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Collimation star shape test - help with strange result please

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#1 mikeoday005

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Posted 08 March 2018 - 10:55 PM

I think I have a problem with alignment somewhere in the image chain of my f4 Newtonian telescope and I am hoping someone recognises the symptom I have discovered.

 

Recently I tweaked the primary mirror tilt of my scope while observing the shape of an unfocused star centred in the field and I observed a curious result.

 

I de-focused the star by moving the focal plane in and then tilted the primary mirror until I obtained a view similar to the left hand ‘sketch’ below.  I then refocused on the star and continued to move the focus out until the star was again de-focused but this time with the focal plane on the other side of the focus point.  The right-hand ‘sketch’ below shows what I saw. 

 

I expected the defocused star to look the same regardless of which way I moved the focuser but apparently this is not the case.

Has anyone come across this before?  Do you know what might be causing it?

 

( From memory, the movement of the center circle was parallel to the telescope tube. )

Attached Thumbnails

  • star test.jpg


#2 zawijava

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Posted 08 March 2018 - 11:09 PM

This may or may not be a collimation issue. What is the focused star test showing? Even with a well collimated scope you can get the image on the right simply due to the defocused star not being precisely centered in the field of view. What is your overall collimation process? Anything like this [from AstroSystems website]?.....

 

https://www.astrosys...iz/COLINST3.pdf

 

also, some good "collimation" Threads recently this Forum, check them out yet?


Edited by zawijava, 08 March 2018 - 11:14 PM.


#3 jim kuhns

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Posted 08 March 2018 - 11:26 PM

It appears that the focuser draw tube,  is tilting away from center of secondary as you are focusing out.

Also check to see if your focuser has a adjustment knob on the back to put some tension against the tube. 

Check and make sure the focuser is secured well to the tube. 


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#4 mikeoday005

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Posted 08 March 2018 - 11:55 PM

 Thanks for your reply - answers in red below.

 

This may or may not be a collimation issue. What is the focused star test showing?

 

A focused star is showing quite strong astigmatism - that is, a centred and focused star is a cross not a circle when the air is very still.  So I possibly have a number of problems frown.gif

 

Even with a well collimated scope you can get the image on the right simply due to the defocused star not being precisely centered in the field of view.

 

For this test the star was very close to the centre of the field of view. 

 

What is your overall collimation process?

 

 I generally use a laser collimator but I think over the weekend I will dig out my Cheshire and try to confirm that the secondary is the right height.

 

Anything like this [from AstroSystems website]?.....

 

https://www.astrosys...iz/COLINST3.pdf

 

also, some good "collimation" Threads recently this Forum, check them out yet?

 



#5 Jason D

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Posted 09 March 2018 - 02:47 AM

What you are observing is a collimation issue. Your scope's axial alignment is off. Unfortunately, collimating using a defocused star is not as reliable and as easy as many would claim. I suggest you use a quality collimation tool to do the job then use star collimation to make just fine adjustments -- if any adjustments are needed.

 

Here is an interesting thread about star collimation.

 

https://www.cloudyni...mation-mistake/

 

Jason


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#6 mikeoday005

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Posted 09 March 2018 - 03:01 AM

What you are observing is a collimation issue. Your scope's axial alignment is off. Unfortunately, collimating using a defocused star is not as reliable and as easy as many would claim. I suggest you use a quality collimation tool to do the job then use star collimation to make just fine adjustments -- if any adjustments are needed.

 

Here is an interesting thread about star collimation.

 

https://www.cloudyni...mation-mistake/

 

Jason

Yes I agree about you comment on using a defocused star; I was just using this to tweak.

 

Would you be able to explain ( in simple terms smile.gif ) what you mean by "... scope's axial alignment is off"?  Do you mean that the secondary is too low / high in the tube or could it be one of a number of problems?  

 

Thanks

 

............

 

updated:

 

I just looked through the thread you linked to ( thanks it is a great help ).

 

In my experiment above I now realise ( just as in your example of a highly de-focused star when the secondary is off ) that I could only see two primary mirror clips and these flipped to the other side when the de-focus was "on the other side".  So I guess this  indicates that I might have a secondary alignment issue (?).  


Edited by mikeoday005, 09 March 2018 - 03:27 AM.


