Another rambling post from me, but I wanted to share some thoughts with you about collimation.
I get the feeling that most people don't realise how critical collimation is - and (like myself) they tend to collimate their scope to the limit of the tools they have on hand and not go the final step with a high-power star test.
If you haven't already read this article, I highly reccommend that you read Thierry Legaults page on collimation:
Just to repeat myself, I think that when most people say their scope is "collimated" what they actually mean is that it looks as good as it can be with their collimation tools.
But there is a world of difference between "close to collimated" and "perfectly collimated". It's easy to get close, and a real test of patience and skill to get "perfect".
So why not stop at "close" to collimation? The answer is clear if you've read that article linked to above - you can still be losing *half* of your resolution if you stop at the limit of your tools. All it takes is for the diffraction rings in the airy disc to overlap a little and you're done for.
The real test of collimation is to look at a not-too-bright star, say mag 2, at ludicrous magnification of say 100x per inch of aperture. If the rings don't look circular then you're not finished collimating.
Practical point - last night I spent my usual ten minutes or so getting the collimation "spot on" according to my tools, and yet when I checked a star this morning just before I started imaging the collimation looked hopeless. I was wishing that I had an extra hour before sunrise to tinker with it and see if I could get it a bit closer to correct.
The tools I use for collimation (barlowed laser, autocollimator, cheshire) can get me very close to collimated, but it was very obvious from the high power view later on that I wasn't really that close after all.
Like so many other parts of this hobby, collimation is somethig that we should all keep working at to improve. It's no secret that one of the skills that sets Damian Peach apart from most others is his insistence of perfect collimation before he starts imaging.
Collimating a scope isn't hard in the way that, say, brain surgery is hard - all it takes is a lot of patience and persistence.
The real test of collimation is to look at a not-too-bright star, say mag 2, at ludicrous magnification of say 100x per inch of aperture. If the rings don't look circular then you're not finished collimating.
I've given up on proper high-mag star testing months ago, lest I kick the OTA senseless in frustration. Trouble is that 1) at those magnifications a star rarely (if ever) looks anything but a fuzzy boiling mess, unless the scope has less than about 100mm of aperture. 2) And for us dobbers, without tracking, it is near impossible to make the adjustments anyway. Anyone got any tips for getting around these problems?
(1) is either poor seeing or the scope is not cooled down, about the only way to fight that is to limit your imaging and collimation to the unmentionable hours just before sunrise, and leave the scope outside from about 8pm onwards to give it the best chance.
(2) is not solvable with a dob, but on the other hand you're not going to imaging anywhere near the
limit with a dob anyway so close collimation is as good as you need.
I guess I should have said that "perfect" collimation only makes sense on an accurately aligned tracking mount.
I got to star test the other night for the first time at around 200x i think, I saw the rings, but an interesting thing was that i put my left hand on the top of scope and fingers inside the tube to move the tube. The heat from my hand was causing viewable currents in the column of air!!!!!
Again an example, how critical is temp for great viewing!!!!!!!
I moved the screws around and thought i got it better, but i will be interested in the light of day where the centred spot now is pointing on the primary!!!!!
DP I'm going to try an "external" collimation system for the secondary on my new scope, the vanes will be fixed onto the secondary holder and where the vanes come out through the tube I'll have an arrangement to move them up and down to tilt the secondary.
As you say, putting your hand inside the tube at high power is not a good thing.
1) Adjust everything up tight and hope nothing moves when you tip the scope around
2) Keep it all a bit loose, and be prepared to recollimate 'cause everything is going to move.
I've chosen (2), mostly to avoid overtightening the optics. I predict where the scope will be pointing when I'm imaging, point it there and collimate it. Leave it sit there with the drives turned off until it's time to start imaging.
for zipping around the sky (but at the tuc47 constant for minimium time to really observe of course!!!), option 1
Also
what seeing conditions do you rate (out of 10) that the airy rings can been seen???? and at what mag do you work at???
That monday night from camp (we all know which one i am talking about). It is now an official national holiday. What seeing out of 10 would you rate that and also when you were star testing my scope, what mag and did you see rings????
Mmmmmmmmm , So what us dobber's need are worm drives, and flexible drive cables, on our primary adjusting screws so that we can go to a star test at any time, and tweak the adjustment while concurrently watching the ring patterns and their movements????? Beats the hell out of the current up and down, back and forth activity, and wondering which one, or way, you tweaked last time. Could be servo-controlled too?
I'll keep a record of this in case some-one tries to beat me to the patent's office!
For the sort of critical collimation required for planetary imaging I'd think most scopes need to be "tweaked" after every major movement, especially across zenith as the weight shift of the primary alone, unless it is bolted down real tight, will shift the collimation from perfect. I use the star test, but it's effectiveness is limited to nights of good seeing (and as Bird stresses good thermal equilibrium).
