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Equipment Discussions >> Reflectors

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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: cheapersleeper]
      #4793184 - 09/06/11 07:22 PM

Quote:

I have also dropped a secondary on a primary





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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: howard929]
      #4793205 - 09/06/11 07:29 PM

Quote:

I'm curious since you didn't mention it, how did you go about verifying those center spot placements?




I thought the photo explained it. I placed the template over my mirror and I oriented it with the existing center spot. I then paid close attention all around the mirror edge to center the template. I even used a magnifier to do that. Finally, I took a photo over the center spot.

The accuracy seems to be in the neighborhood of 0.1mm which is very accurate.

Jason


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howard929
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #4793310 - 09/06/11 08:30 PM

Quote:

Quote:

I'm curious since you didn't mention it, how did you go about verifying those center spot placements?




I thought the photo explained it. I placed the template over my mirror and I oriented it with the existing center spot. I then paid close attention all around the mirror edge to center the template. I even used a magnifier to do that. Finally, I took a photo over the center spot.

The accuracy seems to be in the neighborhood of 0.1mm which is very accurate.

Jason




The point I was getting at is normally, verifying the results of a measurement is considered invalid if the same equipment is used for both the measurement and verification process. Your verification isn't too different then saying you used the same pocket tape measure *twice* to verify a 2" measurement to within .5mm only to have someone hand you a properly calibrated dial caliper.

Howard


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: howard929]
      #4793326 - 09/06/11 08:38 PM

Howard, the Catseye template is accurate and what I did is to verify that when the template is used properly then there is no tangible errors introduced because of slips or mirror sagitta.
Jason


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howard929
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #4793344 - 09/06/11 08:54 PM

Jason, You continue to ignore what I'm trying to tell you. BTW - You don't have any mechanical engineering or machinist training, do you?

Howard


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UmaDog
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Re: Concise thread about autocollimators+improveme new [Re: howard929]
      #4793412 - 09/06/11 09:33 PM

I think the point of the exercise was to check whether the centre spots are being delivered to within the accuracy of the template. Remember that the template is both a measuring device and a tool for placing the centre spot. There are errors in measuring the position of the centre and there are errors in actually using the template to place the spot. The template can shift whilst you press down, etc. When Jason re-uses the template to check spot accuracy, he's not just re-checking a measurement. What he's doing is verifying that spot delivery is as accurate as the template will allow. If his re-measurements consistently showed errors then we'd know that spot placement is less accurate than the template's ability to measure.

Jason's results are important because they show us that the template works very reproducibly. If there were significant random placement errors, we'd see them. In the past I have checked my spot position with a digital photograph. It was accurate to under 0.5 mm.


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: UmaDog]
      #4793440 - 09/06/11 09:53 PM

Couldn’t have said it better. Thank you Rob. That was my point.

Howard, no I do not have mechanical engineering training but is that really necessary!!!!

Jason


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howard929
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #4793519 - 09/06/11 10:33 PM

Quote:

Couldn’t have said it better. Thank you Rob. That was my point.

Howard, no I do not have mechanical engineering training but is that really necessary!!!!

Jason




Jason, just imagine someone continuing to tweak their primary mirror while you continue to point out that the problem is a *serious* rotational error with the secondary...

I'll leave this thread with an answer your question, No. I guess not.

Howard


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UmaDog
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Re: Concise thread about autocollimators+improveme new [Re: howard929]
      #4793609 - 09/06/11 11:24 PM

Howard, I see where you're coming from but you seem to be arguing from a theoretical rather than a practical perspective. Is your concern a large and consistent measurement bias? I don't understand how that might happen and still remain undetected upon careful re-measuring of the deposited spot. For example, if the marking at the centre of the template was off-centre then this would eventually make itself known if the operator re-measures their spots. The circular symmetry of the scenario assures this.

Your analogies are useful, but they describe different situations and I don't think we have those potential pitfalls here. You'd have to frame your concerns in concrete terms within the context of the template measurement procedure. We'd need an example of a significant error that would be missed by Jason's procedure.


