Collimation - again

[bosun21]

You need to loosen the centre screw and slightly rotate the secondary mirror until the mirror appears perfectly circular. It’s oval shaped at the minute and this is why the crosshairs are off centre. Forget about the primary mirror until you sort out the secondary. It’s also not central to the focuser draw tube.

[lawsio]

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.

[Mandy D]

16 minutes ago, 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.

Which scope are you trying to collimate? I did my 200P's secondary mirror yesterday and got all three clips in view. It was a little tricky, but certainly possible. Some Newtonian scopes have an undersized secondary mirror and you will never get all the clips in view simultaneously. The trick is to centre it as best you can, judging the distance from each clip in turn until you end up with none in view. My secondary was a long way out and I use the scope for imaging and could still get sharp images of the lunar surface and sunspots, so it's not going to kill things. I'm now waiting for a clear night (or day) to see how much difference it makes.

[Spile]

41 minutes ago, Mandy D said:

Which scope are you trying to collimate? I did my 200P's secondary mirror yesterday and got all three clips in view. It was a little tricky, but certainly possible. 

Yes but only when after …

1. The outer edges of the primary mirror and secondary mirror are circular (e=f) and concentricwith the edge of the sight tube, so the secondary is centred (a=b and c=d) in respect of the focus tube.

2. The crosshairs of the sight tube intersect with the centre mark and the dark offset secondary reflection (x2) is offset toward the primary mirror. 
 

 

[lawsio]

 

36 minutes ago, Mandy D said:

Which scope are you trying to collimate? I did my 200P's secondary mirror yesterday and got all three clips in view. It was a little tricky, but certainly possible. Some Newtonian scopes have an undersized secondary mirror and you will never get all the clips in view simultaneously. The trick is to centre it as best you can, judging the distance from each clip in turn until you end up with none in view. My secondary was a long way out and I use the scope for imaging and could still get sharp images of the lunar surface and sunspots, so it's not going to kill things. I'm now waiting for a clear night (or day) to see how much difference it makes.

It's the 130p I'm trying to do at the moment.

[bosun21]

19 minutes ago, lawsio said:

 

It's the 130p I'm trying to do at the moment.

I just collimated a 130PDS recently for a friend and you can most definitely get the three primary clips in view. Place a sheet of coloured paper in the tube blocking the primary mirror and a white sheet flat directly opposite the focuser. This highlights the secondary mirror very well. As @Spile has stated you want the secondary mirror perfectly round and concentric with the focuser draw tube. Once you achieve this the crosshairs of the Cheshire will be in the centre of the secondary. Remove the paper sheets and you should be very close to seeing the three primary mirror clips. You can slightly tweak the three screws on the secondary to encompass the primary clips. Nip up the screws on the secondary and now adjust the primary screws to centre the doughnut.

[Spile]

Start by moving the secondary away from the primary so that a=b, and at the same time get the coarse rotation as good as you can. Next, fix the tilt/rotation error; you should be much closer. Ignore the primary tilt until the secondary is sorted.

 

[The60mmKid]

I'm surprised by the "collimation doesn't matter that much" perspective. I'm not the quantifying type, but my experience at the eyepiece has always been that collimation is a major factor in determining the quality of the view. If it's a diffuse object, collimation makes it more defined to my eyes (unless it's some smudge at the threshold of my ability to detect it at all, in which case collimation won't make it appear any sharper). And on stars and planets, the difference between collimated and sort-of-collimated is immediately obvious. Pinpoint stars vs. smudgy comets... impressionistic views of the planets vs. distinct fine details. Unless one spends all their time looking only at smudges at low magnification...

There are comparisons that seem largely pointless to me (e.g., this vs. that diagonal substrate, this vs. that eyepiece), but collimated vs. not-quite-collimated makes a big difference, I think.

I've often met well-intentioned but inexperienced visual astronomers who distress over the "optical quality" of their newtonians and cassegrains that are not collimated and/or thermally acclimated.

Edited August 27, 2023 by The60mmKid

[The60mmKid]

On 26/08/2023 at 16:57, lawsio said:

I need collimation advice (just had my first atyempt!) so will jump on this thread to avoid starting a new one!

Basically, am I collimated yet? After a good hour of farting about everything seems to more or less line up apart from the crosshairs on the cheshire, which I guess wouldn't be there if I used a standard cap for it.

Am I close enough? Need to go back and try again?

I highly recommend sending a PM to @Don Pensack, who created an excellent collimation guide. It was a lifesaver when I got my first newtonian!

