Bahtinov producing unexpected pattern

[m37]

Hello

I got a starsharp Bahtinov for my 102mm refractor and rather than the expected view i.e.

I got this

Any ideas why?!! Spherical Aberration? Chromatic Aberration?

cheers

[Spikey]

What did you expose on and for how long to get the lower image?

[m37]

Hi there. DMK21au04.as, not sure what the exposure was. Probably < 6 seconds... maybe.

[Bizibilder]

This may help you:  http://www.geoastro.co.uk/bahtinov3.htm  and this for more detail and a mask simulator:  http://www.njnoordhoek.com/?m=200907

[Spikey]

The top image looks like something I would expect from a CCD or DSLR with a single, decent exposure on a bright star 5-10 seconds maybe.

I haven't tried a DMK with a mask so can't comment on it's suitability; can it take single long exposures? Your exposure looks a bit like it's moving. 

[Bizibilder]

I've seen a description of those curly patterns somewhere (on the web) but I can't find them at the moment - I'll keep looking.

[Spikey]

I think I've seen one like that before when there was very bad seeing and everything was very 'wobbly' in the atmosphere.

[Spikey]

ah ha ! http://stargazerslounge.com/topic/166972-bahtinov-mask-wavey-diffraction-pattern/

Quick search of the forum works wonders ! why didn't I do this first 

[Bizibilder]

That's the one!  Been all over the web looking!!!

[m37]

Cool, thanks guys. I've also been googling for ages. 

See how we get on tonight...