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Old 24-05-2018, 01:38 AM
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Windston (Dan)
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Advanced methods for Newtonian Collimation

Hi guys,

Just really struggling with collimation of my Newtonian at the moment. I think that I have nailed it through the cheshire, but I go out and its off! Its not off by a huge amount but stars on the edge of the frame are always slightly different to one another. Also I can never seem to get the diffraction spikes perfect! A real eye sore!

Anyway, this is all done with a Chesire/Sight Tube combination from Astrosystems. I think the problem is when they say stuff like, 'make the secondary a perfect circle underneath the focuser', but for imaging, the max error allowed is so small that im sure that I wont be doing it good enough. Same goes for both primary and secondary tilt collimation. The only part of collimation that im at least kinda certain on was centering the spider vanes, because I could use calipers! And that gives a definitive amount to move something by! Not just, awww yea mate, that looks centered to me when I hold my head this way!

'Make sure the clips of the primary are all equal in the secondary', but there is again just so much room for error there without you even knowing it.
I have an Autocollimator coming any day now, which might help, but I feel like I am missing something. Ideally a real time graph of some sort made by the camera would be best, no error introduced by changing from Cheshire to camera etc. Something like the FWHMeccentricty script that PixInsight produces.

I cant find any information online regarding proper, high quality newtonian collimation. And I know that it exists cos I see it in images everywhere! So either someone is holding onto their secret sauce or I just suck at collimation!

Am I crazy or just bad at collimation!?

Dan

EDIT: Here is what I am talking about. https://i.imgur.com/js7EWdZ.png, you can see that the right hand side of the image is causing the troubles, and that might be linked to the spider vanes splitting on the right side as well.

Last edited by Windston; 24-05-2018 at 01:54 AM.
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Old 24-05-2018, 06:34 AM
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erick (Eric)
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Keep reading what is out there Dan. Here are some articles in case you haven't found these:

http://garyseronik.com/a-beginners-g...o-collimation/

https://www.cloudynights.com/article...scope-v4-r2599

A good autocollimater is a good step forward.

https://stargazerslounge.com/topic/1...utocollimator/

Perhaps a set of Catseye tools? Supplement a cheshire with a quality XLK autocollimator? And you might wish to change your primary mirror centre marker if it is the original circular style.

https://www.catseyecollimation.com/

DO you trust Mike? http://www.pbase.com/strongmanmike20...th_the_catseye

As has been said elsewhere, get a strong cup of coffee and tackle Jason Khadder's Cloudy Nights thread:

https://www.cloudynights.com/topic/2.../#entry3189737
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Old 24-05-2018, 08:14 AM
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Quote:
Originally Posted by erick View Post

As has been said elsewhere, get a strong cup of coffee and tackle Jason Khadder's Cloudy Nights thread:

https://www.cloudynights.com/topic/2.../#entry3189737


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Old 24-05-2018, 08:43 AM
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Jason D (Jason)
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Quote:
Originally Posted by Windston View Post
Also I can never seem to get the diffraction spikes perfect! A real eye sore!
Though it could be related to collimation, most likely it is not. A miscollimated scope could push a screw head into the lightpath inducing a spike but even for a perfectly collimated scope any mechanical intrusion into the lightpath will induce a spike.

Quote:
was centering the spider vanes, because I could use calipers! And that gives a definitive amount to move something by! Not just, awww yea mate, that looks centered to me when I hold my head this way!
Centering the spider vanes is less important than ensuring opposite vanes are inline or at least in parallel. If centering the spikes means introducing a small angle between opposite vanes then you will exacerbate spikes in your photo.

Quote:
'Make sure the clips of the primary are all equal in the secondary',
High precision is not required for this alignment. Ideally, you want to evaluate this alignment from a point where the primary apparent size is the same as the secondary mirror. Both should coincide as shown in the attachment. Then again, close enough is good enough for this alignment. Errors in this alignment will be manifested as an uneven illumination intensity in your photo. No spikes and no coma will be introduced with this error.

Quote:
I have an Autocollimator coming any day now, which might help,
Which one are you getting? Are you getting one of the new astro XLK autocollimators from Catseye?


