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Best Practices Using Tri-Bahtinov Mask for Ritchey-Chretien Collimation?

Equipment Ritchey-Chrétien Collimation Help Needed
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Ben Koltenbah avatar
Ben Koltenbah avatar
Greetings!

I've been upping my skills collimating my Teleskop Services 10-inch Truss Ritchey-Chretien scope.  I follow many of the steps detailed in Chris Woodhouse's The Astrophotography Manual, 2nd ed., starting on page 397, particularly the earlier steps.

In addition to all those steps, I find the claims of the Tri-Bahtinov mask to be attractive, where collimation can be finely tuned at focus by centering the six-fold diffraction spikes.  This would be a great final check / fine adjustment for me.  What I'd like to know is if someone has put together a procedure for using this mask for doing final adjustments for both the primary and secondary mirror?  I haven't been able to settle for myself whether or not this mask provides discernment between the two mirrors, or if it's not designed to distinguish movement between the two, just showing the combined state of collimation.

I would appreciate any guidance you could provide on the best use of this mask.  Thank you!

Best Regards,
Ben
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Mike H - Sky View Observatory avatar
Mike H - Sky View Observatory avatar
https://www.youtube.com/watch?v=eJ9_14Jvenk

You have to ignore the part about printing your own mask but I found it informative. I bought mine cheap from Far Point but haven't got a chance to use it yet.

If you've never seen this guy Cuiv, you'll get a kick out of him.
Ben Koltenbah avatar
Ben Koltenbah avatar
https://www.youtube.com/watch?v=eJ9_14Jvenk

You have to ignore the part about printing your own mask but I found it informative. I bought mine cheap from Far Point but haven't got a chance to use it yet.

If you've never seen this guy Cuiv, you'll get a kick out of him.

Thanks for the link, Mike.  I had run across his video before.  He's certainly excited about using his mask!

Unfortunately, he doesn't address my question about whether or not there is procedural difference between primary and secondary mirror adjustment.  This is still an open question for me.  I have a new mask on order, and when it arrives I will examine this issue myself, but my question still stands if someone has accounted for this already.
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Come Raczy avatar
Come Raczy avatar
Hi Ben,

Did you manage to make any progress on this important question?
So far, the only thing I found is ajapanese page that seems to follow this procedure (if I understand correctly): after a coarse traditional collimation (centering the mark with the secondary and adjust baffles concentricity with the primary), uses the tri-bahtinov mask for fine adjustment of the secondary (I guess using a star in the center), and finally uses the tilt plate to re-center the mirror marker. I don't find this really satisfactory.

Re-reading the DSI method (iteratively correcting on-axis coma with primary, then of-axis imbalance with secondary), I have the feeling that the method could benefit from a tri-bahtinov mask. The obvious one is the on-axis coma correction: that's what everyone seems to be doing, but that would be corrected with the primary instead of the secondary. The less obvious is the identification of "pointy" and "flat" stars, from the pattern of spikes (the direction of the major axis should be related to the relative offsets of the three center spikes). One additional capability of the tri-bahtinov mask is that it would also identify shifts in the focus plane and enable adjusting sensor tilt before adjusting the secondary.

So, basically, the algorithm could be to iterate over the following steps, until no additional adjustment is needed:
  1. focus on a bright star in the center to minimize the offset of the center spike in all three directions
  2. adjust the primary until the center spikes are dead center in all three directions
  3. for each of the 8 peripheral regions
    1. adjust the focus to minimize the offset of the center spike in all three directions
    2. keep note of the difference with the original focus position
    3. adjust sensor tilt so that all 8 peripheral regions are in the same plane of focus

  4. for each of the 8 peripheral regions, infer the direction of the elongation of the stars from the relative positions of the center spikes
  5. calculate the imbalance of the image and adjust secondary mirror as needed


Unfortunately, I haven't had enough time experimenting with this so I can't formalize and quantify the objective measurements that can be done at each step.

Best,
Come
DanJunge avatar
DanJunge avatar
Hi Ben. 

I have started lately to use this mask (actually printed one from the link provided by Cuiv smile. -Took some trial and error to print a 14" mask on a Creality Ender 3..:happy-1smile 
How I do it on my C14edge is a combination of both mirrors at the same time. The primary will center the spike on each bahtinov pattern and the secondary will center the 3 patterns on top of each other. I keep adjusting both with small adjustments until all is centered. 

Clear skies. smile
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Ben Koltenbah avatar
Ben Koltenbah avatar
Thank you, Come and Dan for your responses to my posted question.

I have not been able to make progress on the question of how one could use a Tri-Bahtinov mask to assist with full, dual-mirror collimation of an RC scope, or likely any Cassegrain scope.  This is the beginning of the Fall season in the Pacific Northwest, and so my opportunities to image will be reduced.  There are some clear skies at the moment, however I am presently imaging with my small refractor, from which such collimation requirements are blissfully absent.

It is my plan to explore the effects of both secondary and primary mirror adjustments on the spikes from this mask to determine if a suitable procedure could be devised.  I appreciate your inputs and suggestions as they will make a good starting point.  A candidate procedure that I've conjured up is to do something similar to what you said, Dan, to make small adjustments to both primary and secondary mirror iteratively while observing a star at the center of the FOV.  I start with the assumption that collimation is adequately "roughed-in" using daytime and preliminary star test collimation.  This is now the final step for higher precision collimation.  One idea would be to adjust "half-corrections" on each mirror at a time to bring all the spikes to symmetry.  However, this supposes that such dual adjustments would bring collimation to convergence.  That assumption needs to be tested.  My wish is to be able to come up with a procedure that does not require examination of corner starts, however that may not be possible.

