Collimating the Skywatcher 190 MN

[almcl]

The postman delivered my brand new SW 190 MN today, several days earlier than expected.  Yippee!

Great fun unpacking and looking at it, but with no prospect of clear skies I put it on the bench to check collimation.  First with a Concenter then with a Cheshire.

Both indicated that collocation was 'off', but by completely different amounts.  I had hoped that the reportedly more robust collimation of the Mak Newt would have survived the trip from the factory, but alas it was not so. I've never really trusted the Concenter, which appear to ignore Newtonian offset and so proceeded to align  with the Cheshire.

The secondary reflection was round, but the primary spot and reflection of the eyepiece hole were not in the centre.  Now I am no stranger to collimating Newtonians and believe I am reasonably OK at it, so I settled down to line things up.  It didn't take long to get the primary spot and Cheshire eye hole lined up with the crosswires in the centre but on this MN there is also a ring marking the centre of the secondary and this is absolutely resisting my efforts to bring it into the centre.  The diagram below indicates the situation, the magenta ring being the recalcitrant secondary centre mark and the orange lines the copper cross hairs.

I am guessing that the secondary needs moving away from the corrector plate, but am a bit reluctant to try this as I have read that this distance is accurately set at the factory and shouldn't be altered?

If any 190 MN owners who have faced this and solved it would care to share, I'd be very grateful?

[wimvb]

You have figured out collimation by now, but there are a few old threads on this forum and iceinspace, about collimating the MN190 which could be of interest

http://www.iceinspace.com.au/forum/showthread.php?t=140193

 

[almcl]

Ah, thanks for that, Wim.  

I had hoped you might join in, as in part it was the excellent results you have posted with your 190 MN that convinced me to get one.

I should have followed up my results for the benefit of anyone else contemplating a 190 MN. 

I adjusted the secondary mirror position 'down' the OTA and got to this point (sorry to use diagrams but my phone cam resolutely refuses to produce a convincing image):

I could see that the focuser needed tilting, but in the absence of any info about its adjustments, couldn't see how to do this, so resorted to shimming one edge with PTFE strips

This gave me a view like this:
 

 

After paying a good deal more attention to mounting the camera in the focuser and rotating it through 90°, I got the following results for collimation and field flatness:

3d curve tilt:

while FWHM came up as:

This is vastly better than anything I have ever achieved with my 200P and although clear nights have been rather few of late, early results are quite promising.

Edited September 14, 2020 by almcl

[wimvb]

3 hours ago, almcl said:

could see that the focuser needed tilting, but in the absence of any info about its adjustments, couldn't see how to do this, so resorted to shimming one edge with PTFE strips

How about the focuser extension tube? Is that set firmly in the draw tube? I had to fix it with a "shim" of aluminium tape, taped on one side. I wanted to tape three narrow strip evenly distributed around the extension, but one strip already fixed it in place. Clearly not much play to start with, but still too much to leave as it was.

3 hours ago, almcl said:

This gave me a view like this:
 

 

Is that with the TS concentre cap?

 

3 hours ago, almcl said:

After paying a good deal more attention to mounting the camera in the focuser and rotating it through 90°, I got the following results for collimation and field flatness:

What software do you use. Mind you, I only have a small sensor camera on my scope, so it would be difficult to measure flatness over the entire field.

Edited September 14, 2020 by wimvb

[almcl]

2 hours ago, wimvb said:

How about the focuser extension tube? Is that set firmly in the draw tube? I had to fix it with a "shim" of aluminium tape, taped on one side. I wanted to tape three narrow strip evenly distributed around the extension, but one strip already fixed it in place. Clearly not much play to start with, but still too much to leave as it was.

It certainly feels a bit slack and getting everything tight is quite a challenge.  I haven't tried tape yet but may have to.

2 hours ago, wimvb said:

Is that with the TS concentre cap?

Yes indeed.  I wasn't convinced by it on my 200P as it never seemed to agree with the Cheshire eyepiece, but on the 190MN it agrees a lot better and makes getting the secondary centred and squared a bit easier.

2 hours ago, wimvb said:

What software do you use. Mind you, I only have a small sensor camera on my scope, so it would be difficult to measure flatness over the entire field.

That's MaxSelector, available free from http://www.felopaul.com/download.htm 

It's a little bit quirky, but once you discover how the author does things it provides useful data (I think).