Better alternative to Bahtinov Focusing Mask
Posted 21 June 2009 - 08:54 PM
At f/8 and 0.55um the 1/4 wave depth of focus is 0.00554", so the defocus simulated here is less than 1/4 wave.
A central obstruction has no real effect on the shifting of the central spike relative to the X spikes.
This mask is far simpler to make than all the slats a Bahtinov requires, and also gives a brighter image at focus, by about one f-stop.
The Y-Mask is simply the Y-shaped obstruction to the aperture. The blue in the pupil is where light is transmitted.
The Y-Mask will be the first focusing mask I'll make. There is no reason to spend the time cutting out a complicated Bahtinov mask with this innovation.
Posted 21 June 2009 - 11:42 PM
Posted 22 June 2009 - 02:29 AM
Posted 22 June 2009 - 05:28 AM
Posted 22 June 2009 - 02:57 PM
Posted 22 June 2009 - 04:46 PM
Posted 22 June 2009 - 10:37 PM
However, the Y-Mask blocks only a few percent of light to focus in contrast to the Bahtinov, which blocks at least 50% of the pupil. The Y-Mask sidelobes are fainter, but the overall image is brighter by almost an f-stop. The Y-mask sidelobes are also narrower than the Bahtinov, allowing more precise location of focus to be determined, if they can be imaged well.
This analytical work is a good example of real-world results with real imaging equipment, stars of different spectra and brightness, and degrees of turbulence present not necessarily agreeing with the very precise computer calculations under pristine conditions. It is best validated with field testing of Bahtinov and Y-Masks with several different line width and spacing parameters, using direct empirical photographic and visual comparisons. A particular Bahtinov mask design might be superior to a given Y-Mask design in locating best focus when using a particular star under some range of seeing conditions. The Y-mask might do a better job under different conditions and line widths. The two mask configurations could also be a wash, depending on the mask geometries used, the brightness and spectrum of the star being used, and the degree of turbulence present.
Posted 23 June 2009 - 10:11 PM
Posted 23 June 2009 - 11:00 PM
Posted 24 June 2009 - 01:51 AM
Original Bahtinov's idea was to cut slots depending on focal length. The longer the focal length is the fewer slots you need. The formula was f/s=150...200, or 3f/s=150...200 where f - focal length, s - grid period.
So in case of 150mm/1200mm scope the "recommended" period would be either 1200/150=8mm (4mm slots) or 3600/200=18 (9mm slots). Which would require 18 cuts in case of 4 mm slots (not very technological) or merely 8 cuts in case of 9mm slots.
Mike, would you please test how mask with 4mm and 9mm cuts will work in case of your scope? If you have time (and desire) for that of course.
Later people decided that the mask works good even if the original formula was often ignored.
Posted 24 June 2009 - 07:57 AM
Posted 04 July 2009 - 07:10 AM
I found that in practice the Bahtinov mask is superior for visual, since the bright diffraction lobes from the grating are easier to see than the broad lines from a single wire.
Here are some simulated diffraction patterns for different masks.
Posted 10 November 2009 - 09:25 PM
The Y-Mask not only gives a brighter image, it is also a much cleaner one, as it does not generate all the grating sidelobes the Bahtinov makes (similar to a Ronchi grating).
Have you actually _used_ both masks to get precise focus on a telescope ?
Hint, we use the b-mask here all the time. Those sidelobes you seem to dislike, are the way to get the focus PRECISE. first you rough it in using the main spikes, and, when you get that really close, the crispness of the side lobes 'finishes' the job of focussing.
Posted 11 November 2009 - 11:25 PM
Posted 12 November 2009 - 01:59 PM
to the optometrist, the Bahtinov really helped me
getting on the road again! :bow:2Pavel :-))
I'd like to be able to distinguish the difference
each mask offers but for me IMHO it is
useless to try to improve upon what for me has
proven to be the solution
Posted 10 August 2010 - 03:43 AM
Posted 10 August 2010 - 07:47 PM
How about going one step further and making spider vanes of reflector using the same shape?
This would be a built in Bahtinov mask!
Obviously astro pictures with reflectors like this would have a different distinct diffraction pattern from what we normally see, but I think it would look much more interesting than just boring perpendicular spikes we see in regular reflectors.