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Common star collimation mistake

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#1 Jason D

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Posted 02 March 2015 - 01:47 AM

This thread is targeted for beginners who are attempting star collimation.

 

Make sure to:

1- Use magnification >= 20X your scope's aperture

2- Defocus by a little amount -- enough to see 2-3 Fresnel rings

3- Evaluate collimation only with the star is located at the center of the FOV

 

One common mistake that beginners make is defocusing the star too much. Attached composite photo shows the problem. 

 

Left photo is for a star that is defoucsed too much (infocus). Note how all the rings are circular and the secondary shadow looks centered. A beginner will incorrectly conclude that the scope is collimated. 

Middle photo is for the same setup and star but with a little defocus (infocus). Coma is sever which is an indication the scope is badly miscollimated. 

Right photo is the the same setup. Note how far off the Hotspot is located from the Blackcat's ring.

 

In the above experiment, I started off with a well-collimated scope then twisted one of the primary mirror knobs several times. I have a 10" reflector and I used 200X mag in the photo.

 

Jason

 

 

star_collimation.jpg

 

 

 

 



#2 backwoody

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Posted 02 March 2015 - 10:32 AM

Instructive photos, Jason.  Thanks for the post.



#3 George N

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Posted 02 March 2015 - 03:49 PM

If you are using a Televue paracoor to observe, should the lens be used while collimating?



#4 choran

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Posted 02 March 2015 - 03:56 PM

I've always read that the Paracorr should not be used during collimation.  (There is also a non-optical reason: if one is not careful, the collimation tool will extend too far into the paracorr and break it.)



#5 choran

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Posted 02 March 2015 - 03:59 PM

This thread is targeted for beginners who are attempting star collimation.

 

Make sure to:

1- Use magnification >= 20X your scope's aperture

2- Defocus by a little amount -- enough to see 2-3 Fresnel rings

3- Evaluate collimation only with the star is located at the center of the FOV

 

One common mistake that beginners make is defocusing the star too much. Attached composite photo shows the problem. 

 

Left photo is for a star that is defoucsed too much (infocus). Note how all the rings are circular and the secondary shadow looks centered. A beginner will incorrectly conclude that the scope is collimated. 

Middle photo is for the same setup and star but with a little defocus (infocus). Coma is sever which is an indication the scope is badly miscollimated. 

Right photo is the the same setup. Note how far off the Hotspot is located from the Blackcat's ring.

 

In the above experiment, I started off with a well-collimated scope then twisted one of the primary mirror knobs several times. I have a 10" reflector and I used 200X mag in the photo.

 

Jason

 

 

attachicon.gifstar_collimation.jpg

I agree with your post.  I have wondered, however, why the star image, when too far in or our of focus, gives this false appearance of proper collimation?  Just a matter of scale, or what?  Why don't things stay lopsided, but instead falsely appear symmetrical/bullseye?



#6 Brian Carter

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Posted 02 March 2015 - 04:10 PM

Friends don't let friends star collimate.  it is too tedious, it is too hard to make sure the star is exactly centered (assuming you are using Polaris and the star isn't constantly drifting), and there are too many errors like the one you show above.

 

Friends tell their friends buy a cheshire.  Good friends chip in to fund a barlowed laser :)



#7 T1R2

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Posted 02 March 2015 - 04:12 PM

Choran, this is why I stress having a small ring pattern, because it will give a false reading as the OP has shown,  I'm sure you've seen my posts about this when helping newcomers with collimation, because they tend to use a ring pattern that is too large, and tell people the collimations looks good "they Think", but they are still getting blurry views, its either seeing conditions, thermal, too much magnification when viewing in focus images...but more often than not, its usually collimation with maybe a combination of seeing, and thermals.



#8 choran

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Posted 02 March 2015 - 04:19 PM

Yes, I agree with both you and Jason.  I just wonder WHY the large out of focus star gives that false appearance rather than maintaining the accurate picture given with the image that is just barely defocused.


Edited by choran, 02 March 2015 - 04:19 PM.


#9 Kipper-Feet

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Posted 03 March 2015 - 11:10 AM

Choran, I read your question in post #5 and repeated in post #8.

 

Thanks to Jason for starting this informative thread with his composite photos.  I for one have found it to be very instructive particularly in the light of our many PMs pertaining to the use of secondary mirror masks.

 

I cut the LH image out of the composite photo, enlarged it (zoomed-in so to say) and then brightened it up a bit.  When doing so I was also able to see the beginning of the coma effects around the perimeter on right hand side of the image.

