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GeorgeFC
member
Reged: 03/27/09
Posts: 15
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Hi Lawrie. Yes, I came to the same conclusion when figuring out how the Bahtinov mask works. Have a look at http://www.btinternet.com/~gfcarey/bahtinov.htm
and scroll down a little to see my 'Y' diffraction pattern. The Y mask would indeed work, but of course the Bahtinov with many parallel slits produces a much brighter diffraction pattern.
After I had figured out the Bahtinov I realised that my 6" reflector has a built in mask due to it having 3 spider vanes!
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GeorgeFC
member
Reged: 03/27/09
Posts: 15
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Niels had made an even better simulation. This version shows what white light would look like as opposed to discrete wavelengths.
It also shows the focus error in microns.
http://www.youtube.com/watch?v=DVbBVR0PO3Q
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GeorgeFC
member
Reged: 03/27/09
Posts: 15
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Here is a DXF file for a Carey mask for a refractor (no central obstruction).
http://www.btinternet.com/~gfcarey/m...o_obstruct.dxf
Using Niels Noordhoek's simulation software it can be seen that a focus error of only 20 microns is clearly visible using a Carey mask.
The gap between the right hand spikes is larger than that at the left.
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GeorgeFC
member
Reged: 03/27/09
Posts: 15
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Niels Noordhoek's simulation software is now very accurate. This is a Carey mask going in and out of focus:
Carey Mask Simulation
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B. Hebert
member
Reged: 09/04/06
Posts: 61
Loc: Southern California
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Can you print the Carey or Bahtinov masks with an inkjet printer onto transparent overhead sheets and use them without cutting?
B. Hebert
--------------------
SkyView Pro mount
Shoestring GPUSB guiding
ST80 & Meade DSI I
Orion 8" Reflector
Orion 80ED Refractor
Canon 350 (unmoded)
Some eyepieces
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GeorgeFC
member
Reged: 03/27/09
Posts: 15
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I have not tried this myself but I know others have. Most opinions I have heard are that it doesn't work very well.
Bahtinovs are quite easy to cut out of paper card by hand. The first one I made from paper works just as well as the laser cut ones.
Carey masks have a greater number of slits so are quite difficult to make by hand.
Are you in the UK?
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GeorgeFC
member
Reged: 03/27/09
Posts: 15
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The search for the ideal material for Carey masks continues. I have now tried black paper, paper card, HIPS, ABS, aluminium, steel and perspex. The one that gives the best result is the 1.5mm mounting card, as in the picture below.
I recently tried 0.5mm acrylic (perspex), thinking that the thinness would give much better diffraction spikes. It was not too bad, but not as good as the card. Thin acrylic is transparent so needed spray painting matt black.
The 0.5mm acrylic was very thin and needed a 3mm thick acrylic support, as in this image.
I think the success of the card is the sharpness of the cuts and the lack of distortion caused by the heat of the laser.
The added advantage of card is it is cheap!
Edited by GeorgeFC (09/12/09 09:40 AM)
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