However, I have tried this free on-line generator and the .svg file it produced gives me an IO error in my drawing application, Corel Draw.
Try saving it to your desktop and just double-clicking on it. On my computer, it opened in firefox and it looked like I could print it out. Of course, i don't need another one as I have about a dozen aluminum prototypes here at the moment.
Try saving it to your desktop and just double-clicking on it. On my computer, it opened in firefox and it looked like I could print it out. Of course, i don't need another one as I have about a dozen aluminum prototypes here at the moment.
Hi Wunder
Thanks for the tip, but I had already tried that and get the same “I/O read error” event though I can still open other .svg files either via “double clicking” or “drag and drop” or “File Open”.
I’ll check Corel Draw V12 to see if there are any SP’s or updates.
i used the online mask generator - this time i generated the svg files and then used the latest version of the GIMP to read the file and convert it from svg to photoshop - the results are attached. Corel 12 wont read the files even with a patch update Dennis
i used the online mask generator - this time i generated the svg files and then used the latest version of the GIMP to read the file and convert it from svg to photoshop - the results are attached. Corel 12 wont read the files even with a patch update Dennis
Hi Dave
Thanks for the feedback re the .svg files and CorelDraw 12. I already had CDr SP1 and all patches applied and it still gave the I/O error, although I can open .svg files from other sources.
I downloaded and installed Inkscape (free) and that opened the .svg files so I’m okay now.
Cheers
Dennis
PS - Inkscape is an Open Source vector graphics editor
I noticed the ED80 one has signifcantly smaller slits than the one I made up by rescaling your previous post for the 100ed, will this make much difference
I noticed the ED80 one has signifcantly smaller slits than the one I made up by rescaling your previous post for the 100ed, will this make much difference
Cheers
I just followed the formulae and the assumptions - it was either a 5 or 6mm spacing for a 80mm dia and a 600mm focal length
the online mask programme did the rest i am yet to make and test - but i have printed it
Based on the original formula (using my calcualtions) the slit width for an 80ED f/7 should be roughly 3.75mm. The original post of the 100ED mask from Houghy rescaled to 83% produces slit sizes of this width whereas the recent post produces signifcantly smaller slit widths which are not easily workable.
Based on the original formula (using my calcualtions) the slit width for an 80ED f/7 should be roughly 3.75mm. The original post of the 100ED mask from Houghy rescaled to 83% produces slit sizes of this width whereas the recent post produces signifcantly smaller slit widths which are not easily workable.
Cheers
Hi Trevor
The “standard” formula is Focal Length/N(range150 to 200)=(Bar Width + Bar Gap). For shorter focal lengths we multiply the Focal Length by a factor of 3 giving:
3xFL/N=(Width+Gap)
An 80mm F7 has a FL of 560mm.
So, using the denominator at max value of 200, I get a 4.2mm Bar Width.
560x3=1680
1680/200=8.4
8.4/2=4.2mm
And, using the denominator at min value of 150, I get a 5.6mm Bar Width.
560x3=1680
1680/150=11.2
11.2/2=5.6mm
The x3 multiplier for focal lengths of say, less than 800mm means that you don’t have to cut the slits too thin.
We’ve had a few days of cloudy nights (not CN!) so I’ve been playing around in Corel Draw and the layout of the Bahtinov Mask.
In my original design, for the Mewlon 180, I used a denominator of 180 to give me a Bar Widths and Bar Gaps of 6mm. I then laid out the horizontal bars/gaps and 20 degree bars/gaps using this same 6mm result. However, I noticed that due to the offset caused by the 20 degree angle, the angled bars gradually began to move "out of phase" with the horizontal bars the further I went above/below the middle horizontal bar.
So, I did some calcs in Corel Draw and came up with the following:
For my 6mm horizontal bars, to keep the bars in phase (i.e. connected) the angled bar widths and bar gaps need to be 5.6382mm.
I have attached a couple of files illustrating this. In terms of real world use this is probably not important, but if you are going to get these cut at a workshop, it might be worth making sure the master template is accurate? I’ve yet to print and cut the modified mask with the narrower bars/gaps on the angled side, but I’ll probably get around to doing this and testing it over the next few days.
I wonder if the mask generator allows for this offset?
Cheers
Dennis
PS - I know, I know; you’re probably thinking I have way to much time on my hands….
We’ve had a few days of cloudy nights (not CN!) so I’ve been playing around in Corel Draw and the layout of the Bahtinov Mask.
