Why are laser collimators not good enough?

11,96 Andy Wray 8/28/2022 · перегляд переглядів · Поділитися посиланням
I see a lot of articles saying that laser collimators are not good enough to get your Newtonian collimated. I have collimated my laser collimator and it lets me centre the secondary on the primary and then align the primary. I also used a Cheshire to get my secondary aligned properly. I am also using out-of-focus stars to confirm collimation. What am I missing? Відредаговано 8/28/2022 Лайк

11,96

8/28/2022

I see a lot of articles saying that laser collimators are not good enough to get your Newtonian collimated.

I have collimated my laser collimator and it lets me centre the secondary on the primary and then align the primary.

I also used a Cheshire to get my secondary aligned properly.

I am also using out-of-focus stars to confirm collimation.

What am I missing?

Відредаговано 8/28/2022

Björn Arnold 8/28/2022 · перегляд переглядів · 2 лайків · Поділитися посиланням
I'm not an expert on Newtonians but let me give a general string of thoughts: To create a perfect image, optics need to be accurate down to fractions of wavelengths. Therefore, fine alignement of optical elements is done in the order of microns and below. For a SCT a minuscle turn of a screw on the secondary can make a perfect image or screw it up. Take a 10" primary of a Newton and assume a 10 degree turn of a screw is necessary. That will tilt the mirror by a few microns or tens of microns. For a f/5 scope, your collimating laser point will move by a tenth of a millimeter. I don't see a chance to know if alignment is perfect. Besides your laser may also be misaligned. Therefore, final alignment should always be done on a star at critical focus. Björn Лайк

Björn Arnold

8/28/2022

· 2 лайків

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I'm not an expert on Newtonians but let me give a general string of thoughts:
To create a perfect image, optics need to be accurate down to fractions of wavelengths. Therefore, fine alignement of optical elements is done in the order of microns and below. For a SCT a minuscle turn of a screw on the secondary can make a perfect image or screw it up.

Take a 10" primary of a Newton and assume a 10 degree turn of a screw is necessary. That will tilt the mirror by a few microns or tens of microns. For a f/5 scope, your collimating laser point will move by a tenth of a millimeter. I don't see a chance to know if alignment is perfect. Besides your laser may also be misaligned.

Therefore, final alignment should always be done on a star at critical focus.

Björn

8,85 andrea tasselli 8/28/2022 · перегляд переглядів · 3 лайків · Поділитися посиланням
A simple thin-beam laser is rather sensitive to decentering error and it is a single pass collimation tool which is rather imprecise in its own for fast scope (f/5 and below). A barlowed laser beam with a center donut or a holographic one are however much more precise and insensitive to decentering. Besides, how do you check the collimation of your laser collimator and has it got provision to correct its tilt? Final collimation and adjustment should be really be done either with the autocollimator or in-focus at high magnification. Лайк

8,85

andrea tasselli

8/28/2022

· 3 лайків

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A simple thin-beam laser is rather sensitive to decentering error and it is a single pass collimation tool which is rather imprecise in its own for fast scope (f/5 and below). A barlowed laser beam with a center donut or a holographic one are however much more precise and insensitive to decentering.

Besides, how do you check the collimation of your laser collimator and has it got provision to correct its tilt?

Final collimation and adjustment should be really be done either with the autocollimator or in-focus at high magnification.

11,96 Andy Wray Автор теми 8/28/2022 · перегляд переглядів · Поділитися посиланням
FWIW: My laser collimator has grub screws for centering it. I used a target across a 20 foot room to collimate it as best I could. I also rotate the laser collimator through 90 and 180 degrees to get an average offset (although this hardly moves). Лайк

8,85 andrea tasselli 8/28/2022 · перегляд переглядів · Поділитися посиланням
With the typical spread of a collimated laser beam you won't be able to tell the difference with the tolerance required for a fast system on such a baseline. I use a baseline of 10 meters but then I use it only for initial centring. I use a V-block and slowly rotate the collimator on it while adjusting the bolts holding the laser in place. Лайк

7,80 kuechlew 8/28/2022 · перегляд переглядів · Поділитися посиланням
Time and time again I find myself surprised what incredible accuracy is required in this hobby. Not so surprising once you think about what we are trying to achieve. The more so it's amazing that all of this is available to us amateurs in an affordable package these days. Clear skies Wolfgang Відредаговано 8/28/2022 Лайк

