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Old 01-11-2019, 09:38 AM
spaceout
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Bahtmasks

I thought I would share a run of focusing masks that I compared.

I'm sure most of you are familiar with these things, however I have never used one before and was a little confused as to which focusing mask to go with. (Y mask, Bahtinov mask, various screens and sticks.)

The first comparison I did was the coarse Bahtinov mask and the Y mask, as I had read some very convincing articles that suggested the Y mask was just as good. I chose to do a "coarser" Bahtinov mask with a reduced Bahtinov factor of 50, simply because I didn't know what to expect from the printer. I think the slats are in the ballpark of 2mm in this version.

Rather then wait for some good weather I figured it was much more convenient to set up some artificial stars at home with a flashlight and some tin foil. This setup allowed me to hold the two masks directly in front of the camera, switch them back and forth while watching in live view.

My first impression was, oh yeah, the Y mask is awesome, it had nice long monochrome spikes, much cleaner then the Bahtinov mask. However something made me stop and wonder for a moment. I had two artificial stars, a dim on and a bright one. (Bright star had 0.4mm hole, dim star was something smaller.) The Bahtinov mask put spikes on the dim star while the Y mask did not. OK, this is somewhat expected as that is the objective of the multiple slats of the Bahtinov mask.

Then it hit me that I had the ISO set to AUTO. Once I set the ISO to manual it was a totally different outcome. The Bahtinov mask beat the pants off the Y mask.

That was one primary question out of the way, now was to address the Bahtinov factor to see it's effect as some claim that these masks aren't all that critical.

I made another Bahtinov mask with the Bahtinov factor set to 100. (the default in the calculator is 150, says anywhere from 150 to 200 is good unless the slats are too small to cut)

Comparing the 50 against the 100, there was a clear improvement. So then I decided to raise the Bahtinov factor to the default 150 since the 100 printed well. (~1.2mm slats)

Yet again there was a noticeable, substantial improvement. It printed better then I expected. The slats are about 0.6mm in this instance.

OK then I decided to push the Bahtinov factor too 200, it was supposed to give 0.4mm slats which is the same size as the nozzle on my printer. Not Ideal but worth a shot. The problem I ran into is that the SVG outputted by the calculator gave the same slat size as the 150 factor when set to 200. This seems like a good enough place to give up so here we are. Maybe one day I will draw up a finer mask manually, for now I am satisfied. I also have a resin printer which may make a little nicer part if I reduce the outer diameter a bit so I can print it flat, (small printer) however it doesn't really print much finer in that axis anyway. The Polyethylene prints are quite serviceable and easily replaceable. I plan to print up some in black this weekend.

I wanted to share this as despite all my reading about this in 2019, it was unclear to me which mask was better, or how critical it is to remain faithful to the original Bahtinov mask. The Bahtinov is clearly superior to the Y mask and a higher Bahtinov factor is better then a lower factor, worth making the slats as thin as you have patience for.

Having done this comparison, I wouldn't buy a generic mask at all, I wouldn't buy a specific model of mask for my telescope, unless it was in agreement with the calculator at the astrojargon site. If you buy a mask for anything but the smallest scope or lens, it should have a Bahtinov factor of 150 or higher as there isn't really an reason not to on the medium and larger masks.

I didn't think to take any pictures of the spikes, I'm not sure how that would look. Maybe I will try it and post back later.
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Old 01-11-2019, 11:23 AM
spaceout
Fresh Blood

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Here is the comparison of the four masks. These crops should be 100% size when you click on it, if you know what I mean.

ISO 6400, 125 of a second I think, for no particular reason other then it filled the live view screen at ten times zoom but is otherwise arbitrary.

This time I made three stars of various magnitude. These stars are about 10 meters away from the lens.


I'm sure you will find this satisfying if unscientific. The Bahtinov mask is IMO far superior to a simple Y mask by a considerable margin.
Go with a mask recommended Bahtinov factor 150 if you can, if not the 100 and 50 are still much better then the Y mask as you can see.

Ultimately you don't even need the calculator. Make a mask with the same diameter as your clear aperture, make the slats as thin and numerous as practical. Use your judgement as to how wide the "T" part should be the separates the sections of the mask, just wide enough to support the slats is all that is needed. I went with 4mm on my little masks, I may make them a little thinner on a final version.
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