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Equipment Discussions >> Reflectors

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Jarad
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Re: Concise thread about autocollimators+improvements new [Re: Starman1]
      #5232449 - 05/21/12 12:50 PM

On a more philosophical note, can a thread with 20+ pages and over 400 posts be called "Concise"?



Jarad


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Starman1
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Re: Concise thread about autocollimators+improvements new [Re: Jarad]
      #5232487 - 05/21/12 01:14 PM

Jarad,
If you change your preferences to display 99 posts per page, only a small handful of threads will ever be over 1 page.


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Jarad
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Re: Concise thread about autocollimators+improvements new [Re: Starman1]
      #5232597 - 05/21/12 02:27 PM

And this "concise" one will still be one of them...



Jarad


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cmyachtie
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Re: Concise thread about autocollimators+improvements new [Re: EdTheEdge]
      #5232983 - 05/21/12 06:20 PM

Quote:

Wow that's a lot of reading. Unfortunately for me it will take reading this thread quite a few more times before I can begin comprehend it all.

But I applaud you for taking the time to help newbies like myself!

THANKS!




Hear, Hear. me too!!!!

Thanks a million!!! for this effort and your expertise in putting this together....am still trying to understand it all.


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michael_m
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Re: Concise thread about autocollimators+improvements new [Re: cmyachtie]
      #5234596 - 05/22/12 06:49 PM

Jason D, or Don P, or guru surrogate ,

In this post #3635872 from within this very thread, Jason outlines an alternate method from what the catseye website outlines using the 2-pupil XLK.

My question...is the CDP used when performing any of the steps in Jason's alternate method? Or just get a decent collimation, then the hotspot is simply centered as precisely as possible using the cheshire, then insert the XLK and proceed with the steps. There is no CDP while stacking P+2...correct?

p.s. I have 50 posts per page. Less pages, but more scrolling . But seriously, how can anyone who wants to use AC's and cheshires not love this thread. Truly a great thread! My thanks to Jason, Don, Vic and Jim for a really outstanding presentation. It has admittedly taken me a while to absord the intracacies. Still, I learn from a rehashing of it every time. But it is sinking in a lot more lately...the science of it that is. In fact I just sent Jim Fly my Infinity XL to upgrade to the XLK because I am now empowered with confidence that I can do better with collimation, and do it easily. I am believing that Don's words will ring out in the night...it's amazing how much better the seeing got with my better collimation.


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Starman1
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Re: Concise thread about autocollimators+improvements new [Re: michael_m]
      #5234644 - 05/22/12 07:18 PM

Quote:

Jason D, or Don P, or guru surrogate ,

In this post #3635872 from within this very thread, Jason outlines an alternate method from what the catseye website outlines using the 2-pupil XLK.

My question...is the CDP used when performing any of the steps in Jason's alternate method? Or just get a decent collimation, then the hotspot is simply centered as precisely as possible using the cheshire, then insert the XLK and proceed with the steps. There is no CDP while stacking P+2...correct?

p.s. I have 50 posts per page. Less pages, but more scrolling . But seriously, how can anyone who wants to use AC's and cheshires not love this thread. Truly a great thread! My thanks to Jason, Don, Vic and Jim for a really outstanding presentation. It has admittedly taken me a while to absord the intracacies. Still, I learn from a rehashing of it every time. But it is sinking in a lot more lately...the science of it that is. In fact I just sent Jim Fly my Infinity XL to upgrade to the XLK because I am now empowered with confidence that I can do better with collimation, and do it easily. I am believing that Don's words will ring out in the night...it's amazing how much better the seeing got with my better collimation.



Thanks for the kind words.
No, CDP is not used with this technique.
The issue with using the lateral pupil of the AC, as I understand it, is that is makes certain the primary and secondary optical axes are parallel, but not necessarily coincident. Hence, it is possible to see good cheshire alignment, and well-aligned stacks in the lateral pupil, but not have the secondary reflections perfectly stacked in the center pupil's image.
However, if the central pupil, lateral pupil, and cheshire all agree the instrument is collimated, it is.

Vic came up with CDP to make secondary alignment (i.e.the focuser axis) correct BEFORE aligning the primary axis with the cheshire. If CDP is done correctly and accurately, then good alignment can be had without the lateral pupil.

