing, jetstream, collimation, mirror temperature all start to show themselves dramatically!

davidpretorius · #8 21-01-2006, 05:43 PM

Quote:

Originally Posted by cjmarsh81

Asimov. That is exactly the way I see it. If it works don't play with it. .

yes i agree with asiproverbs for astronomy 10:2

as long as it 90% right, you will be fine!

davidpretorius · #9 21-01-2006, 05:47 PM

BUT.....if you are going to do any testing ie an expensive eyepiece you may want to buy, a toucam for imaging, then i would seriously make sure you can control factors that are within your control ie temperature and collimation.

i looked thru a very expensive televue in november and it was worse than my $150 ultra wide. I was out of collimation and then temperature had dropped 2 degrees and i had not waited 1/2 hr at least so that my mirror could catch up. You want to isolate the optics imperfections so you don't come to the wrong conclusions!

Robert_T · #10 21-01-2006, 06:43 PM

Quote:

Originally Posted by cjmarsh81

Asimov. That is exactly the way I see it. If it works don't play with it. I was just trying to establish if I am missing on some quality or detail as I haven't looked through any other telescopes. I think I will not touch the collimation at this point. I will put it off until I know a bit more about it and need to (ie when I get higher magnification eyepieces).

wise move and wise words... you will need to learn this eventually, but it takes a lot of fear out of it if you can watch someone else do it the first time.

great to hear you're getting some nice planetary and lunar views.

cheers,

BerzerkerNerd · #11 21-01-2006, 07:56 PM

Very basically if the mirror is out of perfect diagonal alignment with the tube, and the secondary mirror is not adequately compensated to reflect mirror light to the focuser and EP, very slight two dimensional image distortion of the viewing field will occur amoungst other things- think geometrically in three dimensions for the light path. In cases of quite bad alignment, it could mean that the the tube itself is not pointing exactly at the object of interest, or it is slightly off axis to the target being viewed. This will not aid your focusing of the object across its arc.
I am no optical technician so perhaps the more experienced members can shed greater light here. Collimating a Newt. is nothing to be scared of, and there is heaps of info on the subject just do a google search. Past threads on this site have discussed the subject also. I personally have found the old film canister with a 1/16" hole drilled into its lid and its base cut off to be more than enough to keep things running true in a Newt. Hope this helps a bit.
( we are all Isaacs children)
Steve M

rumples riot · #12 22-01-2006, 05:30 PM

Something to consider is that sooner or later your scope will go out of collimation. If your scope is out of collimation you will notice that your defocused star ie the small donut will not have concentric rings. The rings will touch another somewhere or look like they are nearly touching each other. At low magnification you will not notice too much if your collimation is out. It is when you get up near 400x that you will miss small amounts of detail. Good collimation is a matter of practice and you should not be afraid to undertake this maintenance task. At the very worst you might muff it up. In which case you can either get some help from a local astro club or go to an astronomy shop and get them to help. However, the sooner you get into the habit of doing this task the better. At present you may not have to do anything, knowing when and how to collimate is most important to get the most out of your scope.

cjmarsh81 · #13 22-01-2006, 05:45 PM

The EP's I have at the moment are not powerful enough to give me 400x. When I get some higher power eyepieces I will have to learn how to do it correctly.

I have already got a film canister and put a small hole into it. I just tried playing with the secondary mirror and completely mucked it up! Took me over an hour to get it back to where I think it was. Find out if it still works correctly tonight by checking out Saturn.

rumples riot · #14 22-01-2006, 05:52 PM

That's the way, you will get a lot more satisfaction from doing this yourself and be able to maintain your own scope. Well done on your first try.

cjmarsh81 · #15 22-01-2006, 06:13 PM

Don't congratulate me yet, I don't know if it works yet. I just took it outside and was able to focus on a nearby metal power pole. It was a bit blurry but I think that was simply heat haze.

RAJAH235 · #16 22-01-2006, 08:27 PM

cj, Take your time with it. You are off to a great start, doing the collimation in the daytime when you can 'see' what is happening.
fwiw, lay the tube flat on a lounge or bed, pointing towards an open door or window. Align the focuser/2ndary first, as per the links, then continue on till you reckon it's O.K. Take outside & tweak via a star test.
Don't worry about trying to get over 200 X mag. You'll only get to use that mag & above, when the conditions are good to excellent.
Most of our viewing is done in the range 35 to 150 X ??? (weather permitting).
HTH.

L.

cjmarsh81 · #17 22-01-2006, 11:21 PM

Ok my scope still appears to work after my tinkering today. No noticeable improvement, but at least it is not any worse. I'm pretty sure the primary mirror is a little out, might have a go at fixing it tomorrow.

Question.

My primary mirror has 6 clips holding it on. Everywhere I look, including the manual for my scope say there are only 3 clips. Probably doesn't matter, just wondering if this affects the way I line up the secondary mirror.

When I have lined it up so all mirror clips are visible, there appears to be a small black bar on the top and bottom of the mirror when looking through my 35mm canister. Is this caused by the primary mirror being incorrectly aligned, or is the secondary mirror on the wrong angle or is it normal.

cjmarsh81 · #18 23-01-2006, 10:49 AM

When I was playing with the secondary mirror yesterday I accidentally touched it and left a finger print.

What is the best way to remove it without damaging the mirror?

BerzerkerNerd · #19 25-01-2006, 08:05 PM

Ouch

, how thick is the fingerprint ? if it is only light and not central to the mirror you could ignore it for a while..

Pulling the whole spider vane assembly out by unscrewing the thumb or other screws on the OTA, then carefully washing the mirror with a solution without removing the mirror from the assembly will work, but then diag. mirror collimation will be needed on it. Then it gets a bit more complicated in relation to primary alignment. Are any of the forum members, or an austronomy club near you and could help ??

cjmarsh81 · #20 25-01-2006, 08:32 PM

It is only light and near the edge as my finger wrapped around when I was adjusting the mirror. I am happy to leave it there as it does not affect the view at all (you can only slightly see it on the edge through the eyepiece). The only thing, is I have read somewhere that fingerprints can eat into the reflective coating.