Sky Watcher 130PDS Collimation

[Jamgood]

Hi Guys & Gals.

I seriously need some help. I got my second 130PDS a couple of weeks ago (first one came damaged, not sure of this one!) and the weather being what it is in the UK, haven't had much of a play with it.

Having read loads of different info in on collimation, I'm now at the point where I'm more confused than when I started.  In my amateur knowledge, I mistakenly used a laser collimator until I realised that that was drawing circles when I tried to collimate it. That is impossible to to get a centre beam and has quickly become an expensive toy for the cats.

So today I made a simple collimation cap, (edited) (I've ordered a proper one today) just to have a look at what is going on in there and this is what I see. (see attached photo) Does this look anything near right? Every photo I see shows the secondary in the centre of the focuser and the primary filling the secondary. Mine how looks nothing like what I can see anywhere, hence my confusion.

Last night I did a start focus test (See second photo) and the result was just plain strange and wonky. Nothing like I have seen or expected.

Can someone please give me a bit of advice on where to go from here. 

Thanks in advance

J

Edit. Forgot to add. As things currently stand, I can get a pretty decent star in the centre of the image but all the other look like arrows pointing to the centre. 

Edited July 21, 2020 by Jamgood

[Pixies]

HI. What is the 'cheshire' you made? Is it a collimation cap? If you are sure that the hole is completely central, that's good. We need to be certain that picture is dead centre of the focuser tube.

If yes to the above, it looks like the secondary is out. Can you re-do the Mire de Collimation circles, so that they are concentric with the focuser tube (the large black outer circle).

[Jamgood]

36 minutes ago, Pixies said:

HI. What is the 'cheshire' you made? Is it a collimation cap? If you are sure that the hole is completely central, that's good. We need to be certain that picture is dead centre of the focuser tube.

If yes to the above, it looks like the secondary is out. Can you re-do the Mire de Collimation circles, so that they are concentric with the focuser tube (the large black outer circle).

Hi Pixies. Thank you for your reply. I did indeed mean collimation cap, not cheshire. It's just a simple piece of cardboard to sit on the eyepiece hole. Nothing fancy. I believe it is as central as I could get it. I measured twice and cut once. I didn't want to go drilling a hole in the eyepiece cover just in case I'm sending this back.

Here's the edited image you asked for and the original.

[Pixies]

OK - that secondary needs to be moved, I'm sure. Do others agree?

I'll happily talk you through it. I'm just feeding the kids, so back shortly.

have a read of these two in the meantime:

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

https://garyseronik.com/a-beginners-guide-to-collimation/

 

The next thing will be the coloured sheets of thick paper. Do you have any?

[Moonshane]

It looks to me like the secondary wants to move a smidge towards the primary (i.e. lengthen the central screw a couple of turns - with the usually advised precautions) and it then needs to be rotated slightly. You could just do the latter as you are not that far off. Then tweak the primary.

[Jamgood]

I have tweaked it a bit since the last photo and currently have it like this.

[Pixies]

Jamgood - we'll do: "Collimation Step 2" from Astrobaby's guide, first.

If you can get the coloured paper as she describes, it does make things look clearer. be careful not to touch the secondary mirror.

Keep the tube horizontal, so you don't drop anything down and damage the primary.

If you loosen the central adjuster screw on the secondary, the secondary will move down the tube towards the primary. You will have to take up the slack by tightening the 3 adjuster screws BY EQUAL AMOUNTS, otherwise the secondary will get quite sloppy. Don't over-tighten - just gently finger tight. You are looking to get the image of the secondary to be concentric to the focuser. You can also rotate the secondary so that it shows it's full open face to your eye - it should appear as a circle (concentric) too. If it looks like an ellipse, it needs rotating. 

As long as you do it little by little (DONT OVERTIGHTEN - just finger tight) you should end up with the image of the secondary central and concentric to the focuser . Mire de Collimation will assist in this. 

[Pixies]

3 minutes ago, Jamgood said:

I have tweaked it a bit since the last photo and currently have it like this.

Looking better. If you can do the paper thing, you will be judging the secondary mirror, not the image of the primary, though.

