(member)

03/27/09 01:38 PM

A new focussing mask

I started using a Bahtinov mask last December and found it invaluable. At times I found it difficult to judge the exact point at which the central spike was midway between the other two spikes.
After some experimentation I came up with this design.

Instead of a central spike, this mask generates two 'X' shapes which are symmetrical when perfect focus is reached. Any error in focus causes an asymmetry which is very obvious to see. The shape of the asymmetry indicates which way to alter focus.
Please have a look at this webpage for a clearer explanation.
http://www.btinternet.com/~gfcarey/careymask.htm

I welcome your feedback and/or questions.
Thanks,
George

(Vendor - Deep Space Mods and Composites)

03/27/09 02:19 PM

Re: A new focussing mask

George,

Very nice. I have to say though that this offers up the same problem I had when I used the common "2 String" crossed diffraction mask. As very small movements in focus can be seen it is still a judgement call as to when both spikes are symmetrical. When I switched to the Bahtinov my images were substantially improved as I found it much easier to decipher the split of perfect focus. I have tried the B. Mask by eye and than run several V curves with FocusMax and found it to be redicuously accurate. I will give your mask a try though.

(member)

03/27/09 02:28 PM

Re: A new focussing mask

Thanks Bobby,
I am very keen for someone else to try it. I am obviously a bit biased!
I can provide a CAD file if needed.

(scholastic sledgehammer)

03/27/09 02:31 PM

Re: A new focussing mask

I like the idea behind the mask but I really see two problems that, for me, would be a problem over the B. mask I made by hand:

1) If doing a mask by hand it is going to be really tough to keep the lines straight...and because that 2 degree difference is kinda small, my exacto knife cutting error may be greater than that two degrees. I know my B. Mask isnt perfect when I cut it, bt works really well and gets me as close enough (with the equipment I have).

2) Even with a perfect mask, I think I would have trouble seeing the two spikes on my canon 300d LCD. The resolution isnt that great on the LCD and even at highest zoom, I think I would have problems. I would have to d/l each pic and check with an image viewer to probably see things, and with the transfer rate from my 300d it would be horribly slow.

For others with better rear LCDs, and/or quicker downloads from their cameras it may be easier to see...and work better...but for my camera and laptop, I dont think it would work as well as a B. Mask

(member)

03/27/09 03:04 PM

Re: A new focussing mask

My camera is a Canon 350D and the zoom feature on the lcd screen alows me to see the spikes well. I don't have to involve a PC at all.
I agree that accuracy may be a problem. I have access to a laser cutter so I made the slits very narrow. I think it would work equally well with wider slits.

(Post Laureate)

03/27/09 03:55 PM

Re: A new focussing mask

That is pretty cool but I would have to laser cut since my xacto skills arent that good either.

(member)

03/31/09 08:08 AM

Re: A new focussing mask

George, I just finished reading the information on your website...It is excellent! The graphics and animations are superb teaching tools. I taught physics for 29 years, and still learned a few things! I would point out that the color produced in the diffraction pattern of a mask is not due to chromatic aberration, but is a product of diffraction itself. Diffraction, like refraction, is wavelength dependent. That is, different wavelengths diffract differently. So diffraction can be used to produce a spectrum (think diffraction grating) like a prism can, but through a different mechanism. The color you see in the reflection from the working surface of a CD is caused by this wavelength-dependency of diffraction. Reflection is not wavelength dependent (all wavelengths follow the law of reflection) so reflectors don't suffer from chromatic aberration. Again, your work shows with great clarity just how and why the Bahtinov or Carey mask works. The only question that remains for me is about the formula for determining the width of the bars and slits in the Bahtinov mask. The prescription is for the width to be 1/150 to 1/200 of the focal length of the scope on which is it to be used. I can't think of any reason for this from a physics perspective. Any ideas?

Thanks for the work you did!

Steve

(sage)

03/31/09 09:55 AM

Re: A new focussing mask

I think it's great folks are ALWAYS trying improve things...
My B mask has been a lifesaver (well, more like a time saver, headache saver...etc) but I would be curious to try yours too... It's always fun to try new things. There are always naysayers about everything....especially around here, but that's part of CN's charm!

