amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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Of about 2 cents: simply span 3 wires (actually 2 knotted together) asymmetrically in front of the telescope and a similar pattern will apear as with Bahtinov. To be precise: the wires give a lines pattern, while the Bahtinov mask produces lines consisting of bright individual spots (little rainbows of you look in detail). Bahtinov spots are brighter than the continuous lines produces by single wires, but for photographic use, it still works well.
The asymmetry will result in the middle line in the diffraction pattern to move relative to the crossing of the two other lines (left right in the snapshots).
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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diffraction pattern of the wires:
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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diffraction pattern of a hand-cut paper Bahtinov mask with the same exposure parameters:
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iceman
Post Laureate
Reged: 03/07/04
Posts: 4811
Loc: Gosford, Australia
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Interesting. Can you show us the lines in and out of focus?
--------------------
Mike
. mikesalway.com.au - Astronomy and Photography by Mike Salway
. IceInSpace - The Australian Amateur Astronomy Community
. My Bio | My Jupiter 2007 Gallery | My Image Gallery
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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see post above after edit
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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And the preferred type of knot: 
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Freddy WILLEMS
Carpal Tunnel
Reged: 11/13/05
Posts: 2557
Loc: Hawaii, Honolulu
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Very interesting,
What kind of wire do you use ?
I you use fishing wire does that give you more reflection ?
Freddy
--------------------
Freddy
Meade 14" LX200 GPS UHTC GPS on permanent pier
Celestron C 14" Peltier cooled for planetary imaging.
Meade 10" LX200 & TITAN 50:1 mount Gemini 'go to'
Meade 127 mm f/9 APO & TITAN 50:1 mount Gemini 'go to'
W/O 102 mm f/7 APO doublet
Orion 80 mm f/7 ED
DFK 21AU04.AS
ToUcam 840 II pro
Canon 10D Unmoddified
Canon 40D Hutech moddified
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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I just used what was available: 1mm thick Nylon (white).
Any wire will do, as long as it is not perfectly transparent.
The image you see on your webcam is actually the 2D Fourier transform of the focussing mask.
The Fourier transform of a wire is a line. The Fourier transform of a regular grid is a series of blobs.
See animated gifs.
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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and the three wires plus their Fourier transform
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Freddy WILLEMS
Carpal Tunnel
Reged: 11/13/05
Posts: 2557
Loc: Hawaii, Honolulu
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I'll try tonight with the wires !
I'll keep the template for the different scopes to line up the wires properly.
Freddy
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Freddy
Meade 14" LX200 GPS UHTC GPS on permanent pier
Celestron C 14" Peltier cooled for planetary imaging.
Meade 10" LX200 & TITAN 50:1 mount Gemini 'go to'
Meade 127 mm f/9 APO & TITAN 50:1 mount Gemini 'go to'
W/O 102 mm f/7 APO doublet
Orion 80 mm f/7 ED
DFK 21AU04.AS
ToUcam 840 II pro
Canon 10D Unmoddified
Canon 40D Hutech moddified
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vegasMike
professor emeritus
Reged: 05/24/04
Posts: 605
Loc: Las Vegas
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Super! You learn something new everyday!
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Can't afford an APO but I used to get my mail at one...
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Dennis_Oz
sage
Reged: 08/20/06
Posts: 302
Loc: Brisbane, Australia
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A 3 vane spider will do exactly the same thing. My Tak Mewlon with its 3 vane spider produces the “Bahtinov diffraction pattern” on bright stars. That is, a “stationary” shallow X and a single “moving” line that passes through the centre of the shallow X.
Fitting my Bahtinov Mask makes the diffraction patterns much brighter.
Cheers
Dennis
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Freddy WILLEMS
Carpal Tunnel
Reged: 11/13/05
Posts: 2557
Loc: Hawaii, Honolulu
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I mark the front of the telescopes were the wires need to be. Probably later I will use a lit or so (make a circle) from a can or so to attach the strings.
Freddy
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Freddy
Meade 14" LX200 GPS UHTC GPS on permanent pier
Celestron C 14" Peltier cooled for planetary imaging.
Meade 10" LX200 & TITAN 50:1 mount Gemini 'go to'
Meade 127 mm f/9 APO & TITAN 50:1 mount Gemini 'go to'
W/O 102 mm f/7 APO doublet
Orion 80 mm f/7 ED
DFK 21AU04.AS
ToUcam 840 II pro
Canon 10D Unmoddified
Canon 40D Hutech moddified
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quantumac
sage
Reged: 12/17/07
Posts: 387
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While the string is an interesting idea, a Bahtinov mask can be made out of inexpensive materials, plus it's quicker to put on and take off. I made my mask for under $17 (foam board, cheap wooden needlework loop, foam board cutter, glue). I also overspent because I bought double the foam-board I needed. I could have gotten out of the craft store for about $14.
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Scope: Meade 10" LX200R
Guide/Planetary Camera: Imaging Source DBK41AF02.AS
DSO Camera: QHY8
Guide Hardware: Celestron OAG, Shoestring Astronomy GPUSB
Software: Mac OS X, Starry Night Pro, Nebulosity, PHD Guiding, PixInsight, Astro IIDC. No Windows anything.
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amateur
sage
Reged: 06/15/06
Posts: 301
Loc: The Netherlands
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My idea only came forth from the fascination I have with the wonderful idea of mr Bahtinov.
But still, your 17$ could have been used to give about 2000 people 3 wires to focus 
For visual, small apertures and fast telescopes (DSLR lenses included), the full Bahtinov mask clearly is the winner.
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Dennis_Oz
sage
Reged: 08/20/06
Posts: 302
Loc: Brisbane, Australia
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Hello, amateur
Thanks for the reports and animated gifs describing your experiments – I enjoyed them and learned a lot as well!
Cheers
Dennis
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jgraham
Postmaster
Reged: 12/02/04
Posts: 6760
Loc: Dayton, Ohio
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Neat! I'll have to give this a try. I'll probably make a simple frame using foam core art board and hot-melt glue the wires/strings to the frame.
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-John
================================================
Homebuilt scopes from 4.25-16.5"
Meade LXD75-N6/SN6/SC8, DSX-90, ETX-60BB, ETX-125PE, DS-2130
Orion StarBlast, BinoViewers, Coronado PST
Rebel XT/XTi, DSI Pro (I, II, & III), DSI, LPI, Electronic Eyepiece, Phillips SPC900NC
Tasco 60mm Refractors
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ColoHank
professor emeritus
Reged: 06/07/07
Posts: 516
Loc: western Colorado
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I made a Bahtinov mask out of the black plastic lid from a cashew can. It's just the right diameter inside to fit over the corrector on my Questar. I'm not sure that the plastic left after cutting out the grid is flat enough and rigid enough to hold shape for best accuracy, but then I don't image and can focus satisfactorily for visual use without any mask at all. At least that's what I tell myself. My mask-making efforts were just for fun.
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Questar 3.5 standard - pyrex and BB coatings
Powerguide II
8mm, 12mm, 16mm, 24mm and 32mm Brandons
modified Bogen 3030 w/ homebuilt wedge
Homebuilt Galileo scope and very large and ugly homemade tripod
other odds and ends, including iPod Touch with StarMap Pro (what a marvelous combo)...
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"Nothing exists but atoms and empty space. Everything else is opinion."
Titus Lucretius Carus 99-55 B.C.
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