Bahtinov Mask

[MarkS]

I've just cut a Bahtinov Mask from cardboard and given it a test run on my Celestron C11.
This is the easiest way of focusing a DSLR I have yet come across (much much easier than the Hartmann Mask).

This thread on an astronomical website(and similar ones on the Cloudy Nights site) explains all:
http://www.iceinspace.com.au/forum/showthread.php?t=35676

Here is the one I made tonight:


And here's an animated GIF I created (using the DSLR on the C11) that shows it in action - it is the image of a single star with the Mask placed in the front of the scope:


When the vertical bar is central between the slanted ones then you have perfect focus.  The other great thing is that it even "tells" you which way to turn the focus knob - turn it one way to move the vertical line to the left, the other to move it to the right.   It couldn't be easier!

[Ian]

now that looks scarily useful...

[RobertM]

It's brilliant, I've been trying it out the last couple of sessions and it certainly works much better than the mask I've been using till now.  Mine's rather crudely made out of part of a cardboard box but still works a treat.

[Tony G]

I can feel a Blue Peter moment coming.
Might try this the weekend.

Thanks Mark.

Tony G

[Mike]

Looks interesting. I will give that a go myself.

[Mac]

Found this whilst searching for things astronomical.

http://astrojargon.net/MaskGenerator.aspx

it will generate your masks for you.

[Carole]

I've made the 3 hole version focus mask.  Would any-one say that this mask is even better.  I found the 3 hole version worked perfectly for me when I used it. 
Just wondering whether I should make this one as well?

Carole

[RobertM]

For the extra accuracy it's definitely worth the effort.

[mickw]

How precise does the cutting have to be, or is it the pattern that does the trick ?

Cutting one for a 91 is doing my eyes in

[Mac]

or is it the pattern that does the trick

Its the pattern that works, by using diffraction.
similar to the once you see round stars

cutting dosent have to be that accurate, resonable should do.
all you will get is a slightly different difraction pattern.

[mickw]

Thanks Mac

That's a relief 

[Mike]

Just realised I can get these laser cut at the London Hackspace in perspex or MDF. Will have to make one for my ED80.

[RobertM]

I have a couple that need doing, any chance of getting those done while you're at it ?

[Mike]

Sure. Give me the details.

[RobertM]

Thanks, I'll need to get some perspex and mask layout to you.

When are you getting them done?

[Mike]

Some time next week.

Just give me the numbers for the generator, it will be a lot easier.

I will need to convert the .svg file to .dxf for the laser cutter.

[RobertM]

Ok, will do

[Fay]

I have an 8" one I can sell

[The Thing]

I used the web generator (Mac's link below) and printed it on an A4 self adhesive photo sticker, sprayed it with photo shield stuff, then stuck it on an old plastic folder cover then cut it out with a craft knife. I believe sharp edges are crucial rather than perfectly straight lines as its the edge that causes the diffraction. I have tried a couple of configurations and fewer slots seems to be better. 8" masks must be easier to cut than one for Mick's Burgess.

[mickw]

8" masks must be easier to cut than one for Mick's Burgess.

I still see diffraction spikes when I look at a lightbulb 

At least I know it's in focus 

[Carole]

Wish I'd seen this revived thread a couple of days ago, might have saved some blisters on my fingers from all the pressing with the cutting blade.

[MarkS]

I've just seen the animation at the beginning of this thread.  I was dead impressed until I realised it was my own!

[Carole]

 

[Mac]

 

[Mike]

Note that the maximum size of the cutting bed for the laser is 310mm x 585mm so nothing over 310mm in diameter. Suitable materials - thin MDF sheeting or acrylic.

[mickw]

So the Lightbridge is out then   

When would you want the materials by ?

[Mike]

Acrylic can be cut up to 4mm depth and MDF up to 3mm. I can get materials from Hamar Acrylic which is just a short walk from London Hackspace. I'm doing some for Robert tomorrow (if he gives me the dimensions) so I'll let you know an idea of prices then. I know it is £3 per hour to use the cutter plus cost of materials on top.

[mickw]

PM on it's way

[Mike]

Did a few tests today cutting some of the 3mm MDF Mick gave me:



This was using the 80mm dimensions Robert gave me for his scope.

The acrylic Mick gave me was useless though, it just burnt. It needs to be branded Perspex apparently if you want smooth edges. I shall pick up some in black from Hamar Acrylic some time this/next week and do some more tests.

[Mike]

Just for a laugh I made some planetary gears (tenuous astronomy link) too.

[mickw]

Love the gears 

The reason the acrylic I gave you didn't work was because it was polycarbonate - Sorry 

I'd better get some numbers to you soon as well

[RobertM]

Neat gears 

Those 80mm masks look ok, were they with the original figures I gave?

Robert

[Mike]

Robert, they were with the second set of figures. The original set had the slits so close together that the supporting struts also got burnt away during the cut. You might want to look at the designs for the other two as they may need adjusting too.

Once you are happy with the design I will cut them for real.

[The Thing]

Just for a laugh I made some planetary gears (tenuous astronomy link) too.

What sort of diffraction pattern would that create!

[Rocket Pooch]

I have an 8" one I can sell

Is it an SCT one with the big hole in the middle?

Chris