 

To take a stab at answering your question Choran, and quite possibly subjecting myself to the ridicule of my peers on this forum, I'm going to suggest the following as I think it through for myself.  Please remember that I am not speaking for Jason here.  This is my contention only and I really do understand that the purpose of the intentional mis-collimation in the OP was to instruct, not to deceive.

 

I suspect that we are seeing the shadow of the secondary mirror almost perfectly centred, despite the intentional mis-collimation, due to the actual mis-collimation itself.  You will recall that Jason has intentionally mis-collimated the primary by turning just one of the screws.

 

I am of the opinion that if we were able to see the same defocused image before the intentional mis-collimation, we'd probably see an offset secondary shadow in the image.  In other words, when the single screw intentional mis-collimation was performed, it tilted the mirror in one direction only and that shifted the shadow inwards from an offset position to a more centralized position.  

 

Of course, this does not mean that the single screw mis-colimation could not also have caused the offset shadow to shift outwards the other way, thereby increasing its displacement from the centre.

 

We know from his other posts that Jason is using a secondary mirror in this 10" scope that is configured in accordance with the Modern Method.  In this Modern Method the secondary is configured without a radial offset.  Even so, in a perfectly collimated defocused scope at a much lower power say 60x, i.e. 6x per inch of the 10" mirror, we would still be able to see an apparent radial offset of the secondary which is caused by the tilt of the primary's optical axis away from the centre of the OTA and towards the focuser.  The single screw intentional mis-collimation of such a scope would probably cause the optical axis, and therefore the defocused offset secondary shadow, to shift inwards to a more centralized position.  

 

Perhaps, this low powered view of the defocused secondary shadow is also visible at the higher 200x power that Jason used in these photos.  For me, 200x is about the highest useful power that I can get under my sky conditions.  For others, 200x is only just the beginning as they target 400x - 500x with ease.  For these folk, a low powered star test would be at about 200x, a high powered star test would be at much, much higher powers.  It's all relative is it not.

 

Now, let the flaming begin!  And if not, let our learning continue.  The class awaits.



#10 choran

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Posted 03 March 2015 - 11:28 AM

I've read that the process of refractor collimation should also be carried out by only a minor amount (2-3mm of focuser travel, or whatever it takes to produce two or three rings, roughly).  I wonder if this is because, as in the case of a reflector, too much defocus will present a set of rings that appear concentric even when the scope is out of collimation.



#11 Nils Olof Carlin

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Posted 03 March 2015 - 12:53 PM

Yes, I agree with both you and Jason.  I just wonder WHY the large out of focus star gives that false appearance rather than maintaining the accurate picture given with the image that is just barely defocused.


By de-focusing, you see the image built-up with the contributions from the different mirror zones separated (this is why you defocus more to catch zonal errors!), but centered on a common center. (With so much defocus, we're now largely into the realm of ray optics.)
With coma, the zones no longer are centered on a common center - as you see in Jason's middle image, the zone contributions are shifted so that all contributions overlap at about 4 o'clock but are widely spread out towards 10 o'clock. This will happen in the highly defocused image too, but the shifts of the zonal contributions are much too small compared to the size of the total (highly) defocused image to be eyeballed - at this scale, the secondary offset might swamp the effects of coma (as Richard R suggests).

Not an easy or clear explanation, I know, but the idea is that the effects of (reasonable) coma are so subtle as to be obvious only very near focus. But there, they are indeed visible.

But let me reiterate Jason's message - trying to collimate by star pattern is meaningless if you defocus much (for instance, so much as to see the the secondary shadow clearly).

I have said it before - the only meaningful purpose of star collimation that I can think of is to verify the placement of the primary's spot. You might be the first to find that a spot carefully centered on the blank is not at the optical center.

Nils Olof

#12 Kipper-Feet

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Posted 03 March 2015 - 12:55 PM

Choran (referring to your post #10 since Nils Olof beat me in the line-up), even so, and this is why I went to the trouble of zooming-in on the LH image and then brightening it up a tad, you are not able to see any Fresnel diffraction rings in it, concentric or otherwise.  

 

Notice that even in the original LH image, you are able to see the shadows of the four mirror clips as well as those of the spider vanes.  

 

It's my guess that we're not seeing anything that could be called a high-powered, slightly defocused Airy disc in this LH image.  Rather, we're just seeing the grossly defocused image of the primary mirror's reflection in the secondary mirror. And that was Jason's exact point when pointing out the error typically made by beginning star collimators. And in the middle of that, the blurry reflection of the shadow of the now centralized secondary mirror.


Edited by Richard Roseweir, 03 March 2015 - 01:02 PM.


