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Revolutionary new way of focusing! No less!

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#501 Nate B.

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Posted 20 June 2009 - 04:08 PM

I've been using the Bahtinov masks for a while now. Amazingly easy to focus. I just finished a 24" mask for an OGS RC that our club has. I'm holding a 10" mask next to it for comparison.

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#502 Chris Lord

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Posted 21 June 2009 - 11:30 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>

#503 wb9sat

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Posted 21 June 2009 - 11:35 AM

Sorry. Usually I reply to most messages on the board. Is it the same as a Hartman mask? If so, my answer is no. I focus on the pixel level by watching the FWHM numbers in my capture software getting the best possible focus on a dim star.

Perhaps you could enlighten me as to the focuser you are referring to.

Bill

#504 bill w

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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 Dennis Sakva

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Posted 21 June 2009 - 01:43 PM

Chris, similar designs were already tested. The problem with them is that diffraction spikes are much less bright while the star is much brighter than in Bahtinov's mask. In bahtinov's mask much more energy is pumped into diffraction spikes making them easily visible even on not very bright stars.

Regards,
Dennis.

#506 Mike I. Jones

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Posted 21 June 2009 - 09:11 PM

Chris,
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

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  • 3175386-Y-Mask PSFs 800x205.jpg


#507 Mike I. Jones

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Posted 21 June 2009 - 11:35 PM

Here's the Bahtinov for the same 6" f/8 system at 0.55um and ±0.005" defocus as above. The Y-Mask not only gives a brighter image, it is also a cleaner one, as it does not generate all the grating sidelobes the Bahtinov makes (similar to a Ronchi grating). It's definitely a clever improvement over the Bahtinov.

Mike

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  • 3175620-Bahtinov comparison to Y-Mask 800x210.jpg


#508 Mike I. Jones

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Posted 22 June 2009 - 12:32 AM

Dennis,
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

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  • 3175671-Y-Mask and Bahtinov mask comparisons 800x424.jpg


#509 Chris Lord

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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 glenluceskies

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Posted 03 July 2009 - 05:10 PM

Dennis, has anyone suggested that Mr. Bahtoniv place a Paypal 'Donate' button on his site? I would certainly give a small donation for his wonderful invention.

#511 John Wunderlin

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Posted 06 July 2009 - 11:41 AM

Unfortunately Russians can't receive money from Paypal yet.

#512 Hiram

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Posted 28 December 2010 - 02:54 PM

Hi, I made mine using the SVG file generated with http://www.astrojarg...kGenerator.aspx, I cut it using my small cutting plotter Craft ROBO (just $230.00 USD)
Regards.

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#513 ScubaDude

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Posted 11 February 2011 - 01:46 PM

I made one out of craft foam board. Maybe one day it will stop raining and I'll get to try it out.

#514 ScubaDude

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Posted 11 February 2011 - 01:52 PM

Forgot to add the picture.

Posted Image

#515 chinaknight

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Posted 07 November 2011 - 01:29 AM

After reading through this whole thread, I decided to make my own mask. I made it from a plastic clipboard-binder. The plastic is 0.9 mm (0.0355") thick. It took me two days to cut out. The front side, as you can see, is red. The back side is a dark charcoal grey. I left the central obstruction in place because I found that it provides stability to the mask. I cut it so that it's a nice friction fit inside the front of the meniscus cell. Here it is on my new North Instruments Group 6" f/12.5 Mak. Hopefully, I can get it out tonight and check it out on Jupiter.

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  • 4901913-Focusiing mask_2.jpg


#516 Elliot

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Posted 23 November 2011 - 05:16 PM

Silly question but where do the foam inserts go on the "Spike-A Mask?

#517 John Wunderlin

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Posted 23 November 2011 - 05:37 PM

Hold the mask up the the scope and see where it will touch- put the pads in place so it protects the scope. If you have more questions, feel free to PM me.

#518 Elliot

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Posted 23 November 2011 - 06:32 PM

Thanks John!


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