In my experience you need reasonable seeing (~5-6/10) to usefully star collimate at powers up to 300 or so. While not ideal I find this gets me closer than more mechanical collimation means (eg cheshire ep). To be able to accurately interpret star rings and collimate at powers of 600X or more - needed to get "perfect" collimation - I think really requires exceptional (8/9 out of 10) seeing conditions, so is not really a practical option for every observing session though I'll do it whenever the conditions allow.
Mmmmmmmmm , So what us dobber's need are worm drives, and flexible drive cables, on our primary adjusting screws so that we can go to a star test at any time, and tweak the adjustment while concurrently watching the ring patterns and their movements????? Beats the hell out of the current up and down, back and forth activity, and wondering which one, or way, you tweaked last time. Could be servo-controlled too?
I'll keep a record of this in case some-one tries to beat me to the patent's office!
Graeme
Not just you "dobbers" (your words, not mine ), I've wanted something like this for ages, something to let me turn the collimation adjusters on the primary while my eyeball is looking in the focusser.
Yes, ok, you SCT guys can stop smiling now...
I keep thinking about building something, maybe just 3 cables a-la slow motion controls on scopes without motors, but I haven't quite got there yet.
Now I dont want to lower the high tech tone of this almost ended thread but would like to resurrect it for a mo with a very basic question.
The shift of the central shaded spot on a defocussed star is very marked with my scope and I suspect that the 1st stage of collimation would be a worthwhile exercise for me to carry out. My FL is 1325mm so with my 25mm ep this equates to only 53X and this gives small but clear..er images than my 10mm ep (132X). The scope is a small (102mm dia) Mak (Celestron Nexstar 4 GT) but I dont see any collimation screws as discussed here. I desperately need to sort this out as I have a new ep on the way which will give me maximum mag for this scope (6mm) and I am concerned that the quality of image will nly get worse at the higher magnification.
I am wondering if collimation is posible for this type of scope. No mention made in the manual. Any clues guys???
Hi Muddy, I don't know for sure, but would expect that your secondary will have collimation screws. They may be hiding behind a secondary disk cover - there should be three screws?
Thanks for the heads up. Have looked into it a litle deeper and discovered that the mirror om ,y lens is not adjustable. The lens is secured into the black ring as shown at the end of the unit which, conveniently, unscrews and the whole lens assembley comes off. On the inside, i have a mirror coating and a plastic funnel for directing the light to the eyepiece which appears to be silicon glued to the reflective painted disc. I think a littl primitive but it is only a starter scope. Problem is the colllimation is out. Appreciate the help but will contact Celestron to see if anything can be done.
Muddy, I did quick google search and didnt find anything really useful except for a mention that to collimate your scope isnt an easy task and maybe best left for a dealer.
You might find help on the nexstar 4gt yahoo group.
I've just been reading a couple of articles on similar scopes to yours.(the 5") Apparently the retaining ring on the front is the key. If its up too tight you get pinched optics. If too loose, collimation problems..
Muddy, maks cannot be collimated by the user. It needs to go back to the dealer for repair.
Bird, I have been saying this for nearly a year. I think that I was considered obsessed with collimation. This is one good thing about owning an SCT. Collimation is slightly easier than other scope designs; although much more critical. Barlowed collimation is necessary for ultra tight collimation. Everytime I move my scope it gets a collimation check. Usually, it has moved just a little, enough for my eyes to see it, but not enough for the untrained observer. I also keep records with the toucam with images of out of focus stars. It helps to review your collimation techniques.
Incidently, I would like to stress that collimation should only be performed on a cooled scope. If you collimate before this you will get incorrect difraction rings. Also moving the scope from one side of the meridian to the other will affect collimation. Not a lot, but some movement will be detected. Collimation should be undertaken after a meridian flip also.
Hmmm....I think best left to someone who knows what they are doing in this case. I think York Optical are the people in Sydney who deal with Celestron or should I approach Bintel. I think if i took it back to Australian Geographic where the scope was purchase for me as a gift, they will just want to send it away but this may be best to avoid warranty issues.
Muddy, collimation is something you can learn. It seems daunting but once you do it a couple of times, you will never look back. If your unsure take your scope to a viewing night and I am sure you will get someone to help you collimate. Collimation is a must for all astronomers to learn. A telescope is a tool and you are the tradesman. A good tradesman looks after his tools.
Don't take your scope to someone else at a shop, because by the time you get home your collimation will be out again. You are capable! If you try to collimate yourself and muck it up, then take it to someone. At least you gave it a try.
, So what us dobber's need are worm drives, and flexible drive cables, on our primary adjusting screws so that we can go to a star test at any time, and tweak the adjustment while concurrently watching the ring patterns and their movements????? Beats the hell out of the current up and down, back and forth activity, and wondering which one, or way, you tweaked last time. Could be servo-controlled too? 
