A new focussing mask

[GeorgeC]

Having been very impressed with the Bahtinov mask I did some experiments and have come up with something that may be an improvement on Bahtinov's idea.
I found that there was still a slight uncertainty in judging whether the central spike was truly central and tried to get an arrangement that would have smaller gaps between spikes and afford more critical comparison.
This is what I ended up with:
http://www.btinternet.com/~gfcarey/masks/expt26march/Carey_mask_a.jpg

This gives two 'X' shapes, with slightly different angles. An explanation of what the diffraction pattern looks like is here:
http://www.btinternet.com/~gfcarey/careymask.htm

I am eager for someone else to try it out and let me know if they think it is an improvement.
Regards,
George

[erick]

Thanks George.

Would you be able to put a higher resolution jpg file on your site for download, please.

Thanks
Eric

[GeorgeC]

Hi Eric,
I have put a bigger jpeg on the site, but the edges are very 'jaggy'.
http://www.btinternet.com/~gfcarey/masks/CareyMask6d.jpg (http://www.btinternet.com/~gfcarey/masks/CareyMask6d.jpg)

Are you intending to hand cut a mask?

Update: I managed to create a higher resolution jpeg.
http://www.btinternet.com/~gfcarey/masks/CareyMask.jpg

The DXF file is for those with access to a laser cutter.
http://www.btinternet.com/~gfcarey/masks/CareyMask.dxf (http://www.btinternet.com/~gfcarey/masks/CareyMask.dxf)

[erick]

Thanks George, this is better.

I'm going to try a recommendation I saw elsewhere. Using a photocopier, copy the image (with suitable enlarge/reduce) to overhead transparency film and experiment with that. It'll be a rough test but it should work.

Eric :)

[GeorgeC]

Eric, let me know how you get on. The transparency may need a frame to keep it flat. I suspect that the mask will be sensitive to any bending (but I haven't tested this).

[GeorgeC]

Niels Noordhoek who created the 'Bahtinov Grabber' has made a simulation of what occurs with a Carey mask as you go in and out of focus. Watch the green dots - when they are equally spaced perfect focus is reached.
http://www.youtube.com/watch?v=RXIpqEjAXYY

[GeorgeC]

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

[Dennis]

Thanks George!

Cheers

Dennis

[GeorgeC]

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.

http://www.btinternet.com/~gfcarey/september2009/onepoint5card.jpg

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.

http://www.btinternet.com/~gfcarey/september2009/point5acrylic.jpg


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!