[bosun21]

Do one mirror at a time. Forget the primary mirror when sorting out the secondary, that’s why I recommend blocking the primary with a sheet of coloured paper. A lot of frustration is due to constantly moving from one to the other and endlessly chasing their tail. Once you are experienced and can recognize the individual reflections then the paper is not necessary.

[Spile]

I posted a link to the collimation guide on my site but it appears that the moderators removed it.

[Stu]

12 minutes ago, Spile said:

I posted a link to the collimation guide on my site but it appears that the moderators removed it.

If it is needed this time then please repost it.

[Piero]

On 19/08/2023 at 11:44, Richard N said:

Forums like this always have many threads on collimation of Newtonian reflectors. Endless discussion. All very interesting. What I have never seen is any objective way to assess the effects of various degrees of miscollimation. I suspect that the answers are different for visual and photography? I also suspect (with no evidence at all) that we can obsess over very tiny errors that make no discernable difference. 

 

For an objective way to assess the effects of various degrees of miscollimation, I would suggest to get a copy of Suiter's book (https://www.firstlightoptics.com/books/star-testing-astronomical-telescopes-a-manual-for-optical-evaluation-and-adjustment-second-edition-book.html ). This can also be found in the second hand market.

A mild miscollimation in the primary mirror introduces some degree of coma. A severe miscollimation can also introduce astigmatism. If the optics are diffraction limited (1/4 lambda - which is just to say "commercially acceptable"), you want to collimate your telescope better than that to avoid introducing a larger error. In other words, you don't want that your collimation worsens the views delivered by your optics. To do that you need good collimation tools (=robust and collimated themselves) and you need to use them properly. These tools have tolerances, so the key point is to do the best job as possible so that your optics are collimated up to the tolerance of your tool.

Assuming that you have collimated your Newtonian as good as possible with your trusted tool, what is enough? Well, if you draw something technically, what scale of ruler is good enough? It depends on the image size you observe. The higher the magnification is, the larger the diffraction disc becomes, the more you are challenging your optics, the more you can see imperfections. Take a 10" dobson. Below 150x possibly any tool can correct well enough. You likely need a good tool for higher magnifications tough. Above possibly 400x, you might even want to use an autocollimator. Of course you can just use a cheap collimation tool for any magnification you want to reach, but that tool will not offer a reading which is precise enough and these imperfections can be detected at high magnifications.

 

Regarding "obsessing about collimation", well.. this is a generic comment. It really depends case by case. Personally, I find that if the optics are not close to ambient temperature, the introduced error is visibly larger than a slight miscollimation. If the mirrors are not supported properly, the introduced error is even larger. Reading comments by members it seems to me that a large number of people simply accepts the mirror supports as they are, meaning without questioning whether these work well or not. In addition to this, it seems to me that very few people use a fan to cool down their optics, justifying this with terms like (it does not have any effect, again, maybe the questions should be - is this fan working okay? Was it installed properly?).

Often, the most basic and cheapest collimation tools are also employed (oddly, as cases of Televue and other high end eyepieces are often used).

Finally, the statement often  reported is: 150-250x is the highest magnification due to UK seeing (the only culprit of the equation!).

In agreement with Suiter, collimation is a factor in the stack. There are many other factors to take into account, some of which are out of control, whereas other can be controlled.

Unfortunately what people tend to get really fussy about are eyepieces, whereas the rest seems mostly neglected in my opinion.

Eyepieces are just another step in the stack. The best eyepieces deliver miserably if other steps in that stack do not perform satisfactory (and yes, I write this by experience).

I hope this helps a bit.

Edited August 29, 2023 by Piero

[lawsio]

Right, after too many hours of fettling I'm back. I think it's better now but not 100%.

A couple of things;

- I am absolutely 100% adamant I can't see more than one primary clip at a time in the secondary. If I take the cheshire out and look directly onto the secondary I can see where all three are if my move my eye about but never all at the same time.

- I live in Cheshire and I'm going to have to move because the whole place reminds me of collimators now.

- If anyone's wondering if telescopes float, I might have the answer for you very soon!

 

[Spile]

2 hours ago, lawsio said:

Right, after too many hours of fettling I'm back. I think it's better now but not 100%.

A couple of things;

- I am absolutely 100% adamant I can't see more than one primary clip at a time in the secondary. If I take the cheshire out and look directly onto the secondary I can see where all three are if my move my eye about but never all at the same time.

- I live in Cheshire and I'm going to have to move because the whole place reminds me of collimators now.

