Collimation - again

[Spile]

47 minutes ago, Richard N said:

And just to worry folks, if you collimate with the scope horizontal, does the collimation remain accurate for all elevation angles? 

Yes - if it doesn't I'd be checking the tightness of your lock bolds/stiffness of your springs for the primary and tightness of the spider and 3 outer bolts for the secondary.

[Mr Spock]

51 minutes ago, Richard N said:

And just to worry folks, if you collimate with the scope horizontal, does the collimation remain accurate for all elevation angles? 

With Newtonians and correct springs, yes.

Some SCTs suffer from mirror flop, so no / yes depending on the scope. My C9.25 never had a problem, but others report serious problems.

[Piero]

2 hours ago, Richard N said:

And just to worry folks, if you collimate with the scope horizontal, does the collimation remain accurate for all elevation angles? 

Only if you have excellent mechanics.

I would suggest to collimate at 60 degree. If you are doing some "serious" planetary observation, you might want to collimate at an angle closer to the planet altitude.

-- 

EDIT: Reason for the 60 degrees

The primary mirror is supported at the back and laterally. When the telescope points to the zenith, it is 100% supported at the back. When it points to the horizon, it is 100 % supported laterally. Assuming the angle x is the telescope altitude (0=pointing at the horizon, 1/2 pi=90 degree or pointing at the zenith), you could model the back and lateral supports with the functions sine and cosine, respectively (see plot below). The two forces intersect at 1/4 pi, which is 45 degrees. Therefore, one could simply collimate at 45 degrees, as a balance point. Practically, it is pointless to observe at the horizon and just above it as the atmosphere is thicker and the views are more affected by the seeing. The vast majority of observations is done between 30 and 90 degrees, therefore, my suggestion to collimate at 60 degrees. 

 

Edited August 29, 2023 by Piero

[Piero]

40 minutes ago, Mr Spock said:

With Newtonians and correct springs, yes.

Some SCTs suffer from mirror flop, so no / yes depending on the scope. My C9.25 never had a problem, but others report serious problems.

 

It is not just "newtonians and correct springs". You also need to have good secondary support (spider and holder), focuser (e.g. the bearings support the drawtube differently at different angles) and primary mirror lateral support. Then, if the telescope is a truss one, it also depends on pole length / diameters and pole attachments.

With a laser collimator it is quite easy to see whether collimation changes at different levels of altitude. 

Edited August 29, 2023 by Piero

[Spile]

11 hours ago, Richard N said:

And just to worry folks, if you collimate with the scope horizontal, does the collimation remain accurate for all elevation angles? 

Yes it should.

[inFINNity Deck]

Well, my SkyWatcher Explorer 300PDS does show slight deformation when tilted from horizontal to vertical. It is not much, and perhaps even insignificant, but it sure does happen. Best is indeed to collimate it somewhere around 60 degrees or, even better, at the altitude where it is being used.

Nicolàs

[Mr Spock]

45 minutes ago, inFINNity Deck said:

Well, my SkyWatcher Explorer 300PDS does show slight deformation when tilted from horizontal to vertical.

Have you upgraded the springs?

[alacant]

22 hours ago, Richard N said:

And just to worry folks, if you collimate with the scope horizontal, does the collimation remain accurate for all elevation angles?

Not in our experience.

None of the low-end Newtonian Telescopes we have tested hold collimation out of the box, especially if collimated horizontally, due mainly to lateral movement of the main mirror, tendency of the same to 'fall forward' toward the retaining clips, tube flex etc. as the tube is tilted.

Fortunately, there are simple modifications which allow collimation to hold at all angles. We outline them here:

https://linuxcb.blogspot.com/2022/06/gso-tube-support_22.html

Collimation wise, be sure to read Telia's collimation myths and Seronik's no nonsense guide, both of whom outline the simplicity of a much misunderstood procedure.

Cheers and HTH

Edited August 30, 2023 by alacant
Gramática

[Richard N]

10 hours ago, Spile said:

Yes it should.

It should of course; but does it? Comments suggest not in all cases. 