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mmclure
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Re: Concise thread about autocollimators+improveme new [Re: UmaDog]
      #4793706 - 09/07/11 12:45 AM

Quote:

The circular symmetry of the scenario assures this.




I think you just hit the nail on the head. Since the Hotspot and center triangle have radial symmetry in three positions, it suffices to measure the location of the hotspot with the template rotated at 0, 120 and 240 degrees. It is geometrically impossible for all three measurements to show that the center spot is in the right place unless the template is also correct.


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: mmclure]
      #4793819 - 09/07/11 03:14 AM

When I placed the Hotspot, I used the template upside-down. In my recent evaluation, I used the template the other way. Therefore, I definitely did not repeat the same template placement.

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Ned Smith
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Re: Concise thread about autocollimators+improvements new [Re: EdTheEdge]
      #4962877 - 12/12/11 12:37 PM

Is there a dual pupil autocollimator with CAM commercially available? I have a new 24" f3.5 mirror that needs better collimation than my present tools can deliver.

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Jason D
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Re: Concise thread about autocollimators+improvements new [Re: Ned Smith]
      #4963107 - 12/12/11 02:57 PM

No, it is not available commercially. I do not know if Jim Fly is planning to offer the CAM but contact him privately and ask him. He might make one for you.
Jason


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Gianluca67
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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #5231196 - 05/20/12 04:23 PM

I am going to resume this old thread to relate of my experience of removing the old Catseye triangle and placing the hotspot on my mirror. I have decided to do so as my triangle turned out to be 0.35 mm away from the true center of the mirror and a white hotspot seems to provide better readability. To remove the triangle I had to pry the three edges with my fingernails and then I used a tweezers to peel it off. Then I followed the Jason method of placing blocks of the same height of the mirror. I set 4 blocks 90 degree apart and secured 2 of them with weights. Then I pressed the Catseye template all the way down the mirror and used a Taylor's measuring tape to check the position where the hotspot would be. I also used a magnifier to check the position of the template against the bevel edge of the mirror, the position of the hotspot on the template and to read the Taylor's measuring tape results. Finally I was able to apply the hotspot very precisely on the mirror. I double checked its position with both a Taylor's measuring tape and a calliper together with the aid of a magnifier and the error in any direction is negligible, perhaps below 0.1 mm. The whole process took me about 4 hours in two steps but now I am very happy to have a correctly placed centermark that provides a much better readability compared to the triangle. By the way, I used the old Catseye template with the triangle as I found out that the apexes of the triangle match the apexes of the hotspot so no need for a new template.

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1stScope
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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #5231250 - 05/20/12 05:00 PM

Is the center of the mirror the center of the OD of the mirror blank? If so some calipers or a depth gage can be used, with an attachment secured for different sizes of mirrors. In this case we are looking for a consistent measurement with adequate precision and repeatability, accuracy is not needed as the measurement will be performed from different locations around the mirror. One could even attach a cross hair eyepiece to the end of the attachment.

Is the center of the reflective portion of the mirror to be used? The portion defined in part by a coarse bevel? The technique above could be used for a first measurement and then a second measurement to the start of the reflective portion could be used, but now we are adding another measurement error.

Or is the desired center the 'optical center', regardless of the above. I think that the 'optical center is what is desired, but at the moment it seems that it will be more difficult to determine (fun for you guys). The question presented is 'is there a practical (observable) difference between using the physical center of the mirror for collimation and using an 'optical center'? In my case I am using the center applied at the Coulter factory with my autocollimator and during some of my best observing sessions the in focus stars in the field had a single centered diffraction ring that moved with the seeing, like some sort of cosmic dance, so I don't think that I could expect much more from such a scope. But, it does leave a nagging doubt.