- If anyone's wondering if telescopes float, I might have the answer for you very soon!

 

This is where you need to end up...

 

Ignore the mirror clips (step 3) until you have confirmed that e=f , a=b and c=d (step 1) and that the cross hairs of the sight tube intersect with the centre mark and dark offset secondary reflection (step 2).

Ref  https://astro.catshill.com/collimation-guide/

[bosun21]

Your secondary mirror is still not round and is clearly oval. Did you try the sheets of white and coloured paper i previously described. This isolates the secondary mirror allowing you to rotate the secondary mirror holder until it’s perfectly round and concentric with the focuser draw tube.

[lawsio]

3 hours ago, bosun21 said:

Your secondary mirror is still not round and is clearly oval. Did you try the sheets of white and coloured paper i previously described. This isolates the secondary mirror allowing you to rotate the secondary mirror holder until it’s perfectly round and concentric with the focuser draw tube.

Harsh but fair!

It's interesting because so much of this seems to be down to personal perception. For example, you say my image shows it as being 'clearly oval' - I'm a Chartered Engineer and I cant see that as oval without the guide lines drawn on in the post above, and even then apart from a bit in the top right it looks pretty circular to me. The guide linked above has the following quote in bold letters - 

"The tolerances for this step are larger than those for alignment of the primary mirror so I do not obsess about this check – good is good enough."

So my good is obviously different to yours! Not saying you're wrong at all - it's just a frustrating thing.

I've used so many guides and youtube videos and they all seem pretty consistent but what I'm doing isn't giving the expected result. I am using the paper in the tube, have it facing a white wall, using a proper collimator, etc.

One thing I will say is that I find it very difficult to make micro adjustments to the secondary, the slightest turn on the 3 small bolts and it drops a lot. I can hold it in position when loose but trying to tighten it back up into that position never gets it to where I want it to be. It is a second hand scope from eBay, and the image below shows the underside of it has seen better days, so I'm not ruling out the possibility it could be knackered but it doesn't seem mechanically complicated enough to be that and it's far more likely to be me!

Edited August 28, 2023 by lawsio

[Dan_Paris]

On 19/08/2023 at 12:44, Richard N said:

I suspect that the answers are different for visual and photography?

 

Indeed. For visual you "just" need to ensure that the optical axis of the primary (reflected by the secondary) and the mechanical axis of the focuser intersect at the focal plane, in order to minimize coma in the center of the field. This can be ensured, for instance, by collimating on a star once rough alignment has been done.

However for deep-sky imaging this is not sufficient. You need, on top of this, to have the optical axis orthogonal to the focal plane, otherwise tilt occurs. This cannot be assessed with a star test on-axis.

 

Edited August 28, 2023 by Dan_Paris

[bosun21]

41 minutes ago, lawsio said:

"The tolerances for this step are larger than those for alignment of the primary mirror so I do not obsess about this check – good is good enough."

So my good is obviously different to yours!

Well why post the photo and ask for opinions? If you are satisfied with it the way it is then that's all that matters.

[lawsio]

20 minutes ago, bosun21 said:

Well why post the photo and ask for opinions? If you are satisfied with it the way it is then that's all that matters.

For that very reason mate, if I think it looks OK but people who know what they're talking about don't then its a good job I asked 🤣

[wimvb]

22 hours ago, Piero said:

This can also be found in the second hand market.

I bought it a month or so ago, directly from the publisher. Don't buy from the great river company, they charge 3× the price for some reason.

[Spile]

My suggestion is that you find a guide that makes sense to you and stick with that. Trying to combine advice from different authors would be like following a recipe from Delia Smith, Ken Hom and Mrs Beeton at the same time. I wrote my guide because I wanted to put into words and most importantly diagrams, the words of people I trust to get it right. Aka Vic, Jason and Don. If it helps someone else then that’s a bonus but the prime reason for doing it was for myself. I’ve used letters and diagrams rather than descriptions because it’s clearer to me.

[bottletopburly]

Another thing to check out is the Center ring on primary central on my 200pds it was 6mm out so never assume it is , Newtonians don’t you just love them .

[Richard N]

1 hour ago, bottletopburly said:

Another thing to check out is the Center ring on primary central on my 200pds it was 6mm out so never assume it is , Newtonians don’t you just love them .

That’s bad. I kind of assumed that the reference point would be accurate.  Might the optical centre be offset from the physical centre for some obscure reason?

Edited August 29, 2023 by Richard N

[Richard N]

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