292 replies to this topic

[Vic Menard]

I wish you guys would translate all this high tech info into simple terms and how the Howie Glatter & Blug, or the Catseye, would apply to these problems or conditions you describe.

First, I apologize for continuing the autocollimator theory discussion in this thread (I did suggest moving it to another thread back on page 5, but...) Understanding autocollimator theory isn't necessary to use the carefully decollimated primary procedure (which, incidentally, works very well with the Glatter 1mm aperture stop too!) But the theory does explain why the carefully decollimated primary procedure works, so...

When you ask how the Howie Glatter and Blug compares to the Catseye, I suppose you mean the Catseye trio--sight tube, Cheshire and autocollimator. I think this has been covered already in the discussion, but I'll take a few minutes to review the pros and cons again, specifically targeting the three Newtonian alignments.

Primary mirror axial alignment:
The Glatter laser Barlow attachments (includes the 1mm aperture stop, standard Barlow attachment, Blug, and tuBlug) all magnify the primary mirror error 2X with zero parallax error and all are easily visible after dark. The Blug and tuBlug additionally make the error visible from the rear of the scope where adjustments can be made without moving from the back to the front repeatedly (although good vision may be necessary to see the alignment from a distance).
The Catseye BlackCat XL Cheshire eyepiece also magnifies the primary mirror error 2X and the smaller XL series pupil reduces parallax effects. The tool is easily illuminated after dark, but some people have difficulty reading the alignment to a high precision for various reasons (issues with depth of field, triangle to circle calibration, etc.) Cheshire eyepieces are often found combined with sight tubes (the Catseye TeleCat) and this makes them economical and more useful for the other Newtonian alignments.
The Catseye Infinity XL can magnify the primary mirror error 2X, 4X or 8X depending on how it's used and the latency of the reflections. Since it's usually used in conjunction with a Cheshire eyepiece or Barlowed laser, I'll defer to the Catseye Backcat Cheshire for the head to head comparison.
Advantage--too dependent on personal preference to call.

Focuser axial alignment:
The Glatter laser point attachments (includes the 1mm aperture stop and the holographic reticle) magnify the focuser axial error 1X with zero parallax. Using the 1mm aperture stop and carefully decollimating the primary mirror allows the central beam to be collimated to the silhouette reflection of the primary mirror center spot (carried back to the 1mm target face by the laser diffraction pattern) which simplifies focuser axial adjustment similar to the Blug (no need to head back and forth from the front of the scope to the primary mirror for a closer look). The alignment is also easily visible after dark.
The Catseye TeleCat and Infinity XL alignment tools magnify the focuser axial error 1X and 2X (with a CDP) respectively. The read precision with the TeleCat can be compromised because of the focal distance differences, especially when compared to the Infinity XL, and the performance of either tool after dark is diminished to some degree (depending on one's vision and the amount of illumination available).
Advantage--before dark, the Infinity XL. After dark, the Glatter laser with 1mm aperture stop.

Secondary mirror alignment:
The Glatter laser holographic attachment is an excellent tool for evaluating secondary mirror alignment (especially after dark), but can be difficult to read when a combined (tilt/rotation/offset) secondary mirror is present.
The Catseye TeleCat is an excellent tool for evaluating and correcting all types of secondary mirror alignment errors.
Advantage--the Catseye TeleCat combo tool.

I'm going to be using the HG & Blug. What can I expect? Will I have almost perfect collimation?

You should have almost perfect axial collimation (focuser and primary mirror axes) especially if you use the 1mm aperture stop (assuming it's easily visible from the front aperture of your scope). If you include a TeleCat with the Glatter, and you're willing to take the time to get the specific alignments right, you shouldn't have any problems assessing and correcting the three potential alignment errors.

[Nils Olof Carlin]

Here is the explanation of the Vanishing Act

When you come close to convergence, the "inverted" reflections (2 and 3) seem to vanish - you would expect all reflections to add up to a David's star pattern, but instead you only get one triangle (P, actually). Why?

Look at the figure: we will trace a sample ray from the tip of the triangle to its expected (but missing) reflection 2 and further to the observer's eye. Call R= the radius of the AC pupil.

