Quote:
Originally Posted by Moon
Sorry for digging up this old thread - but I have an update.
I did some tests with my refractor and the Bahtinov mask with a CCD camera.
It shows clearly the blue filter has a different focus point to the R and G filters - which makes sense for a refractor. In addition - with the blue filter in place, you can see curvature in the diffraction spike, which indicates that the different wavelengths of blue light have different focus points and perfect focus of blue light will never be possible.
By the same token, the Bahtinov mask gives a visual clue to the the width of the pass band width for all filters, include narrowband.
LINK
It would be great to repeat this test with a few achromats and top end APOs as well.
Overall a very useful tool!
James |
Hi James - I think we had the same discusion on Yahoo! My results agree with yours (and H's)...to find the optimum focus on a Triplet refractor you cannot just use a Bahtinov mask, I think this is because the focus points for RG and B are not identical. The optician can get a zone of good focus with no CA but not a point of perfect focus with no CA. If you go to best focus one of the colours will be out - typically B. I find the FWHM measures in AA4 to be more reliable.
As others have found you can use the mask - after all it gets you close, go to best focus (centralise bar) then rack out 1/4 turn (on the fine focus of the ED 127) and you should be there, sharp stars no colours.
For the very best results with NB you do need to re-focus with each filter change, I am looking at adding Robofocus to do this, I think thats is the only way to get it always perfect.