1,81 Mike Dobres 8/28/2022 · перегляд переглядів · Поділитися посиланням
Hi Andy I've found that an artificial star is another useful 'cloudy night' tool for collimating the scope. I use the Geoptik model. Even with my 14 inch Dob, I can get focus with a Barlow at 60 yards (distance from garage to back wall of orchard). I can then check collimation in-focus and out-focus and compare that to the laser collimator. It's the next level up from using a laser, and you dont have to worry about clouds or tracking the star! One chap on Stargazer Lounge mentioned he just sticks the artificial star on a tripod and walks it far enough down the street for to get focus. Apparently leads to some interesting conversations with neighbors. As with star-alignment, I've also found the artificial star useful for detecting sources and cures for thermal currents within the tube. Andrea - thanks for the Laser-Barlow tip - just googled it. Will give that a go as well! Mike Відредаговано 8/29/2022 Лайк

1,81

Mike Dobres

8/28/2022

· Поділитися посиланням

Hi Andy
I've found that an artificial star is another useful 'cloudy night' tool for collimating the scope. I use the Geoptik model. Even with my 14 inch Dob, I can get focus with a Barlow at 60 yards (distance from garage to back wall of orchard). I can then check collimation in-focus and out-focus and compare that to the laser collimator. It's the next level up from using a laser, and you dont have to worry about clouds or tracking the star! One chap on Stargazer Lounge mentioned he just sticks the artificial star on a tripod and walks it far enough down the street for to get focus. Apparently leads to some interesting conversations with neighbors. As with star-alignment, I've also found the artificial star useful for detecting sources and cures for thermal currents within the tube.

Andrea - thanks for the Laser-Barlow tip - just googled it. Will give that a go as well!

Mike

Відредаговано 8/29/2022

13,46 Arun H 8/28/2022 · перегляд переглядів · 3 лайків · Поділитися посиланням
A lot of this also depends on what your definition of good collimation is. If your definition is round stars at excellent FWHM through the entirety of the frame when viewed at 100% or more, then laser collimators, which are prone to collimation error themselves, are certainly inadequate for reasons already discussed. Even Cheshires or barlowed lasers (for which laser collimation is irrelevant) will only correct one of the two forms of collimation error (primary axial error). Focuser axial error will need to be corrected using tools such as autocollimators using the correct protocol. It takes owning and using a fast Newtonian and being picky about your stars to realize just how sensitive these systems are to collimation and tilt. This old but very nice thread goes though the complexity involved: https://www.cloudynights.com/topic/251778-concise-thread-about-autocollimatorsimprovements/#entry3189737 See the second post by Jason that talks about the fundamentals of collimation. Відредаговано 8/28/2022 Лайк

13,46

Arun H

8/28/2022

· 3 лайків

· Поділитися посиланням

A lot of this also depends on what your definition of good collimation is. If your definition is round stars at excellent FWHM through the entirety of the frame when viewed at 100% or more, then laser collimators, which are prone to collimation error themselves, are certainly inadequate for reasons already discussed. Even Cheshires or barlowed lasers (for which laser collimation is irrelevant) will only correct one of the two forms of collimation error (primary axial error). Focuser axial error will need to be corrected using tools such as autocollimators using the correct protocol. It takes owning and using a fast Newtonian and being picky about your stars to realize just how sensitive these systems are to collimation and tilt. This old but very nice thread goes though the complexity involved:

https://www.cloudynights.com/topic/251778-concise-thread-about-autocollimatorsimprovements/#entry3189737

See the second post by Jason that talks about the fundamentals of collimation.

Відредаговано 8/28/2022

Tim Hawkes 8/28/2022 · перегляд переглядів · Поділитися посиланням
Hi Andy, I do exactly as you describe for my PDS200 . In my personal view that is perfectly adequate for an F 5.0 telescope. The importance of perfect stars also varies with the object , the scale of imaging and the angular scale of the detail that you are trying to resolve. and of course the seeing. With a coma corrector (Baader III in my case) the stars are near round across the field at ~ 1.0 arcsec - certainly at normal exposure times and seeing. But push it down to < 0.5 arcsec , good seeing and short frames (lucky maging) and any imperfections in the set up will become more obvious - but then everything matters including the mount - imperfections in the movement of that at the subsecond scale can also cause distorted stars. F 4.0 Newtonian ? -- much more sensitive and difficult to get right. Tim Лайк

Tim Hawkes

8/28/2022

· Поділитися посиланням

Hi Andy, I do exactly as you describe for my PDS200 . In my personal view that is perfectly adequate for an F 5.0 telescope. The importance of perfect stars also varies with the object , the scale of imaging and the angular scale of the detail that you are trying to resolve. and of course the seeing. With a coma corrector (Baader III in my case) the stars are near round across the field at ~ 1.0 arcsec - certainly at normal exposure times and seeing. But push it down to < 0.5 arcsec , good seeing and short frames (lucky maging) and any imperfections in the set up will become more obvious - but then everything matters including the mount - imperfections in the movement of that at the subsecond scale can also cause distorted stars. F 4.0 Newtonian ? -- much more sensitive and difficult to get right. Tim

