57 replies to this topic

[carver2011]

Hello

Does anyone know of a commercial seller of Tri Bahtinov Masks. I would like to purchase one for an 8” SCT. 

Regards

Ed

[cytan299]

Hi Ed

 

Sadly, the answer is there are no commercially available masks. You'll have to make one yourself.

 

cytan

Edited by cytan299, 06 June 2018 - 06:23 PM.

[carver2011]

Thank you cytan

I was hoping that wasn’t the case. 

Regards

  Ed

[Scott Beith]

https://www.highpoin...KMaAuxaEALw_wcB

 

This is for a 10" but you might be able to find an 8".

[cytan299]

https://www.highpoin...KMaAuxaEALw_wcB

 

This is for a 10" but you might be able to find an 8".

Hi Scott,

   That's a Bahtinov cut onto a telescope cover and not a Tri-Bahtinov. Here's a link to the Tri-Bahtinov if you're not familiar with it:

 

https://github.com/c...299/tribahtinov

 

It's used for both collimation and focusing.

 

cytan

[Scott Beith]

I was definitely not familiar.  Thanks for the link.  

[jhayes_tucson]

Oh boy...I don't think that's going to work very well.  The first problem is astigmatic error in the optical system.  Putting multiple masks around the aperture will definitely show any anamorphic errors really well, but it won't provide unambiguous guidance for optimum alignment.  I think that the other problem is one of sensitivity.  Secondary misalignment in a SCT produces on-axis coma--not astigmatism.  I've done some analysis of the standard mask and found that it's possible to detect focus changes to about 1/10 wave over the whole aperture.  I don't think that that's going to be sensitive enough to detect small amounts of coma by sub-sampling the pupil.  I'd have to do some analysis to confirm it but I'll be surprised if you get enough sensitivity to achieve very good alignment using this idea.  It might work to get the alignment roughed in but I doubt that it will be much better than simply centering the secondary shadow, which in itself, isn't sufficient to achieve good alignment.

 

John

[cytan299]

Oh boy...I don't think that's going to work very well.  The first problem is astigmatic error in the optical system.  Putting multiple masks around the aperture will definitely show any anamorphic errors really well, but it won't provide unambiguous guidance for optimum alignment.  I think that the other problem is one of sensitivity.  Secondary misalignment in a SCT produces on-axis coma--not astigmatism.  I've done some analysis of the standard mask and found that it's possible to detect focus changes to about 1/10 wave over the whole aperture.  I don't think that that's going to be sensitive enough to detect small amounts of coma by sub-sampling the pupil.  I'd have to do some analysis to confirm it but I'll be surprised if you get enough sensitivity to achieve very good alignment using this idea.  It might work to get the alignment roughed in but I doubt that it will be much better than simply centering the secondary shadow, which in itself, isn't sufficient to achieve good alignment.

 

John

Hi John,

 

   I know from your posts that you are one of the resident experts on optics. I don't have the required expertise to even come close to rebutting any of your criticisms. There are three points I'd like to make in support of the Tribahtinov, the first is that  empirical evidence shows that the Tri-Bahtinov does, in fact, find misalignments, for example:

https://blog.goo.ne....0a428092c877abc

 

where the author describes finding his camera tilt error which was not found with a Cheshire.

 

The second is that the idea behind the Tri is trivial: it's looking at the focus error for three orientations from the three different path lengths the light rays travel. If what you say about the Bahtinov is true, that the focus with the Bahtinov allows you to get to 1/10 wavelength then the Tribahtinov should be able to detect this much error as well. This means that if you believe that the Bahtinov works, then the TriBahtinov should help you to focus properly on all three orientations by tweaking on the collimating screws. Again, anecdotal evidence from this link shows how a C14 was finally collimated:

 

http://buffaloastron...-April-2018.pdf

 

 

Finally, the best part about the Tribahtinov is that's it's "free". Try it out. There's sufficient anecdotal evidence from a google search to show that it does help with getting "good" collimation. Perhaps not good enough to your standards though

 

cytan

Edited by cytan299, 07 June 2018 - 07:30 AM.

[Stu Todd]

John, I started a thread asking if anyone had used a Duncan mask for collimating and focussing a SCT, the results of using one and the Tribahtinov debate came into it.