Jason
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Old 24-05-2018, 12:34 PM
SteveInNZ
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If you want to go down the "what the camera sees" approach, the standard seems to be CCDInspector. I haven't used it myself but I'm considering it. There are similar functions in MaxPilote (free) and Prism ($$$). I don't think it replaces the tools and techniques above in determining what to adjust but does give you a quantified result at the end.

Steve.
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Old 24-05-2018, 07:14 PM
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Windston (Dan)
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Quote:
Originally Posted by Jason D View Post
Though it could be related to collimation, most likely it is not. A miscollimated scope could push a screw head into the lightpath inducing a spike but even for a perfectly collimated scope any mechanical intrusion into the lightpath will induce a spike.


Centering the spider vanes is less important than ensuring opposite vanes are inline or at least in parallel. If centering the spikes means introducing a small angle between opposite vanes then you will exacerbate spikes in your photo.

High precision is not required for this alignment. Ideally, you want to evaluate this alignment from a point where the primary apparent size is the same as the secondary mirror. Both should coincide as shown in the attachment. Then again, close enough is good enough for this alignment. Errors in this alignment will be manifested as an uneven illumination intensity in your photo. No spikes and no coma will be introduced with this error.


Which one are you getting? Are you getting one of the new astro XLK autocollimators from Catseye?


Jason
Wow! I am making my way though that thread now! A lot of information that is easily understood! Thanks!

I have just taken the scope arart to flock it, and during that time I gave the primary and secondary a good hit of air to get rid of some loose dust. Putting it back together, I didnt center the secondary, but I used a square sitting infront of the scope to make sure that the vanes were as close to parallel as can be, which will probably help, but I need to check some more.

I have a sighttube/cheshire combo, one question I have is, is it the same to align the crosshairs on the sighttube to the primary mirror centre dot as it is to make the 3 clips even around the outside? I did that today, the crosshair of the sightube is right over the primary dot, however when I rack the focuser in, the primary does not look centered at all on the secondary. I am thinking that it could be from a misalignment in secondary rotation?

And one last thing that I found when disassembling, was that the primary was actually loose in the cell! I could easily move it around with my hands, which confirmed the nagging feeling I had. When I moved it before, I always thought I could feel and hear the primary moving but never knew for sure! So now it is clipped in tight and shouldn't go anywhere!

I ended up going with an Autocollimator made by a local guy, it is just a single pupil as far as I know, but it also was only 40$ delivered (I think, it was somewhere around that much) so I figured I would give it a go. The Catseye tools were a lot of cash to spend, and I am just a uni student who left high school so I cant really justify that extra cost, yet!

I think it should work out OK, but that thread is amazing, again, thankyou! I dont know how i didnt see it before!

Dan
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Old 25-05-2018, 03:45 AM
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Quote:
Originally Posted by Windston View Post
.
I have a sighttube/cheshire combo, one question I have is, is it the same to align the crosshairs on the sighttube to the primary mirror centre dot as it is to make the 3 clips even around the outside?
Aligning the cross-hairs of the sight-tube with the primary center implies that the primary mirror is centered with respect to the focuser edge. It does not necessarily mean it is centered with respect to the secondary mirror. Can you take a photo using your cell phone through your sight-tube and post the photo in this thread?

Quote:
I did that today, the crosshair of the sightube is right over the primary dot, however when I rack the focuser in, the primary does not look centered at all on the secondary. I am thinking that it could be from a misalignment in secondary rotation?
Start off with a perfectly collimated scope with a well-centered primary mirror with respect to the secondary mirror. When you start racking in the sight-tube you will notice that the primary mirror reflection will slowly shift in the direction of the OTA bottom -- it is geometry. Is that consistent with what you noticed?


Quote:
So now it is clipped in tight and shouldn't go anywhere!
Be careful, when you tighten too much it is possible to temporarily deform the mirror paraboloid shape and end up introducing temporary astigmatism. A star test will tell if it is too tight. If the star test shows distortions around the clips then you will need to untighten the clips.

