Collimation - That Old Chestnut...

[Guest]

2 hours ago, Alan64 said:

Whilst straying away, I did think that you might convey those aspects to your friend.  The best of luck to both of you.

To be honest, we seldom use that scope. We prefer the large Dobsonian... 

[Guest]

Here's 2 pictures looking down the eyepiece holder of the 8" Dobsonian, one without an eyepiece, the other looking through the Cheshire peep hole.  @Alan64

Edited October 10, 2020 by Guest

[Guest]

Here's a view from the eyepiece holder of the 4.5" Newtonian reflector, without and with the Cheshire eyepiece...  This telescope doesn't have a doughnut or anything similar in the centre of the primary mirror. 

[Guest]

Here's a look down the eyepiece holder, using the brown and white paper method suggested by Astro Baby. 

Any thoughts?  @Alan64

Edited October 10, 2020 by Guest

[ScouseSpaceCadet]

3 hours ago, merlin100 said:

Here's 2 pictures looking down the eyepiece holder of the 8" Dobsonian, one without an eyepiece, the other looking through the Cheshire peep hole.  @Alan64

I

Due to the angle it's not easy to tell for sure imho.

For accuracy take a photo through the the cheshire or collimation cap using the white paper behind tbe secondary method.

That's my 150mm newtonian's perfect primary with a very slightly **miscollimated secondary, taken through a collimation cap.

 

(**Zoom in - note the position of the central dot, it's just barely right of centre - a 30 second job to correct.)

Edited October 10, 2020 by ScouseSpaceCadet

[Guest]

16 minutes ago, ScouseSpaceCadet said:

Due to the angle it's not easy to tell for sure imho.

For accuracy take a photo through the the cheshire or collimation cap using the white paper behind tbe secondary method.

That's my 150mm newtonian's perfect primary with a very slightly **miscollimated secondary, taken through a collimation cap.

 

(**Zoom in - note the position of the central dot, it's just barely right of centre - a 30 second job to correct.)

My above later posts, I've done exactly that. 

[Pixies]

You need to do it with the Cheshire acting as a 'sight-tube'. Get it closer to the secondary - then you can see how close to round and centered it is. For example:

[Guest]

I'm out just now, but will try it again when I'm back in. 

[Ricochet]

3 hours ago, Pixies said:

You need to do it with the Cheshire acting as a 'sight-tube'. Get it closer to the secondary - then you can see how close to round and centered it is. For example:

This ^. 

Once you've (@merlin100) done that, adjust the secondary so that the doughnut is centred under the cheshire crosshairs (the big fuzzy ones in your photo). After that, adjust the primary so that the dot is centred in the doughnut.

 

[Guest]

36 minutes ago, Ricochet said:

This ^. 

Once you've (@merlin100) done that, adjust the secondary so that the doughnut is centred under the cheshire crosshairs (the big fuzzy ones in your photo). After that, adjust the primary so that the dot is centred in the doughnut.

 

Here's what the view looks like. Unfortunately, light was fading when I got back in. 

[Pixies]

OK. You need to bring the secondary back up the tube (away from the primary) and rotate it a wee bit, too I think.

As you are using paper, and there are no reflections, you can point it at a lamp and get a nice contrasty view.

[Guest]

34 minutes ago, Pixies said:

OK. You need to bring the secondary back up the tube (away from the primary) and rotate it a wee bit, too I think.

As you are using paper, and there are no reflections, you can point it at a lamp and get a nice contrasty view.

Here's what I've got inserted... 

[ScouseSpaceCadet]

1 hour ago, merlin100 said:

Here's what I've got inserted... 

Swap the white and brown... 

 

 

[Guest]

14 hours ago, Pixies said:

OK. You need to bring the secondary back up the tube (away from the primary) and rotate it a wee bit, too I think.

As you are using paper, and there are no reflections, you can point it at a lamp and get a nice contrasty view.

How does this look? 

 

 

Edited October 11, 2020 by Guest
Update...