[inFINNity Deck]

9 hours ago, Mr Spock said:

Have you upgraded the springs?

I was not aware that there is an upgrade-set, so no. But for me that is no issue, I use it mainly as a collimator for other scopes and once a year for reach-out where I collimate it at about 45-60° altitude.

Still I would like to learn more about those springs, is there a set off-the-shelf available?

Nicolàs

[bottletopburly]

3 minutes ago, inFINNity Deck said:

I was not aware that there is an upgrade-set, so no. But for me that is no issue, I use it mainly as a collimator for other scopes and once a year for reach-out where I collimate it at about 45-60° altitude.

Still I would like to learn more about those springs, is there a set off-the-shelf available?

Nicolàs

I used these on my 200pds https://www.bay.co.uk/itm/124783634360?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=8wq7Z9toR0m&sssrc=4429486&ssuid=xQsxeo45SGa&var=&widget_ver=artemis&media=COPY

[Spile]

12 hours ago, Richard N said:

It should of course; but does it? Comments suggest not in all cases. 

What I have found is that as long as the collimation bolts and spider fixings are tight then the mirrors don’t move. I haven’t changed the springs on my primary because I see no reason to.

[Mandy D]

On 26/08/2023 at 20:59, lawsio said:

Thanks! Had a nightmare with the secondary, the [removed word] went all over the place.

One thing I wasn't sure about is the instructions I have said I needed to get it so all three primary clips were visible, but it doesn't seem  like the secondary is big enough to get more than one on at a time. Not sure if that makes sense but I spent ages trying to move it further away from the primary and even with the screw all the way out i still couldn't fit all three clips in the view.

I am just in the process of checking the collimation on my 200P and noticed that I could not see any of the secondary mirror clips, despite having got all three into view a few days ago. I then realised that, today, I had the focuser racked out a considerable way. Racking it back in brought all three into clear view as before, but now I could see the focuser tube in the Cheshire.

Edited August 31, 2023 by Mandy D

[lawsio]

2 hours ago, Mandy D said:

I am just in the process of checking the collimation on my 200P and noticed that I could not see any of the secondary mirror clips, despite having got all three into view a few days ago. I then realised that, today, I had the focuser racked out a considerable way. Racking it back in brought all three into clear view as before, but now I could see the focuser tube in the Cheshire.

Thanks for this, it's something for me to check when I try it again! It's definitely the point I'm falling down at as I feel like I'm following instructions to the letter but not getting the same effect.

Only thing I might need to do is lose the Cheshire and make a collimating cap instead maybe?

[Mandy D]

1 hour ago, lawsio said:

Thanks for this, it's something for me to check when I try it again! It's definitely the point I'm falling down at as I feel like I'm following instructions to the letter but not getting the same effect.

Only thing I might need to do is lose the Cheshire and make a collimating cap instead maybe?

I think the collimation process is not simple until you fully understand it and have done it dozens of times at which point it becomes second nature, like riding a bike. Unfortunately, It appears that there are subtle differences in the design of Newtonian reflectors which makes a "catch all" guide difficult to write and even more difficult to follow. I suspect that a major problem I am experiencing, and likely you, too, is that the secondary collimation is an iterative process and one thing disturbs another. I currently have the secondary aligned so the clips on the primary are evenly visible and I can get the spot in the Cheshire into the centre of the primary marker. What I am not getting, so far, is the complete alignment with everything (that should be) concentric. I am guessing that I need to play with the secondary some more, which may well throw the primary out, but I can deal with that. I've attached a photo of where I am currently at. You cannot see the mirror clips in my photo, but they are clear to the eye and all equally visible.

I don't, honestly, think that the Cheshire is the problem and would not recommend abandoning it, certainly not in favour of a collimating cap.

