How critical is collimation?

[osbourne one-nil]

The other night, I collimated my scope at the start of the session, but I felt when I was looking at Jupiter, things weren't quite right. I checked the collimation again and it was perfect. Since then I've been playing about inside and it seems that when the scope is perfectly collimated in the upright position, it's loses collimation when it's slewed about. 

The three photos show this - vertical and then either side. 

Now, does this matter in an f6 scope? 

 

    

[Northern Soul man]

Hi Ozzy, seems that there is some flex in the optical system when the scope is moved around to different parts of the sky, I think its hard to get a balance between rigidity/portability - all the manufacturers say that they have the next best design to the previous one.  How secure is the main mirror - I know you have to cut a fine line when tightening the main mirror in its cell - I've always thought that the mirror should be just tight enough as not to pinch the mirror but tight enough as not to have any movement - as said again - a very fine line.

If it were me, I would just point the scope at say Jupiter - or in that general direction - then collimate - should only take a minute or so - but there is going to be movement over time  -so may just need to check on a regular basis during the observing sessions.

From what I have read, critical collimation, when perfect, gives very good views as if the atmosphere was really steady, but as you move away from critical collimation, you get the same effect as if the atmosphere was giving poor seeing - so maybe a slight blurring of fine detail - and I think Collimation has to be way off before the views become so poor.  But I think its very hard to evaluate critical collimation in poor seeing conditions and critical seeing conditions when your collimation is out.

May be worth you experimenting to find how secure your main mirror is whilst moving  the scope in Dec and Alt/Az - also check your top cradle - I think the ultra light dob users will pitch in - as said a very fine line between having the optical system securely lined up - but not so tight as it gives the "pinched"optics tell tale signs in a star test.

Paul.

[Ricochet]

I'd probably start with checking the tightness of the truss bolts as that would be an obvious place for a bit of movement to creep in as the weight shifts.

[Moonshane]

I'd try to find where the movement is and correct it.

[osbourne one-nil]

Thanks Paul - the mirror cell in this is actually very very ingenious. It is secured to a base board with silicon and that base board attaches to the collimation bolts; I don't think it's going to be that.

Excuse the dust! 

 

I suspect it's originating from the base of the trusses which sit in ball sockets. 

 

I like the sound of just tweaking when it's done - I've only ever thought I've seen something on Jupiter. 

[Moonshane]

do the truss ends clip in or is there capacity to tighten the pink ring ?

[osbourne one-nil]

They slot in there and then the rings pull up to just hold everything tight. They don't themselves tighten. I'm wondering about some PTFE tape to make it a bit tighter; PTFE tape makes everything better. 

[Sporadic Dobstronomer]

To answer your question "does it matter" the answer is yes.

I picked up somewhere that the diffraction limited field of view of a Newtonian is:

0.022mm x f/number^3

-which is about 5mm at f/6.  You are showing more movement than this.

By the way, are you sure your centre mark and your laser are accurate?  You might find that final collimation by star test is a better bet.

[osbourne one-nil]

I'm fairly confident everything's accurate but I completely agree that star tests are more accurate. I'm probably not a lot over 5mm to be fair, but that's a very interesting formula! Always good to be able to have definitive boundaries to aim for.

[Astrobits]

Try gently lifting the secondary cage when pointing at 45 deg with the laser on. If the spot moves before the telescope then it is flex in the truss system. I must admit that I don't like what looks like ball ends to the truss poles. It is extremely difficult to truly lock ball sockets rigid.

[Tim]

How critical? AAarrrgh, think you are going to set my OCD off

Personally I like to collimate as precisely as possible, especially when imaging, but also when viewing. Seems a shame to spend all that on equipment and then not use it at it's best configuration.

It's on things like planets that the extra attention to detail will start to show the most.

Hope you get it sorted.

 

[bomberbaz]

Ok I read what you said about the primary fitting and also can see the tube fittings, for anyone else reading this with a similar problem though, I had a similar problem sorted caused by the primary not being clamped down properly. The screws holding it in place were only part screwed down and so the mirror wobbled from side to side when it was slewed around. 

Crazy thing is it took me 6 months to figure this out as I had not realised due to me collimating the scope at zenith and then not re-checking at a different angle.

Fixed by removing the cheapo screws used and replacing with quality, high tensile steel hex screw/bolts.