1,91 David Nozadze 8/29/2022 · перегляд переглядів · Поділитися посиланням
Andy Wray: I see a lot of articles saying that laser collimators are not good enough to get your Newtonian collimated. I have collimated my laser collimator and it lets me centre the secondary on the primary and then align the primary. I also used a Cheshire to get my secondary aligned properly. I am also using out-of-focus stars to confirm collimation. What am I missing? Hi Andy, Since you are using a laser AND a cheshire, then I don't think you are missing anything. As I understand, you can't correctly collimate a newtonian with ONLY a laser because: a) you can't align a secondary mirror with the focuser with a laser. It will show you correct centering but will not detect mirror tilt. b) laser itself may not be collimated. Лайк

1,91

David Nozadze

8/29/2022

· Поділитися посиланням

Andy Wray:
I see a lot of articles saying that laser collimators are not good enough to get your Newtonian collimated.

I have collimated my laser collimator and it lets me centre the secondary on the primary and then align the primary.

I also used a Cheshire to get my secondary aligned properly.

I am also using out-of-focus stars to confirm collimation.

What am I missing?

Hi Andy,

Since you are using a laser AND a cheshire, then I don't think you are missing anything. As I understand, you can't correctly collimate a newtonian with ONLY a laser because: a) you can't align a secondary mirror with the focuser with a laser. It will show you correct centering but will not detect mirror tilt. b) laser itself may not be collimated.

13,46 Arun H 8/29/2022 · перегляд переглядів · Поділитися посиланням
The attached shows the level of tolerances required to get maximum performance from a coma corrector for PAE and FAE. The tolerances with the coma corrector are tighter, because the coma is lower (i.e., the definition of a defect is tighter): https://www.catseyecollimation.com/Newtonian%20Axial%20Tolerances.pdf PAE tolerances are in fractions of a millimeter for fast Newtonians, FAE tolerances on the order of 1 mm or so for the systems we use. Our analysis and measurement tools should have corresponding precision. Visual judgement of a reasonably large diameter laser spot is not very useful here even if collimated. Відредаговано 8/29/2022 Лайк

13,46

Arun H

8/29/2022

· Поділитися посиланням

The attached shows the level of tolerances required to get maximum performance from a coma corrector for PAE and FAE. The tolerances with the coma corrector are tighter, because the coma is lower (i.e., the definition of a defect is tighter):

https://www.catseyecollimation.com/Newtonian%20Axial%20Tolerances.pdf

PAE tolerances are in fractions of a millimeter for fast Newtonians, FAE tolerances on the order of 1 mm or so for the systems we use. Our analysis and measurement tools should have corresponding precision. Visual judgement of a reasonably large diameter laser spot is not very useful here even if collimated.

Відредаговано 8/29/2022

13,46 Arun H 8/29/2022 · перегляд переглядів · Поділитися посиланням
I guess I should add - I think that graph in the linked article above shows nicely how imaging system tilt and collimation interact. If you go through a collimation exercise, but your sensor is tilted, that is indistinguishable from not properly correcting Focuser Axial Error during collimation (the imaging plane is tilted with respect to the focal plane). This is why it is important to be completely confident of collimation, using tools you can trust, before attempting to address tilt. Also, larger sensors and smaller pixels will make these errors more apparent. Відредаговано 8/29/2022 Лайк

13,46

Arun H

8/29/2022

· Поділитися посиланням

I guess I should add - I think that graph in the linked article above shows nicely how imaging system tilt and collimation interact.

If you go through a collimation exercise, but your sensor is tilted, that is indistinguishable from not properly correcting Focuser Axial Error during collimation (the imaging plane is tilted with respect to the focal plane). This is why it is important to be completely confident of collimation, using tools you can trust, before attempting to address tilt. Also, larger sensors and smaller pixels will make these errors more apparent.

Відредаговано 8/29/2022

25,47 John Hayes 8/29/2022 · перегляд переглядів · 1 лайк · Поділитися посиланням
A lot of this also depends on what your definition of good collimation is. If your definition is round stars at excellent FWHM through the entirety of the frame when viewed at 100% or more, then laser collimators, which are prone to collimation error themselves, are certainly inadequate for reasons already discussed. Even Cheshires or barlowed lasers (for which laser collimation is irrelevant) will only correct one of the two forms of collimation error (primary axial error). Focuser axial error will need to be corrected using tools such as autocollimators using the correct protocol. It takes owning and using a fast Newtonian and being picky about your stars to realize just how sensitive these systems are to collimation and tilt. This old but very nice thread goes though the complexity involved: https://www.cloudynights.com/topic/251778-concise-thread-about-autocollimatorsimprovements/#entry3189737 See the second post by Jason that talks about the fundamentals of collimation. From an optical engineering standpoint, alignment is achieved when the wavefront errors over the field are minimized. Seeing is what puts a tolerance on how accurately the system needs to be aligned but in general, if you can align the system to be within about a quarter of a wave (+/- 1/8 wv) from the performance achieved by the design, the system will work well under any seeing conditions. Aligning a simple Newtonian telescope is purely a geometric exercise so it is among the easiest optical systems to align--provided that you have access to the required mechanical adjustments. The diagrams in the CN thread that you referenced didn't show up when I checked it out but at first glance, that whole thread seems unnecessary complicated. The challenge boils down to positioning the secondary mirror, making the axis of the parabola collinear with the axis of the focuser, and minimized sensor tilt. Adding a coma corrector will almost certainly increase the challenge of getting properly everything aligned. It's a shame that most of the laser collimators that are sold for optical alignment into the amateur market are so poorly made. That's really just about all you need to properly align a Newtonian--along with some software to measure sensor tilt. John Лайк

