Sky Watcher 130PDS Collimation

[Jamgood]

Ok, I think this is right. Hopefully.

[Pixies]

Cool. Now the secondary is aligned so that the line of sight from the hole in the colimation cap hits the centre of the secondary and reflects down to the centre of the primary (the doughnut is dead centre  on the concentric circles). You won't have to do this again unless your scope takes a whack, or suffers some serious bouncing in the back of a car. Check it now and again though, with a Cheshire.

The next step is to align the primary so that the reflection down that  line of sight comes straight back up to the hole in the cap. So adjust the primary to move the black dot into the centre of the doughnut. This is "Collimation Step 4" in Astrobaby's guide. 

Slacken off the locknuts at the primary mirror end and adjust the main adjuster screws the same as before to centre the dot in the doughnut. You can't be too fussy here - this is what makes the real difference. When you're done, tighten up the lock screws. These will have an effect on the primary alignment too! Use them like a fine-tuning setting and keep the dot centred. You will have to learn how tight to lock them down. Some people hardly bother with them and adjust the primary very regularly. I just do them gently finger tight.

As I said - this last stage is the one that makes the difference and you'll need to check this each time you get the scope out, or at least when you suspect the collimation has slipped slightly. You can do the star test to make sure it's all good.

[Jamgood]

Brilliant. Thank you. I shall give that a go and report back.

Thank you for all your help. Very clear and straight forward. 😀

[Pixies]

I do have a suspicion that the secondary might need some more rotating to get it perfect. Looking at the shadow of the secondary, the offset isn't quite in line with the focuser.

[Pixies]

2 minutes ago, Jamgood said:

Brilliant. Thank you. I shall give that a go and report back.

Thank you for all your help. Very clear and straight forward. 😀

[Moonshane]

Well done!

[Jamgood]

13 hours ago, Pixies said:

I do have a suspicion that the secondary might need some more rotating to get it perfect. Looking at the shadow of the secondary, the offset isn't quite in line with the focuser.

You are right. It's not spot on yet but with some gentle tinkering, hopefully, I should be able to get it bang on. I have a Cheshire arriving from FLO tomorrow so that will help. Should have bough a collimation cap as well really. 

Here's a star focus test from last night. I was decent enough for viewing. I'll have another go at it later and report back.

Thanks again for your help yesterday. Made everything nice and clear. 😀

[Moonshane]

Just to be clear on star tests, what you have above is a defocused image of the view. A star text is a slightly defocused star at high magnification. e.g. https://astunit.com/astunit_tutorial.php?topic=startest

[Jamgood]

11 minutes ago, Moonshane said:

Just to be clear on star tests, what you have above is a defocused image of the view. A star text is a slightly defocused star at high magnification. e.g. https://astunit.com/astunit_tutorial.php?topic=startest

Thank you. I'll have a good read of that.

[Moonshane]

It's a good way to accurately determine various things but you naturally have to ensure the star is very central  and stays there 👍

Some people use a ball bearing glued on a piece of black card in which the sun's reflection is a pinpoint. 

 

[Jamgood]

I think this is probably about as close as I can get it without tightening the centre screw a bit and bring the primary forward a touch. The problem there being, everytime I touch that screw, the mirrow rotates and I have to start from scratch getting a good circle.

Is this looking OK?

[Pixies]

Hi.

It looks great. Have you got the shot with the paper removed?

Don't try and tighten the centre screw - you make things secure by tightening up the 3 adjuster screws GENTLY. Be careful and don't go too far. I learnt the hard way - they can actually dig holes into the secondary holder and make rotation impossible.

It does look like there is a 'flat' edge on the mirror (top right in the above picture) - do you see what I mean? Now, your secondary is pretty large, so you aren't worrying about losing part of the image of the primary. I assume the focuser isn't fully in as there was no shadow of it in last night's images.

As per the article by Gary Seronik - as long as you have an image of the whole of the primary and it is correctly centred, that's all you need to worry about for visual astronomy. Any further improvement trying to get the secondary located EXACTLY won't provide any visual improvement. But as you are busy getting the secondary adjusted, you might as well get it as close as possible.

Edited July 21, 2020 by Pixies

[Jamgood]

12 minutes ago, Pixies said:

Hi.

It looks great. Have you got the shot with the paper removed?

Don't try and tighten the centre screw - you make things secure by tightening up the 3 adjuster screws GENTLY. Be careful and don't go too far. I learnt the hard way - they can actually dig holes into the secondary holder and make rotation impossible.

It does look like there is a 'flat' edge on the mirror (top right in the above picture) - do you see what I mean? Now, your secondary is pretty large, so you aren't worrying about losing part of the image of the primary. I assume the focuser isn't fully in as there was no shadow of it in last night's images.

