Collimation woes

[ispookie666]

No end to my various problems.  

I have a Sky-watcher 200PDS and the views were a not very sharp.  

Been trying to collimate the scope using a cheshire, for the life of me cannot get the mirror clips in sight, at the most I can get two.  I was following @Astro_Baby guide and got the Secondary mirror to be a circle (it was pretty much elliptical at the start).  have been stuck trying to get the clips in view.  

Any help appreciated, even better - anyone close to cambridge wiling to help. 

😐

[wookie1965]

I have a sight which may help but I am out at the moment. 

What you can do is place a white piece of paper opposite the focuser and use a another to block the primary mirror reflections. You can then concentrate on the secondary. 

Once you have done this remove the paper blocking the primary reflections and adjust the 3 collimation screws to get the mirror clips in. 

Make sure you wind the focuser in and out to get the best view. 

Once you have done this it's just a case of tweaking the primary then you are done. 

[malc-c]

If you use the search function and type in "collimation" you'll came across 100s of posts on how to collimate a newt.  It's probably in the top 5 of frequently asked questions so there is a lot of coverage, including tools such as self centring adapters and laser 

[banjaxed]

Since I watched collimating with a laser by Astronomy and Nature on YouTube I have never looked back. It now only takes me a couple of minutes.

[wulfrun]

Here's one of the best guides, in my opinion. He keeps is as simple as possible:

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

[alacant]

2 hours ago, ispookie666 said:

cannot get the mirror clips in sight

If you have a long Cheshire sight tube, you won't be able to. Anyway, just remember that apart from reflection, the secondary mirror has no optical properties. 

+1 for @wulfrun's recommendation. To make life even easier, understandable and to reinforce Seronik's simplicity, preface with Telia's plain English collimation myths.

 

**EDIT. Important.

So that the collimation holds at all angles, fit a long (at least 50cm) Losmandy dovetail plate, a rigid box section to tie the top of the tube rings and fit six strong springs to the main mirror. Then remove the mirror clips and fix the main mirror to the cell using three generous applications of neutral silicone.

 

Edited July 24, 2021 by alacant

[Mr Spock]

Almost the first thing I did when I got my 250 was fettle the collimation. Like you the secondary wasn't quite round. Touching the secondary can be daunting but here's what I did. In my case I also had to move the vanes as well as the secondary mirror to mounting wasn't quite square leading to the oval shape.

I didn't have a sight handy so I made one from one of those plastic end caps used for focusers. Identified the centre and drilled a 2mm hole in it. This allowed me to get my eye central over the focuser. I then loosened the secondary screws and held it in approximately the right position. I had a piece of card under the secondary so it stood out against the black. From there I was able to make small adjustments to the vanes to get the image circular. It was then a case of minor adjustments to the secondary screws to get the mirror clips in view. Once I'd done that it was time to take it to the next step and fit my laser collimator. Showing my 'by sight only' method was good, the laser was shining on the primary centre ring - not bad  A slight tweak to the secondary screws got the laser central.

Once that is done (and it shouldn't need touching again), adjusting the primary with a laser is easy.

Out of the box the scope was good, but once properly collimated lunar features were as sharp as I'd seen. It is worth the effort! Near enough isn't good enough if you want bitingly sharp views.

[Spile]

I use a cap and Cheshire sight tube to first align the secondary and then the primary.
This is what I do when it comes to the secondary…

1) SPIDER VANE: UP/DOWN/LEFT RIGHT - Spider vane screws(4) - Check with ruler/pair of compasses/circle of paper to ensure centre screw is equidistant from wall of OTA. 

The next adjustments will take into account any minor adjustment errors made above.

2) AXIAL: UP/DOWN OUTER BOLTS (3) - Loosen each bolt equally to move the secondary mirror toward or away from the primary until it is centred in the sight tube.

3) ROTATION: CENTRE SCREW - Turn the secondary until the secondary mirror is facing you and is circular and not elliptical.