#13 Vic Menard

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Posted 03 March 2015 - 01:03 PM

Looking at Jason's star images and comparing them to those found in Suiter's Star Testing Astronomical Telescopes, the image showing the misalignment appears to be about 10 wavelengths defocused. For an f/5 scope, that's about 1mm focuser travel. At this distance, while gross misalignments are visible, it's almost impossible to fine align the primary mirror collimation (you would need to be much closer to focus, perhaps 1 to 3 wavelengths (0.11mm to 0.33mm defocus).

 

The image to the left clearly shows the silhouette shadows of the mirror clips, spider vanes, and the secondary mirror offset--way too much defocus. The diffraction visible in this image is caused by the various edges contributed by the primary mirror, secondary mirror and spider. The diffraction we're interested in is from a point source (in this case, a star). If you've ever used a Ronchi grating to test a sphere or parabola, both kinds of diffraction are visible (unless you use a slit instead of a pinhole or a star for the light source). And like the star test, the Ronchi reading is magnified and surface errors are much more obvious closer to focus...


Edited by Vic Menard, 03 March 2015 - 01:04 PM.


#14 choran

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Posted 03 March 2015 - 01:03 PM

All very interesting, and I wonder if the same concepts translate directly to the case of refractors.  On the rare occasions that I collimate a refractor, I use a cheshire and am concerned with merging circles, in the case of a doublet.  And, I have used a barely defocused star as well in cases where, due to lens design, seeing the reflections is made difficult.  I do not, that in my reflector a star that is defocused far too much makes seeing rings difficult, where in the case of the refractor (while probably useless) they persist.  I wonder why that is?  Thank you all for the information, the posts are very good and quite understandable.



#15 Vic Menard

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Posted 03 March 2015 - 01:09 PM

...we're just seeing the grossly defocused image of the primary mirror's reflection in the secondary mirror. And that was Jason's exact point when pointing out the error typically made by beginning star collimators. And in the middle of that, the blurry reflection of the shadow of the now centralized secondary mirror.

 

Yep!  



#16 Vic Menard

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Posted 03 March 2015 - 01:22 PM

...in my reflector a star that is defocused far too much makes seeing rings difficult, where in the case of the refractor (while probably useless) they persist.  I wonder why that is?

 

The central obstruction impacts the diffraction ring structure in a reflector.



#17 choran

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Posted 03 March 2015 - 01:54 PM

Yep, makes sense gentlemen.



#18 Mike Lockwood

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Posted 03 March 2015 - 03:55 PM

My article "Why aren't my stars round?" covers the topic of star collimation.  See the second section for how I do it:

  http://www.loptics.c.../starshape.html



#19 jtsenghas

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Posted 03 March 2015 - 08:47 PM

Thank you, gentlemen, for this collaborative and mutually respectful discussion. My preliminary attempts at star collimation many years ago were with too much defocus and it took a while for me to discover this on my own. Thank you, Jason, for offering this advice so clearly. Thank you, everyone else, for contributing further detail.



#20 Jason D

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Posted 04 March 2015 - 12:44 PM

When the star is defocused too much, it will show "mechanical" alignment -- NOT "optical" alignment.

In other words, a star that is too defocused shows what the primary mirror sees "mechanically". A star that is defocused my a small amount shows and magnifies optical alignment (coma).

 

Refer to the following attachment:

A too-defocused star is shown in the left photo. Note how all "mechanical" intrusions seen by the primary mirror are clearly visible (primary clips, spider vanes, secondary mirror shadow). Event the secondary mirror offset is visible (enhanced by the middle photo). I asked my son to place his hand in front of the OTA (right photo). The silhouette of his hand is crisp. At this defoucs level, there is little to no information available about collimation.

 

star_collimation_3.jpg

 

Refer to the following attachment:

Left photo shows about the right amount of defocus needed at high magnification to evaluate collimation. Middle photo is the same photo but magnified. Right photo shows the concentric Fresnel rings for a well-collimated scope. Note there are no "mechanical" intrusions visible. No clear secondary mirror silhouette, no clips, and no spider vanes. Only optical alignment is visible at this level of defocus.

 

star_collimation_2.jpg

 

Jason


Edited by Jason D, 04 March 2015 - 02:21 PM.


#21 Brian Carter

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Posted 04 March 2015 - 12:57 PM

"Right photo shows about the right amount of defocus needed at high magnification to evaluate collimation. Middle photo is the same photo but magnified. Left photo shows the concentric Fresnel rings for a well-collimated scope. Note there are no "mechanical" intrusions visible. No clear secondary mirror silhouette, no clips, and no spider vanes. Only optical alignment is visible at this level of defocus."

 

I think you have your right and left reversed for these?

 

Regardless, really excellent photos.  I don't think I've seen photos of star collimation so nicely before (usually they are just aberrator images).

 

Thanks!