#7 Jason D

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Posted 09 March 2018 - 03:56 AM

Secondary mirror being little too low or little too high should not have any impact on star collimation.

 

Axial alignment is made up of two alignments:

1- Focuser axial alignment is achieved when the secondary mirror is adjusted in such a way to deflect he focuser axis to strike the primary center.

2- Optical axial alignment is achieved when the primary mirror is adjusted in such a way that places the focal point of the primary mirror along the focuser axis (after deflecting the optical axis via the secondary mirror)

 

Focuser axial alignment eliminates any focal planar tilt between the primary mirror and eyepiece .

Optical axial alignment ensures both the primary mirror and eyepiece foci points coincide to minimize coma.

 

What you are encountering is a scope with bad focuser axial alignment. At one side of focus, both the foci points of the eyepiece and the primary mirror are very close hence the acceptable defocused star you have shown. At the other side of focus, both foci points are far apart hence the bad defocused star you have shown.

 

What you need to do is use either a laser collimator or a sight-tube with cross-hairs to fine adjust the secondary mirror first then use star collimation.

 

Jason



#8 PETER DREW

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Posted 09 March 2018 - 04:24 AM

I would be more concerned about the in focus astigmatism that you describe @Mikeyoday005. this would have a significant affect on the performance. Check if the primary mirror is held too tight, you should be able to rotate it without effort.



#9 mikeoday005

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Posted 09 March 2018 - 08:41 PM

Secondary mirror being little too low or little too high should not have any impact on star collimation.

 

Axial alignment is made up of two alignments:

1- Focuser axial alignment is achieved when the secondary mirror is adjusted in such a way to deflect he focuser axis to strike the primary center.

2- Optical axial alignment is achieved when the primary mirror is adjusted in such a way that places the focal point of the primary mirror along the focuser axis (after deflecting the optical axis via the secondary mirror)

 

Focuser axial alignment eliminates any focal planar tilt between the primary mirror and eyepiece .

Optical axial alignment ensures both the primary mirror and eyepiece foci points coincide to minimize coma.

 

What you are encountering is a scope with bad focuser axial alignment. At one side of focus, both the foci points of the eyepiece and the primary mirror are very close hence the acceptable defocused star you have shown. At the other side of focus, both foci points are far apart hence the bad defocused star you have shown.

 

What you need to do is use either a laser collimator or a sight-tube with cross-hairs to fine adjust the secondary mirror first then use star collimation.

 

Jason

Thanks Jason for your detailed reply.  I have readjusted the secondary and re-collimated with my laser.  I will check it tonight to see if I have managed to Improve things ( or made them worse, which would not be the first time :) ).  



#10 mikeoday005

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Posted 09 March 2018 - 08:45 PM

I would be more concerned about the in focus astigmatism that you describe @Mikeyoday005. this would have a significant affect on the performance. Check if the primary mirror is held too tight, you should be able to rotate it without effort.

 

Thank Peter.  Yes, I am more concerned about the astigmatism, which seems to be quite bad.  I am hoping that releasing the pressure on the secondary adjustment screws might have helped ( I read somewhere that this was one person's main issue ).  Removing/rotating the main mirror single handed is a major undertaking ( I can't remember its exact weight but it is a significant percentage of the 26kg OTA weight ) and I would like to eliminate everything else first.



#11 SeaBee1

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Posted 10 March 2018 - 07:07 PM

Are you sure it's astigmatism? Could it be diffraction spikes from the spider?

 

CB



#12 Asbytec

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Posted 10 March 2018 - 07:59 PM

Focuser axial alignment eliminates any focal planar tilt between the primary mirror and eyepiece .

 

This. 

 

Adjust the secondary tilt so the focuser axis is pointing at the center of the primary mirror to within 3% of the primary aperture. Your center spot may be a little off or the laser, if any, is not well collimated. This way, when you focus, the eyepiece moves along the collimated primary optical axis. It may be some registration problem in the eyepiece adapter, as well. The eyepiece tilts a little when the set screw is tightened or the adapter has some slop to it. 



#13 mikeoday005

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Posted 10 March 2018 - 08:46 PM

Are you sure it's astigmatism? Could it be diffraction spikes from the spider?

 

CB

Thanks CB.  

