Skywatcher Dobsonian - collimation

[RichardK]

Hi all,

I've recently bought a second-hand Skywatcher 200P - my first telescope. I've had some success with viewing the moon, but I think it could do with collimating. Having looked at some other posts on the subject, I've looked at Astro Baby's guide and will be using this when I make my first attempt.

I have one question that I'm not clear on - Step 1 in Astro Baby's guide suggests that the secondary mirror should be central. The secondary mirror in my telescope is off-centre (around 3mm off of the centre point). Should I leave as-is and have a go at collimating, or should I ensure that the secondary mirror is perfectly centred first? I don't want to cause myself problems that I'll struggle to fix by trying to do too much first time round.

Any thoughts/experience appreciated...

[johninderby]

Seconday mirrors are very slightly offset on purpose. The hub the mirror is mounted on should be dead centre though as the mirror will have been attached off centre to give the necessary offset.

[RichardK]

Sorry, I should have been clearer - the centre point of the spider vanes (and therefore, the hub) is 3mm off centre. Does this need adjusting before I attempt collimation? 

[johninderby]

The hub should be centered. I have seen people read about offset and adjust the hub off centre without realising the mirror is already attached off centre.

[Dr Strange]

A Cheshire collimating eyepiece will really help with collimation. And collimation needs to be done each time you move the scope. It is very easy to do once you get the hang of it.  

[RichardK]

On 10/05/2020 at 22:56, johninderby said:

The hub should be centered. I have seen people read about offset and adjust the hub off centre without realising the mirror is already attached off centre.

Thanks - will centre the hub as I give this a go.

 

51 minutes ago, Dr Strange said:

A Cheshire collimating eyepiece will really help with collimation. And collimation needs to be done each time you move the scope. It is very easy to do once you get the hang of it.  

The telescope came with a Cheshire collimating eyepiece; I just haven't got as far as trying it out yet. A job for later in the week!

[Pixies]

49 minutes ago, Dr Strange said:

A Cheshire collimating eyepiece will really help with collimation. And collimation needs to be done each time you move the scope. It is very easy to do once you get the hang of it.  

Sorry to hijack. But as a newbie, I'm unsure of how frequently collimation is required (and what type). Most of us will be moving our scopes each viewing session - and in my case I have to move it depending on my target, since the back garden is surrounded by various obstructions. Are you saying we should be thinking about re-collimating each time? And are you talking about just collimating the primary, as the secondary (as I understand it) needs collimation much less frequently?

Thanks

[johninderby]

Basicly if just moving from house to garden and back again shouldn’t need collimating very often. Haven’t needed to colimate mine for a good six months or more.  If transporting it in a car will need a collimation check every time before using it.

Edited May 11, 2020 by johninderby

[Dr Strange]

41 minutes ago, Pixies said:

Sorry to hijack. But as a newbie, I'm unsure of how frequently collimation is required (and what type). Most of us will be moving our scopes each viewing session - and in my case I have to move it depending on my target, since the back garden is surrounded by various obstructions. Are you saying we should be thinking about re-collimating each time? And are you talking about just collimating the primary, as the secondary (as I understand it) needs collimation much less frequently?

Thanks

In my experience every move it should have its collimation checked. I had two high end Dob's (Obsession UC 15" and Teeter Journey 12.5") as well as a Skywatcher 203mm and each one needed collimation each time I moved it from garage to driveway. The UC and Teeter because each time I had to assemble them since they were truss Dob's and compact so the Secondary moved but the SW was a solid tube. 

Generally speaking check it each time you move it or set it up. Take action as needed. Good collimation does make a difference in what you see. Some are more militant than others about it though. A observing friend was fanatical about his collimation. He would use a TuBlug then an auto collimator and would touch up collimation throughout the night. I am no where near that level. But I do notice a difference between slightly out of collimation and well collimated. It doesn't take more than 5 minutes to collimate a Dob to the "good" level so it isn't that much extra to do.

[John]

I check my collimation each time I set the scope up. It rarely needs more than a tweak but I do it anyway.

 

[Pixies]

Is that with a star test when out in the dark?

[John]

14 minutes ago, Pixies said:

Is that with a star test when out in the dark?

Cheshire eyepiece while there is still some light in the sky, usually followed by a star test on Polaris. Quite often no adjustment is needed - like tonight for example.

 

 

[Ricochet]

7 hours ago, Pixies said:

Sorry to hijack. But as a newbie, I'm unsure of how frequently collimation is required (and what type). Most of us will be moving our scopes each viewing session - and in my case I have to move it depending on my target, since the back garden is surrounded by various obstructions. Are you saying we should be thinking about re-collimating each time? And are you talking about just collimating the primary, as the secondary (as I understand it) needs collimation much less frequently?

Thanks

With my Bresser, I check collimation each time I get the scope out, but I haven't had to touch anything for quite a few sessions. If the collimation moves when you lift the scope from one point in the garden to another or from house/shed to garden then you might need to tighten the collimation bolts a touch more. For the secondary that would just be doing the bolts up more tightly, but for the primary I would tighten all three collimation (not locking) knobs a full turn or more so that the springs are more tightly compressed, and then recollimate. 

[Stardaze]

4 hours ago, Ricochet said:

With my Bresser, I check collimation each time I get the scope out, but I haven't had to touch anything for quite a few sessions. If the collimation moves when you lift the scope from one point in the garden to another or from house/shed to garden then you might need to tighten the collimation bolts a touch more. For the secondary that would just be doing the bolts up more tightly, but for the primary I would tighten all three collimation (not locking) knobs a full turn or more so that the springs are more tightly compressed, and then recollimate. 

How tight do you tighten the locking knobs on the primary? Do you back off a bit or as finger tight as possible? Mine seems to have moved a little again?

[John]

Another thing I've learned is not to use the locking screws unless you are transporting the scope.

With the newtonians that I've owned, once you have got collimation right with the collimation adjustment screws, applying the locking screws could move it out again. Now I leave my locking screws loose unless I'm taking the scope somewhere in the car.

 

[Ricochet]

6 hours ago, Stardaze said:

How tight do you tighten the locking knobs on the primary? Do you back off a bit or as finger tight as possible? Mine seems to have moved a little again?

I think, until they just touch. It's difficult to remember now as I just do it naturally. 

[johninderby]

I tighten the locking knobs lightly and then check collimation again. Further tightening of the appropriate locking knobs allows correcting any miscollimation caused by tightening the knobs. Basicly I’m using the locking screws as part of the collimation process. Has the bennifit that the scope stays in collimation when moved. 

Edited May 12, 2020 by johninderby