Quote:
Originally Posted by citivolus
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An observation/speculation on focus being slightly out after using the mask: Collimation can dramatically impact the performance of these masks, as could slight imperfections in the curvature of your optics. Based on information in a few Astrophotography books that I have, the Bahtinov design is similar to mirror testing Hartmann masks which are designed to highlight the imperfections in curvature of lenses and mirrors. Having open regions on such a large portion of the mask could possibly have this negative side effect, as a trade-off for brightness and contrast.
Try rotating the mask 90 or 120 degrees on the OTA and see if it still shows focus as perfect. If it does, then my speculation above is likely incorrect  |
Quote:
Originally Posted by jerry3672
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However, if your mask is not centered or lines are not a mirror, then this will also magnify the error.
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Hi Guys,
I think that I have bottomed out the reason for the slight focus variation in my original A4 transparency cut-out, Bahtinov style mask.
- The mask wasn’t glued around the entire circumference as the narrow dimension of the A4 sheet of transparency did not quite stretch all the way to the edges and the unsupported edges introduced some flexing into the mask.
- I inspected the templates I drew and discovered that the “snap to” function had aligned the bottom and top edges of the bars at 20° and not their centre lines, so as Jerry comments, the two sides were not a mirror reflection.
- The focusing error did not appear to be noticeable by eyeball inspection. It only became apparent when I fitted the SBIG ST7 and used the CCDSoft focusing routine which I includes a FWHM and maximum value function.
I have since made a new mask from A4 Transparency that fixes the above issues and I am just waiting on an opportunity to test it out.
I have found that with poor seeing (I had some 4-5/10) the Bahtinov mask appears to still perform very effectively, even with exposures of around 2 secs to average out the seeing.
Cheers
Dennis