26th March 2009
The Bahtinov Mask requires you to judge when the central spike is midway between the two angle spikes.
This is quite easy to do, but there is still a little uncertainty in determining the exact centre alignment, especially when viewing in a DSLR viewfinder.
After some experimentation, I have come up with this mask, which is perhaps different enough from the Bahtinov mask to warrant a new name.
The central spike idea has been abandoned. The angle between spikes has been considerably reduced. Instead of two sloping sets of slits and one central set, there are four sets of slits, with a small angle difference on each side of the mask. In the prototype I used 10 degrees and 12 degrees.
The diffraction pattern produced is two overlapping 'X' shapes, with slightly differing angles.
At perfect focus the Xs will be symmetrical.
Inside and outside focus the Xs will be displaced. Because the angle of each X is slightly different, it will be easy to spot when out of focus, and also which way focus should be altered.
What is the best material from which to make the mask?
3mm black perpex did not work well for the Carey mask because diffraction spikes were very indistinct.
(This comment applies to smaller aperture scopes. See here for results of experiments with a 10" reflector)My prototype cut from photographic mounting card was much better.
I then assumed that the thinner the better, and tried a variety of different materials. It does not seems possible to get black Perspex thinner than 3mm. I tried HIPS (high impact polystyrene) and ABS (acrylonitrile butadiene styrene) and both were subject to warping and melting problems caused by the laser cutting.
I had steel and aluminium ones cut but in both cases the heat caused severe warping.
I recently tried 0.5mm clear perspex and the initial results looked promising. I had to spray paint the mask with matt black paint, and because the perspex was very thin I made a 3mm perspex support.
This version also had rounded ends to the slits and I anticipated very sharp diffraction spikes.
However, the star tests were disappointing. The mask is perfectly usable, but the diffraction spikes are not as bright nor as sharp as those with the 1.5mm card!
The cuts appear to be quite straight and clean, and the mask with its 3mm thick support is actually flatter than the card.
A possible cause is that perhaps the spray paint adhering between the slits causes a rounding effect.
Certainly the edges of the paper card are very sharp.
Conclusion: I will stick with card for the time being, and carry out a few experiments on different thicknesses. I have seen a source for 1mm thick black mounting card and will give that a try.
Watch this space!
How accurately should the mask be centred?
It is quite likely that the mask will be placed on the front of the telescope and the centre will be a few millimeters from the true optical centre.
What effect will this produce in the final image?
I used the maskulator software to simulate this.
Photoshop was used to offset the mask by an amount which is greater than that likely to be encountered in reality.
The animated images below show the two positions of the mask, and the resulting diffraction patterns.