One method of verifying that the mirror is collimated might be verifying that focus does not change as an off axis aperture is used at the edge of the mirror at four or more different locations. Seeing will need to be very good and steady, or a terrestrial target might need to be used. I have a helical focuser on my scope and I've calculated that with fine focusing I am focusing in the range of a few ten thousandths of an inch based upon how much I am rotating the focuser. This seems to be adequate focusing precision, but the technique might not be sensitive enough as any measured differences in focus would seem to be a measurement of mirror tilt, and the tilt would need to be compared to the focal length.

Any other ideas on how to determine an 'optical collimation' as opposed to a 'physical collimation'?


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Jason D
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Re: Concise thread about autocollimators+improvements new [Re: Gianluca67]
      #5231279 - 05/20/12 05:23 PM

First, congrats on the successful procedure though the 4 hours seemed little lengthy but I guess being patient and taking your time is much better than being regretful and making a mistake.

Quote:

To remove the triangle I had to pry the three edges with my fingernails and then I used a tweezers to peel it off.



That is how I did it too. After soaking the old triangle in isopropyl alcohol then acetone to no avail, I used my fingernail to pry one edge then tweezers to peel it off.

I hope you will find switching to the Hotspot was worth it.

Jason


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Starman1
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Reged: 06/24/03

Loc: Los Angeles
Re: Concise thread about autocollimators+improvements new [Re: 1stScope]
      #5231411 - 05/20/12 07:22 PM

Quote:

Is the center of the mirror the center of the OD of the mirror blank? If so some calipers or a depth gage can be used, with an attachment secured for different sizes of mirrors. In this case we are looking for a consistent measurement with adequate precision and repeatability, accuracy is not needed as the measurement will be performed from different locations around the mirror. One could even attach a cross hair eyepiece to the end of the attachment.

Is the center of the reflective portion of the mirror to be used? The portion defined in part by a coarse bevel? The technique above could be used for a first measurement and then a second measurement to the start of the reflective portion could be used, but now we are adding another measurement error.

Or is the desired center the 'optical center', regardless of the above. I think that the 'optical center is what is desired, but at the moment it seems that it will be more difficult to determine (fun for you guys). The question presented is 'is there a practical (observable) difference between using the physical center of the mirror for collimation and using an 'optical center'? In my case I am using the center applied at the Coulter factory with my autocollimator and during some of my best observing sessions the in focus stars in the field had a single centered diffraction ring that moved with the seeing, like some sort of cosmic dance, so I don't think that I could expect much more from such a scope. But, it does leave a nagging doubt.

One method of verifying that the mirror is collimated might be verifying that focus does not change as an off axis aperture is used at the edge of the mirror at four or more different locations. Seeing will need to be very good and steady, or a terrestrial target might need to be used. I have a helical focuser on my scope and I've calculated that with fine focusing I am focusing in the range of a few ten thousandths of an inch based upon how much I am rotating the focuser. This seems to be adequate focusing precision, but the technique might not be sensitive enough as any measured differences in focus would seem to be a measurement of mirror tilt, and the tilt would need to be compared to the focal length.

Any other ideas on how to determine an 'optical collimation' as opposed to a 'physical collimation'?



There does not appear to be any easy way to make the center of the mirror NOT be the optical center because of the way mirrors are made. So I think we can safely assume the geometric center of the primary IS the optical center.

As for your last question, collimation is the process of optical collimation. The alignment of the secondary under the focuser and the centering of the primary in the tube are physical steps, I guess, but basically you are aligning the optical axis of the system to the focuser axis in the process of collimation. I don't mean that to sound obtuse, but the tools we use for collimation of the mirrors and focuser ARE doing optical, as well as physical, collimation.


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Nils Olof Carlin
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Re: Concise thread about autocollimators+improvements new [Re: 1stScope]
      #5231903 - 05/21/12 02:54 AM

Quote:

One method of verifying that the mirror is collimated might be verifying that focus does not change as an off axis aperture is used at the edge of the mirror at four or more different locations. Seeing will need to be very good and steady, or a terrestrial target might need to be used. I have a helical focuser on my scope and I've calculated that with fine focusing I am focusing in the range of a few ten thousandths of an inch based upon how much I am rotating the focuser. This seems to be adequate focusing precision, but the technique might not be sensitive enough as any measured differences in focus would seem to be a measurement of mirror tilt, and the tilt would need to be compared to the focal length.