Here is a link to a guide that seems to discuss things in a lot more depth than most. Try not to get too bogged down in the technical details.

https://mikehotka.com/Collimation/CollimationNewtonianTelescope.htm

Edited August 31, 2023 by Mandy D

[bomberbaz]

1 hour ago, Mandy D said:

I think the collimation process is not simple until you fully understand it and have done it dozens of times at which point it becomes second nature, like riding a bike. Unfortunately, It appears that there are subtle differences in the design of Newtonian reflectors which makes a "catch all" guide difficult to write and even more difficult to follow. I suspect that a major problem I am experiencing, and likely you, too, is that the secondary collimation is an iterative process and one thing disturbs another. I currently have the secondary aligned so the clips on the primary are evenly visible and I can get the spot in the Cheshire into the centre of the primary marker. What I am not getting, so far, is the complete alignment with everything (that should be) concentric. I am guessing that I need to play with the secondary some more, which may well throw the primary out, but I can deal with that. I've attached a photo of where I am currently at. You cannot see the mirror clips in my photo, but they are clear to the eye and all equally visible.

I don't, honestly, think that the Cheshire is the problem and would not recommend abandoning it, certainly not in favour of a collimating cap.

Here is a link to a guide that seems to discuss things in a lot more depth than most. Try not to get too bogged down in the technical details.

https://mikehotka.com/Collimation/CollimationNewtonianTelescope.htm

 

oh but you can have hours of "fun" collimating and I am entirely with you Mandy.

I had a similar journey of fun with a Heritage 130p using first the OCAL collimator to get the secondary beautifully collimated, I was so happy at doing what I thought was  a model of collimation.  Upon removing my coloured card blanking sheet, the primary was a country mile off. 

So I put in my new and perfectly collimated baader laser and in a few minutes the whole system looked perfectly aligned. Until I popped the OCAL back in which showed my secondary was now out!

HOWEVER, a laser recheck showed a superb alignment so we are full circle and I decided to leave well alone!

Steve.

[John]

2 hours ago, bomberbaz said:

oh but you can have hours of "fun" collimating and I am entirely with you Mandy.

I had a similar journey of fun with a Heritage 130p using first the OCAL collimator to get the secondary beautifully collimated, I was so happy at doing what I thought was  a model of collimation.  Upon removing my coloured card blanking sheet, the primary was a country mile off. 

So I put in my new and perfectly collimated baader laser and in a few minutes the whole system looked perfectly aligned. Until I popped the OCAL back in which showed my secondary was now out!

HOWEVER, a laser recheck showed a superb alignment so we are full circle and I decided to leave well alone!

Steve.

It does cause headaches and confusion if different collimation aids simply don't agree with each other. 

Fortunately, if it's dark, a simple star test can settle the matter. I ended up going with my cheshire eyepiece more often than I did my laser collimator, despite carefully collimating the latter !

 

 

[Mr Spock]

How I do my 12" is as follows. The secondary needs to be done only once usually, but it is useful to check occasionally.

1.  Check the vanes on the secondary to make sure they are equidistant.
2.  Use a concentre to get the secondary in the correct place under the focuser.
3.  Use a collimation cap to get the mirror clips equal.
4.  Check the above with a (collimated) laser; the laser was on the primary doughnut so only a minor tweak required.
5. Use a laser to adjust the primary. Note: my laser results are the same as my Cheshire results.
6.  Use the laser to check primary collimation prior to each session.
7.  Do a star test to confirm collimation is spot on. Collimation has never required adjustment from a star test after performing step 6.

Once complete this is how I can view the moon at high magnification and the image be sharp.

There is no mystery, secret or voodoo involved in obtaining high resolution viewing. If your optics are aligned perfectly you will have it; if they are not perfectly aligned you will not.

[inFINNity Deck]

7 hours ago, Mr Spock said:

How I do my 12" is as follows. The secondary needs to be done only once usually, but it is useful to check occasionally.

1.  Check the vanes on the secondary to make sure they are equidistant.
2.  Use a concentre to get the secondary in the correct place under the focuser.
3.  Use a collimation cap to get the mirror clips equal.
4.  Check the above with a (collimated) laser; the laser was on the primary doughnut so only a minor tweak required.
5. Use a laser to adjust the primary. Note: my laser results are the same as my Cheshire results.
6.  Use the laser to check primary collimation prior to each session.
7.  Do a star test to confirm collimation is spot on. Collimation has never required adjustment from a star test after performing step 6.

