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Old 16-05-2019, 07:29 PM
Averton (P and C)
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Bahtinov mask questions

A member having looked at one of our images suggested that a Bahtinov mask to improve focus would be a good idea. We do need all the help we can get in the focus department
We looked up on-line details and there appeared to be discussion about the sharpness of the edge of the mask influencing its effectiveness. We couldn't find any suitable dxf files on line, so made up our own based on someones calculations and made two examples. One is 3D printed and the other is CNC routed from 0.8mm aircraft birch ply mounted on a 3D printed holder.
Questions-
Are the edges sharp enough?
Which is the better version?
Does it need to be matt black? The 3D printed PLA is definitely glossy and presently the aircraft ply is unpainted but we did see some on-line in unpainted wood.

Thanks for your assistance.
Clare & Peter
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Old 16-05-2019, 07:37 PM
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Both will work.
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Old 16-05-2019, 07:46 PM
Imme (Jon)
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Both will work fine
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Old 17-05-2019, 07:40 AM
Averton (P and C)
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Thanks guys. We'll give them a try next time we have clear skies.
C&P
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Old 17-05-2019, 09:14 AM
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The_bluester (Paul)
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I have one for my SCT that is in laser cut high density particle board and is unpainted, it works fine. I have been meaning to paint it just to protect it from moisture though I don't use it very much any more, using the scope for imaging with effective auto focus in Sequence Generator has made it a bit redundant.
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Old 17-05-2019, 08:44 PM
Averton (P and C)
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Hi Paul, Thanks. We are actually just in from giving the unpainted ply wood one a try. It worked well even though there is light high cloud about in Melbourne tonight. Very happy - it can only improve our focusing.
P&C
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Old 25-05-2019, 07:26 AM
Sunfish (Ray)
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I have two bahtinov masks. One cut with a stencil knife in black plastic binder cover for the SCT and one made from fishing line , like a tennis racket , for the 100mm refractor or 800 lens which provides more light and so works on a small aperture.

Both work well and I find essential.
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Old 25-05-2019, 08:37 AM
Averton (P and C)
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Hi Ray, It appears that the mask construction isn't as critical as some of the information we had read indicated as both our attempts are working. Your fishing line version sounds very interesting. If you get a chance, a pic would be great.
Peter & Clare
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Old 25-05-2019, 10:51 AM
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billdan (Bill)
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Fly screen wire works good too.
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Old 25-05-2019, 11:19 AM
Sunfish (Ray)
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Reading tells me that the Fraunhofer diffraction pattern exploited by the Bahtinov simply relies on three differing direction linear obstructions of any size or number, the limiting factor being simply the amount of light available to create a large and bright enough pattern to see at the magnification you are using.

I read that you can use three sticks tied together in any distribution , shape and width if the aperture is sufficient , the magnification is sufficient to reveal the pattern and the star is bright enough. In focus the diffraction pattern will cross. There are lots of different masks that exploit this effect.

The fishing line opens the pattern and increases the amount of light for a 100mm retractor while the number of lines add up to a visible pattern on a bright star at full live zoom on my DSLR. I imagine you could cnc cut a very open 1mm wide line pattern in aluminium and achieve a similar or better result. A formula can be downloaded to produce an image of the required mask, imported into cad and then copied at the correct scale for the material used, which is what I have done.

Quote:
Originally Posted by Averton View Post
Hi Ray, It appears that the mask construction isn't as critical as some of the information we had read indicated as both our attempts are working. Your fishing line version sounds very interesting. If you get a chance, a pic would be great.
Peter & Clare
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Old 25-05-2019, 12:45 PM
Averton (P and C)
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Hi Ray, thanks for the picture. The fishing line version is very interesting. The idea of letting more light through is of interest to us as we have small aperture scopes.
Peter & Clare
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Old 25-05-2019, 12:47 PM
Averton (P and C)
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Thanks Bill. Another idea to try
Peter & Clare
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Old 25-05-2019, 02:17 PM
Sunfish (Ray)
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Have to say that heavy duty wire mesh looks simpler than the fishing line weaving and probably just as effective.

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Fly screen wire works good too.
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Old 26-05-2019, 10:05 PM
I.C.D (Ian)
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Hi C&P check this sit out it may help
http://astrojargon.net/maskgenerator
Most sigh writers have laser cutter, when you make your mask save it as a cad file or something like that (ask them which file they would like )or they may take it as pdf file and change it to what they can use.I had the local signwriter make mine
Ian
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Old 26-05-2019, 11:00 PM
Ukastronomer (Jeremy)
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Quote:
Originally Posted by I.C.D View Post
Hi C&P check this sit out it may help
http://astrojargon.net/maskgenerator
Most sigh writers have laser cutter, when you make your mask save it as a cad file or something like that (ask them which file they would like )or they may take it as pdf file and change it to what they can use.I had the local signwriter make mine
Ian


check link, not working
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Old 27-05-2019, 07:29 AM
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Outcast (Carlton)
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http://astrojargon.net/maskgen.aspx
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Old 27-05-2019, 12:05 PM
Sunfish (Ray)
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Yep. That is the generator I used and then copied in cad so I can print with differing scales and constructions.

I can try and post a DXF cad file if anyone likes for my 8 inch cut out version and my four inch line versions. If you have a basic cad program you can edit or simply print at differing scales or send to someone with a cutter.

Here is the effect of the fishing line diffraction without zooming in.
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Old 27-05-2019, 01:15 PM
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billdan (Bill)
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That looks pretty cool - they look like spaceships from StarWars.
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Old 27-05-2019, 04:04 PM
Sunfish (Ray)
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Bahtinov DXF

Attached is a DXF file containing a bahtinov mask for a C8 and one for a 100mm refractor in line. Both are vector line drawings which can be edited and scale in mmx100.

So scale down to 1% for actual size. .

A PDF image of the mask from the generator used as a model also attached.

I think this will work with less bars if required due to cutting constraints.
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File Type: zip bahtinov 2.dxf.zip (19.4 KB, 10 views)
File Type: pdf Bahtinov c8.pdf (379.3 KB, 19 views)
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  #20  
Old 27-05-2019, 07:24 PM
Averton (P and C)
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Hi Ian, Thanks for the link. We actually tried to use this generator however it generates svg files which are not industry standard for CNC machines. We tried to convert the svg to dxf using Inkscape but it was such a mess that we simply re-drew it in LibreCAD using the generator's basic dimensions. We then CNC routed it on our home-made router. dxf file attached for 5 inch mask.
Clare & Peter
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