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Checking and performing conditional collimation using Bahtinov masks!


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Hi Everyone.

I have devised a quick way to check binocular miscollimation but also to conditional alignment binoculars and I would like to further field-test the procedure with more people. We have had preliminary tests of this alignment procedure submitted by individuals and the feedback looks very promising. There may be some points of deficiency in this procedure that we currently cannot see and will need to be addressed. I would appreciate hearing from other individuals who have used this procedure and to hear what results they obtained. Can somebody try and review the procedure, to indicate if it's was easy for you to find misalignments errors or the need for some refinements? Please, I must ask, to stay on subject and not make a general conversation. I am interested in the test results and of any improvements in the procedure.

Advantages of Bahtinov mask for collimation:

It is simple!

You can test it using a star, which is the best method for checking collimation by yourself. Or you can use street lights. This method not only lets you know if there is a problem, but is also a very good method for conditional alignment your binoculars.

Having tried different techniques over the years, I have never achieved total satisfaction from any of them. For example, using the "relaxed eye method", eyes have the tendency to accommodate for misalignments. So sometimes it's not that easy in locating the misalignments. Especially for horizontal misalignments, where eyes work better and fast to eliminate the problem, finding the error is a pain.

The "defocus one eyepiece" method, although it is the most easy and reliable, has a small disadvantage. Eyes try to accommodate the problem if this exists, and the star in focus looks like "fishing around" the defocus image. Furthermore we have noticed that this is dependent of the eyes position on binos axis. So sometimes you are UNCERTAIN if a problem exists or not.

With Bahtinov mask,

  • You can observe even the small divergences of vertical and horizontal miscollimation, because of the spikes addition.
  • You can tell very easy, what is the problem, Vertical, Horizontal or both?
  • Especially when you want to fix small aberrations of miscollimation.
  • Bahtinov mask spikes reduce the tendency of eyes to accommodate for misalignments. When eyes are trying to accommodate, spikes are easier to separate. Eyes CANNOT accommodate the misalignments EVEN IF YOU TRY HARD. And this is the advantage from the use of the mask.

How to test:

As it costs nothing, download a mask just to give it a try, using parameters only for Aperture and Focal Length, from:

astrojargon - Bahtinov Focusing Mask Generator: Version 0.4

- First print out 2 copies of the generated file from your browser on a paper, cut it, and uses a rubber band to hold it in place.

- Turn the second mask vertical 90 degrees in relation of the 1st mask axis. Then watch the diffraction spikes pattern around a star or a strong long distance street light, and adjust the focus.

That's it!

If you see one star everything is OK. If you see two stars then you have a miscollimation problem. You can find even small misalignments in collimation. Using a small screwdriver turn the appropriate screws (usually 2) to adjust the prism until the two stars overlapped and see only one star. It is an easy way for conditional alignment of your binoculars.

In the photo I used a laser cut mask and one from paper. You can make both from paper (or from inkjet transparency as you will read at the end of the page). Also use a tripod.


IMG_2166 b.jpg


1. First focus each eyepiece.


Bahtinov Diffraction Spikes.png

Sirius through the Bahtinov Mask. At left, the image is out of focus. At right, the star is in focus.


2. If you see two stars separated you have a collimation misalignment. Using a small screwdriver turn the appropriate screws to adjust the prism.

Bahtinov collimation 1.jpg


3. Closer but not in center yet.

Bahtinov collimation 2.jpg


4. Dead center. Conditional collimation is OK.

Bahtinov collimation 3.jpg


As I said, I am interesting in the test results and any improvements in the procedure will be helpful.

Try it, I hope you will find it useful.

You will find small misalignment that you did not know you already had.


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Thank you Steve. I will wait for the results. Pm me if you need anything to explain.

And don't forget to print from your browser so the the print scale to be at 100% and not to "Shrink to fit", so the dimensions prints at right size.

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How can one determine, accuratley, the focal lenght of the optics such as my 10x50's.

Hi David,

I don't know if there is an accurate method without opening the binoculars and measure the light path through prisms. Anyway to create a Bahtinov mask for this test, it is not necessary for the focal length to be very accurate.

So for a general measurement, we can measure

A) the length from the center of objective to the eyepiece. Then measure approximately

:) the length of light path through the prism. (Must be something around 90mm for 8X40 - to 130mm for 20X80. Divide the light path through the prism length by the glass refractive index of 1.57 i.e. for a 130mm/1.57=82.80mm.

When these two measurements A) + :) added together, is the total focal length of the binoculars. I have to repeat here that the accurate number is not necessary. And unfortunately, I don't know an easier method to measure the focal length in binoculars.

One more example A) measurement is 160mm for a 10X50. :) is 100mm/1.57= 64mm

A+B=224mm this is the focal length of those 10X50 binoculars.

I hope I didn't make it very complicate.

@Moonshane @Rik Thanks

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How can one determine, accuratley, the focal lenght of the optics such as my 10x50's.

As Konstantinos has already said, it doesn't need to be particularly precise. As a rule of thumb, most common binoculars have objectives of "around f/5" -- i.e. somewhere between f/4 and f/6) -- just multiply the aperture by 5, and you should be close enough. The 10x50 in Konstantinos's example have objectives with a focal ratio of f/4.5.

