
Revolutionary new way of focusing! No less!
#501
Posted 20 June 2009 - 04:08 PM
#502
Posted 21 June 2009 - 11:30 AM
Would you all please take a look at Forum Article #27 on my website <http://www.brayebroo...Forum.html#TOP>
#503
Posted 21 June 2009 - 11:35 AM
Perhaps you could enlighten me as to the focuser you are referring to.
Bill
#504
Posted 21 June 2009 - 12:40 PM
I assume the lack of response to my question, "Has anyone run an FFT on the Bahtinov mask?" means nobody has - correct?
Would you all please take a look at Forum Article #27 on my website <http://www.brayebroo...Forum.html#TOP>
great article chris
thanks for posting it!
why don't you submit it as a formal review to cloudy nights?
fwiw, i confirmed empirically that my B mask focal point doesn't shift when it's moved off axis.
others have reported results similar to the B mask with a y made out of fishing wire, so this brings it together nicely.
perhaps the lord-y mask

should work especially well on sundays
#505
Posted 21 June 2009 - 01:43 PM
Regards,
Dennis.
#506
Posted 21 June 2009 - 09:11 PM
I ran ZEMAX FFT and Huyghens analysis on the Bahtinov when it first came out, and had planned an article in Astronomy Technology Today on it. But my day job in optics has become very demanding in the last several months so I didn't get the article written.
Roger Ceragioli sent me your paper on your new innovation, and I wanted to get it up here and over in the ATM Forum.
This is my ZEMAX simulation of your Y-Mask, a much simpler and better alternative to the very complex Bahtinov mask. Your Y-Mask gives the same basic through-focus diffraction behavior as the Bahtinov, is FAR simpler to make, and gives an image that is at least one f-stop brighter.
Well done! This is a very good innovation. There is no reason to go to the trouble cutting all the slats in a Bahtinov with your new mask design.
For those not familiar with ZEMAX, the blue part of the pupil function on the left is where light comes through. Chris's new Y-shaped mask is an obstruction. The through-focus diffraction pattern behavior is not significantly changed by the introduction of a central obstruction.
Congratulations!
Mike
#507
Posted 21 June 2009 - 11:35 PM
Mike
#508
Posted 22 June 2009 - 12:32 AM
Your point about the diffraction energy in the sidelobes relative to the central peak may be a good one. I combined my two ZEMAX predictions into the one graphic attached, all plotted at the same X-Y scale, and it is easy to see that the Bahtinov sidelobes are brighter than those produced by the Y-Mask. 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.
Guess it gets down to direct empirical photographic and visual comparisons in the field rather than just studying numerical analysis. If the Bahtinov is still superior in locating best focus to the Y-Mask in real-world practical applications, then the Y-mask still goes down as a good idea but just not as good. They 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.
Mike
#509
Posted 22 June 2009 - 04:15 AM
I assume the lack of response to my question, "Has anyone run an FFT on the Bahtinov mask?" means nobody has - correct?
Would you all please take a look at Forum Article #27 on my website <http://www.brayebroo...Forum.html#TOP>
great article chris
thanks for posting it!
why don't you submit it as a formal review to cloudy nights?
fwiw, i confirmed empirically that my B mask focal point doesn't shift when it's moved off axis.
others have reported results similar to the B mask with a y made out of fishing wire, so this brings it together nicely.
perhaps the lord-y mask
should work especially well on sundays
I wrote the article in rich text and then converted it to a pdf. I'll have a bash at writing it in html, but I'm really slow with html, I find it so confusing, it makes my brain ache. Get it to look right in Safari & its not OK in Opera, &c, &c. But I'll have a go later today.
The notion that the overlapping nth order image spectra make the spikes appear almost as bright as the zero order image occurred to me. I was about to make one after the webmaster in our local astro. soc. brought it to my attention. I have the foam board, the craft knife, and the steel straight edge, and I'm a trained draughtsman. No problem in drawing one and making it. But it was a beautiful sunny day, so I spent it on my sun lounger instead, pondering its design function.
The first amateur astronomer to make use of a coarse diffraction grating to my knowledge was Colin Pither, a member of the Webb Society who invented the diffraction grating micrometer for measuring double stars, back in the 1960's. There is a relationship between the bar/slit spacing and the useful angular separation range of measurable pairs. (Its described in Bob Argyle's Vol.1 Webb Society Double Star handbook).
That's when it occurred to me that all that was required were the three elements of the mask. So why had Pavel turned it into a coarse diffraction grating? Because that would put more energy into the spikes. But at the cost of lower throughput. Does the extra energy in the spikes more than compensate for the lower throughput? That's why I tinkered with the FFT PSF. The normalized PSF certainly confirms Dennis' argument, but when you allow for the energy loss, the spikes are slightly fainter, roughly by a factor of root 2.
Dennis has pointed out Pavel found that the lower throughput did not result in fainter spikes. However I'm not convinced by his argument. That's why I've nominated our local astro. soc. imager, who has run off a mask pattern using the svg file generator, to make his own Bahtinov mask, and then my 'Y' mask, using the same bar width. Maybe the Bahtinov mask will yield brighter diffraction spikes, maybe not. Certainly the 'Y' mask will yield finer spikes, which ought to look more intense as a consequence, plus you have about 90%+ throughput as opposed to 50%-.
What some of you maybe overlooking though is you don't have to make the bars in the 'Y' mask the same width as the equivalent Bahtinov mask, you could make them slightly wider, which makes the spikes shorter but brighter, but still plenty long enough for you to see the same effect as the full Bahtinov mask.
Maybe in average seeing (±2" - ±4") the bright knots that make up the diffraction spikes are more noticeable. But I would have thought that finer spikes would still appear finer in average seeing, and enable a better judgement as to focus. Bear in mind you have a depth of focus dependent on your 'scope f/ratio and the wavefront error of the objective in any case. It will be interesting to see how the two masks compare in excellent, good and indifferent seeing, on the same telescope.
Anyway, I hope my little Fraunhofer diffraction primer has cleared up a few misconceptions about diffraction and how patterns are produced, and how the Bahtinov mask functions. Whichever focusing mask you prefer, just bear in mind it is the bar/slit width to aperture ratio that determines the brightness and length of the spikes. If you make your Bahtinov mask with wide bars/slits you will get shorter, brighter spikes (with fewer knots) than if you make it with narrow bars/slits. If the slit pitch is twice the slit width, as it ought to be for a coarse diffraction grating, the more slits you have, the greater the number of orders produced, and the greater the extent to which they overlap.
#510
Posted 03 July 2009 - 05:10 PM
#511
Posted 06 July 2009 - 11:41 AM
#512
Posted 28 December 2010 - 02:54 PM
Regards.
#513
Posted 11 February 2011 - 01:46 PM
#514
Posted 11 February 2011 - 01:52 PM

#515
Posted 07 November 2011 - 01:29 AM
#516
Posted 23 November 2011 - 05:16 PM
#517
Posted 23 November 2011 - 05:37 PM
#518
Posted 23 November 2011 - 06:32 PM