The tip is at -S, and we will use an arbitrary parameter Y, such that the first reflection (1) in the AC occurs at -S+Y.The next reflection (2) is at the primary, at (-S+2Y).

Next reflection again (3) is at the AC, at (S-Y) - and the next in turn (4) is at the primary at (you may have guessed) S. But from here, it goes back to the AC (5) and hopefully out the pupil, to let you see it.

Now, there are several hurdles to pass: the first reflection must not hit the AC pupil, in order to go on:

(-S+Y)<-R i.e. Y<(S-R) if to the left of it in the figure, or

(-S+Y)>R i.e. Y>(S+R) if to the right.


The next hurdle is of course hitting the triangle itself, but we'll leave this for a while...

Then the ray must hit the AC pupil in order for us to see it: -R<(S-Y)< R , i.e.

Y<(S+R) and Y>(S-R)

- in other words, when collimation is done, no light at all can go from the triangle to form the real reflection 2 and still reach your eye. This is independent of the size of the triangle, as well as of the autocollimator pupil.

Or in yet other words - if the reflections hit the AC pupil in the first place, they won't be reflected - if they miss it in the first place, they will miss the pupil the second time, too, and you will see nothing of them.

In fact with no pupil at all, the simulation shows that they do NOT dissapear and a Star of David is formed at axial convergence.

Yes - only drawback is since there is no pupil at all, you'll see nothing whatsoever of that star

Is this the solution to the last mystery of the autocollimator ?

Nils Olof

Attached Thumbnails

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[Vic Menard]

...Is this the solution to the last mystery of the autocollimator ?

I don't know. From the bow tie 3D simulation on my website, the reflections disappear when the offset between P and 1 is less than 1/2 the radius of the autocollimator pupil. I have another 3D simulation of the CDP and this latency appears to be the same as in the bow tie simulation. P to 1 magnifies the primary error 4X with a CDP, and if the latency is 1/2 of the autocollimator pupil, and the pupil is 0.1-inch, the latent primary mirror error with a CDP (which has already been adjusted to zero the focuser axial error) that has been adjusted to cause all but the front most primary mirror center spot reflection to disappear should be no more than 0.0125-inch. A primary mirror error greater than that would cause the other reflections to reappear (assuming a CDP). This gives us a value for the primary mirror axial resolution of the tool using the CDP.

[Jason D]

But what if you shift your eye to one side of the pupil hole, wouldn't you catch a glimpse of the second reflection?

EDIT: I answered my own question. This scenario is feasible; however, it would require a huge pupil hole (comparable to the size of the triangle) which is NOT a reasonable assumption.

Attached Thumbnails

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[Jason D]

Back to the "bow tie" scenario, thanks to the many good explanations in this thread, now it is clear to me why reflections "2" and "3" disappeared due to the pupil hole. I am having fun with my new little Excel ray tracer. All of a sudden, the mysteries of the 4 AC reflections started to unravel before my eyes . Interpreting the cryptic 4 AC images helps in visualizing the relationship between the focuser/optical axes in 3D. It is kind of like interpreting the cryptic star test images to visualize the quality of the primary mirror surface.

The attachment shows the scenario right before reflections "2" and "3" disappear. Note that "P" and "1" are close but have not stacked up yet.
Jason

EDIT: I updated the attachment to closely resemble the "bow tie" video included in Vic's collimation page
http://homepage.mac....s/NPaddend.html
The attachment captures the ray traces right before reflection images "2" and "3" disappear due to the pupil hole. Images "2" and "3" will disappear if the AC is minutely rotated towards the optical axis around the pupil hole or if the AC is minutely shifted closer to the primary optical axis.
Jason

EDIT: Just to be picky, reflection image "3" will disappear fractionally before reflection image "2". The reason is because reflection image "3" is closer to the primary center; therefore, it has a slightly lower ROC than the ROC of image "2". Keep in mind that paraboloid does not have a fixed ROC in space like a sphere. The minute difference can be seen in the attachment though it might not be clear. Image “3” reflection point is at the edge of the pupil hole (shown by the red arrow) but image “2” reflection point is minutely farther away from the pupil edge which mean it will persist little longer than image “3” before it disappears. In addition, images "2" and "3" are slightly spread apart compared to images "P" and "1" for the same reason.
Jason

Attached Thumbnails

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[Substorm]

Ok, after all the inquire, is there a conclusion to the question?