4) TILT: OUTER BOLTS (3) - Make small adjustments to each bolt until the secondary edge is concentric with the primary mirror (r) edge.

Repeat procedure (3,4) as necessary. Get all circles (except the secondary reflection which should be offset in the direction of the primary) centred on the cross hairs and concentric. Get the above as close as you can but you have a wider degree of tolerance when adjusting the secondary. With the focuser sufficiently in, I am looking to see all three mirror clips standing equally proud. 

For the primary, it is just a case of getting the centre mark in the middle of the ring if using a cap or between the sight tube cross hairs by adjusting the tilt of the primary mirror in small steps. This is the last step but the most important in terms of being accurate.
 

Edited July 24, 2021 by Spile

[wookie1965]

http://www.schlatter.org/Dad/Astronomy/collimate.htm

Here is that link I used this in conjunction with "Astrobaby`s"  helped enormously. 

[Pixies]

OK - here's my bit of advice. It's essential that you get the secondary mirror base level across the tube. You don't want it tilted at all, otherwise it's impossible to get it adjusted correctly when you are trying to line it up with the centre of the primary. Astro baby mentions it briefly - saying use a mirror to check that it is not excessively tilted - but here's how I suggest you do it. It should only be a one-off adjustment.

Keep loosening the 3 adjuster screws and tightening the centre screw. This brings the mirror back up the tube, away from the primary. Keep doing this until the mirror has been pulled all the way back to the top of the tube and is flush against the vane-assembly, with the adjusters just barely loose. This means that the base of the secondary holder is now flush with the assembly. Now you can bring the secondary back down the tube by loosening the centre screw by a turn and taking up the slack with the adjusters by screwing them all in by equal amounts. If you repeat this until the mirror is directly under the focuser, making sure the 3 adjusters are always rotated by equal amounts, you can be sure that the secondary mirror is not tilted. Now, all you need to do is make sure it is rotated and facing the focuser correctly, forming a perfect circle in the Cheshire/cap.

And it should be an exact fit:

After that, you should have only very little tilt adjustment to make to align it with the primary, and on you go....

Also, don't miss out the early stage where you check (and adjust if necessary) the central alignment of the secondary assembly within the tube, by adjusting the vane tensions. They should be tight, too - otherwise the secondary can flex under different altitude angles.

But all the above adjustments are pretty much a one-off. Unless you are very unfortunate, you won't have to redo them unless you have been performing some maintenance.

[NGC 1502]

As mentioned by others, the link to Gary Seronic is brilliant. Gary cuts through all the information and mis-information that’s unfortunately widely available by so called experts.

Ed.

[ispookie666]

Thank you everyone.  

Slowly getting the hang of it.  Looks like the spider vanes were off , adjusted them. Tweaked a bit of primary mirror and have finally got things kind of collimated. 

Will check again tomorrow

[Sabalias]

Hi there,

It's also worth mentioning that you may need to collimate the focus tube. Mine was off and, as a result, I couldn't get the whole of the primary in view when the secondary was true (the problem was diagnosed by others on this forum). Useful video (doesn't show a 200PDS but it is still relevant) -

Stu

[iapa]

On 25/07/2021 at 07:32, Sabalias said:

Hi there,

It's also worth mentioning that you may need to collimate the focus tube. Mine was off and, as a result, I couldn't get the whole of the primary in view when the secondary was true (the problem was diagnosed by others on this forum). Useful video (doesn't show a 200PDS but it is still relevant) -

Stu

I used this successfully with my Quattro 10” CF and the 200P-DS

 

[Spile]

For a well-aligned secondary, I am looking for 

a=b  (or b>a for a centred alignment with no offset)* 
c=d  
e=f   
g=h  
i < j    IF a=b

or i=j  IF *b>a by the correct offset amount

In terms of the view - https://astro.catshill.com/through-the-eyepiece-tube/

[barbulo]

I read this thread carefully, but I have a question regarding the final adjustment using a laser collimator: What about the play the collimator has in the 1,25" hole and the play the 2"-to-1,25" has in the focuser? Can they be avoided? Would those lead to a misalignment of the primary?

