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Concise thread about autocollimators+improvements

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#401 Jarad

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Posted 21 May 2012 - 11:50 AM

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

:question:

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#402 Starman1

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Posted 21 May 2012 - 12:14 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.

#403 Jarad

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Posted 21 May 2012 - 01:27 PM

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

:lol:

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#404 cmyachtie

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Posted 21 May 2012 - 05:20 PM

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. :confused:

#405 michael_m

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Posted 22 May 2012 - 05:49 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.

#406 Starman1

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Posted 22 May 2012 - 06:18 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.

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.

#407 Vic Menard

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Posted 23 May 2012 - 07:47 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.

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.

...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.

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.

#408 Vic Menard

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Posted 23 May 2012 - 08:30 AM

...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.

...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).

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! :waytogo:

...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...

#409 Gianluca67

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Posted 27 May 2012 - 04:01 PM

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.

#410 1stScope

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Posted 01 June 2012 - 11:35 AM

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.

#411 bilgebay

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Posted 10 June 2012 - 02:32 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.

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#412 bilgebay

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Posted 10 June 2012 - 02:55 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.

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

All c/c are welcome.

Sedat

#413 UmaDog

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Posted 10 June 2012 - 11:26 PM

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.

#414 bilgebay

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Posted 11 June 2012 - 07:40 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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#415 howard929

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Posted 11 June 2012 - 09:06 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

#416 Mark Peterman

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Posted 21 November 2012 - 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

#417 Jason D

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Posted 21 November 2012 - 10:54 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


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

#418 Nils Olof Carlin

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Posted 22 November 2012 - 02:33 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).


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

#419 rlmxracer

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Posted 18 January 2013 - 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.

#420 precaud

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Posted 18 January 2013 - 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.

#421 Jason D

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Posted 18 January 2013 - 10:04 AM

What am I doing wrong?

It does not sound you are doing anything wrong. Based on your steps, collimation should be close enough to see the center spot reflections via the autocollimator pupil. I assisted few members in other forums who own the same brand – Farpoint autocollimator s. They reported when conducted the 180 degrees rotation test, the center spot reflections moved significantly. This is an indication the autocollimator mirror is not precisely squared with its case. Since you have the same brand, it could be the reason why you can't see the reflections. According to the same members, the Farpoint mirror was placed on cork pads which apparently shifted and caused the mirror to tip slightly on one side. They took their autocollimators apart and adjusted the cork pads placement. According to them, that improved the tool but they still noticed a large center spot reflections shift when conducted the 180 degrees rotation test. I do not know if your Farpoint autocollimator has the same issues.

The 180 degree rotation test does NOT apply to reflection P since it is the only reflection that does not reflect off the autocollimator mirror. It is best to reference reflection 2 for this test.

Jason

#422 rlmxracer

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Posted 18 January 2013 - 11:11 AM

Thanks for the quick reply Jason, I'll give the AC a 180 turn and see if that changes anything. Another possible issue that I forgot to mention is the Farpoint cheshire seems to have a very loose fit in my focuser. I cant tell if this is causing it to shift when I snug up the brass compression ring to hold it. Thanks Rob.

#423 Jason D

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Posted 18 January 2013 - 11:38 AM

Cheshire loose fit has no impact since the cheshire is only sensitive to the primary mirror alignment -- as in your case. On the other hand, the sight-tube/cheshire combo is senstive to the loose fit.

#424 rlmxracer

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Posted 18 January 2013 - 12:16 PM

Ok good. I have placed a lap of scotch tape around my combo tool to make for a mor precise fit in the 1.25 to 2" adapter. When I remove and replace the combo tool and adapter the crosshairs remain aligned with the center spot so that seems good. You were right (as usual)the Farpoint AC I purchased was poorly made. the mirror is glued to a thin plexiglass disk and sandwiched in between the cap and body of the AC with the small cork pices there to keep it from rattling around. My AC mirror was glued off center on the plexi disk and the hole in the center of it has small chips in the mirror. I did the 180 deg turn you suggested and I can see half of one of the centerspot reflections, but thats it.
So it looks like at this point the best I can do is use my combo tool to align the secondary and the 2" cheshire for the primary. I'm kind of bummed cause I'm going out to the desert for the weekend and was hoping for the best collimation I've had yet. At least it will be close. Rob

#425 Jason D

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Posted 18 January 2013 - 01:31 PM

You were right (as usual)the Farpoint AC I purchased was poorly made. the mirror is glued to a thin plexiglass disk and sandwiched in between the cap and body of the AC with the small cork pices there to keep it from rattling around

Squaring the AC mirror to its case is critical. Without precise squaring, the AC will not be useful. I do not see how Farpoint design can achieve and maintain this a critical alignment.

I need to clarify that some jittery movement of reflection 2 with respect to reflection P when conducting the 180 degree rotation test is OK since this test is highly sensitive. However, dramatic movement is unacceptable.

Jason


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