Certainly, the diffraction images in the Tri thread look to show misalignment of the secondary in one axis and correction applied quite nicely.

 

Would you go with the Tribahtinov over a Duncan mask, prefer the Duncan or forget both?

 

Stu

[jhayes_tucson]

cytan and Stu,

I understand the idea of this mask, which is why I think that it will have big problems if the system has any astigmatic error.  In that case, it may indicate an alignment error when there is none.  Have you ever rotated a B-mask on a system with anamorphic components?  It will (correctly) show the best focus position changing with the mask angle.  In fact, this is a very simple test that you can do on your system to demonstrate that it does not have astigmatic-type optical errors (to within about 1/10 wave.)

 

Assuming perfectly made optics, my concern is about the sensitivity of the measurement relative to the required alignment tolerance.  Ideally, the sensitivity should be at least good enough to minimize alignment induced errors in the wavefront to less than 0.07 waves rms.  That's the requirement for a diffraction limited system.  As I said, I haven't done any analysis but I suspect that even with a perfect system, that might be hard to do with this method.  On top of that, when you add in the normal surface errors present in any optical system, I have a concern that this kind of mask may provide conflicting or a false signal with regard to how to best align the secondary, but I'll have to give it a bit more thought.

 

Let me emphasize that my concern is about the usefulness of this mask for aligning the secondary.  I am a big believer in these types of diffraction masks for focusing.  Bahtinov had a very clever idea!

 

John

[cytan299]

cytan and Stu,

I understand the idea of this mask, which is why I think that it will have big problems if the system has any astigmatic error.  In that case, it may indicate an alignment error when there is none.  Have you ever rotated a B-mask on a system with anamorphic components?  It will (correctly) show the best focus position changing with the mask angle.  In fact, this is a very simple test that you can do on your system to demonstrate that it does not have astigmatic-type optical errors (to within about 1/10 wave.)

 

Assuming perfectly made optics, my concern is about the sensitivity of the measurement relative to the required alignment tolerance.  Ideally, the sensitivity should be at least good enough to minimize alignment induced errors in the wavefront to less than 0.07 waves rms.  That's the requirement for a diffraction limited system.  As I said, I haven't done any analysis but I suspect that even with a perfect system, that might be hard to do with this method.  On top of that, when you add in the normal surface errors present in any optical system, I have a concern that this kind of mask may provide conflicting or a false signal with regard to how to best align the secondary, but I'll have to give it a bit more thought.

 

Let me emphasize that my concern is about the usefulness of this mask for aligning the secondary.  I am a big believer in these types of diffraction masks for focusing.  Bahtinov had a very clever idea!

 

John

Hi John,

  Practically for most of us, it's the defocus star method with the obstruction to check for collimation. If there are other imperfections like astigmatism (unless it is pretty obvious), it is usually ignored by most of us when we do simple collimation because we _assume_ perfect optics! I understand your concern about astigmatism. However, collimation will not correct astimagtism anyway, so I believe this argument is moot.

 

  My idea for the Tri-Bahtinov is to leverage our use of the Bahtinov. And, IMO, for all practical purposes, it works as described to first order. The dominant error is the seeing when you look at a star to collimate. I just couldn't reliably collimate with the defocused star method, but with the Tri-Bahtinov it is very easy. And its results can always be confirmed by the defocus star method.

 

  

 

 Like I had said above, it costs next to nothing to try it out. If you don't like it, or it doesn't work, it's less than $50 out of pocket. And if people don't find that the Tri-Bahtinov mask works, I'd like to hear about it. It's nice to have data!

 

cytan

Edited by cytan299, 07 June 2018 - 12:40 PM.

[jhayes_tucson]

That's all fine but there's one thing I want to point out.  Simply centering defocused shadows only gets you started.  In order to precisely align the secondary mirror, you need to examine the Airy disk.  A lot of folks stress over getting the best possible optics; but, it only takes a very small amount of misalignment to pull the Strehl below 0.8.  I would wager that a large fraction of SCTs operate in the range of 0.5 - 0.75 Strehl simply because of poor secondary alignment.  

 

You are 100% correct that astigmatic errors can't be corrected with alignment, but that's not what I'm talking about.  The problem is that an astigmatic system may provide a false alignment signal which would make it hard to minimize alignment error...and that does count.