[Jason D]

Chuck, this thread is no longer about the questioned stated in the title. Vic suggested many posts ago to start another thread for the autocollimation discussion. In retrospect, we should have done that.
Jason

[CatseyeMan]

...

In fact with no pupil at all, the simulation shows that they do NOT dissapear and a Star of David is formed at axial convergence.

Yes - only drawback is since there is no pupil at all, you'll see nothing whatsoever of that star

Sounds like a one-way mirror is what the doctor has orderd

Is this the solution to the last mystery of the autocollimator ?

I have run the PovRay simulations out to 7 decimal places of axial error to the point where images 3 & 4 are eclipsed/disappear and have empirically determined the image disappearance tolerance relationships of PAE and FAE as a funtion of pupil diameter (PD):

PAE disappearance tolerance: PD/8
FAE disappearance tolerance: PD/4

Example: Using an AC with a .125" diameter pupil (approximate size for the INFINITY XL mirror which is slightly bigger than the eyepiece pupil at .099"), with no FAE, images 3 & 4 will disappear at a distance of .125/8 = .015625" Primary axial displacement from the focuser axis origin at the center of the eyepiece field stop. In a 12" f4.5 socpe with a 54" fl, this translates to a 59.7 arcsec Primary axis error.

With no PAE, images 3 & 4 will disappear at a distance of .125/4 = .03125" Focuser axial displacement from the Primary axis origin at the mirror center. In a 12" f4.5 socpe with a 54" fl, this translates to a 119.4 arcsec Focuser axis error.

Inside these tolerances, the "real" inverted image (image 3)formed at the ROC passes through the AC pupil and thus it is not reflected back to the Primary (disappears); Without image 3, its virtual twin (image 4) disappears as well.

[Vic Menard]

Ok, after all the inquire, is there a conclusion to the question?

(Eight posts (and eight hours) before your post, I posted a conclusion of sorts. You may have missed it, so here it is again.)

First, I apologize for continuing the autocollimator theory discussion in this thread (I did suggest moving it to another thread back on page 5, but...) Understanding autocollimator theory isn't necessary to use the carefully decollimated primary procedure (which, incidentally, works very well with the Glatter 1mm aperture stop too!) But the theory does explain why the carefully decollimated primary procedure works, so...

When you ask how the Howie Glatter and Blug compares to the Catseye, I suppose you mean the Catseye trio--sight tube, Cheshire and autocollimator. I think this has been covered already in the discussion, but I'll take a few minutes to review the pros and cons again, specifically targeting the three Newtonian alignments.

Primary mirror axial alignment:
The Glatter laser Barlow attachments (includes the 1mm aperture stop, standard Barlow attachment, Blug, and tuBlug) all magnify the primary mirror error 2X with zero parallax error and all are easily visible after dark. The Blug and tuBlug additionally make the error visible from the rear of the scope where adjustments can be made without moving from the back to the front repeatedly (although good vision may be necessary to see the alignment from a distance).
The Catseye BlackCat XL Cheshire eyepiece also magnifies the primary mirror error 2X and the smaller XL series pupil reduces parallax effects. The tool is easily illuminated after dark, but some people have difficulty reading the alignment to a high precision for various reasons (issues with depth of field, triangle to circle calibration, etc.) Cheshire eyepieces are often found combined with sight tubes (the Catseye TeleCat) and this makes them economical and more useful for the other Newtonian alignments.
The Catseye Infinity XL can magnify the primary mirror error 2X, 4X or 8X depending on how it's used and the latency of the reflections. Since it's usually used in conjunction with a Cheshire eyepiece or Barlowed laser, I'll defer to the Catseye Backcat Cheshire for the head to head comparison.
Advantage--too dependent on personal preference to call.

