Jon’s Project Blog

After making and using the Paper Bahtinov Mask for awhile, I decided to try to make a better, cheaper, and perhaps more robust mask. The original Paper Bahtinov Mask still works, however, it’s still a little too delicate for my tastes, and I would rather have a more resilient foldable solution for the times when I go on hiking or camping trips in the wilderness where compactness is important.

When mulling it over, there were several key considerations that lead to the idea of using screen material as a base to build a Bahtinov Mask. The first is that screen material is fairly robust and cloth-like and so it’s relatively easy to store and it’s decently strong. The second is that the unique features of the traditional Bahtinov mask rely on the existence of the particular set of parallel slits in each direction. It’s the obstruction of light in a particular orientation which creates the strong signal (“light streak” that can be seen when zoomed in on the view finder.

By the same line of reasoning, using a screen as a base in theory shouldn’t have much effect on the diffraction pattern, other than reducing incoming light by a tad bit. The reasoning is that the screen pattern obstructs light “equally” in horizontal and vertical directions. On average if the obstruction is small, the directionality of the bands is somewhat mitigated. If we align the portion of the Bahtinov mask that corresponds to the “large streak” (the largest portion that goes in a vertical or horizontal direction) with the screen door, we can ensure that that signal doesn’t get attenuated. The perpendicular orientation of the screen will then attenuate the symmetric portions of the mask equally; therefore the overall effect is that a stronger middle streak should occur., but the overall characteristics of the Bahtinov mask are retained.

With that in mind, I decided to use a sheet of extra screen I had laying around and use painters tape ask the masking material to create the pattern. I did not generate a mask this time. From the post on the Paper Bahtinov Mask, the important insight about the Bahtinov Mask is that the symmetric smaller portions remain parallel and symmetric to create the banding patterns that yield the diagonal light streaks. Therefore, I chose to keep a “slope” for each of the slanted portions that was easy to eyeball, and looked aesthetically pleasing, which ended up being a 3:1 vertical to horizontal ratio.

(Note: I understand of course that the angle of the smaller symmetric portions affects the angle of the diagonal lines, but as long as it’s visually appealing – I’m not using a computer to aid my focusing – I think it will be more than sufficient. The angle doesn’t affect the construction anyways.)

Construction

The construction is relatively simple. First trace a circle with a sharpie or some other marking utensil that covers the appropriate mask area. Next decide how thin you want each of the slots and bands of the mask. In this trial, I chose 2mm, because I can cut fairly straight slivers of painters tape at that accuracy, and because the more slots I have, hopefully the better signal I can get. The screen material was also spaced at roughly 2mm.

Start of construction

After marking the area which I want to cover and then using a slightly thicker piece of tape (4mm) to signify two halves, I placed a third vertical 2mm band over the middle to section the smaller symmetrical diagonal portions. I then proceeded to mark and cut 2mm long bands of painters tape and then applied them to the screen material. Diagonal bands were placed at a 3:1 vertical to horizontal ratio, and the large bottom portion contained vertical bands. The diagonal bands were more or less “eyeballed” at an even spacing and checked with symmetry. The vertical bands were placed 2mm (one screen grid spacing) apart. The mask took about 20-30 minutes to cut and piece together.

Cutting the bands made of painters tape

Mask used for the demosntration

Demonstration Test

The Screen door Bahtinov Mask is attached to the camera by centering the mask then folding the remaining material around the lens. A rubber band or temporary zip-tie is used to hold the screen in place while one focuses manually. After focus is achieved, the zip ties or rubber band is removed.

The following pictures show the mask in action. In this test I unfortunately used a headlamp that has two LEDs thus if you zoom in you will see not one, but two sets of light streaks. The pictures were taken from a chair with a 2 second delay.

original image

focused image

And the in-focus image.

focused image

After usage, I noticed that some of the bands of painter tape weren’t reinforced onto the screen too strongly, so I added some thicker bands of painters tape on the periphery as well as in the main horizontal and center vertical band

Conclusion

As you can see here, the Screen door Bahtinov Mask seems to work fairly well. I’d warrant to say it works as well as my Paper Bahtinov Mask which I used for a few nice panoramas at Sequoia National Park. Here’s one of my 500px photos where I used the Paper Bahtinov Mask for focusing.

There are definitely a few improvements I could make. Placing painter’s tape on a screen naturally evokes thoughts about screen printing. I don’t have a lot of experience in this area, and you would have to go through the screen printing masking and etching process for every few Bahtinov masks you want to make. A similar idea that I am proposing is to 3d print a relatively large yet fine Bahtinov Mask at a thin thickness (say 4-5mm). Place a screen over the thin 3d printed mask. Next use a spray on plastic like PlastiDip and spray the mask several times. Replace the sacrificial spray base material until a thin layer of plastic forms atop the screen which is atop the Bahtinov mask. Lastly, peal the screen from the mask, and hopefully one can achieve a beautifully opaque bahtinov mask. If one prints a large 3d mask that covers up to 77-80mm, that should pretty much cover most camera lenses. Making smaller lenses is simple, as the large mask can be reused. One simply covers part of the screen such that a smaller portion of the mask and screen are exposed to Plastidip. If anyone tries this out, please let me know if it works.

Lately, I’ve been getting into astrophotography, and so I’ve been spending several hours every month or so outside at night. The thing about astrophotography is that you have to be somewhat familiar with identifying constellations and then on top of that you have to be able to master the technicalities of acquiring enough light in the proper fashion to produce acceptable and interesting photos.