[Pixies]

4 hours ago, barbulo said:

I read this thread carefully, but I have a question regarding the final adjustment using a laser collimator: What about the play the collimator has in the 1,25" hole and the play the 2"-to-1,25" has in the focuser? Can they be avoided? Would those lead to a misalignment of the primary?

Yes, that's all relevant. Collimation will get you as close as your system allows you too. The final test has to be a proper star test: the highest arbiter of collimation.

[ispookie666]

Thank you once again everyone.  Decided to revisit collimation and got it done.  🙂

[Jason D]

On 30/07/2021 at 01:54, Spile said:

For a well-aligned secondary, I am looking for 

a=b  (or b>a for a centred alignment with no offset)* 
c=d  
e=f   
g=h  
i < j    IF a=b

or i=j  IF *b>a by the correct offset amount

In terms of the view - https://astro.catshill.com/through-the-eyepiece-tube/

 

 

a=b  (or b>a for a centred alignment with no offset)*     [[ a=b should be the goal regardless whether the secondary mirror was mounted centrally or with an offset ]]
c=d  
e=f                                                                                     [[ This will always be true no matter what you do. ]]
g=h                                                                                    [[ This should be ignored. If collimation is done correctly for a mechanically aligned scope then g=h will be true. That is, there is no collimation step specifically for g=h. It is an automatic result of a good collimation ]]
i < j    IF a=b                                                                      [[ a=b should always be the target. i<j will always be true for a good collimated scope regardless whether the secondary was mounted with an offset or centrally.]]

or i=j  IF *b>a by the correct offset amount                     [[ This statement should be dropped. It is an incorrect target ]]

Jason

 

[Spile]

Thanks Jason.

a=b - That is my goal but IF a user had chosen a non-offset arrangement then it is right to say that IF  i=j THEN a≠b SHOULD be true? 

e=f - Incorrect labelling on my diagram. See below

g=h  - Added to represent a non-centred secondary and therefore highlight an error.

I look forward to your comments.

Edited August 13, 2021 by Spile

[Jason D]

You stated: a=b - That is my goal but IF a user had chosen a non-offset arrangement then it is right to say that IF  i=j THEN a≠b SHOULD be true? 

It is true if i=j then a≠b. But this statement is true regardless whether the secondary mirror was mounted centrally or with an offset. 

 

You stated: e=f - Incorrect labelling on my diagram. See below

We are in agreement. 

You stated: g=h  - Added to represent a non-centred secondary and therefore highlight an error.

Above statement is true regardless whether the secondary mirror was mounted centrally or with an offset.

For the most part, there is no difference in the collimation view between a scope with its secondary mirror mounted centrally or with an offset. Interestingly, the only difference is the apparent relative placement of one set of the vanes. For a scope with a secondary mirror mounted centrally, the vanes will appear centered with respect to the secondary shadow (figure 2 below). For a scope with a secondary mirror mounted with an offset, the vanes will appear centered with respect to the secondary mirror (figure 1 below). For example, I can tell the secondary mirror was mounted with an offset in your illustration.

Below is the view through my scope with the secondary mirror mounted centrally:

Whether the secondary mirror mounted centrally or with an offset, the optical axis of the primary mirror will intersect the secondary mirror at the same spot, hence, the view (with the exception of the vanes) will look the same.

"New Model" figure corresponds to a scope with a secondary mirror mounted centrally

"Classical Offset" figure corresponds to a scope with a secondary mirror mounted with an offset.

 

Edited August 15, 2021 by Jason D

[ispookie666]

Wowsers 🤪

[Alan64]

I do hope you get the telescope collimated to where you might enjoy powers in excess of 300x, and not just on the Moon.  The 200P-DS is a really nice Newtonian. 

