Is my 150P secondary collimation good enough?

[jonathan]

Trying my best to follow AstroBaby's collimation advice for my Skywatcher 150P reflector...  Would love to hear some advice on whether I should just stop adjusting, or which screws I should concentrate on next as I'm feeling a bit overwhealmed / exhausted by this secondary collimation malarky.  To me it looks like it could do with another turn or two, but I'm not sure which way to do it.

Thanks!

 

Edited January 21, 2021 by jonathan

[malc-c]

A better picture might help, but looking at what you've provided I would agree that it's still in need of a little tweaking to get the secondary more central. 

[Ricochet]

It's fine as far as being central is concerned. What you really need to do is to look through a Cheshire and see if the cross hairs line up with the doughnut on the primary, as that is the important bit. Once you have that done it is a case of adjusting the primary to get the shadow of the eye hole of your Cheshire or collimation cap inside the doughnut. 

[malc-c]

29 minutes ago, Ricochet said:

It's fine as far as being central is concerned. 

 

Really? - I've labeled them just for reference - nothing to do with orientation etc.   Clearly, if anything the secondary is low and needs to be moved upwards towards N slightly  to make it more central

Edited January 21, 2021 by malc-c

[jonathan]

1 hour ago, malc-c said:

A better picture might help, but looking at what you've provided I would agree that it's still in need of a little tweaking to get the secondary more central. 

This is the best I can do with my iPad through the home-made collimation cap (plastic film cannister with a hole in it).

Which screw would I adjust though, or is it back to random guesswork?

I'll need a diagram to explain why it needs moving towards N, I must be missing something.

Edited January 21, 2021 by jonathan

[Pixies]

Looks OK to me. The red circles are pretty concentric with the outer 'circle' of the focuser draw-tube. The outer edge of the secondary (showing white paper) is circular and concentric too.

I'd say that the next stage is to remove the paper and start looking at the reflections. Without a Cheshire, you can use the red reticle to align the doughnut (when you adjust the secondary tilt). Otherwise the traditional "can you see the mirror clips?" will do.  Then on to the primary.

@malc-c - can you indicate what you think is misaligned?

[Ricochet]

3 hours ago, malc-c said:

 

Really? - I've labeled them just for reference - nothing to do with orientation etc.   Clearly, if anything the secondary is low and needs to be moved upwards towards N slightly  to make it more central

The only thing of importance is the brightest circle, which is pretty well centred in the overlaid red circles. The darker shadow within the brighter circle had nothing to do with the collimation. There must be a piece of card/paper in the tube blocking the reflection of the primary and the Newtonian is in a location with a light source slightly off centre to the axis of the OTA, so the spider is throwing a shadow onto the card, but this just signifies the angle of the tube compared to the light source, not the collimation of the telescope. 

2 hours ago, jonathan said:

 

I'll need a diagram to explain why it needs moving towards N, I must be missing something.

There is no need to move anything. 

[malc-c]

I was thinking the darker circle was an out of focused secondary ??

[Ricochet]

 

11 minutes ago, malc-c said:

I was thinking the darker circle was an out of focused secondary ??

I think it is too big for that. Even if it was, that shadow would be offset in roughly the direction it is offset. (Which suggests I might be wrong about the relation to the light source direction). See the picture below of the view through a collimation cap in my 8" dob. Notice how much smaller the secondary shadow is. (Not the best picture as it is too dark to see the edge of the secondary, but you get the idea.) 

Edited January 21, 2021 by Ricochet

[malc-c]

Yeah, looking at it and having a rethink, that would be quite an obstruction if it were the secondary

[Pixies]

Yep- it looks like the shadow of the secondary falling on the paper baffle that's been placed down the tube.

@jonathan - whip the paper out. It's collimation time!

[jonathan]

Yes, absolutely that's the shadow of the secondary being cast onto the paper, it didn't cross my mind that that's what @malc-c was talking about.

You guys don't think there's tweaking to be done in the NW / SE direction?  Looks like the secondary is still not perfectly circular to me, a couple of small tweaks of the mirror angle might just sort it.

Btw I have black felt on my secondary glass edge, that's why it looks so dark in the photo (in the NW direction).

[Pixies]

16 hours ago, jonathan said:

Yes, absolutely that's the shadow of the secondary being cast onto the paper, it didn't cross my mind that that's what @malc-c was talking about.

You guys don't think there's tweaking to be done in the NW / SE direction?  Looks like the secondary is still not perfectly circular to me, a couple of small tweaks of the mirror angle might just sort it.

Btw I have black felt on my secondary glass edge, that's why it looks so dark in the photo (in the NW direction).

