More DIY Bahtinov Testing.

[StuartJPP]

Following on from a previous thread, I engraved the largest Bahtinov Mask that my little miller/engraver could handle, just large enough for my 500mm lens hood (although the aperture of the lens is 125mm the hood is ~150mm in diameter).

The results seem quite good on a real star (Arcturus and Dubhe) but there is a funky rainbow effect happening, though this doesn't seem to be an issue in reality. What I liked about the diffraction spikes was that even in live view they were really large and it is easy to focus even by hand. I will try APT's Bahtinov grabber next time, but I am sure it will be okay. I compared it to my previous Bahtinov mask which can be seen in the images below. All exposure times were the same and the images are zoomed to 100%.

 

The one on the right is an 82mm step up ring version.

 

The two pegs make mounting very easy.

 

My previous Bahtinov Mask.

 

The two compared, taken within a couple of seconds of each other (100% crop, 1/4sec exposure at ISO6400 on Canon 5D Mark III, 500mm f/4. Left-hand is my old Bahtinov Mask and the right-hand one is the new one, lets in a lot more light:

The custom software to design the mask and cut it on the engraver/miller:

 

Quite pleased with the result, but it may need tweaking.

 

On a side note, I tried my previous "Y-mask", but it performed dismally, even on Arcturus. Using it with a fake star it performs as well as, if not better than the old Bahtinov mask but not on a real star...not sure what is happening there, I can only conclude that my fake star is much, much brighter than an actual star.

[wimvb]

Diffraction is wavelength dependent. So rainbow colours are good in this case. Long spikes is also a sign of quality. 

[RichLD]

Excellent result!

[Jonk]

I'd like some masks for my various prime lenses (14mm, 24mm, 85mm).

The shorter the fl the harder it is to get a diffraction pattern of course, but magic lantern and live view should help at 10x magnification. 

My 6d will allow iso 25600 and is great for framing so perhaps a b mask will work?

I'd be happy to purchase a set if you have the time?

Perhaps you could bulk buy some filter rings for various sizes and make a small bit of profit for your time?