Once complete this is how I can view the moon at high magnification and the image be sharp.

There is no mystery, secret or voodoo involved in obtaining high resolution viewing. If your optics are aligned perfectly you will have it; if they are not perfectly aligned you will not.

At a first-time serious collimation one should also check whether the focuser is pointing towards the centre of the tube. Best way is to mark a point (or line) opposite of the focuser and temporarily remove the secondary (finding that spot kan be done using a strip of paper wrapped against the inside of the tube with a mark halfway the circumference of the tube. If you wrap it first clockwsie, then counter clockwise, and take over the mark on the inside of the tube, the oppositie location is halfway between those two marks). Alternative method is using a hand-held mirror to see if the laser-spot of a laser-collimator falls in the centre of the secondary (but for that you need to test whether that is already properly centred, but that can be checked with a tape measure).

First time I properly tested my 300PDS I found the focuser to be seriously off, here is an image after adjustment (at the left it is higher than at the right):

Luckily that focuser has adjustment-screws for this lateral tilt, which is not always the case in other Newtons:

HTH

Nicolàs

[Mr Spock]

I should add that once you have the vanes equidistant a concentre will show if the secondary is off centre left to right. That would mean the focuser isn't aligned and would need adjusting. 

Overall though as long as the optics are aligned the position of the secondary isn't too critical as it's a flat. If it is slightly off centre it will affect where vignetting occurs but that is all. 

[Mandy D]

3 hours ago, inFINNity Deck said:

At a first-time serious collimation one should also check whether the focuser is pointing towards the centre of the tube. Best way is to mark a point (or line) opposite of the focuser and temporarily remove the secondary (finding that spot kan be done using a strip of paper wrapped against the inside of the tube with a mark halfway the circumference of the tube. If you wrap it first clockwsie, then counter clockwise, and take over the mark on the inside of the tube, the oppositie location is halfway between those two marks). Alternative method is using a hand-held mirror to see if the laser-spot of a laser-collimator falls in the centre of the secondary (but for that you need to test whether that is already properly centred, but that can be checked with a tape measure).

First time I properly tested my 300PDS I found the focuser to be seriously off, here is an image after adjustment (at the left it is higher than at the right):

Luckily that focuser has adjustment-screws for this lateral tilt, which is not always the case in other Newtons:

HTH

Nicolàs

Thank you for this interesting perspective. I've just looked at the focuser on my 300PDS which is identical to the one on yours and the single speed stock focusers on my 200P and 250PX are similarly adjustable, so that is good to know.

However, what we are dealing with here is first-time collimation attempts, not precision final alignments for astrometry or high resolution astro-imaging. So, whilst your comments and notes on focuser alignment are perfectly valid and correct (and appreciated), it appears that the amount of effect they are going to have on good (maybe not in the limit perfect) collimation is minimal, hence, I think the basics matter rather more at this point. I've always achieved what I thought was acceptable collimation of my 200P and 250PX by simple adjustment of their primary mirrors alone using a Cheshire and have photos to prove it. My next step is to improve this incrementally to gain higher resolution and improved image scale to match the awesome performance and resolution of my Nikon D800. It appears from my reading elsewhere on the internet that squaring the focuser is an unnecessary complication at this stage and only leads to confusion amongst beginners.

I'm going to link to the most rational and level-headed guide I have yet managed to find: "A Beginner's Guide to Collimation" by Gary Seronik, which tells us that squaring the focuser is probably the least important of all the possible stages of collimation. Now, obviously, it may be vitally important to you with everything else perfectly set and looking to squeeze the last few hundredths of an arc-second of resolution from your telescope, but to the beginner it is probably an unnecessary incumberance.

@lawsio This is for you. I hope it helps:

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

I make no apologies if this article has already been linked to in this discussion, as I think it is such a practical and level-headed guide that does not take the beginner on unnecessary detours.

Edited September 1, 2023 by Mandy D
edited for clarity