25,47

John Hayes

8/29/2022

· 1 лайк

· Поділитися посиланням

A lot of this also depends on what your definition of good collimation is. If your definition is round stars at excellent FWHM through the entirety of the frame when viewed at 100% or more, then laser collimators, which are prone to collimation error themselves, are certainly inadequate for reasons already discussed. Even Cheshires or barlowed lasers (for which laser collimation is irrelevant) will only correct one of the two forms of collimation error (primary axial error). Focuser axial error will need to be corrected using tools such as autocollimators using the correct protocol. It takes owning and using a fast Newtonian and being picky about your stars to realize just how sensitive these systems are to collimation and tilt. This old but very nice thread goes though the complexity involved:

https://www.cloudynights.com/topic/251778-concise-thread-about-autocollimatorsimprovements/#entry3189737

See the second post by Jason that talks about the fundamentals of collimation.

From an optical engineering standpoint, alignment is achieved when the wavefront errors over the field are minimized. Seeing is what puts a tolerance on how accurately the system needs to be aligned but in general, if you can align the system to be within about a quarter of a wave (+/- 1/8 wv) from the performance achieved by the design, the system will work well under any seeing conditions. Aligning a simple Newtonian telescope is purely a geometric exercise so it is among the easiest optical systems to align--provided that you have access to the required mechanical adjustments. The diagrams in the CN thread that you referenced didn't show up when I checked it out but at first glance, that whole thread seems unnecessary complicated. The challenge boils down to positioning the secondary mirror, making the axis of the parabola collinear with the axis of the focuser, and minimized sensor tilt. Adding a coma corrector will almost certainly increase the challenge of getting properly everything aligned. It's a shame that most of the laser collimators that are sold for optical alignment into the amateur market are so poorly made. That's really just about all you need to properly align a Newtonian--along with some software to measure sensor tilt.

John

1,20 Phil Cambre 8/29/2022 · перегляд переглядів · Поділитися посиланням
Andy Wray: I see a lot of articles saying that laser collimators are not good enough to get your Newtonian collimated. I have collimated my laser collimator and it lets me centre the secondary on the primary and then align the primary. I also used a Cheshire to get my secondary aligned properly. I am also using out-of-focus stars to confirm collimation. What am I missing? Usually it is true, though it depends on the precision of the laser you use . In my case, I do all my high resolution planetary/moon imaging with my 15" F4.5 dobson collimated only with a laser in 2min. BUT... it is the Howie glatter laser (for the secondary collimation) + Tublug (integrated barlow) for the primary collimation. This laser is a very expensive beast but this is also the most precise laser on the market, period. I also have the Catseye complete set just in case and they both say the same thing. I also triple checked with airy pattern on a star test on several nights after collimating with the laser, and find out that there was no fine tuning to do, so I decided to only make laser collimation ever since. But it is in my case and it will not work as good with cheaper/less precise laser (such as Baader, TS etc.). Лайк

1,20

Phil Cambre

8/29/2022

· Поділитися посиланням

Andy Wray:
I see a lot of articles saying that laser collimators are not good enough to get your Newtonian collimated.

I have collimated my laser collimator and it lets me centre the secondary on the primary and then align the primary.

I also used a Cheshire to get my secondary aligned properly.

I am also using out-of-focus stars to confirm collimation.

What am I missing?

Usually it is true, though it depends on the precision of the laser you use

.
In my case, I do all my high resolution planetary/moon imaging with my 15" F4.5 dobson collimated only with a laser in 2min.
BUT... it is the Howie glatter laser (for the secondary collimation) + Tublug (integrated barlow) for the primary collimation. This laser is a very expensive beast but this is also the most precise laser on the market, period. I also have the Catseye complete set just in case and they both say the same thing. I also triple checked with airy pattern on a star test on several nights after collimating with the laser, and find out that there was no fine tuning to do, so I decided to only make laser collimation ever since. But it is in my case and it will not work as good with cheaper/less precise laser (such as Baader, TS etc.).

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