[osbourne one-nil]

Now that makes a lot of sense because the secondary cage has so little weight in it I'd be surprised if it caused the flexing. I'll take the primary assembly out and take a look later. It feels secure, but no harm checking. Thanks. 

[osbourne one-nil]

Well I've taken the mirror cell out and checked everything; all is tight. The problem is still there though. Dave Juckem (Portaball's owner) is very good though and says he's improved the sockets the truss poles sit in and is sending a new set. As long as they arrive sometime in the next 6 months I'm unlikely to miss any clear nights. 

[patomlin76]

I'd just like to add that I received a second hand scope last week, and immediately went to use it on the first clear night. I enjoyed it and thought I'd seen some pretty sharp and contrasty views, especially of some star clusters and the moon. Then a couple of days later I decided to see how collimated it actually was. The initial red dot from the laser hit the primary mirror something like 10cms off centre, and the secondary mirror wasn't centred in the focuser.

On the next clear night I'm going to see what it's like properly collimated, but the point above is that if you're non the wiser, the views still look very good!

[Moonshane]

it really depends what's off. The secondary can be quite a bit off with no obvious effects, the primary can effect the views massively if slightly mis-aligned with the optical axis, especially in fast newts. Better to get both right but the primary matters most.

[osbourne one-nil]

I agree with no fretting on stuff like this too much - perhaps with this scope/mirror more than most though it deserves to be set up perfectly!

[Paul M]

I'll buck the trend and maybe even start a war! 

I think that, after a point, necessary collimation accuracy is proportional to observing skill. A seasoned and skilled observer might very well notice the image degredation that might result from collimation error demonstrated by the op's images.

I suspect many of us would not. I doubt I would and I'd be not one bit worried. I'd be worried if something was loose as appears to be the case here. Don't want things falling about!

I only ever collimated ye olde fullerscope by rack-of-the-eye. No tools or gismos. Just sighting down the focuser draw tube. My newer 10inch Skywatcher gets the same treatment. I've then checked it with a cheap laser collimator and its been within the ballpark and plenty good enough for me!

Am I alone in thinking people get a bit hung up over collimating?

P.s. I've read a bit about it over the years so  not approaching the subject as a greenhorn

[patomlin76]

Yes I certainly get hung up over trying to get a perfect collimation, which isn't ever possible.

Yes the op has issues with the flex that is causing the movement, but after getting collimation as close as possible last night, focuser slop meant that actually it's between spot-on and near-enough depending on how extended the focuser is. I can't control that and I'm certainly not replacing the focuser, so I'm happy that it's as close as I can get. I don't think in any other of my scopes I've actually noticed any effects of focuser-slop in any observing.

Given the physics of the reflections from the rather large parabolic mirror at the base, I'd say collimation would have to be massively out to affect most observing, for the casual astronomer that is...

[osbourne one-nil]

Well, seeing as I spend very little time observing planets, and if I were to I'd probably use my Tak anyway, I'd tend to go along with those views - they're quite convenient right now as well!

[ollypenrice]

If I have this right the spot from the return beam is deviating from the secondary centre spot by the distance we see in the pictures just because you are slewing the scope?

That's crazy and surely indicates a structural problem in the truss or the cell - or, of course, both.

I have a farily rustic truss pole 20 inch F4.1 and it doesn't show any such effect. I really don't like the look of those ball joints. I mean, what are ball joints designed to do??

Olly

[osbourne one-nil]

I think it's the push-fit convenience perhaps? It wasn't like this before it went off to be refurbished, but I thought it looked a good idea personally; by the time everything is secure together it feels absolutely rock solid. 

[osbourne one-nil]

Anyway, I think we owe the ball joints an apology....think I've sussed it. If you look at the attached photo, you'll see the bezel around the sphere is secure using screws (which go into holes within the fibre glass of the sphere).

These were very loose - don't know how it escaped my attention before). Since tightening them up, it doesn't budge at all. I still think it could be improved by perhaps using nuts and bolts rather than what appear to be self-tapping screws as I think it wouldn't be fairly easy to overtighten, but I could sort that easily enough. 

You'll see I've wrapped the balls at the end of the truss poles in PTFE tape just to make the fit a bit more snug, but as I say, I think that side of things is pretty spot-on. 

I'll see how I get on now.