 

I don't know if the issue I described in the first post above is related to or impacted by astigmatism.  The consensus above is that it is a "simple" alignment problem.  Having said that, I definitely have a astigmatism problem as well.  On one side of focus all stars throughout the field are oval in the same direction and on the other side of focus they are oval but 90deg rotated.  At focus the stars usually appear as circles but this is mainly due to poor seeing.  The other night I had a very rare night of excellent seeing and the stars had definite cross like shape.  The image below is not mine - but it could have been as this is pretty much what I see.

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  • start-test-astigmatism-c.jpg

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#14 mikeoday005

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Posted 10 March 2018 - 09:04 PM

This. 

 

Adjust the secondary tilt so the focuser axis is pointing at the center of the primary mirror to within 3% of the primary aperture. Your center spot may be a little off or the laser, if any, is not well collimated. This way, when you focus, the eyepiece moves along the collimated primary optical axis. It may be some registration problem in the eyepiece adapter, as well. The eyepiece tilts a little when the set screw is tightened or the adapter has some slop to it. 

Thanks for that.

 

I adjusted the secondary yesterday and re-collimated using my laser.  If anything I think I made it a little worse (  as if often the case when I start tinkering frown.gif ).  No impact on the astigmatism ( I was hoping that reducing the pressure on the secondary adjustment bolts might help, but no ) and the weird effect above is still there.

 

The contour plot below of the flat I produced last night clearly shows a very strong offset.  The camera's long axis is parallel with the tube with the left hand side of the image below nearest the primary mirror.  Am I right in thinking that I need to lower the secondary?

 

With regard to the astigmatism,I pulled off the primary mirror today it is not bound up on the clips.  I am beginning to think that the primary mirror may have been put out of shape slightly by storing the telescope with the Dec axis rotated 180deg so that the open end of the tube pointed down by 30deg (I did this so it would fit in the shed ) .  I have now found a combination of RA and DEC rotation that (just) allows the scope to fit in the shed with it pointing nearly 30deg up.  Maybe this will help over time.

 

( contour plot from master flat )

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  • integration_CFA2_G_contourPlot.jpg

Edited by mikeoday005, 11 March 2018 - 12:19 AM.


#15 Asbytec

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Posted 10 March 2018 - 09:35 PM

Secondary off set, lowering the secondary, will improve field illumination. Its likely not a cause of the collimation problem. Any astigmatism may be in the diagonal itself, stressed by it's attachment to the spider. I doubt collimation screws are causing it as they dont contact the glass.

#16 SeaBee1

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Posted 10 March 2018 - 10:21 PM

Yeah, looking at those images, Mike, I would probably say astigmatism... definitely not diffraction spikes... that stinks!

 

I would suspect the primary being astigmatic before the secondary... although I have heard that a poor secondary can be astigmatic as well.

 

Maybe consider having the primary tested and re-figured if found bad? Just throwing it out there...

 

CB



#17 mikeoday005

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Posted 11 March 2018 - 12:14 AM

Secondary off set, lowering the secondary, will improve field illumination. Its likely not a cause of the collimation problem. Any astigmatism may be in the diagonal itself, stressed by it's attachment to the spider. I doubt collimation screws are causing it as they dont contact the glass.

I just lowered the secondary ( thanks ) and re-collimated.  Centring is much improved smile.gif

 

Collimation with the laser seems ok and looking through the Cheshire everything seems aligned so fingers crossed for tonight.

 

( contour plot from a single flat frame )

Attached Thumbnails

  • DSC_0288_CFA2_G_contourPlot.jpg

Edited by mikeoday005, 11 March 2018 - 12:19 AM.


#18 mikeoday005

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Posted 11 March 2018 - 12:18 AM

Yeah, looking at those images, Mike, I would probably say astigmatism... definitely not diffraction spikes... that stinks!

 

I would suspect the primary being astigmatic before the secondary... although I have heard that a poor secondary can be astigmatic as well.

 

Maybe consider having the primary tested and re-figured if found bad? Just throwing it out there...

 

CB

I pulled off the primary and it was not bound up on the clips.  However, when I just collimated I had to turn one of the bolts two complete turns to re-point the primary so maybe something freed up just by removing it.  Here's hoping ...


