Collimatian thread #7475

[swiss75]

Evening all, should the secondary mirror on my heritage 130p appear circular when viewed via a cheshire?

 

At present it is oval which concerns me as for all of the research I've done thus far no images show this

[Ricochet]

Yes, it should appear to be circular. It is best to block off the view of the primary mirror when checking the shape of the secondary as the offset reflections confuse things. 

[Charic]

The secondary mirror is physically oval in shape, but when adjusted into the correct position and tilt it should appear circular looking down the focuser, or down the Cheshire, and if its central to the focuser ( which it should be ) it will be concentric and equally spaced in comparison to the  wall/edge of the focuser tube.

In simple terms, the process involves getting the secondary mirror central to the focuser tube, then rotation and tilt if required to see the face of the secondary, which in-turn will show the mirror clips, when viewed down the focuser tube, then the primary is aligned as the last step in the process.

This  link is a good example of the procedures to follow  http://www.astro-baby.com/collimation/astro babys collimation guide.htm

Welcome to the SGL.

[MarsG76]

YES, definitely circular.... otherwise it's not exactly under the focuser at 45 degrees.

[Ally8446]

I have practiced a little plagiarism here but this may give you an idea of what you're trying to achieve. 

[swiss75]

Thanks all, after 2 hours last night the secondary is less oval than it was before

 Just cannot seem to get the mirror circular and the three clips together, it is either one or the other then a compromise...slightly oval but with apparent alignment as per crosses in the third reflection pic above

 

Do you have to 'tune' the depth of the cheshire to the secondary? As when it is pushed in as far as it will go the area in view is too close as to have a bottom edge like the first reflection pic above, I did think I'd be clever and just adjust it out but the holder allows movement and to my mind cannot be trusted

 

The scope itself still gives good views, well, as good as the class 7 skies I live under will allow but it is niggling me that it could be better.

[Charic]

41 minutes ago, swiss75 said:

 

Do you have to 'tune' the depth of the cheshire to the secondary? As when it is pushed in as far as it will go the area in view is too close as to have a bottom edge like the first reflection pic above, I did think I'd be clever and just adjust it out but the holder allows movement and to my mind cannot be trusted

 

I prefer a long Cheshire for the tighter accuracy it provides, but nothing wrong with adjusting the focuser out so that it raises the Cheshire, or raise the Cheshire alone if there's a way to secure it. There is often mention of a 'shorty' Barlow, its just a shorter tube!

There's no optics in a Cheshire, its just a tool to align your eye to the axis, and to align the axis of the mirrors, so raising the Cheshire should not be a big issue. 

[Ricochet]

I would set the Cheshire at a height out of the focuser that allows you to see the whole secondary. If you're worried about it being tilted in the focuser then finding a way to set the telescope up with the focuser pointing vertically is probably the best bet. 

[25585]

I find collimating the secondary more frustrating than the primary.

[Charic]

23 minutes ago, 25585 said:

I find collimating the secondary more frustrating than the primary.

Frustrating maybe, but once done, its unlikely it will need adjusting for some time thereafter. There is more to adjust with the secondary, but its normally a case of set and forget!

I outlined above but the method I adopt is to loosen sufficiently or remove  completely the three tilt adjusters, then tighten the centre bolt 'clockwise' until no more movement is possible. This procedure pulls the secondary to the face of the spider, and squares the mirror to the spider.

Then, slowly turn the central bolt 'anticlockwise', this does the opposite, by moving  the secondary towards the primary mirror, I  continue with this  adjustment only as far as necessary, which is the point  where the secondary mirror appears to be central when viewed through the focuser.

I continue by tightening or re-inserting the tilt adjusters  until they just touch the back of the secondary mirror, this ensures me that the mirror is still squared. It may  need a  slight amount of  rotation  by hand (without touching the mirrors surface) in order to produce that circular effect ( the mirror  is oval by design )  when you view the mirror through the focuser tube. Now the mirror should be pretty central to the focuser and concentric with the focuser tube, circular in appearance.

Further, by  gently adjusting the tilt adjusters ( doesn't matter which one of the three I start with ) I'll just try one, then another and so on! The task is to tilt the mirror in order to see the reflection of the primary mirror, and to visually see the  three primary mirror clips in the outer field of the reflected view.

Once aligned, taking the above into consideration, the secondary is  now 'set' and should need no further adjustments for some time if your cautious and gentle with the handling of your scope. 

Collimation then continues by further aligning the tilt of the primary to the secondary.
Owning a Newtonian comes with a requirement to understand and carry out this task without fear, and once you've done it once or twice, it does become easier to understand, and quicker to complete?

[swiss75]

This is the absolute best I can get it, one pic is via the cheshire and the other via a collimation cap, this is also probably the 10th attempt tonight and took an hour 

 

Would be great if folk can tell me what I'm doing wrong because if I read another guide saying it's quite simple I will most likely implode

[swiss75]

3 minutes ago, Charic said:

Frustrating maybe, but once done, its unlikely it will need adjusting for some time thereafter. There is more to adjust with the secondary, but its normally a case of set and forget!

I outlined above but the method I adopt is to loosen sufficiently or remove  completely the three tilt adjusters, then tighten the centre bolt 'clockwise' until no more movement is possible. This procedure pulls the secondary to the face of the spider, and squares the mirror to the spider.

Then, slowly turn the central bolt 'anticlockwise', this does the opposite, by moving  the secondary towards the primary mirror, I  continue with this  adjustment only as far as necessary, which is the point  where the secondary mirror appears to be central when viewed through the focuser.