 

Maybe this idea works so I agree that it would be nice to have some data and a bit of analysis so see how well it works.

 

John

[cytan299]

That's all fine but there's one thing I want to point out.  Simply centering defocused shadows only gets you started.  In order to precisely align the secondary mirror, you need to examine the Airy disk.  A lot of folks stress over getting the best possible optics; but, it only takes a very small amount of misalignment to pull the Strehl below 0.8.  I would wager that a large fraction of SCTs operate in the range of 0.5 - 0.75 Strehl simply because of poor secondary alignment.  

 

You are 100% correct that astigmatic errors can't be corrected with alignment, but that's not what I'm talking about.  The problem is that an astigmatic system may provide a false alignment signal which would make it hard to minimize alignment error...and that does count.

 

Maybe this idea works so I agree that it would be nice to have some data and a bit of analysis so see how well it works.

 

John

Hi John,

  I agree that absolutely on how well the Tri-Bahtinov works will need to be tested. But I think you're the expert in this area and have the tools and experience to do the calculations to definitively pronounce how well it works or doesn't. One supportive data point is that a tilted camera plane was diagnosed with the Tri-Bahtinov and so there is hope that this mask does indeed detect misalignments.

 

  I agree that defocused shadows only gets you started and IMO is as good as most of us can do given the economics of doing better.

 

cytan

[jhayes_tucson]

You can easily diagnose a tilted camera relative to the image plane with a standard B-mask.  Each mask component in a B3-mask covers a smaller portion of the aperture so it will be inherently less sensitive to focus errors relative to a standard B-mask so I don't see any reason why a B3-mask would be better at detecting a tilted image plane.  Furthermore, the ability to detect a tilted image plane isn't relevant to detecting secondary misalignment.

 

John

[Psittacula]

Hi John and cytan,

For sensitivity,
By scintillation, the star image through the mask appears to vibrate as if the focus fluctuates. This prevents confirmation of focusing.
And, focus confirmation in three directions by rotating the existing bahtinov mask may be more difficult because there is no simultaneity.
On the other hand, since the tri-bahtinov mask can simultaneously confirm focus in three directions, I felt it relatively easy.

If you have knowledge, please tell me the relationship between diffraction grating parameters and sensitivity. I merely cite the formula in the discussion of the original Bartinov mask. *1

Regarding alignment,
I think the tri-bahtinov mask basically gives information on three degrees of freedom. Therefore, for an optical axis adjustment mechanism with more degrees of freedom, it will show several appropriate focus points. And I actually experienced it in Newtonian. Even when the laser collimator does not pass through the center mark of the primary mirror, it will show focus points in three directions. This would be forming an off-axis optical system.

Here, the optical axis adjustment mechanism in the SCT has only three degrees of freedom for ordinary users. Since it matches the degree of freedom of the tri-bahtinov mask, I think that the optical axis adjustment is reasonable within that restriction.
By the way, is this optical adjustment mechanism with low degrees of freedom of SCT due to tolerance to alignment?

Also, I noticed the possibility of changing the thickness of the diffracted light streak, but it may be information that does not become an advantage in the sensitivity of this mask.

*1:https://www.cloudyni...s/#entry2309839

Satoru

[jhayes_tucson]

Satoru,

 

1)  You can very easily detect astigmatic error to within about 1/10 wave in a scope under the sky by rotating a regular B-mask.

 

2)  I don't understand how the tri-Bahtinov mask gives information about three degrees of freedom.  There are only two degrees of freedom that are adjustable on a SCT secondary mirror (tip and tilt.)  Just because the manufacturers (mistakenly) use three screws does not add a degree of freedom to the adjustment.  Using three screws just creates cross-talk between each axis as you adjust the screws.  (BTW, the 6 degrees of freedom on any part are:  tip, tilt, clock angle, x, y, and z.)

 

3)  The sub aperture masks in the tri-mask cover roughly 1/3 of the full aperture, which means that the sensitivity of each sub-aperture will be roughy 1/3 that of the full aperture.  So the ability to minimize defocus in each sub-aperture will be roughly about 1/3 wave (relative to the whole aperture,) which isn't very good.

 

John

Edited by jhayes_tucson, 08 June 2018 - 11:53 PM.