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#19 Asbytec

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Posted 11 March 2018 - 12:37 AM

Thanks CB.  

 

I don't know if the issue I described in the first post above is related to or impacted by astigmatism.  The consensus above is that it is a "simple" alignment problem.  Having said that, I definitely have a astigmatism problem as well.  On one side of focus all stars throughout the field are oval in the same direction and on the other side of focus they are oval but 90deg rotated.  At focus the stars usually appear as circles but this is mainly due to poor seeing.  The other night I had a very rare night of excellent seeing and the stars had definite cross like shape.  The image below is not mine - but it could have been as this is pretty much what I see.

The image above is a separate problem from your axial alignment and field illumination, the latter seems fixed and you can recheck the former. I'd start with the diagonal mounting on the spider. It may be applying some stress to the secondary mirror. 



#20 mikeoday005

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Posted 11 March 2018 - 03:05 AM

The image above is a separate problem from your axial alignment and field illumination, the latter seems fixed and you can recheck the former. I'd start with the diagonal mounting on the spider. It may be applying some stress to the secondary mirror. 

Yes thanks, I know they are two problems that I am talking about at the same time and confusingly interchangeably,  smile.gif

 

I will have a look tonight and see what the astigmatism looks like ( cross-fingers, maybe removing and replacing the primary may have relieved some pressure and the astigmatism will be miraculously gone ).  If it is the spider then I have a bigger problem;  my lack of mechanical skills - correcting it would be well beyond me and I would have to send it to someone.

 

By the way, back to the alignment issue ...

 

Am I reading the Cheshire results correctly, does this look reasonably aligned to you?

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  • Cheshire.jpg


#21 Asbytec

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Posted 11 March 2018 - 03:40 AM

From what I can tell, it looks good. But, I am suspicious. I do not see the secondary offset. At f/4, your secondary mirror reflection should be visibly offset to one side with all other circles concentric to the cross hairs. With f/4 offset, the secondary shadow will not be centered on the crosshairs. Being that you showed an image of a centered fully illuminated field above, and no visible offset to account for it, something is not right. Somehow you aimed the fully illuminated field to the center of the FOV without the required secondary offset toward the primary and away from the focuser. One of the axes is tilted. Maybe both. 


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#22 mikeoday005

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Posted 11 March 2018 - 08:35 AM

From what I can tell, it looks good. But, I am suspicious. I do not see the secondary offset. At f/4, your secondary mirror reflection should be visibly offset to one side with all other circles concentric to the cross hairs. With f/4 offset, the secondary shadow will not be centered on the crosshairs. Being that you showed an image of a centered fully illuminated field above, and no visible offset to account for it, something is not right. Somehow you aimed the fully illuminated field to the center of the FOV without the required secondary offset toward the primary and away from the focuser. One of the axes is tilted. Maybe both. 

 

Mmm ...

 

I have never altered the offset - it was set in the factory.  I might see how I go with what I have for now - at least it is better than I had before smile.gif .  Messing with the spider vanes or worse still, pulling apart the secondary to change the offset, fills me with dread.

 

I have been out tonight.  The star shape seems pretty good throughout the field.  The astigmatism is still there, although, it might just be my imagination but I think it is somewhat less than before.  Perhaps just wishful thinking.  

 

Cheers

Mike


Edited by mikeoday005, 11 March 2018 - 10:57 PM.


#23 Starman1

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Posted 13 March 2018 - 01:25 PM

Your image should look like this through the focuser.  Note the offset to the secondary shadow:

 

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#24 mikeoday005

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Posted 14 March 2018 - 02:56 AM

Your image should look like this through the focuser.  Note the offset to the secondary shadow:

Thanks for that.  How do I adjust the scope to get that view?  I do not want ( know how ) to remove the secondary mirror from the spider and re-glue it place.  So other than that, is there a way to offset the secondary?  I suppose I could try to adjust the spider vane bolts to move the centre of the spider but won't that cause the vanes to become out of alignment and hence produce split diffraction spikes?



#25 mikeoday005

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Posted 14 March 2018 - 03:02 AM

Here is the contour plot of the master flat.  The cross shows the geometric centre of the frame.  The horizontal axis of the image is aligned with long axis of the tube.

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  • integration_CFA2_contourPlot.jpg



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