In my original design, for the Mewlon 180, I used a denominator of 180 to give me a Bar Widths and Bar Gaps of 6mm. I then laid out the horizontal bars/gaps and 20 degree bars/gaps using this same 6mm result. However, I noticed that due to the offset caused by the 20 degree angle, the angled bars gradually began to move "out of phase" with the horizontal bars the further I went above/below the middle horizontal bar.
So, I did some calcs in Corel Draw and came up with the following:
For my 6mm horizontal bars, to keep the bars in phase (i.e. connected) the angled bar widths and bar gaps need to be 5.6382mm.
I have attached a couple of files illustrating this. In terms of real world use this is probably not important, but if you are going to get these cut at a workshop, it might be worth making sure the master template is accurate? I’ve yet to print and cut the modified mask with the narrower bars/gaps on the angled side, but I’ll probably get around to doing this and testing it over the next few days.
I wonder if the mask generator allows for this offset?
Cheers
Dennis
PS - I know, I know; you’re probably thinking I have way to much time on my hands….
No need to keep the angled side in phase. You only need to make sure that whatever angle you use is repeated in a mirror across the center axis. Remember a diffraction spike is created when the light wave is disrupted by a sharp edge. It will always be in the center of the light path when it is in focus. This is the root of how the Bahtinov masks works. The diffraction spikes on the horizontal side will shoot out 90 degrees from the cutouts, and always in the center of the light. the angled side will also shoot diffraction spikes out 90 degrees from the cutouts. So the mirror image design is important so as you get all of the light in focus the diffraction spike from the horizontal side will be in the center of the two angled sides. The number of slots and distance of the cutouts are only minor details.
Thanks for the explanation, I suspected as much. Apart from a basic understanding of the central Airy disc and the accompanying 1st and 2nd order diffraction rings (which I can usually seen in-focus with the Mewlon 180 and my Vixen ED102 refractor at high magnifications), I’m still a little hazy on some of the wider aspects of diffraction, other than the more edges, the more I get!
Dennis- Good point! The more edges you have the more diffraction you get. However as our friends in Russia have found also too many slots will blur the spikes and make it a bit more difficult to focus. Pavels formula is designed to catch the peak of each light wave and stack them on top of each other to get the maximum diffraction spike that is clear. Farmer Dave posted his mask generator that works fabulously.
When I sat down and plotted a curve for different focal ratio scopes I saw a curve in the results. I drew a straight line down the middle and came up with a simple formula that works on every scope we tested. These were as small as an ED80mm up to a 16 inch Meade LX200. From F4 to F15. 26 different scopes so far. All of the work with the following math:
Aperture divided by 35 will give you the spacing for a center cutout and 8 up and 8 down. The other tip we did was when you get down less than 130mm we added one extra slot on the angled side only. this helped the spikes to show up a bit brighter.
Dennis- Good point! The more edges you have the more diffraction you get. However as our friends in Russia have found also too many slots will blur the spikes and make it a bit more difficult to focus. Pavels formula is designed to catch the peak of each light wave and stack them on top of each other to get the maximum diffraction spike that is clear. Farmer Dave posted his mask generator that works fabulously.
When I sat down and plotted a curve for different focal ratio scopes I saw a curve in the results. I drew a straight line down the middle and came up with a simple formula that works on every scope we tested. These were as small as an ED80mm up to a 16 inch Meade LX200. From F4 to F15. 26 different scopes so far. All of the work with the following math:
Aperture divided by 35 will give you the spacing for a center cutout and 8 up and 8 down. The other tip we did was when you get down less than 130mm we added one extra slot on the angled side only. this helped the spikes to show up a bit brighter.
Hi Jerry
Thanks again for providing your insights and some experimental results from the research and development efforts of developing the mask for commercial release.
Not withstanding Pavel Bahtinov’s original talent, intuition and generosity in publishing his design, I think you have been very generous in providing guidance and helpful information on several forums that I have enjoyed visiting on this topic.
I note that your posts have provided encouragement for others to better understand the principles involved, as well as providing helpful “nudges” to the army of DIY mask makers, helping them focus on the key design issues for optimising their own home made designs.
I hope that once the home made masks begin to un-stick, wilt and fall apart, their owners will visit your website and purchase the “last a lifetime” model! I feel certain I’ll be one of those after our next humid summer season down under!