Any other ideas on how to determine an 'optical collimation' as opposed to a 'physical collimation'?




The only way, I believe, to accurately decide the optical center is by careful star collimation using the full aperture - an off axis aperture will have a large Airy disk and be very insensitive, and the depth of focus of the narrow cone of light would make reading very uncertain.

If the seeing is very good (depending on aperture), you tweak the collimation until the ring(s) around the Airy disk is as symmetric as possible, making sure the image is accurately centered in the FOV. Then use any device that shows only the collimation of the primary (Cheshire or equivalent, Barlowed laser, Krupa - not autocollimator) and see if the marker spot is indeed centered. If it is, you have verified the optical center. If not, and the offset is consistent over several tries, you may conclude that the marker is actually misplaced, and you may move it or note the offset to use for instrument collimation in the future.

An artificial star can be used, of course, if you can mount it sufficiently far away, and at some altitude (altaz Newtonians can be tricky to use near the horizon).

Other than this one-time sacrifice of a night of excellent seeing, I can see no meaningful use of star collimation. The optical center, I trust, will not wander around, and instrument collimation is much quicker and more precise.

Then again, I have yet to hear of a well documented instance of an optical center being noticeably displaced from the mechanical center. The symmetry of the figuring process should ensure this - and I trust that if there is an asymmetry of the bevel, it is the edge of the blank you should center by.

On the other hand, it seems that factory-centered spots are not so seldom significantly displaced, so it would be prudent to check.

Nils Olof


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1stScope
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Re: Concise thread about autocollimators+improvements new [Re: Nils Olof Carlin]
      #5231995 - 05/21/12 07:04 AM

"I don't mean that to sound obtuse, but the tools we use for collimation of the mirrors and focuser ARE doing optical, as well as physical, collimation."

No, because with the collimation techniques discussed one starts by placing a center spot on the mirror and then proceeds to use the center spot for the remainder of the procedure. I am asking this question as the claims for accuracy for placing the center spot are getting down into the 0.1mm range, so it seems prudent to ask center of what? Are the mirror blanks round enough for such levels? If the reflective portion of the mirror is being used is the bevel consistent enough on the round enough mirror for such levels of accuracy?

My personal opinion is that a decently placed center works fine provided one obtains a good star test when done, but that's just my opinion.


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Starman1
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Re: Concise thread about autocollimators+improvements new [Re: 1stScope]
      #5232293 - 05/21/12 11:24 AM

Quote:

"I don't mean that to sound obtuse, but the tools we use for collimation of the mirrors and focuser ARE doing optical, as well as physical, collimation."

No, because with the collimation techniques discussed one starts by placing a center spot on the mirror and then proceeds to use the center spot for the remainder of the procedure. I am asking this question as the claims for accuracy for placing the center spot are getting down into the 0.1mm range, so it seems prudent to ask center of what? Are the mirror blanks round enough for such levels? If the reflective portion of the mirror is being used is the bevel consistent enough on the round enough mirror for such levels of accuracy?

My personal opinion is that a decently placed center works fine provided one obtains a good star test when done, but that's just my opinion.



Or, another way to state it:
If the tools used to collimate yield good star images, round with concentric diffraction rings, then collimation is good, and no improvement will be made by collimating using a star test.

My only issues with the star test when used for collimation are that you cannot collimate the secondary using it, and that the seeing is very rarely good enough to use a high-power star image for collimation. Good collimation tools can easily do the former, and daylight collimation using an artificial star could easily do the latter. But we who have scopes that require collimation every time they are assembled, and who must travel long distances to observe, want to spend as close as possible to zero observing time to collimate, and that simply rules out the use of star collimation.

The good news is that the high quality collimation tools we have today are accurate enough that, so long as the center marker is accurately placed, they will do the job without necessitating more seeing-dependent techniques.


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