However, I have used CDP to align the scope and taken my time only to find the lateral pupil still shows some unresolved secondary/focuser axis anomalies. Using Jason's technique in that post (and I start from much closer, where the central pupil jumble is almost collimated), a couple iterations and all tools and pupils agree the scope is collimated.

Alas, that lateral pupil's image is REALLY sensitive to the changes that occur as the scope cools: pole shrinkage, UTA contractions, focuser shrinkage with temperature. As my scope cools, I can watch the lateral pupil collimation go all over the place (well, that's an exaggeration--let's just say uncollimate and recollimate slightly in a wandering way), while the central pupil shows no changes.

So there is no question in my mind the lateral pupil reveals secondary/focuser axis errors of smaller size than the central pupil. And that's why the technique jason outlined in that post results in better alignment for the focuser axis than does CDP. If you had an autocollimator with only the central hole, you'd use CDP. With the XLK, Jason's method is just a bit easier.


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Vic Menard
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Re: Concise thread about autocollimators+improvements new [Re: Starman1]
      #5235310 - 05/23/12 08:47 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.



Although we are collimating the primary mirror, or optical, axis when we use a Cheshire eyepiece or Barlowed laser, the axis itself is defined by the center spot placement, which is measured relative to the edge of the primary mirror.

Quote:

...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?



I agree with your concerns. Below f/5, I usually try to place the center spot to <0.5mm, which is a relatively easy read with a precision scale or template. Below f/4 (for high magnification performance), you're really pushing the mechanicals and the optical (optimal?) center of the parabola. Above f/5 or f/6, the tolerances relax significantly.

Quote:

Quote:

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.



I agree with both of these statements. Having spent countless hours observing at relatively high magnifications, I constantly scrutinize star images and other image performance markers. Since the tool alignment I use has consistently provided excellent performance, if the image begins to deteriorate during an observing session, it's simple to use a collimation tool to determine whether or not the cause is a change in the axial alignment. Because my scope holds alignment quite well, the change in performance is usually attributable to other factors.


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Vic Menard
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Re: Concise thread about autocollimators+improvements new [Re: Starman1]
      #5235351 - 05/23/12 09:30 AM

Quote:

...The issue with using the lateral pupil of the AC, as I understand it, is that is makes certain the primary and secondary optical axes are parallel, but not necessarily coincident. Hence, it is possible to see good cheshire alignment, and well-aligned stacks in the lateral pupil, but not have the secondary reflections perfectly stacked in the center pupil's image.
However, if the central pupil, lateral pupil, and cheshire all agree the instrument is collimated, it is.



This is unclear since you've included Cheshire alignment in the first statement. From my experience, when the Cheshire and the XLK offset pupil alignments are both good, the XLK center pupil alignment will also be good. If you leave the Cheshire alignment out of the first statement, and systematically align the primary mirror axis with the XLK center pupil and the focuser axis with the XLK offset pupil, you may find a residual primary mirror axial error if you finish with a Cheshire eyepiece.

Quote:

...I have used CDP to align the scope and taken my time only to find the lateral pupil still shows some unresolved secondary/focuser axis anomalies.
...So there is no question in my mind the lateral pupil reveals secondary/focuser axis errors of smaller size than the central pupil.



Considering P-3 (used with a CDP) magnifies the focuser axial error 2X and P-2 (used in the XLK offset pupil) magnifies the focuser axial error 4X (assuming a "zeroed" primary mirror axial error via the Cheshire), and adding P-3 is defocused and subject to parallax, I would expect P-2 to show some residual error. P-3 is still an important signature because it uniquely separates the focuser axial alignment (the reference axis) from the jumbled reflections, where P-2 normally magnifies both the focuser and primary mirror axial errors 4X (the reason for including the Cheshire as part of the alignment procedure).

Quote:

Using Jason's technique in that post (and I start from much closer, where the central pupil jumble is almost collimated), a couple iterations and all tools and pupils agree the scope is collimated.



And that's what matters!

Quote:

...And that's why the technique jason outlined in that post results in better alignment for the focuser axis than does CDP. If you had an autocollimator with only the central hole, you'd use CDP. With the XLK, Jason's method is just a bit easier.



I use both. I find that when I get P-3 sorted out first, it takes fewer steps to get the P-2/Cheshire alignment fully corrected. Of course, my routine alignment is at 88-inches focal length and my mechanicals are excellent, so I have to add, ymmv...