Quote:
I ended up going with an Autocollimator made by a local guy, it is just a single pupil as far as I know, but it also was only 40$ delivered (I think, it was somewhere around that much) so I figured I would give it a go. The Catseye tools were a lot of cash to spend, and I am just a uni student who left high school so I cant really justify that extra cost, yet!
An autocollimator is one of these tools that has to be of high quality; otherwise, it is not worth it. Catseye is well-known to provide high quality collimation tools. That does not mean the autocollimator you have does not meet the high standards of what is expected from an autocollimator. Do this simple test:
Collimate your scope using other tools. Insert the autocollimator in your focuser. Do you see multiple reflections of the center spot against a dark background? If yes, rotate the autocollimator slowly. If the locations of the multiple center spot reflections gitter or slighly move, that is OK. If reflections move wildly and all over the place or possibly disappear then the quality of your autocollimator is questionable.
Simply put, the autocollimator is supposed to give you that additional collimation precision; therefore, it has to meet a much higher quality standard than other collimation tools.

Jason
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Old 25-05-2018, 09:35 AM
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Hi Dan.
I hope by now your collimation woes are over. I just thought I'd add a few more bits to consider.
Primary mirror springs. All is good when you have the scope collimated standing still, then you move it and it's out again. The springs should be reasonably tight.
Primary mirror cell. Check all screw threads. Use blue Teflon tape on the screws which are a tad loose.
Focuser. Ensure it's seated firmly against the tube. Check the motion so it moves smoothly in and out without play.
Hope this helps. Cheers!
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Old 25-05-2018, 09:44 AM
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Thanks for asking this Dan!

I'm learning heaps from the links others have posted - I think it needs more than a strong cup of coffee though. And I had no idea about autocollimators etc. Might have to move on from my orion cheshire and ebay laser at some stage - once I figure it all out
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Old 29-05-2018, 10:47 PM
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Windston (Dan)
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Quote:
Originally Posted by Jason D View Post
Aligning the cross-hairs of the sight-tube with the primary center implies that the primary mirror is centered with respect to the focuser edge. It does not necessarily mean it is centered with respect to the secondary mirror. Can you take a photo using your cell phone through your sight-tube and post the photo in this thread?



Start off with a perfectly collimated scope with a well-centered primary mirror with respect to the secondary mirror. When you start racking in the sight-tube you will notice that the primary mirror reflection will slowly shift in the direction of the OTA bottom -- it is geometry. Is that consistent with what you noticed?



Be careful, when you tighten too much it is possible to temporarily deform the mirror paraboloid shape and end up introducing temporary astigmatism. A star test will tell if it is too tight. If the star test shows distortions around the clips then you will need to untighten the clips.



An autocollimator is one of these tools that has to be of high quality; otherwise, it is not worth it. Catseye is well-known to provide high quality collimation tools. That does not mean the autocollimator you have does not meet the high standards of what is expected from an autocollimator. Do this simple test:
Collimate your scope using other tools. Insert the autocollimator in your focuser. Do you see multiple reflections of the center spot against a dark background? If yes, rotate the autocollimator slowly. If the locations of the multiple center spot reflections gitter or slighly move, that is OK. If reflections move wildly and all over the place or possibly disappear then the quality of your autocollimator is questionable.
Simply put, the autocollimator is supposed to give you that additional collimation precision; therefore, it has to meet a much higher quality standard than other collimation tools.

Jason
I made sure not to pinch the mirror! I should be ok to test when the clouds leave me alone!

I also am still waiting on the auto-collimator, which im getting pretty sick of, so I might just get a catseye at this rate and be done with it!
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Old 29-05-2018, 11:52 PM
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floyd_2 (Dean)
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Just like everyone else has mentioned, you need a decent autocollimator. That's the point where you fine tune collimation generally with very small tweaks. Collimation needs to be fairly decent before you use the autocollimator. I used to place a match head into the light path during fine tuning with the autocollimator and tweak until the centre reflection flushed with the colour of the match head. Usually you'll see converging match stick reflections that don't quite meet in the centre when collimation is very close. Minor Secondary tweaking will generally solve this problem.

I didn't see it mentioned previously but it probably was - make sure to check that your focuser is spot on. If it's not, you may need to shim it until it's perfectly perpendicular to the light path. Some people will spot the tube immediately opposite the focuser to check this with the secondary removed.

Good collimation is very rewarding. Stick with it.
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