The Dave Lane Astrophotography Reveal Focus Filter

Hi, I’m Dave Lane of Dave Lane Astrophotography, and Focus (note capital F) has been a focus of mine since I began in taking landscape astrophotography images four years ago. Last fall/winter I spent 100s of hours researching how to automate and computerize focus. I developed software, bought four different Windows based tablets so I could carry a PC in my backpack to digitally focus my Milky Way images.
I was looking for several things in my efforts. My goals were to make focusing a quick, easy and repeatable process.  After hundreds of trials, brackets, stepper motors, belts, gears and software tweaks I failed. It actually worked, I tried using it one or two sessions and got outstanding focus out of the camera, but it was kludgy, hard to keep attached reliably and was neither quick nor easy.
So after 100s of hours and 1000s of attempts I set the concept of precise digital focusing aside.
The thought never left me, though, how to get accurate focus and the resulting superb star control that comes with it. I experimented further with Bahtinov masks and at one point even tried stretching fishing line over a filter to produce star spikes. I openly admit that one was a low point in my thought process, but I was desperate. There had to be a way.
You see I’m really picky when it comes to getting the focus exactly right. Having first learned to be an astrophotographer of deep space objects brought me a need for round, in-focus stars. I wanted stars that exhibited themselves like diamond dust scattered across black velvet. The deep space photography I had done demanded this of me in a way not easily understood by daytime photographers.
So as the 2016 imaging season wound down, my mind turned increasingly to solving this problem. It takes me 5 minutes or so before each shoot to get the focus just right. It’s a painful, slow process because of my need to get it perfect. I usually succeeded in very tight focus, but at times it was just off this much > <. Standing next to drops of 1000s of feet while doing this and giving total concentration to focusing was not only time consuming but dangerous.
So once the season was over my efforts got intense, it was time to solve the conundrum. This offseason I would overcome the hurdles that had plagued my digital focusing attempts. I went back to the Bahtinov mask idea, one that is very common in the astronomy community but not workable in the DSLR world. The question I asked myself was why was that?
It turns out after 100s of tests that the real issue is the amount of light that was blocked by the slots of the mask. So the slots needed to b
e much finer, and I assumed many more of them. It turns out I had at least part of that correct.
So without going into the finer details of the 47 different versions of the masks, at least the ones I remember making, I present you with a year and a half of research and the final product.

The Reveal Focus Filter
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Here is a very quick video that shows the ease of use of the Reveal Filter

 

So how does it work? Let’s go through an example. Here are a series of images I did on the star Vega. Vega is a bright star that is always visible when its Milky Way season. Vega rises before the Milky Way so you can focus on it even when the Milky Way is below the horizon.

How it works: The Goal of the Reveal Focus Filter is to create six spikes (really there are three spikes that pass through the star but if you are counting them one-by-one I don’t want you confused) on bright stars. There is a main large spike then an X of four smaller spikes. The goal is to get the main large spike centered between the X and centered on the star itself. As soon as that is done BOOM you have perfect focus in as little as 2-3 seconds.

Using the Reveal Focuser Filter and 10x Liveview here was my first attempt at focus.

It took me 2-3 seconds to get this result

This is a test shot to see if I got it right. This is a 5 second shot straight out of the camera.

Click any image for an enlarged version.

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That result is very very close to accurate, but it’s just slightly to the left of centered on Vega, I’m sure the resulting image would be outstanding, but that is not good enough for me. So let’s make a slight adjustment and take another 5-second exposure and see what happens.

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Opps went the wrong way. Even this move of maybe a half a millimeter on the focuser changed it this much. Let’s move it back a bit and try again.

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The focus is closer. Still, it is worse than what I could do with my naked eye and live view. Let’s adjust the focus ring and try another 5-second shot.


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Yikes! Boy, just a tiny amount of adjustment is way too much. Time for another focus adjustment.

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There we go! Perfect result. Splits the X and the central spike is centered on Vega. Notice even the minor stars around Vega have spikes. The Reveal filter mask is THAT sensitive.
(Click any of the images for larger versions if you can’t see the minor star spikes.)

Here’s a 5-second image of the area around Vega after the mask is removed.

Now THAT is super tight focus, all in under 30 seconds.


It’s the world’s easiest to use focusing system for DSLRs. It is fast, easy to use and gives the very best results you can get. All for a very low price, which is maybe the best part

Now let us see what kind of Milky Way results we get from this focusing session.

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