As per the article by Gary Seronik - as long as you have an image of the whole of the primary and it is correctly centred, that's all you need to worry about for visual astronomy. Any further improvement trying to get the secondary located EXACTLY won't provide any visual improvement. But as you are busy getting the secondary adjusted, you might as well get it as close as possible.

Here's a shot with the paper removed. I've left the yellow in as it makes it easier for me to see. 

I haven't tightened the screw. I did a little turn earlier to bring the mirror up a bit but I loosened the 3 adjusters first. Nothing is tight, just finger tight. A little pinch.

I can see the flat edge. Is that a defect in the mirror or due to the rotation? It's hard to see in there. How it was set before, you could see that flat edge in a zoomed out star. I don't know if it will alter the image in any way.

I'm planning on using this scope mainly for photography, so I'm guessing I need to get it as bang on as possible. I'm close in the image below. Just needs some fine tuning, to my eye anyway.

[Pixies]

Hi. 

It looks like the mirror needs to be rotated a little more. Can you see how the image of the secondary in the primary is a little off line? The secondary is offset when viewed from the primary and the offset means that it overhangs away from the focuser (which is on the left of your picture assuming it is in line with the spider vanes). So the offset of your secondary should be on the right, in line with the spider vane.

See:

ie - rotate the top of the secondary towards you (in your picture)

Edited July 21, 2020 by Pixies

[Jamgood]

This is where I'm very confused about what I'm looking at. The image above, I assume is looking through the focuser? So I need to move the image of my primary to the left? and round off that flat edge?

[Pixies]

No. You're almost there. I just think you could rotate the secondary a wee bit. Looking from the adjusters, that would be slightly clockwise. Looking from the focuser, the top of the secondary would move towards you.

That would mean, that in your pic, the image of the secondary would rotate slightly anticlockwise:

 

Does that make sense?

[Jamgood]

Ok thanks, that makes sense. That other image is confusing to me. (Too many circles) I'll give that a go in a short while and post results. 

[Jamgood]

I think I've progressively made it worse tonight. Right now my brain, my eyes and my back aches. Time to quit for today. If I carry on tonight the thing will probably end up in the bin!

I actually believe me and 130PDS are never destined to get along. First one I ordered never made it as it was damaged in store. Second came to and was damaged and this one. ARRRRGGGGHHHH!!!! I should've just bought a lens for my camera.

 

[Pixies]

That looks great? What's the problem?

[Jamgood]

I thought the all had to be concentric circles? Equally distance etc.

[Pixies]

And they are. The only thing that isn't is the image of the secondary in the primary, and that's because of the offset. That's normal. 

The doughnut is in the centre of the cross hairs, isn't it?

[Jamgood]

Right I get it now. The doughnut needs to be in the centre, not the bigger black circle, which is the shadow of the secondary.

I seriously just had a "Duh" moment! I've been trying to align all the circles in the centre. I did say at the beginning I needed help.

[Pixies]

I know - it can be a bit brain-bending. This offset is only there for fast scopes. Ones with longer focal lengths don't have them and their secondary silhouettes are central.

So - stage 1. You've aligned the secondary under the focuser (length and rotation), so that the outer circumference of the secondary is concentric with the focuser tube. You won't have to do this again unless you've had to loosen the secondary for some reason.

Stage 2. You then adjust the tilt of the secondary so that the outside edge of the image of the primary is concentric with the above - which should mean that the doughnut is centred too. This only needs checked occasionally, esp after the scope gets bumped. Ignore the silhouette of the secondary in the primary mirror.

Stage 3. You adjust the primary so that the eye-hole 'dot' is centred in the doughnut. You'll do this a lot, and get quick at it.

 

All the above with the Mire de Collimation can be done with a Cheshire. The final stage too. @johninderbywill be along later to sing the virtues of the 'Concenter' collimation tool.

 

Now - beer time!

 

[Jamgood]

15 minutes ago, Pixies said:

This offset is only there for fast scopes. Ones with longer focal lengths don't have them and their secondary silhouettes are central.

That was the bit that confused me the most. I had read that fast scopes have an offset but didn't really know what that mean't (lots of pictures show lots of things) and as mine from the factory was so far out, I thought that was the offset and along I went merrily confused.

It'll be interesting to see what it looks like through the Cheshire when that arrives. If nothing else, I feel more comfortable adjusting things now and have more of an understanding of how it all works. You explained it better than any guide I read or video I watched. They basically say "get a laser and you're done". I did and I was! Haha.

Thank you for your time and patience Pixies. I'll be eternally grateful and hopefully, someone else along the way will find this thread and learn something, as I did. Can't test the scope tonight as it's cloudy but I'll update when I can and when the Cheshire arrives.

Now......Guinness. Cheers!

[Pixies]

No problem! It was good to get my understanding of it down on 'paper'. I made some mistakes when I got my new scope and nearly mucked things up well-and-truly!

 

Cheers!