[Pixies]

That's a lot better! Did you back off the 3 adjust screws before you tried to bring the secondary back up the tube (by turning the centre screw clockwise)?

[Guest]

Any better? I just remembered about them... 

Edited October 11, 2020 by Guest

[Pixies]

That looks great. Did you edit you previous post?

Now you can remove the paper and see what's involved for the next stage - adjusting the secondary tilt so that the cross-hairs align with the doughnut. 

[Guest]

1 minute ago, Pixies said:

That looks great. Did you edit you previous post?

Now you can remove the paper and see what's involved for the next stage - adjusting the secondary tilt so that the cross-hairs align with the doughnut. 

I did, when I remembered about the 3 Allen screws...  I was editing it at the time you replied! LOL  Do I need to retighten the Allen screws at all?

[Pixies]

Wait until you've finished with the secondary. And even then - just snug enough to make sure nothing moves. You can damage things if you overtighten - even end up scoring dimples into the secondary holder so it's hard to do any future rotation changes. If you can't rotate the secondary by hand, it's tight enough.

But you can worry about that after you have done the next stage. You probably haven't finished with those adjusters yet.

[Guest]

19 minutes ago, Pixies said:

Wait until you've finished with the secondary. And even then - just snug enough to make sure nothing moves. You can damage things if you overtighten - even end up scoring dimples into the secondary holder so it's hard to do any future rotation changes. If you can't rotate the secondary by hand, it's tight enough.

But you can worry about that after you have done the next stage. You probably haven't finished with those adjusters yet.

I could feel a small dimple in the back of the secondary holder. I loosened it off just to visually confirm it. I've slacked the Allen screws off further. 

Edited October 11, 2020 by Guest

[Pixies]

Been there, done that. As long as you can get the secondary in the positions you need, you're fine. I ended up in a situation where whenever I tightened the secondary, it slipped back into the same position because of these (a won't say 'dimples', more like craters). One can fit a steel washer between the secondary adjust surface and the screws - but hopefully you don't need that.

OK - let us know how things go with the next stage.

 

[Guest]

14 minutes ago, Pixies said:

Been there, done that. As long as you can get the secondary in the positions you need, you're fine. I ended up in a situation where whenever I tightened the secondary, it slipped back into the same position because of these (a won't say 'dimples', more like craters). One can fit a steel washer between the secondary adjust surface and the screws - but hopefully you don't need that.

OK - let us know how things go with the next stage.

 

Erm? 🤔😯

[Pixies]

No problem - that just shows you how far out you were to start with.

Ignore all the reflections - just get the doughnut under the cross-hairs. It might take a few shots to work out which adjusters you need to use, it's tricky when you can't just keeps your eye looking down the Cheshire while you do it - that's why so many replace the allen screws with thumbscrews.

Anyway - get stuck in, but make sure you don't loosen things off enough the secondary rotates. You'll find you will have to loosen one a 1/4 turn, say, and tighten another the same amount (or the other 2 a wee bit) in order to keep the overall tension the same. Don't go backing one off a large amount, or tightening. Baby steps and keep the overall tension as equal as possible.

[Guest]

18 minutes ago, Pixies said:

No problem - that just shows you how far out you were to start with.

Ignore all the reflections - just get the doughnut under the cross-hairs. It might take a few shots to work out which adjusters you need to use, it's tricky when you can't just keeps your eye looking down the Cheshire while you do it - that's why so many replace the allen screws with thumbscrews.

Anyway - get stuck in, but make sure you don't loosen things off enough the secondary rotates. You'll find you will have to loosen one a 1/4 turn, say, and tighten another the same amount (or the other 2 a wee bit) in order to keep the overall tension the same. Don't go backing one off a large amount, or tightening. Baby steps and keep the overall tension as equal as possible.

How's this? 🤔

[Pixies]

It's so hard to say with photos. The different focus between the doughtnut and the crosshairs make it hard to get an image to use here. Can try to get a pic focussed on the doughnut, not the crosshairs?  If you can see that the doughtnut is under the crosshairs, then great.

But looking at the top pic - just make sure the secondary hasn't rotated.