[Psittacula]

Hi John,

 

Thank you for your comment.

 

 

Regarding degree of freedom,
 Looking at the secondary mirror structure of the SCT, the secondary mirror seems to be supported by the pivot at the center of the plastic cell and the three adjusting screws. And due to the effect of the spring by plastic strain, it seemed that each screw could be adjusted independently within a certain range. As a result, it seemed that tip, tilt and movement in the optical axis direction (z?) Could be adjusted with a combination of three screws.
However, this will not be a very sophisticated adjustment mechanism.

 

 

Well, as for sensitivity, I have not understood well yet. I want to know the relationship between sensitivity and diffraction image.

 

My imagination is like this.

Compared to regular bahtinov mask, the area of ​​individual bahtinov mask of tri-bahtinov mask is about one-third the area. As a result, I think that the intensity of the diffraction image weakens and the streak becomes thick. The simulation result by maskulator of normal Bartinoff mask and Bartinoff mask with radius halved is attached. How should we evaluate the difference in sensitivity against such image difference?

 

: reslut of normal Bahtinov mask

: Pattern of normal Bahtinov mask

 

: reslut of half sized Bahtinov mask

: Pattern of half sized Bahtinov mask

 

Satoru

Edited by Psittacula, 10 June 2018 - 04:26 AM.

[jhayes_tucson]

Hi John,

 

Thank you for your comment.

 

 

Regarding degree of freedom,
 Looking at the secondary mirror structure of the SCT, the secondary mirror seems to be supported by the pivot at the center of the plastic cell and the three adjusting screws. And due to the effect of the spring by plastic strain, it seemed that each screw could be adjusted independently within a certain range. As a result, it seemed that tip, tilt and movement in the optical axis direction (z?) Could be adjusted with a combination of three screws.
However, this will not be a very sophisticated adjustment mechanism.

 

 

Well, as for sensitivity, I have not understood well yet. I want to know the relationship between sensitivity and diffraction image.

 

My imagination is like this.

Compared to regular bahtinov mask, the area of ​​individual bahtinov mask of tri-bahtinov mask is about one-third the area. As a result, I think that the intensity of the diffraction image weakens and the streak becomes thick. The simulation result by maskulator of normal Bartinoff mask and Bartinoff mask with radius halved is attached. How should we evaluate the difference in sensitivity against such image difference?

 

 

Satoru

 

 

Satoru,

The degrees of freedom must be orthogonal (i.e. at right angles) such that they are independent.  The three adjustment screws at 120 degrees on the secondary are NOT orthogonal.  Adjusting any one screw affects two axis in the output.  There are only two degrees of freedom adjustable on the secondary mount and they are called "tip" and "tilt."  Any change in the "z" axis is very, very tiny and unintended due to the way the mount pivots about a point behind the mirror surface.  I've redesigned the secondary on my mount on my telescope to have true orthogonal adjustments, which makes alignment very easy. Each screw is configured to change only one axis at a time.  This is something that all manufacturers should do.

 

The sensitivity of any measurement system refers to the minimum detectable change--ignoring any external influences such as seeing.  A while back I did a study of the B-mask to look at this issue.  I've attached a monochromatic diffraction calculation showing how the pattern changes with various amounts of 3rd order defocus.  Remember that defocus is given in terms of waves per pupil radius.  So these patterns are valid for any size aperture with the specified waves of defocus per pupil radius.  I also looked at the smallest amount of defocus that I could introduce with a reasonable expectation that an observer could reasonably detect the change and found that it's about 0.1 waves/radius.  Obviously if you throw poor seeing on top of the measurement, the sensitivity will be further decreased.

 

The fact that the sub apertures in the triple mask are roughly 1/3 the size of the full aperture makes them 1/3 as sensitive to defocus in the full aperture.  That means that the triple mask will have a measurement sensitivity of roughy 3/10 wave ~ 1/3 wave over the full aperture.   That is insufficient to guarantee diffraction limited performance over the entire pupil.  I haven't looked that the effect relative to trying to detect coma but I suspect that you'll have the same problem.  Will it be able to help dial out coma?  Probably.  Will it have enough sensitivity to achieve really good, unambiguous secondary alignment?  Probably not.

 

John

Attached Thumbnails

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[cytan299]

Hi John,

  Then does tripling the number of slits, or subapertures, recover sensitivity?