Focuser axial alignment:
The Glatter laser point attachments (includes the 1mm aperture stop and the holographic reticle) magnify the focuser axial error 1X with zero parallax. Using the 1mm aperture stop and carefully decollimating the primary mirror allows the central beam to be collimated to the silhouette reflection of the primary mirror center spot (carried back to the 1mm target face by the laser diffraction pattern) which simplifies focuser axial adjustment similar to the Blug (no need to head back and forth from the front of the scope to the primary mirror for a closer look). The alignment is also easily visible after dark.
The Catseye TeleCat and Infinity XL alignment tools magnify the focuser axial error 1X and 2X (with a CDP) respectively. The read precision with the TeleCat can be compromised because of the focal distance differences, especially when compared to the Infinity XL, and the performance of either tool after dark is diminished to some degree (depending on one's vision and the amount of illumination available).
Advantage--before dark, the Catseye Infinity XL. After dark, the Glatter laser with 1mm aperture stop.

Secondary mirror alignment:
The Glatter laser holographic attachment is an excellent tool for evaluating secondary mirror alignment (especially after dark), but can be difficult to read when a combined (tilt/rotation/offset) secondary mirror error is present.
The Catseye TeleCat is an excellent tool for evaluating and correcting all types of secondary mirror alignment errors.
Advantage--the Catseye TeleCat combo tool.

(...from JR1650) ...I'm going to be using the HG & Blug. What can I expect? Will I have almost perfect collimation?

You should have almost perfect axial collimation (focuser and primary mirror axes) especially if you use the 1mm aperture stop (assuming it's easily visible from the front aperture of your scope). If you include a TeleCat with the Glatter, and you're willing to take the time to get the specific alignments right, you shouldn't have any problems assessing and correcting the three potential alignment errors.

[BozemanWalt]

This has been a VERY interesting thread, albeit one that's a bit of an extension of a previous one. I was hesitant to chime in once it got a bit, um, contentious, but I'll say this.

While there are several times I don't know what the heck Don, Vic, Jason, et al are talking about it has gotten me thinking about how I can get better performance out of my scope (an Orion XT12). And it's made a big difference.

I started out trying to collimate my scope with a mis-collimated laser. I then spent the money on the Catseye tools and Glatter laser (including the tuBlug).

The learning curve on how to use the Catseye tools was steep (I'm a slow learner), but once I figured it out it all came together and now I can use all three tools and collimate in less than five minutes.

I don't get what amounted to almost personal attacks in this thread. Agreed, it expanded probably way beyond what the OP intended, but it has been a fun, learning thread. I have enjoyed it. To me it goes beyond the obscure technical stuff and gets to the bottom line of getting the best performance out of your scope. And reading threads like this one gives me ideas I would never have thought of otherwise.

Just my two cents.

--Walt

[DeepSpaceTour]

...[snip] I was hesitant to chime in once it got a bit, um, contentious, but I'll say this.

...[snip]I don't get what amounted to almost personal attacks in this thread. Agreed, it expanded probably way beyond what the OP intended, but it has been a fun, learning thread. I have enjoyed it. To me it goes beyond the obscure technical stuff and gets to the bottom line of getting the best performance out of your scope. And reading threads like this one gives me ideas I would never have thought of otherwise.

Just my two cents.

--Walt

...Bravo,couldn't have said it better myself.And that indeed was my point earlier in this thread, that this thread has something for everyone's state of collimation comprehension from answering the OP's question to more in depth discussions,from which we "ALL" can learn!! .I have thoroughly enjoyed the technical discussions in this thread and have learned much from it.Thanks to all the collimation experts who have contributed,for the benefit of all!!

Clear skies.

[Don W]

Let's avoid the personal jabs. We all know what happened and it's been dealt with. THE END

[LcJ]

One thing I do know from when I shot a lot of pool. When I only played with people I could beat, I never got any better. But when I started getting beaten a lot, I got a lot better.

If you always do what you've always done, you will always have what you've already got.

Learning isn't always fun, but it is always good! And this collimation thing is a lot more involved and the more I learn, the more I don't know.

Thanks to all who share from their abundance of knowledge and experience. We all garner something even if only the crumbs. But some of us, like me, learn then forget then have to do it all over again.

Peace,
Lyle

[Substorm]

thanks Vic and Jason and everybody for your time and input on this subject. After reading all the info i decided on my personal need for the collimation tool

[hudson_yak]

Sounds like a one-way mirror is what the doctor has orderd

So, are you going to try this Jim? It does seem to me that the AC would be at least a little bit better if some of the images weren't going AWOL just before the moment of truth.