I don't think there's much in it. You could try adjusting the Mire de Collimation circles so that you have the outer one lined-up with the inside edge of the draw-tube, then you can be exactly certain that the reticle is centered properly. But it looks pretty circular to me.

A Cheshire being used as a sight-tube makes this stage so much quicker and easier. The next step, too.

[jonathan]

I did some more fine adjustment to the secondary tilt, looks perfect to me now (near as dammit perfect circle when the Mire de Collimation circles are overlaid).  So I removed the green felt and paper that was blocking the tube, and I'm left scratching my head because my next view down the collimation cap doesn't match Astro-Baby's, and doesn't match what she says (or am I reading it wrong again?)  I don't know how to interpret what's going on here, my collimation looks so far out compared to the photos in the guide, I can't understand why.

First photo is with the collimation cap, second is with the Cheshire (which looks miles out).  I measured the spider vanes meticulously, all the same length.  I spent an age getting the secondary mirror positioned correctly in the focuser tube circle, and making sure it was a perfect circle too.

Edited February 2, 2021 by jonathan

[jetstream]

When collimating we go back and forth between secondary and primary collimation? Try putting the primary dot in the cheshire ring, then go back to the sec, then back to primary etc etc. IMHO.

[malc-c]

Due to personal reasons my scope wasn't used for some time, but last summer I opted to strip and clean it and start using it again.  Prior to doing so I checked the collimation this was the result, which compares to the image in Ricochets post (Ignore the cobweb  )

 

The only true way to test the collimation is with a star test where you ramp the focuser in and out and check for concentric rings in the image like this

 

 

Hope this helps

[jonathan]

1 hour ago, jetstream said:

When collimating we go back and forth between secondary and primary collimation? Try putting the primary dot in the cheshire ring, then go back to the sec, then back to primary etc etc. IMHO.

How do I put the primary dot in the Cheshire ring?  That sentence makes almost no sense to me, sorry.

[jetstream]

Just collimate the primary with the 3 mirror cell screws. I have 3 cheshires, including the Catseye. Image from their site. Hope this makes more sense.

[Ricochet]

With your Cheshire in the focuser, adjust the secondary so that the doughnut on the primary appears directly under the cross hairs of the Cheshire. 

Once that is done switch to the collimation cap and adjust the primary so that the dark spot of the hole in the collimation cap is inside the doughnut. 

These are the only two things that matter. Don't worry about anything else. 

[jonathan]

19 minutes ago, Ricochet said:

With your Cheshire in the focuser, adjust the secondary so that the doughnut on the primary appears directly under the cross hairs of the Cheshire. 

Once that is done switch to the collimation cap and adjust the primary so that the dark spot of the hole in the collimation cap is inside the doughnut. 

These are the only two things that matter. Don't worry about anything else. 

I think I need lots of pictures, instructional diagrams, and probably a video showing the exact procedure because at the moment my mind is in major confusion mode!  Just wish I was able to take it to my local astro society (of which I am a member), I know someone there would probably be able to show me in a few minutes.  Damn you, lockdown!!

[jetstream]

9 minutes ago, jonathan said:

I think I need lots of pictures,

Its easy

1- make the secondary look round and centered under the focuser. Loosen stalk nut a bit for this, then tighten back up.

2-put the cheshire cross hairs on the primary dot. Use the 3 secondary adjustment screws for this

3- primary collimation- put the dot in the donut of the cheshire. Use 3 primary adjusting screws on the mirror cell for this.

This 1 iteration gets close, maybe right on. Repeat if needed

 

Edited February 2, 2021 by jetstream

[Paz]

If you are struggling with collimating it's worth just star testing - defocus on a bright star and see if the rings look concentric. If they are you are good enough to carry on and not worry about it. Eventually you will likely need to collimate but it may not be necessary and a star test may save you the hassle at least for a while.

[Paz]

P.s. I have a collimation cap and Cheshire and have practiced with them at home but in the field I cheat and use the laser through a Barlow approach. Thankfully that has always done the business, I bet if I had to use a collimation cap and cheshire for real in the field I would probably mess it up, throw a temper tantrum and go home early!

[Ricochet]

1 hour ago, jonathan said:

I think I need lots of pictures, instructional diagrams, and probably a video showing the exact procedure because at the moment my mind is in major confusion mode!  Just wish I was able to take it to my local astro society (of which I am a member), I know someone there would probably be able to show me in a few minutes.  Damn you, lockdown!!

Does this help?

[jonathan]

14 hours ago, Ricochet said:

Does this help?

A lot, thanks! 

Couldn't help reading 'Does this help?' in the deep voice Blackadder does in the episode 'Head' when he's pretending to be Farrow!! 

Edited February 3, 2021 by jonathan