I have been following a post on Cloudy Nights about a revolutionary focusing mask designed by a Russian amateur astronomer, Pavel Bahtinov. The mask consists of a pattern of solid bars with equal width spaces between the bars. One side of the mask has horizontal bars, the other side has bars tilted at angles of ±20°. I decided to make one and give it a test drive.
Using Pavel’s formula: Focal Length/(Range 150-200) = Bar Thickness + Inter-Bar Gap, I came up with the following design for my Tak Mewlon 180mm F12 with a focal length of 2160mm.
2160/180=12mm, giving a Bar Thickness of 6mm and an Inter-Bar Gap of 6mm.
NOTE: The divisor range of 150-200 allows for a Bar Thickness/Bar Gap dimension that doesn’t come out as an awkward value of, say 6.5735mm.
I drew up the mask in Corel Draw (phew!) and printed it off on a sheet of overhead transparency film as a prototype for testing. I then designed and built a foam collar which I glued to the transparency mask and finally, trimmed the rectangular transparency.
Last night I was able to conduct some testing with the following results.
The Bahtinov Mask does work.
It works very well indeed!
As you approach focus, you can easily see a horizontal “moving diffraction bar” approaching the shallow “X” of the two “fixed diffraction bars”.
At focus, the “moving diffraction bar” sits nicely centred between the shallow “X” of the two “fixed diffraction bars”.
When past focus, the horizontal “moving diffraction bar” begins to leave the shallow “X” of the two “fixed diffraction bars”.
Transparency film is not suited to making the mask as it does change the focus due to the thickness of the transparency film; visually, this was easy to see. Using Fomalhaut (1.2m), focusing without the mask was relatively precise by watching the 6 diffraction spikes produced by the Mewlon 3 vane spider snap into focus. When I fitted the mask, I had to adjust the focus by 3 or 4 short stabs on the Moonlite motor focus button – the difference definitely noticeable visually. Also, the transparency film has a sort of ground glass texture which dims the incident light and makes things a little smudgy.
So, it looks like I’ll have to make a mask with clear gaps! Many thanks to Pavel for sharing his design for this elegant, incredibly useful astro-accessory.
Cheers
Thanks for that Dennis, this design is doing the rounds on the UK boards at the moment its a brilliant bit of kit. I don't agree tho with whats happening with this company http://www.spike-a.com/
The guy is ripping Pavel off plus NOT even mentioning him as the inventor, this is shameful beyond belief and will only put off people in the future from sharing ideas.
I’ve just trialled my Mk II mask, with the clear transparency slots cut out and wow, what a difference. The results are simply astonishing. There is absolutely no doubt about the point of focus and I think this will even beat software assisted focusing.
My next trial (not tonight) will be to try it on “Remote Live View” on the Canon 40D.
Wow, wow, wow and wow again – it even works with a TeleVue x2.5 Powermate inserted into the system. Using my DMK21AF04.AS I had no problems achieving perfect focus, despite the somewhat iffy seeing conditions! That’s using the Mewlon 180 F12 with TVx2.5 giving a nominal F30.
What an ingenious design; so elegant and effective – great stuff Pavel, I hope you gain some benefit (hopefully financial!) from releasing this design into the amateur astronomy community.
Here is an animated gif of a focusing run on Fomalhaut (1.2mag) at the F12 prime focus of a Mewlon 180 using a DMK21AF04.AS. Note the jitters are due to my lack of skills in generating the gif and are nothing to do with the operation or functioning of the mask.
This is similar to Rod Wodaski's diffraction spike focusing.
Rods is much easier, and just as accurate as it uses the same method (Diffraction spikes). An example here http://home.comcast.net/~dandkmccrad.../FocusMask.htm can be seen.
No measurements, photocopying etc, so much easier.
Theres plenty more ways to use diffraction spikes for focussing, just need to look. As was found here by Dennis.
Theo, yep thats true, but It seems with this method that it's easier to tell when its at exact focus.
I have trouble with the three-triangle style Hartmann that I use, can never really tell if the spikes are lined up nicely.