And, without thinking it through very much, given what was said earlier about parallax, maybe you wouldn't need a peephole at all?

Mike

[CatseyeMan]

Sounds like a one-way mirror is what the doctor has orderd

So, are you going to try this Jim? It does seem to me that the AC would be at least a little bit better of some of the images weren't going AWOL just before the moment of truth.

Actually, I've pondered the concept of a "no-pupil" AC mirror for a long time. The compromise is that typical 1-way (sometimes called 2-way) mirrors are made by actually reducing the reflective film deposit thickness sufficiently to allow light transmission (for viewing through the mirror) which of course will reduce the reflectivity. We certainly don't want to (literally) "loose sight" of that 4th image so important to the CDP.

And, without thinking it through very much, given what was said earlier about parallax, maybe you wouldn't need a peephole at all?

It's possible a 1-way mirror scenario may act as a "projection screen" much like the Barlowed laser return screen eliminating parallax error and the need for a small pupil.

[Jason D]

which of course will reduce the reflectivity. We certainly don't want to (literally) "loose sight" of that 4th image so important to the CDP.

Not to mention stacked images will have to pass through the center triangle at least once (except for the 1st) which will reduce their intensity significantly (especially the 4th image) -- unless the center triangle is a flat mirror
Jason

[LcJ]

Due to my Christmas generosity, I will volunteer to try one of the 1 (2) way mirrors

[Jason D]

Just curious, sometimes the 4 images are referred to as "P", "1", "2", and "3" and sometimes as "1", "2", "3", and "4". I do understand the first image is special because it does not undergo any reflections.
Which one of these two conventions is commonly used?
Jason

[CatseyeMan]

Just curious, sometimes the 4 images are referred to as "P", "1", "2", and "3" and sometimes as "1", "2", "3", and "4". I do understand the first image is special because it does not undergo any reflections.
Which one of these two conventions is commonly used?
Jason

The original "P,1,2,3" convention was started in the original discussions 4 years ago - I think "P" was for "Primary" (reflection). The discussion here using the "1,2,3,4" notation just sort of happened in the context of distinguishing them in the reflection sequence they are generated in the total group of "4".

Do we need to take a forum vote on the preferred notation?

[CatseyeMan]

which of course will reduce the reflectivity. We certainly don't want to (literally) "loose sight" of that 4th image so important to the CDP.

Not to mention stacked images will have to pass through the center triangle at least once (except for the 1st) which will reduce their intensity significantly (especially the 4th image) -- unless the center triangle is a flat mirror
Jason

... not quite sure what you are thinking here, Jason. In terms of reflection path, an AC with no peep hole behaves the same as a conventional AC with the following exceptions: There's no peephole for the 3rd "real" image to fall through (facilitating the Star of David formed at convergence) and the "visibility" of the stacked images becomes a function of the "transmission" property of the 1-way mirror (rather than a clear shot through the mirror). This of course is not considering the potential "projection screen" possibility.

[Jason D]

Jim, I was referring to reflections at the Primary end -- not at the AC end. When all images are stacked, it means the last rays (for images “2”, “3”, and “4”) have to emanate from the primary center where the triangle sticker is located.
With a peep hole, that last 3 images are invisible therefore the above will not come to play. With a peep hole, the above will reduce the intensity of the last 3 images significantly – perhaps to nil.
Jason

[Starman1]

Perhaps I'm not following this.
What problems result from having a peephole that are solved by a 1-way mirror?
I see some problems that might be solved by having a smaller hole or a thicker top plate (longer tunnel on the peephole), but it seems that a 1-way mirror might cause more problems.

[Jason D]

Don, the 1-way discussion is not to solve the parallax issue but rather to solve the disappearance act of the last 3 images. The peep hole is the reason why the last 3 images disappear under "perfect" collimation. The 1-way mirror will allow all stacked images to remain – at least in theory.
Jason

[hudson_yak]

Right. Although this is probably just another academic point, considering the calculations Jim presented earlier on this page, it does seem to me that the dissappearing compromises the 2x-4x-6x-8x accuracy claim of the AC at least a little. If that didn't happen its accuracy superiority to a cheshire would be more convincing. I'd buy one, anyway.

Mike