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As a rule of thumb, most common binoculars have objectives of "around f/5" -- i.e. somewhere between f/4 and f/6) -- just multiply the aperture by 5, and you should be close enough.

Easy and simple! Never thought about this.

I suggest to include a lot of info like this in your site Steve, which is difficult to find elsewhere.

Thank you for the tip.

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Easy and simple! Never thought about this.

Ah, the value of open collaboration.... :)

I suggest to include a lot of info like this in your site Steve, which is difficult to find elsewhere.

Will do. I want to do my own Bahtinov experiments, and see what goes (I will, of course, attribute the idea to you), but first I have another 8 image maps to make, and they are taking me over an hour each...

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I have just realised that I could also remove the objective (it unscrews easily) and measure the focus the old fashioned way of projecting the moon onto a card. :-))

if you do that, make sure it goes back exactly as it was previously (mark it before you unscrew it). if it does not go back as it was, you will change the collimation.

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Could you use an artificial star with the masks to do this or would the shorter distance between the star and the bins give rise to problems....

Good question! Because I observe stars, usually I test collimation with stars. But for just checking the method here, of course you can use an artificial star as far away as you can, or farther a pinpoint strong street light, or farther away a star. The only thing we need is the light source to be bright enough to create spikes.

In any case at the end check with a star. But to adjust collimation definitely use a star.

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I think this may be way to short for bins :)

I think you're probably right. Our eyes converge when we believe we are looking at something close and, with a 10x??,, you will "believe" yourself to be looking at something 3m away. I suspect (but haven't yet tested) that you need something at least a few hundred metres away. (When I test binoc collimation during daylight, I usually go to a nearby hill and use electricity pylons about 4km away.)

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Thank you for the warm words!

I hope you will find the time a cloudy night to make 2 masks from paper and review it on another clear night. I have to know if you propose some improvements, or if it was easy for you too to find miscollimation and/or fix conditional alignment.

Try it, you will not regret it. :)

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I've not done the Bahtinov masks yet, but yesterday evening I tried a "poor man's natural method". I was travelling between jobs at dusk, the clouds were rolling in, but the Moon and Jupiter were still clear of the cloud, so I stopped to try it.

Preamble: Decades ago (i.e. pre-Bahtinov), when I used to dabble in astrophotography and spectroscopy, in winter and autumn when the trees are leafless, instead of using a Hartmann mask to focus, I'd just focus on a star that was visible among the twigs of a tree; like a Hartmann, it gave multiple images if it was not focused.

Tried the same thing yesterday with the 10x50 binocular that suffers a lifetime of abuse in my van. Sure enough, like Konstantinos reported with the Bahtinov, miscollimation shows as separate images (in this case of Jupiter), whose orientation changes as you change the IPD. Away from the tree, they instantly merged into one image but, back among the twigs, they split again. It was possible to "force" them to merge -- there was about 5 arcmin of step (vertical misalgnment) and 10 arcmin of convergence at my IPD, which is within acceptable tolerances, but it has convinced me that I must get those Bahtinovs made so I can test and (conditionally) align all my binoculars.

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Thanks -- 4 pairs now printed, cut and ready to roll -- of course, the sky is now overcast...

Later: No point hanging about -- used a distant street lamp a couple of hundred metres away, binoculars on a monopod. Because the streetlight is a relatively large extended object, the diffraction spikes aren't as crisp as with a star (or Jupiter), so it's impossible to estimate the degree of misalignment, but:

Opticron 10x42BGA: Very slight convergence and even slighter (almost imperceptible) step.

Strathspey 10x50 Marine: Confirmed the "arborial" method above .

Helios Apollo 15x70: As far as I can tell, spot on.

Strathspey 15x70 (I bought this 2nd hand a few weeks ago, primarily to experiment on; it was about 2 deg out of alignment, when I got it, so I tweaked it back into collimation using Betelgeuse and the "relaxed eye" method): A few arcmin convergence and step. Probably luck that I got it that close.

I didn't do my Miyauchi 25/37x100 because it is too heavy for the monopod and I want to wait for actual stars if I'm going to cart out the big tripod and P-mount.

First off, this method of collimation checking does seem to have merit. I can confirm what Konstantinos (pleiades) has said, i.e. that it is probably impossible to "force" the centres of the spike-patterns into alignment if there is miscollimation. If a binocular is degrees out of alignment, anyone with two working eyes can tell. If it is within standard tolerances, miscollimation is not a problem. In between there is a "miscollimation zone" where we can "force" the images to merge, but where doing so causes eye-strain, followed by headaches, tiredness, or nausea (or some combination thereof). The "crossed Bahtinov" technique reveals this. It works with masks printed onto transparencies, so there is no great expense in buying, or time in cutting, the masks.

I'd like to do this more rigorously under stars, and to miscollimate the Strath 15x70 again and see how easy it is to get good conditional alignment.

Konstantinos, I think you have hit on something valuable here; I think it is your privilege to give the method a name of your choosing.:icon_salut:

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Sounds great. Just been out splitting doubles..my 7 x 50 russians can get down to just under 40" (16 Cygni) and my 10 x 50 Japs to 31" (Psi UMI)...My 20 x 80 are still easily splitting the 31" so I must go closer!

On Collimation I find that even if the coll is not spot on it does not cause me much of a strain...but I think your method is facinating...and I am sure it is accurate from your description...


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