[Moonshane]

Assuming your camera is central, it looks like your secondary adjusters are different lengths. Is the secondary holder on a slant when you look from the top of the tube? 

Bear in mind the secondary has a minor effect on image quality.

[Jamgood]

I have blue and yellow card in the tube. The screw on the secondary is really tight? Feels like it will bend everything.

[Pixies]

You're turning the central screw anti-clockwise, yes? Don't force it if it's stiff.

[Jamgood]

Yeah. Lefty Loosey. It doesn't feel like it wants to budge.

[Pixies]

Can you try backing off the 3 smaller adjustment screws very slightly, just in case everything has been tightened up too much

[Jamgood]

Yeah that worked. Gave it a small turn and it's definitely moved. Repeat?

[Pixies]

Yep. Slowly does it. Use the Mire de thingy to confirm when it's all concentric. Also, as moonshane said, check that the secondary holder isn't squint. Are the 3 small secondary adjusters all at the same depth, roughly?

[Jamgood]

The 3 adjusters are all roughly the same depth. (Using a finger/thumb pinch allen key depth test.

Getting closer.

[Jamgood]

The secondary mirror is starting to turn as I turn the screw now. I'm trying to correct it.

[Pixies]

Toy don't have to be dead accurate, just as close as reasonably possible. 

[Jamgood]

Nearly there......

[Jamgood]

Extremely close now. I'll stop until you tell me what to do next. 

[bomberbaz]

Leave it there, that is absolutely fine

[bomberbaz]

Next a picture through a cheshire or similar down the drawtube. 

[Pixies]

Yep - well done! You are better at this than me - and I like the teal/yellow ochre colour scheme too!

That's the first stage done. You won't have to do that again unless your are doing some maintenance.

Check the three secondary adjusters aren't slack. Also, remove the paper (don't touch the mirror). Now we can move on to the 2nd stage and adjust the tilt of the secondary and align it to the primary mirror. Your secondary is nice and large, so we don't have to be too exact. Just try your best to get the image of the primary concentric with the edge of the secondary and the focuser.

This is "Collimation Step 3" in the Astro baby guide. You adjust the 3 secondary adjusters by small amounts until the primary image is concentric (ignore the reflections in it for now).

You only need to make small adjustments, and be careful not to start tightening things up. For example: back off on adjuster 1/8th of a turn and tighten another by an 1/8th. Or - back off one by 1/8th of a turn and tighten the other 2 by 1/16th of a turn each. You are trying to tilt the base of the secondary mirror holder around the central screw, without tightening or loosening the whole shebang. You'll eventually see how the adjustments move the image of the primary. 

Let us know when you think you have the secondary aligned.
 

[Jamgood]

Just now, bomberbaz said:

Next a picture through a cheshire or similar down the drawtube. 

I don't have a cheshire yet. It's in the post.

[Jamgood]

1 minute ago, Pixies said:

Yep - well done! You are better at this than me - and I like the teal/yellow ochre colour scheme too!

That's the first stage done. You won't have to do that again unless your are doing some maintenance.

Check the three secondary adjusters aren't slack. Also, remove the paper (don't touch the mirror). Now we can move on to the 2nd stage and adjust the tilt of the secondary and align it to the primary mirror. Your secondary is nice and large, so we don't have to be too exact. Just try your best to get the image of the primary concentric with the edge of the secondary and the focuser.

This is "Collimation Step 3" in the Astro baby guide. You adjust the 3 secondary adjusters by small amounts until the primary image is concentric (ignore the reflections in it for now).

You only need to make small adjustments, and be careful not to start tightening things up. For example: back off on adjuster 1/8th of a turn and tighten another by an 1/8th. Or - back off one by 1/8th of a turn and tighten the other 2 by 1/16th of a turn each. You are trying to tilt the base of the secondary mirror holder around the central screw, without tightening or loosening the whole shebang. You'll eventually see how the adjustments move the image of the primary. 

Let us know when you think you have the secondary aligned.
 

Gotcha. This bit I've read about a lot. More confidence here.

I'll make you a thank you card in that colour scheme. 👍