[Spile]

On 14/08/2021 at 06:35, Jason D said:

You stated: a=b - That is my goal but IF a user had chosen a non-offset arrangement then it is right to say that IF  i=j THEN a≠b SHOULD be true? 

It is true if i=j then a≠b. But this statement is true regardless whether the secondary mirror was mounted centrally or with an offset. 

 

You stated: e=f - Incorrect labelling on my diagram. See below

We are in agreement. 

You stated: g=h  - Added to represent a non-centred secondary and therefore highlight an error.

Above statement is true regardless whether the secondary mirror was mounted centrally or with an offset.

For the most part, there is no difference in the collimation view between a scope with its secondary mirror mounted centrally or with an offset. Interestingly, the only difference is the apparent relative placement of one set of the vanes. For a scope with a secondary mirror mounted centrally, the vanes will appear centered with respect to the secondary shadow (figure 2 below). For a scope with a secondary mirror mounted with an offset, the vanes will appear centered with respect to the secondary mirror (figure 1 below). For example, I can tell the secondary mirror was mounted with an offset in your illustration.

 

Below is the view through my scope with the secondary mirror mounted centrally:

 

Whether the secondary mirror mounted centrally or with an offset, the optical axis of the primary mirror will intersect the secondary mirror at the same spot, hence, the view (with the exception of the vanes) will look the same.

"New Model" figure corresponds to a scope with a secondary mirror mounted centrally

"Classical Offset" figure corresponds to a scope with a secondary mirror mounted with an offset.

 

 

 

Thanks Jason
In future when I refer to the diagram I will mention that this is for an offset secondary setup. I think trying to show a non-centered secondary and an offset secondary on a single diagram is going to take away the simplicity I was trying to achieve.

Regarding...  g=h  - Added to represent a non-centred secondary and therefore highlight an error.

"Above statement is true regardless whether the secondary mirror was mounted centrally or with an offset."

But not if the secondary reflection (x2) were elliptical indicating a secondary  error?

 

Edited August 16, 2021 by Spile

[Jason D]

"single diagram is going to take away the simplicity I was trying to achieve"

My suggestion is to show a diagram without mentioning whether the secondary was mounted centrally or with an offset. Just don't mention how the secondary mirror was mounted since such an info will have no significance on collimation. That will keep it simple.

"But not if the secondary reflection (x2) were elliptical indicating a secondary  error?"

Well, the "error" has to be taken within context. Achieving axial alignment is the most important goal of collimation. Axial alignment is achieved when the eyepiece axis points directly at the primary mirror center and the optical axis of the primary mirror points back at the eyepiece center. The secondary shadow elliptical "error" you have referenced has nothing to do whatsoever with axial alignment. In that sense, the "error" impact could range from nothing to something. If the elliptical shape is minor and the scope is used for visual observation then the "error" can be safely ignored. If it is major and/or the scope is used for astrophotography then it needs to be corrected. The elliptical "error" only impacts the illumination of the FOV which impacts astrophotography more than visual observation.

But sometimes the elliptical "error" is not an indication of a secondary mirror rotation/tilt error but rather an indication of a slightly off-centered secondary mirror in the OTA and/or a non-squared focuser. In this case, achieving axial alignment will demand rotating/tilting the secondary mirror to achieve axial alignment which will give its shadow an elliptical shape. 

Check the attached animation. In each frame, my scope has achieved axial alignment yet only one frame includes the optimal placement of the secondary mirror. My scope happened to be mechanically aligned so the optimal placement will not show an elliptical shadowy shape of the secondary mirror. If I intentionally adjust the vanes of my scope to shift the whole secondary mirror assembly to the left or right perpendicular to the focuser axis then the optimal placement of the secondary mirror will show a slight elliptical shadowy shape.

Jason