#22 Jason D

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Posted 04 March 2015 - 01:04 PM

 

I think you have your right and left reversed for these?

 

 

 

I originally had the left/right photos swapped but after writing the paragraph I decided to swap them but forgot to update the associated text.

Thanks for catching the error. I edited my post.

Jason



#23 Jason D

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Posted 04 March 2015 - 09:09 PM

Another issue with star collimation is the misalignment of the secondary mirror. Star collimation can't be used to adjust the secondary mirror. To illustrate the issue with secondary mirror misalignment, I conducted the following experiment:

 

I started off with my well-collimated scope then I misaligned the secondary mirror to have the laser beam hit the primary mirror 2.5" away from the center. I do realize the misalignment is excessive but I did it to illustrate my points.

 

star_collimation_7.jpg

 

Then I star collimated by only adjusting the primary mirror. I did my best to collimate even though the final defocused star had an oval shape.

 

star_collimation_8.jpg

 

Then I defocused too much on both sides (in and out). The shapes were distorted and elongated. Many run into similar issues when they just can't get a nice circular defocused star collimation. In the following attachment, left hand and middle photos correspond to in&out defocused star (too defocused). One is the mirror image of the other. The right hand photo is the same as the middle one but annotated (more info in the next paragraph)

 

star_collimation_4.jpg

 

When I misalignment the secondary mirror to shift the laser beam away from the primary center and away from the focuser, I had to tilt the secondary mirror forward towards the focuser. When I star collimated, the only way to reflect the primary optical axis back to the focuser center is by tilting the primary mirror forward excessively. Doing so, intruded the OTA edge to the light path. The oval shape was caused by the clipping/vignetting of the OTA edge. Referring to the previous attachment, note how only diffraction of two primary clips are visible. The other two are masked by the OTA edge. The illustration below explains how the OTA edge is vignetting the light path.

 

star_collimation_5.jpg

 

Unless the secondary mirror is well-aligned before star collimation, overall good collimation might not be able to be achievable by only star collimation.

 

Jason


Edited by Jason D, 04 March 2015 - 10:23 PM.


#24 Jason D

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Posted 05 March 2015 - 03:09 AM

Here is an interesting experiment:

 

1- Use your favorite and trusted collimation tools to collimate your scope

2- Randomly,  use any two primary mirror adjustment knobs and twist them random number of turns in random direction -- ONLY two knobs

3- Use ONLY two primary mirror knobs to star collimate -- one of the knobs has to be the unused one from step # 2. This is meant to encourage fairness. 

4- When you believe primary collimation is achieved via star collimation, re-use your collimation tools to assess the primary alignment. Make a note of what you see via the collimation tools.

5- Repeat/iterate steps 2 to 4 many times

 

First, below are two examples showing how badly I miscollimated the primary mirror at the start of each iteration:

 

star_collimation_10.jpg

 

I must have done the experiment around 20 times. I have captured the last 6 iterations:

 

star_collimation_9.jpg

 

My conclusions:

1- Star collimation result was slightly inconsistent between iterations. Inconsistency came from the difficulty to read the defocused star when collimation is almost achieved. Seeing conditions causes the defocused star to continuously distort shape. It is difficult to judge when perfect collimation is achieved. Small tweaks did not appear to make a difference. Interestingly, seeing in my area is average to above average.

2- Star collimation takes skills and knowledge. Unfortunately many do it the wrong way. If the defocused star appears as a donut even with a small amount of defocus, the secondary offset will impact the accuracy of collimation. That is, if the defocused star looks like the following, secondary offset will have an impact:

 

star_test.jpg

 

It should look like:

 

star_collimation_2.jpg

 

3- I found out that star collimation was always consistent with my tool-based collimation.

 

Jason


Edited by Jason D, 05 March 2015 - 09:19 AM.


#25 howard929

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Posted 05 March 2015 - 07:08 AM

 

My conclusions:

1- Star collimation was slightly inconsistent between iterations. Inconsistency came from the difficulty to read the defocused star when collimation is almost achieved.

2- Star collimation takes skills and knowledge. Unfortunately many do it the wrong way. 

3- I found out that star collimation was always consistent with my tool-based collimation.

Jason

 

 

The last 6 photos are very telling. Three of those results would unlikely be anywhere close enough to "good enough" by any definition in a sub f/5 telescope and would be questionable at f/6. OTHO:

 

3- I found out that star collimation was always consistent with my tool-based collimation.

 

Seems to be a valid cut-to-the-chase "take away" concerning star collimation. Well done and in the face of your conclusions I'm liking Brian Carters "friends don't let friends star collimate" even more.


Edited by howard929, 05 March 2015 - 07:21 AM.



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