focusing mask

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Bruce D
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focusing mask

Unread post by Bruce D »

As I slowly wend my way towards semi-serious imaging, focus is a concern as it is probably the root of planetary imaging failures I've had in the past. Has anyone tried an oleshko 2 frequency mask?

I find the concept appealing as they seem easy to make and don't require tedious cutting of the angle slots as with a regular bahtinov mask. An article in the January 2020 S&T describes them as an "excellent focus mask". Made by using 2 different size mesh materials, the larger mesh material covers half the mask, the other half is divided into 2 parts for 2 pieces of the smaller mesh each set at slight, opposite angles to the large mesh. The angle doesn't appear to be too critical, the article recommends between 10* and 20*.

The S&T article says the perfect mesh materials are 5 and 7 count "plastic canvas" which I have never heard of but I actually stumbled upon it while scouring joann fabrics for fly tying materials.
https://www.joann.com/joann-plastic-can ... 57093.html
Both the 5 count and 7 count were in stock in single sheets, the stuff is cheap enough I can make custom masks for each of my scopes for less than $12.

Here's a video showing a similar mask made with ordinary window screen, one side of the mask set to a 45* angle to the other- but the plastic canvas seems very robust while window screen is easily deformed.
https://youtu.be/IeUs7pFwMJw
Bruce D
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Pete
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Re: focusing mask

Unread post by Pete »

That's a new one for me Bruce. Wonder what kind of a focusing pattern it makes?

The Bahtinov mask has become the focusing standard. Picked one up this summer for my Orion 80mm scopes. And wasn't greatly impressed.

Back around 2005 Dennis di Cicco had an article on a super simple home-made mask design consisting of just 2 triangular cutouts. The images below are of a $0 cardboard focusing mask I've been using on the 14" for many years. The second triangular cutout is rotated 90° from orientation of the first. Corners must be sharp. And triangle centers equidistant from the center of the mask.

It works very similar to the Bahtiov. Point at a bright star. When unfocused you have 2 stars that are separated and have 3 spikes. As you focus the stars merge. And the 6 merged spikes flare out significantly. The point where the stars are merged into the smallest form and the spikes are the longest is your point of focus.

In my opinion the elongated rays are more accurate than centering the bar in the Bahtiov as the Bahtiov center zone is pretty small and precise centering is pretty much guess work.

The di Cicco mask can be used in the daylight as it darkens everything except the star itself. I've found that to be very handy. And in my opinion the mask is more sensitive than the purchased Bahtiov as the length of the spikes shifts significantly with just touching the focuser. I'd even considered selling these masks, but they're so damned simple that no one with a piece of cardboard lying around would buy one.

Try it. You'll like it.
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Pete P.
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NGC7000
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Re: focusing mask

Unread post by NGC7000 »

Bruce, that one you posted about the screen mesh looks like it might work really well with camera lenses. I have a smaller plastic purchased Bahtinov, but it is very hard to determine if I'm in focus with it. I think I might give this one a try. I think you're right about the lack of sturdiness, but for smaller lenses, and so small mask diameter, this could work really well. Glad you posted it.

Tom
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Bruce D
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Re: focusing mask

Unread post by Bruce D »

Pete- that's another interesting variant!

Tom- yes that might be perfect for a smaller aperture camera lens, unfortunately the only info I have for the window screen mask is the youtube link but it seems pretty simple. Not sure if I'd try the metal screening or the plastic first but I'm very interested in hearing how it works for you, I have a small refractor it would probably work well for.
Bruce D
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