Should you come up with anything new to add, I would appreciate hearing about it!

Until then, here's an example of my latest use of the mask to at least check my alignment following adjustments of the primary mirror on a central star.  This was done on a bright star with fast exposures.  In the future, I should use longer exposures on a dimmer star to average over seeing, which causes the spikes to wiggle around.

Best Regards,
Ben

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Ashraf AbuSara avatar
Ashraf AbuSara avatar
Have you had any success figuring out how to use a tri-bhatinov mask to get things collimated for both the primary and the secondary? Really interested in this approach.
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Ben Koltenbah avatar
Ben Koltenbah avatar
Ashraf:

So sorry for not seeing your post until now.  I've been busy with non-astrophotography related things including work, travel, family, taxes, etc!

To answer your question: "No, unfortunately!"  We've had a long run of typical winter weather for several months with dark clouds, rain, some snow.  This is usually my off season.  I do see that I may finally get some clear nights starting later this week, so let's keep our fingers crossed.  Maybe I'll have some time to test things, particularly this weekend.  However, I may wish to get my collimation in place using all techniques at my disposal, and then at long last get into some imaging.

Best Regards,
Ben
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Ben Koltenbah avatar
Ben Koltenbah avatar
Greetings from the Pacific Northwest, briefly emerging from a long, dark, wet winter at long last!

I've  finally had opportunity to set up my RC10 scope once again.  In the previous months I did a tear down of my scope in order to install new fans on the back plate.  This went well, and my new fans are whisper quiet.  The previous were noisy and periodically failing.

Now with the telescope reassembled, mirror spacing set using a bright star and Ronchi grating, I worked on collimation of the focuser, secondary mirror and primary.

I did not have time to make the concerted study of the Bahtinov mask that I desired as the mirror spacing task was my greatest priority.  However, I did spend quite a bit of time aligning the focuser and  then secondary with a laser and collimation telescope.  This went well.

Lastly, I aligned the primary with a defocused star.  Checking with the Bahtinov mask, this looked good.  I was most pleased to be getting nice circular stars both inside and outside of focus.  These would be elliptical, showing astigmatism when the mirror spacing is not set correctly.

At this point, I have only used my mask as a double check after eyeing in the collimation using standard methods.  However, I did want to post that I may be back in business after a long dreary season, and perhaps I can make this a higher priority next time.

Until then, I'd love to hear what you have been up to, particularly if you have fiddled with your mask.

Best Regards,
Ben
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John Hayes avatar
John Hayes avatar
Ben,
I don’t understand how this mask will show 3rd order coma, which is a major thing to correct in a RC by tilting the primary.  Have you done a far field diffraction calculation to show what the pattern looks like with coma?

Frankly, I don’t believe that this will turn out to be a very good way to align a RC.  I just had great success aligning my new RC using SKW.  It was quite repeatable and once you get the alignment screws correctly identified between the scope and the software, it was super easy to use.  I could dial the on-axis coma down to a reported 0.02 waves rms (or less).  A Takahashi alignment scope can be used to set the secondary tilt and that’s what got my scope really close to final alignment.

John
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Ben Koltenbah avatar
Ben Koltenbah avatar
John:

Thanks for chiming in!  It's good to hear from you!

Several months ago I was musing about whether or not adjustments of the primary mirror vs. secondary mirror could be discerned with a Bahtinov mask.  My suspicions all along have been that they are probably not differentiable, but I wanted to either test this myself or perhaps calculate it.  I threw it out there to see if anybody had tried this, and as you can see no one had.  However, the fact is I get so little imaging time that I haven't been willing to commit any time to it.

Is there some hopefully free software out there that would calculate diffraction patterns from an obscured aperture, including a mask like this?  I know how to set up the calculation myself (intersection of my day job), but my method is way overkill for this problem.  Any suggestions?

Best Regards,
Ben
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Francis Moreau avatar
Francis Moreau avatar
Ben Koltenbah:
Is there some hopefully free software out there that would calculate diffraction patterns from an obscured aperture, including a mask like this? I know how to set up the calculation myself (intersection of my day job), but my method is way overkill for this problem. Any suggestions?

I think Spherofront can help you. Unfortunatly, i've no idea how to use it.
https://diffractionlimit.com/

The discovery of this subject prompted me to do some research to improve the way I collimate my RC8.
I've just printed out the tri-bahtinov. I used the modified version. I'll be testing it in the next few days on my RC8.
Ben Koltenbah avatar
Ben Koltenbah avatar
Thank you, Francis!

This is the very thing I would like to delve into when I get the time.  I'll post back here if anything comes out of it.

Ben
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John Hayes avatar
John Hayes avatar
Ben Koltenbah:
John:

Thanks for chiming in!  It's good to hear from you!

Several months ago I was musing about whether or not adjustments of the primary mirror vs. secondary mirror could be discerned with a Bahtinov mask.  My suspicions all along have been that they are probably not differentiable, but I wanted to either test this myself or perhaps calculate it.  I threw it out there to see if anybody had tried this, and as you can see no one had.  However, the fact is I get so little imaging time that I haven't been willing to commit any time to it.

Is there some hopefully free software out there that would calculate diffraction patterns from an obscured aperture, including a mask like this?  I know how to set up the calculation myself (intersection of my day job), but my method is way overkill for this problem.  Any suggestions?

Best Regards,
Ben

Ben,
Unfortunately, I'm not aware of any free software that will do just what you want.  I once wrote the code to do it long ago but my programing skills are now way too out of date to reproduce that work.  My problem with the idea of using any of these masks to do optical alignment is that it's not at all intuitively obvious what they do to help align the optics.  It might somehow help or maybe I'm not understanding something but at first glance, it seems like just a random idea of something to try.

John