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Gianluca67
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Re: Concise thread about autocollimators+improvements new [Re: Jason D]
      #5242535 - 05/27/12 05:01 PM

Quote:

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




Yes indeed. The hotspot is definitely a better choice over the triangle as regards to the readability of the error. In fact I find critical collimation to be MUCH easier with the hotspot.

For 1stscope: the hotspot can be placed very precisely at the center of the mirror. I believe we are talking about the optical center of the mirror. I find very useful the use of blocks with weights to secure the acetate template and a magnifier to correctly register the template over the mirror. My mirror has a blank of 202 mm and the reflective portion is 200.5 mm in diameter. With a magnifier and a caliper you can assess a very small error in placing the reference circles of the template. Over mine there are 2 circles that indicate 200 mm and 203.2 mm and I was able to correctly register the template to a very tiny fraction of a millimiter over the reflective portion of the mirror. Next step was to press the template all the way down to the mirror to assess the position of the hotspot very carefully. I haven't done a star test yet but I will do that soon just to make sure that also this step is in agreement with the other measures.


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1stScope
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Re: Concise thread about autocollimators+improvements new [Re: Gianluca67]
      #5250402 - 06/01/12 12:35 PM

Regarding the center spot Nils and Vic understand the difference between the mechanical center and the optical center, but I don't know how many others do. With a spherical surface there is no optical center, it can be anywhere, but using the mechanical center of the reflective area will take full advantage of the optical surface area available. An optical center is defined once the mirror is parabolized. I think that the parabolizing process will be centered around the low point of the spherical surface, and that the low point will be defined by the back of the mirror and the surface that it is resting upon, but others more familiar with the mirror making process can correct me on this.

A few people have commented that as long as the collimation process is consistent, has adequate precision and results in a good star test it meets our needs, which I agree with.


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bilgebay
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Re: Concise thread about autocollimators+improvements new [Re: Starman1]
      #5264920 - 06/10/12 03:32 PM

Hi everyone,

I have been following this very informative thread from the beginning. I still need to go through the procedures and grab the technique. I cannot digest things just by reading, I need to practice as well.

I am very new to the reflectors. I have collected my Cat's eye collimation tools and Glatter Laser, together with Tublug only at NEAF this year.

Today, I decided to center spot my 8" Orion SVP. However, the bevel was not in line with the blanks outer diameter. The bevel is at least 1 mm off.

Although it is stated here, a few posts ago, that we should rely on the outer diameter of the blank I am still not very comfortable with this. To rely on anything, we should know manufacturer's method of production.

If they are finishing grinding the parabola in one machine, releasing the blank, then clamping it again and chamfering the mirror in an other machine I guess we cannot rely on anything. The surface quality, roughness and less than perfect roundness of my 8" mirror will render this process unreliable.

If, on the other hand, they are finishing the mirror and beveling it in a single machine, without ever releasing the mirror from the fixture, we can rely on the bevel/chamfer area more than the outer diameter.

What do you think ?

Despite my concerns above, I decided to follow the route suggested by the experts here and used the blank's outer diameter as the reference for my Cat's Eye template.

Following Jim's instructions I tried to fix the hot spot onto the template as precisely as possible.

My first tries looked fine in my observatory but when I took the template to outside I was able to see that the template and the hotspot were not aligned completely.

After a few more tries, I thought I would use my EL Panel to help me with this job. EL Panel would provide enough backlighting to allow me to do a better job... and it really helped. I was successful the first time I used this method. There was some discrepancy between the template and the hotspot. I did my best to average the differences.



Full size photo



Full size photo


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bilgebay
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Re: Concise thread about autocollimators+improvements new [Re: bilgebay]
      #5264948 - 06/10/12 03:55 PM

Then I discovered the 8" circle was smaller than the blank's diameter, which is quite normal I believe. However, this made centering the template even more difficult. After a few unsuccesful attempts, I have decided to use 3 pins to help with centering the template. I have pinned them just outside of the 8" circle, equally distant from the circle. The first 2 tries were too small a circle but the third iteration fit the mirror very nicely, without distorting the template. I have also used a rubber band to press the pins against the periphery of the mirror.

Then I continued with Jim's instructions to press the hotspot onto the the mirror.

I made several measurements with my caliper and it seems the center spot is centered with a 0.3mm precision. Of course, the coarse surface of the blank is not allowing a better measurement or better application.



Full size photo



Full size photo

I will be able to continue my CEC journey next weekend.