(member)

04/01/09 04:16 AM

Re: A new focussing mask

Quote:

Steve: I would point out that the color produced in the diffraction pattern of a mask is not due to chromatic aberration, but is a product of diffraction itself.

Thanks for the comments Steve. I was also a Physics teacher, which probably explains why I could not rest until I had figured out exactly how the Bahtinov mask worked.

When I mentioned spherical abberration I was not referring to the colours seen in the spikes as you move away from the central diffraction blob (the zero order). As you say, those are a product of the diffraction. What I meant was the fact that when spherical aberration is present, those colours will be diplaced laterally from the spike. My reflector always show the colours in a neat straight line, but the refractor shows the red displaced to one side and the blue to the other. This is because they are brought to focus above and below the green position. The effect is only visible at short exposures like 1/40 sec, but it is a useful test to see if a refractor really is 'APO'.
This shows the effect

Note how the blue is shifted upwards and the red downwards.

As for the calculations of slit spacing I am not too sure. Closer spacing and more slits will cause the diffraction orders to be spread further from the centre. The Russian website where Bahtinov first showed his mask goes into detail about spacing, but I think that is only important if you use the 2nd or 3rd order to focus with, using a short exposure. Most people just use the elongated spike with a longer exposure.
Very narrow slits are hard to cut by hand, and even the laser cutter has problems because heat tends to make plastic melt and distort.
I am hoping someone will give my design a try. It needs a longer exposure and a fairly bright star, but the results seem very good.

(member)

04/01/09 04:30 AM

Re: A new focussing mask

Quote:

Jade: but I would be curious to try yours too... It's always fun to try new things.

Hi Jade, I hope you give it a try. I have found that the mask needs to be made from a sheet as thin as possible. I tried a 3mm perspex one and it caused blurred spikes which wasn't so good. I got good results from photographic mounting card which was about 1.5mm thick.

(member)

04/01/09 07:50 AM

Re: A new focussing mask

After reading your explanation, I would have bet money that you were a teacher! Please forgive my attempt to explain diffraction to you! I should have read more carefully. Also, the idea that narrower slits would spread the higher order spots more makes sense. Perhaps the 1/150 to 1/200 FL "rule" is just a practical compromise. As for materials, I have made a few masks out of Formica, cutting the pattern with a router. It is not too hard to set up a jig to cut nice straight lines at the proper angles. Thanks again.

(journeyman)

05/29/09 02:33 AM

Re: A new focussing mask

I want to buy a Carey mask... how can I get one?

(member)

05/29/09 07:30 AM

Re: A new focussing mask

Quote:

I want to buy a Carey mask... how can I get one?

Nobody is making these commercially.
If you have access to a laser cutter, here is a DXF file that you can scale to suit your scope.
http://www.btinternet.com/~gfcarey/masks/CareyMask.dxf

I am still trying to find the ideal material for the masks. Black perpex is unavailable less than 3mm thick, and the mask ideally should be much thinner. I ordered a range of clear perspex today from 0.25 to 1.5mm thickness and will experiment with cutting a mask and then spray painting black.
When I have found the best material I would be happy to send a few to people to try out.
(The one I use is cut from 1.9mm mounting card and works well, but it is a bit fragile).

(scholastic sledgehammer)

05/29/09 08:17 AM

Re: A new focussing mask

George, have you tried black sheet styrene? My other hobby is model railroading and I hand cut my Bahtinov masks from a 1mm thick 8"x16" sheet of black Evergreen sheet styrene(otherwise used to make roofs for model buildings). Styrene is pretty stiff and is readily available at a good hobby shop and the black styrene eliminates the need for painting.
I imagine sizes larger than 8" would be available.

(member)

05/29/09 10:33 AM

Re: A new focussing mask

Thanks Warren, that is worth a try. The question is, can it be laser cut? I have tried numerous other plastics like high impact polystyrene and the heat from the laser causes too much melting.
Are the styrene sheets a 'foam' plastic?

(scholastic sledgehammer)

05/29/09 12:11 PM

Re: A new focussing mask

Sorry George, I have no familiarity with laser cutting but now that you mention it the heat from a laser may be problematical. I obviously hand cut my masks with an exacto knife (essential tool for model railroading). Try this link to http://www.evergreenscalemodels.com/ and maybe they can shed some light.