I continue by tightening or re-inserting the tilt adjusters  until they just touch the back of the secondary mirror, this ensures me that the mirror is still squared. It may  need a  slight amount of  rotation  by hand (without touching the mirrors surface) in order to produce that circular effect ( the mirror  is oval by design )  when you view the mirror through the focuser tube. Now the mirror should be pretty central to the focuser and concentric with the focuser tube, circular in appearance.

Further, by  gently adjusting the tilt adjusters ( doesn't matter which one of the three I start with ) I'll just try one, then another and so on! The task is to tilt the mirror in order to see the reflection of the primary mirror, and to visually see the  three primary mirror clips in the outer field of the reflected view.

Once aligned, taking the above into consideration, the secondary is  now 'set' and should need no further adjustments for some time if your cautious and gentle with the handling of your scope. 

Collimation then continues by further aligning the tilt of the primary to the secondary.
Owning a Newtonian comes with a requirement to understand and carry out this task without fear, and once you've done it once or twice, it does become easier to understand, and quicker to complete?

I did try this method but the mirror flops slightly and tighten the screws to try and eliminate this. I'll try again tomorrow as my wife is now getting sick of me and the scope on the kitchen table

[Charic]

Its quite likely that you will need to hold the mirror while adjusting or tightening screws.

Before you do much more with your present setup ( which is looking good) Check out  'Airy Disk' and 'Star test'. Carry out a Star test, just to see how much out the setup is (or not).

I don't recommend you strip it right down tonight, but a full strip of the spider and secondary helped me to better understand how that assembly works. 

This video shows a modification that can be done by the end user, especially  if your into DIY, but forget the mod itself, just take a look and note the parts of the spider and the mirror itself. It helps to better understand the assembly and how to assemble and adjust things. https://www.youtube.com/watch?v=MUE8fxqH3Qc

[swiss75]

Cheers Charic, I have seen that video* - and am a metal worker  - so would be easy for me to do. I have checked the central position of the secondary holder with a set of dividers and it was off slightly so re-centered it and also adjusted it so the face is square to the front of the scope also, I had the secondary off last night to 'paint' the edges and back of the mirror with a black marker pen 

 

* In amongst hundreds i've watched trying to understand collimation

[cletrac1922]

Never used a Cheshire

Use a laser collimator

Align secondary mirror so laser is located dot on primary

Then slightly loosen mounting screws and adjust primary

Ensure when retightening mounting screws, primary does not move out of focus

When come to secondary, never, never loosen centre screw

Entire secondary mirror can swivel out of alignment

 

 

[JOC]

I found it was 100% necessary to hold onto the secondary whilst trying to tighten the screws - if nature had provided me with three arms and shorter legs it may have been easier.  What I found was that I could hold the secondary perfectly in place, but as I tightened the screws the whole thing moved out of alignment.  I too recall many tries and much (MUCH!) time spent trying to overcome this issue (I also recall near tears at the thought I had wrecked this expensive object I had bought) - your story very much mirrors (Sorry !!) my first and, so far, only attempt.  In the finish I found, through trial and error, how much extra movement I got when tightening the screws and found I succeeded when I accommodated this extra movement into the position I held the mirror at before I finally moved and tightened the screws - then I allowed the screws to make the final tightened position and the mirror ended up where I wanted it.  In the finish mine looked identical to Astrobaby's picture - even down the slight oval-ness that she notes she always got with her telescope.  I guess I was lucky that I was doing it on the same type of scope.

[david_taurus83]

I too have had a mare of a time collimating over the last couple of nights. I removed both mirrors to give them a much needed wash and because I've just bought a ZWO ASI120 camera I thought it would be easy to place this in the focuser, fire up Sharpcap on the laptop and use the reticle function to perfectly align the secondary mirror. Oh no!! Camera, Cheshire collimator and laser all give different results! There's simply too much play at the top of the focuser between all 3 to accurately judge the exact centre, and I have a focuser with compression ring tighter at the top. I gave up at 1am Monday night! I got better results by simply aligning the mirror by eye in the focuser tube getting it as central as possible, used Sharpcap and camera then by moving the circular reticle over the edge of the mirror making sure it was as round as possible and finished off the process by using the Cheshire. It's not perfect and I don't think it ever will be but if I can get sharp stars and refocus to get round concentric rings then I'll be happy with that. What a fun hobby we have embarked upon!!

[25585]

More critical too for shorter FL mirrors I believe, though they have larger secondary mirrors. At least big mirrors have larger nuts and bolts.

Reminds me of one reason I have a SCT and fracs. 

[swiss75]

On 16/01/2018 at 23:10, cletrac1922 said:

Never used a Cheshire

Use a laser collimator

Align secondary mirror so laser is located dot on primary

Then slightly loosen mounting screws and adjust primary

Ensure when retightening mounting screws, primary does not move out of focus

When come to secondary, never, never loosen centre screw

Entire secondary mirror can swivel out of alignment

 

 

Inspiring stuff mate 

 

Seems I got it right on my own anyway

[alanjgreen]

13 hours ago, swiss75 said:

Seems I got it right on my own anyway

Now you are happy, here is a tip to keep the secondary and primary in about the same place when you re-collimation in future.

Both mirrors have three adjustment screws. Choose 2 of them and only fine tune the collimation using those two from now on. Because you never touch the third screw, the mirrors should not ride up or down the tube.

(adjusting both of the chosen two is the same as adjusting the one you are choosing to avoid - if that makes sense)

works for me  

Alan