 

cytan

[dmdouglass]

Now THIS has become a **FANTASTIC** discussion.

Good information flowing here.

[jhayes_tucson]

Hi John,

  Then does tripling the number of slits, or subapertures, recover sensitivity?

 

cytan

Short answer:  no.

 

Making the slits narrower simply increases the length of the diffracted lines and does nothing for the sensitivity.  Increasing the number of sub-apertures makes them smaller and that further decreases the sensitivity.   The B-mask is interesting in that it relies on geometric optics for operation and diffraction to create the "marker" lines.  It is not a true diffractive optical element in that it's not modifying the wavefront in the way that a hologram might do.

 

John

[cytan299]

Short answer:  no.

 

Making the slits narrower simply increases the length of the diffracted lines and does nothing for the sensitivity.  Increasing the number of sub-apertures makes them smaller and that further decreases the sensitivity.   The B-mask is interesting in that it relies on geometric optics for operation and diffraction to create the "marker" lines.  It is not a true diffractive optical element in that it's not modifying the wavefront in the way that a hologram might do.

 

John

Hi John,

   Thanks for the reply. 

 

   Anecdotally, at least from all the posts that I have seen, the Tri-Bahtinov seems to work quite well. However, it may be because in real life, we collimate with a real star when seeing isn't that great. Personally, I've never had good luck with collimation just by looking at the Airy disk alone. To my it's like focusing -- without the Bahtinov, it's a lot more difficult in unsteady skies.

 

More responses from users will definitely help determine its usefulness.

 

cytan

Edited by cytan299, 10 June 2018 - 08:34 PM.

[jhayes_tucson]

I guess the question is how do you know how well the system is aligned after you use the tri-mask for alignment?

 

John

[cytan299]

I guess the question is how do you know how well the system is aligned after you use the tri-mask for alignment?

 

John

 

Hi John,

 

IMO, if the spikes are not symmetric before adjustment and it is after adjustment. I believe, the statement, it is better collimated after adjustment applies. If you want perfection by looking at the focused Airy disk then, unless the seeing is very good, I don't believe it can be done most of the time.

 

cytan

Edited by cytan299, 10 June 2018 - 09:15 PM.

[jhayes_tucson]

I think that you are missing the point.  If you measure the length of a piece of machined aluminum to be 138.42 mm, it's a perfectly valid question to ask how good is that measurement?  The "goodness" of any measurement can be broken down into three components:

 

1) The repeatability of the measurement--meaning if you measure it multiple times, what is the uncertainty in the result?

 

2) The reproducibility of the measurement--meaning if multiple operators make the measurement, how much uncertainty or bias is introduced across operators?

 

3)  The absolute accuracy of the measurement--meaning how well does the average result compare to a known standard?

 

In this case, you are using a modified B-mask that has at best only a vague underlying theory of operation to measure the alignment of a secondary mirror.  So it's valid to ask some simple questions:

 

a)  How do you know that your system is better aligned after using the mask to make the adjustment?  Just because you believe that it is better aligned, does not make it so.  Is the imaging better or worse?  How does the secondary shadow look after alignment?  How does the Airy disk appear?  Can you corroborate results in the wavefront?  These are all possible questions that can be used to approach an answer.

 

b)  How do you know how close it gets to producing a well aligned system?  And here "a well aligned system" refers to the absolute standard of perfect alignment.  A key part of this question is how repeatable is the method?  Do you get the same result every time or is it perfect one time and then all over the place the next time around?  This is a harder question to answer but it is answerable with some care and a bit of methodology.

 

c)  Can you come up with a theory of operation?  The theory is what allows the limits of the method to be explored and understood.  I can understand how this method can show different focus positions as a function of clock angle but I'm not so sure that I understand how this method relates to precisely aligning the secondary mirror.

 

A lot of folks claim that they use a star test to regularly achieve perfect alignment and I could ask the same questions about that method.  In that case there are some good ways to approach answering those questions--and Suiter wrote a whole book about analyzing star images.  I am trying to get you to understand that simply because you can turn the screws to center up all of the diffraction patterns using this mask does not necessarily mean that this method works to align a scope.  Maybe it does; but, wishful thinking isn't a part of how you demonstrate that it works.

 

John