All c/c are welcome.

Sedat


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UmaDog
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Re: Concise thread about autocollimators+improvements new [Re: bilgebay]
      #5265598 - 06/11/12 12:26 AM

I remember having similar problems with my XX12's mirror. Like you, I tried my best and it turned out to be good enough. You seem to have done it carefully so I'm sure you'll be fine.

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bilgebay
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Re: Concise thread about autocollimators+improvements new [Re: UmaDog]
      #5265873 - 06/11/12 08:40 AM

Hope so Thanks!

I just realized that I forgot to post a picture of the center spotted mirror. Here it is:



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howard929
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Re: Concise thread about autocollimators+improvements new [Re: bilgebay]
      #5266006 - 06/11/12 10:06 AM

Since the entire spot is behind the central obstruction, it's not a dangerous operation with a bit of care. (drawing a short line with...) A .5mm marker then very careful measuring of a pin point along that short line segment to multiple points at the edges to ensure centering also works well.

Howard

Edited by howard929 (06/11/12 01:47 PM)


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Mark Peterman
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Re: Concise thread about autocollimators+improveme new [Re: Jason D]
      #5533060 - 11/21/12 10:37 PM

Could one simply take, say a 'BB' or small bearing, and place it softly very near the center of the mirror and let it come to rest in the exact center of the parabola (bowl).

Then, using a template or tweezers, place the donut, triangle, or hotspot over the said object.

Am I missing something?

Mark


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Jason D
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Re: Concise thread about autocollimators+improveme new [Re: Mark Peterman]
      #5533092 - 11/21/12 10:54 PM

Quote:

Could one simply take, say a 'BB' or small bearing, and place it softly very near the center of the mirror and let it come to rest in the exact center of the parabola (bowl).

Then, using a template or tweezers, place the donut, triangle, or hotspot over the said object.

Am I missing something?

Mark




1) You need to ensure the mirror is precisely horizontal -- super precision.
2) You need to ensure static friction coefficient between the BB bearing and the glass is virtually zero.
3) Do you really feel comfortable having a metallic object rolling on you mirror?
In theory, it should work. In practice, I would not recommend it.

Do an experiment on any glass surface such as a glass bowl. Place the BB bearing on the surface and wait for it to settle. If you tip the glass minutely and the BB bearing does not move then there is too much static friction to use as you have suggested.

Jason


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Nils Olof Carlin
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Re: Concise thread about autocollimators+improveme new [Re: Mark Peterman]
      #5534049 - 11/22/12 02:33 PM

Quote:

Could one simply take, say a 'BB' or small bearing, and place it softly very near the center of the mirror and let it come to rest in the exact center of the parabola (bowl).




If you mean a small ball will roll to the optical center of the mirror, marking it for you - no, it won't!
Imagine the mirror tilted a fraction of a degree (perhaps due to some wedge, or a table that is ever so slightly uneven), and the ball will roll somewhere else, away from the optical and geometric center. If you want to mark the geometric center of the mirror, it is better done with other means (template, rulers) - most likely, it will be close enough for collimation purposes.

Nils Olof


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rlmxracer
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Re: Concise thread about autocollimators+improveme new [Re: Nils Olof Carlin]
      #5629333 - 01/18/13 04:15 AM

After reading Jason and Vic's various posts on collimation I have gained a good understanding of using my 1.25" combo tool. With the combo tool I have been able to get my 10" f4.9 dob collimated pretty well, but I know it could be better. I have just recieved a 2" Farpoint cheshire and also 2" auto collimator. After recenterspoting my primary with the suplied reflective triangle centerspot I went to work. First I centered the secondary in the focuser then adjusted its tilt to align the combo tools crosshairs with the center of the primary centerspot. Then I placed the Farpoint 2" cheshire in the focuser and adjusted the primary to center the triangle in the cheshires ring. Finally I placed the 2" AC in the focuser and all I see is P (centerspot reflection) the other reflections are not there. What am I doing wrong? Eventually I'll get a 2" sight tube could my 1.25" combo tool not be accurate enough to use the AC? Thanks in advance Rob.

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precaud
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Re: Concise thread about autocollimators+improveme new [Re: rlmxracer]
      #5629532 - 01/18/13 09:18 AM

Do you have the Agena 1/25" combo tool? I've been wrestling with mine and I now see it has problems down at the crosshairs end.

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