Like all beginners, I made the mistake of taking a bunch of pictures in a wonderful place (Lone Pine, CA) where the light pollution is almost non-existent during a new moon for 2 hours and I neglected to really check the most important thing, which is to CHECK the FOCUS!.

Unfortunately, as you may know, I am using a kit auto-focusing zoom lens (SEL1855) with my NEX 5R. The focus ring is manual (which is great for astrophotography) but it has no markers, because the focus ring is not a manual ring! It’s more or less a potentiometer used to tell the motor how much to change the focus, and so it spins around indefinitely. As you can tell, this can make focusing a tad harder, as you can never turn the ring until it stops at infinity or more than infinity focus. Also, because I have a NEX 5R it doesn’t tell you anywhere on the camera, what your approximate focus distance is. Even if it did, astrophotography is relatively unforgiving when it comes to nailing down the focus.

Reading up on focus methods for astrophotography brought up a nifty diffraction mask called the Bahtinov mask. It was invented by Pavel Bahtinov (although I can’t really find too much about this guy) for the purpose of accurate focusing of telescopes. You can read more about it on wikipedia, but the the diffraction pattern creates 3 “streaks” of light that intersect each other (although not necessarily at the same location) are emitted when you zoom in and focus on a bright light source at night with the mask attached. When the image is out of focus, the intersection point will not be in the middle. The convenience of the Bahtinov mask is because the mask is symmetric about one axis, the two diagonal streaks are symmetrical. The position of the “middle” streak can then be judged quite easily by the human eye. When the image is in focus, the middle streak should be exactly in between the two diagonal streaks and all 3 should intersect at one point!

Mask Creation

There are several ways to obtain a mask. The simplest way (although perhaps the most expensive) is to order one online. The second is to use the free Bahtinov Focusing Mask Generator provided by astrojargon. However, I could never get it to properly work given the clear aperture settings of my tiny MILC NEX 5R. It seems the mask generator was designed for “actual telescopes”. If you scrounge around online, you’ll also see that people have built their own custom bahtinov masks using spare pieces of plastic, wood, and metal.

Also online, there have been mentions of creating a simpler “Y” mask. I created one, but it didn’t seemed a little more finicky and a little harder to setup, so since I don’t really understand it, and it didn’t really work for me, I decided to just keep myself to the standard Bahtinov Mask.

However, being an engineer and having a penchant for what my friends refer to as “ghetto engineering”, I decided to make one out of paper. After talking to my friend Daniel who is finishing up his Ph.D. in lasers and optics at UCLA, about how much construction precision actually matters for a diffraction mask like this one, the overall conclusion was, that as long as the lines are fairly parallel, it should create a relatively strong signal with which the Bahtinov focusing effect should be good enough. The construction precision overall shouldn’t be that important as long as the average orientation of the mask slits are accurate enough. Therefore, I decided to first try out a bahtinov mask made of just paper and paper based products I had lying around at home. I pretty much eye-balled the mask holder construction, so it’s definitely not pretty.

Cutting the Mask

The first step was to generate a mask that looked possible to cut and didn’t require too much work on astrojargon. So I basically generated a large template that looked good and then shrunk it down to a diameter that is slightly larger than my 49mm lenses.

Cutting out the mask.

I taped the mask to the front page of a spare musician catalog (I actually like the retailer, I just haven’t had the need to buy another instrument recently, and why let a perfectly good piece of paper go to waste!). Next I pulled a trusty X-acto knife and started scoring away with a ruler. The mask probably took 30-40 minutes to cut out.

Mounting the Mask

After cutting the mask, I decided to create a mask holder out of a manilla folder I had that was already slightly bent on one side. Using a ruler, I created a flange which I then cut into segments to create a crown with which to hold the paper Bahtinov Mask.

Creating the flange for the crown.

Separating the flange for the crown

I also cut out a larger cardboard ring to mount the mask onto. The ring also serves as something sturdy for me to attach the flanges of the crown to also.

Reinforcement ring

Using a bit of Elmer’s rubber cement (just in case I needed to make adjustments) I assembled the paper bahtinov mask.

Assembled mask

Top view of assembled mask

Going out on a test run revealed a few things. The first is that it seemed that I couldn’t see the 3 light streaks, but instead saw 3 images of the light I was focusing on. Aligning all 3 images yielded an very in-focus image. I later found out that the mask actually works as prescribed. However on the LCD viewfinder on the NEX 5R, what is initially displayed is not 1:1 pixel density. The light streaks are actually small, and so I need to zoom into 4.3x or 9.6x to see the 3 light streak focusing phenomenon.

The other thing I noticed was that I had a hard time rotating the focus ring. On the SEL1855, the focus ring is the front  ring closer to the lens, and the zoom ring is behind that. The crown of the mask holder covers up the focus ring. As a compromise in order to retain some semblance of material strength, I cut two slots into the crown so I could rotate the focus ring while allowing the mask to grip the lens fairly well.

SEL1855 Adjusted Paper Bahtinov Mask

Conclusion

Overall, I’d call this work of “ghetto engineering” a success. It gives me confidence in nailing the focus for photos at night and it helps me visually determine how out-of-focus I am. When using it with really dark skies, I’ve noticed that it’s hard to find bright stars to use this on. One trick for focusing with dark skies that I read about online is to walk 40-100ft away and leave a bright LED flashlight. Then walk back and then proceed to focus on the light with the mask on. I’ve found this to work quite well especially because I’m shooting slightly wide (about 24mm equivalent).