Looks like it's very easy to see with this one, will have to try it
So far, from the two relatively short testing sessions that I have conducted, my impression is that Pavel’s mask seems more “certain” and “absolute” compared to the on-screen example I have just looked at on the Ron Woadski link provided.
My Mewlon has a 3 vane spider and so all bright stars have 6 diffraction spikes without any mask. Although these diffraction spikes are a very useful aid in getting a good focus, I can still “dither” with the motor movement keys and “interpret” the “best” focus using the built in 6 diffraction spikes.
When I fit Pavel’s mask, I experience something quite different. There is no dithering or interpretation; the spot of best focus just jumps out due to the “moving bar” being located between the arms of the shallow X.
Note that here, I am comparing Pavel’s mask to my “native” diffraction spikes as I have not used the Wodaski Mask and Method.
Here is an example using Altair. The central “moving diffraction bar” is either in between the “stationary” shallow “X diffraction bars” or it is not, so the positioning can be quite precise.
Theo, yep thats true, but It seems with this method that it's easier to tell when its at exact focus.
I have trouble with the three-triangle style Hartmann that I use, can never really tell if the spikes are lined up nicely.
Looks like it's very easy to see with this one, will have to try it
Thats why the chap in my post used circles with a strip thru them instead of Rod's method, so he can remove most of the extra diffraction spikes.
I use this method, and its easy and accurate.
Still, Dennis is on a crusade with this one.
i am with you on this Dennis, i think this method looks far better then the Hartman mask with diffraction gratings. I downloaded the files off cloudy nights but do not have Corel draw. might have a go at a CAD drawing of this and send it to a CAD/Cam cutter to make it up for the 8" vixen, the 127ED and the 8" SCT and the ED80, oh and the 10" lx200R.
i am with you on this Dennis, i think this method looks far better then the Hartman mask with diffraction gratings. I downloaded the files off cloudy nights but do not have Corel draw. might have a go at a CAD drawing of this and send it to a CAD/Cam cutter to make it up for the 8" vixen, the 127ED and the 8" SCT and the ED80, oh and the 10" lx200R.
Hi Dave
So far, I have found that the Bahtinov Mask works at the prime focus of the Mewlon and also with a TeleVue x2.5 PowerMate in the optical train, both visually and with a DMK21AF04.AS as the sensor.
So, even though the calculation as written, of Focal Length/(Range 150-200) = Bar + Space dimension gave me a Bar Width of 6mm and a corresponding Bar Gap of 6mm using a fl of 2160mm, the same mask seems to hold up even at a fl of 5400mm.
At shorter focal lengths of say, 800mm and less, the formula changes to 3xF/(Range 150-200) so the bars don’t get too thin.
Therefore, for your larger ‘scopes, you may only require the largest mask with some means of fitting it to the smaller scopes, rather than a mask for each?
Cheers
Dennis
PS – I drew up my mask in Corel Draw and would be happy to send you a high res file if you want it.
might have a go at a CAD drawing of this and send it to a CAD/Cam cutter to make it up for the 8" vixen, the 127ED and the 8" SCT and the ED80, oh and the 10" lx200R
Dave, if you go down that route I'd be interested as well, I expect that the price per unit would be better for a quantity buy. What would be really nice is for someone to write a simple program to automatically generate the drawing.
Quote:
Originally Posted by cavalier blanc Dennis
PS – I drew up my mask in Corel Draw and would be happy to send you a high res file if you want it.
Here are some quick pics of the Bahtinov Mask that I made to give an idea of its construction. I drew up the mask in Corel Draw with a bar Width of 6mm and an inter-Bar Gap of 6mm based on my focal length of 2160mm.
The mask was printed onto an A4 overhead transparency which was slightly too narrow, although wide enough for the 180mm aperture of the Mewlon.
I cut a strip of camping mat foam (closed cells) and made a ring which just fitted over the end of the OTA. I then cut the transparent gaps out of the transparency film using a steel rule and a Stanley knife and glued the mask to the foam ring.
Where I accidentally scratched the ink during the cutting process, I used a permanent marker pen to blot out the scratches.
and mount your trust steed
, go forth and vanquish the unbelievers and convert them to Bahtinovism.....................:he lp:
Can't wait to see the pics, Dennis.