(journeyman)

05/29/09 02:44 PM

Re: A new focussing mask

Quote:

If you have access to a laser cutter, here is a DXF file that you can scale to suit your scope.
http://www.btinternet.com/~gfcarey/masks/CareyMask.dxf

This link does not work...

(member)

05/29/09 03:56 PM

Re: A new focussing mask

Quote:

Quote:

If you have access to a laser cutter, here is a DXF file that you can scale to suit your scope.
http://www.btinternet.com/~gfcarey/masks/CareyMask.dxf

This link does not work...

Try right-clicking on it and then 'Save target as...'

You should then be able to save it to a folder.
If you still have trouble I can email a copy.
My email address is gfcarey at gmail.com

(member)

07/04/09 07:07 AM

Re: A new focussing mask

Niels Noordhoek who created the 'Bahtinov Grabber' has made a simulation of what occurs with a Carey mask as you go in and out of focus. Watch the green dots - when they are equally spaced perfect focus is reached.
http://www.youtube.com/watch?v=RXIpqEjAXYY

(Carpal Tunnel)

07/04/09 11:41 AM

Re: A new focussing mask

There are many different designs (like the Bahtinov) in masks that make diffraction spikes.
One that I am going to try is a Y shaped mask,
read #27 at the bottom of the page here;
http://www.brayebrookobservatory.org/BrayObsWebSite/HOMEPAGE/Forum.html#TOP
This corroborates what you are doing George.

(member)

07/04/09 12:01 PM

Re: A new focussing mask

Hi Lawrie. Yes, I came to the same conclusion when figuring out how the Bahtinov mask works. Have a look at http://www.btinternet.com/~gfcarey/bahtinov.htm
and scroll down a little to see my 'Y' diffraction pattern. The Y mask would indeed work, but of course the Bahtinov with many parallel slits produces a much brighter diffraction pattern.
After I had figured out the Bahtinov I realised that my 6" reflector has a built in mask due to it having 3 spider vanes!

(member)

07/09/09 08:07 AM

Re: A new focussing mask

Niels had made an even better simulation. This version shows what white light would look like as opposed to discrete wavelengths.
It also shows the focus error in microns.
http://www.youtube.com/watch?v=DVbBVR0PO3Q

(member)

07/22/09 03:15 PM

Re: A new focussing mask

Here is a DXF file for a Carey mask for a refractor (no central obstruction).
http://www.btinternet.com/~gfcarey/m...o_obstruct.dxf
Using Niels Noordhoek's simulation software it can be seen that a focus error of only 20 microns is clearly visible using a Carey mask.

The gap between the right hand spikes is larger than that at the left.

(member)

08/09/09 11:56 AM

Re: A new focussing mask

Niels Noordhoek's simulation software is now very accurate. This is a Carey mask going in and out of focus:
Carey Mask Simulation

(member)

08/10/09 02:39 AM

A new focussing mask shortcut?

Can you print the Carey or Bahtinov masks with an inkjet printer onto transparent overhead sheets and use them without cutting?

B. Hebert

(member)

08/10/09 11:11 AM

Re: A new focussing mask shortcut?

I have not tried this myself but I know others have. Most opinions I have heard are that it doesn't work very well.
Bahtinovs are quite easy to cut out of paper card by hand. The first one I made from paper works just as well as the laser cut ones.
Carey masks have a greater number of slits so are quite difficult to make by hand.
Are you in the UK?

(member)

09/12/09 09:39 AM

Re: A new focussing mask shortcut?

The search for the ideal material for Carey masks continues. I have now tried black paper, paper card, HIPS, ABS, aluminium, steel and perspex. The one that gives the best result is the 1.5mm mounting card, as in the picture below.

I recently tried 0.5mm acrylic (perspex), thinking that the thinness would give much better diffraction spikes. It was not too bad, but not as good as the card. Thin acrylic is transparent so needed spray painting matt black.
The 0.5mm acrylic was very thin and needed a 3mm thick acrylic support, as in this image.

I think the success of the card is the sharpness of the cuts and the lack of distortion caused by the heat of the laser.
The added advantage of card is it is cheap!

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