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The Wire Mesh Focuser  (Primitive....)

UPDATE 2008:  I now know that a Bahtinov Mask is a much, much better tool than the Chicken Wire Focuser.  Click here to read all about the exciting new Bahtinov Mask.

I did this experiment with the Chicken Wire Diffraction focuser back in the distant past of - 2005 A.D.  If you're really keen to know more about the chicken wire then read on if you must.

Star Date 2005 AD.  Blog entry....

Getting nice focus is hard enough when you are viewing objects in your telescope by eye.  When you need to photograph objects with a CCD camera or even 35 mm film then getting good focus is CRITICAL.  So many otherwise excellent astronomical photos that were captured at great time and expense are noticeably out of focus.  If they were properly in focus they would turn from being good shots to sensational shots.

Focusing through a camera's viewfinder with your eye is not good enough because your eye can deal with seeing things that are 3% out of focus.  Your eye and brain compensate, so you will think that the view is in focus when in reality (as captured by the camera) it is not.

So if your eyes can't tell you if a view is in focus how can you tell?

Introducing the Chicken Wire Focuser...

Diffraction Focuser
Chicken Wire Mesh Diffraction Focuser


Making a Wire Mesh Focuser

So what is a Wire Mesh focuser?  I'd seen them mentioned once or twice when searching the internet for tips on focusing but they were mentioned only in passing.  No one described how to make or use one.  So I gave it a try. 

Creating one is simplicity in itself.  The easiest way is to cut a flat square of chicken wire and just hold it in front of the telescope objective.   Use wire that has a small grid and which is arranged in parallel lines.  Having the lines in a square grid will maximise the strength of the diffraction pattern.  Watch out for those expensive optics!  Don't scratch the lens or corrector plate if you have a refractor or a CAT.   Cover the edges of the wire with tape or better still make a permanent holder for your focuser.

chickenDiffractionFocussed.jpg (2843 bytes)
An in-focus star as seen by a Mintron
CCD camera with the Mask on.

The best way to construct the focuser is to cut a circle of flat chicken wire and strap it with string onto a rigid ring (something like the old turntable ring from a microwave that I used).  Use a couple of bits of string to hold it in place on the ring, then draw the mesh flat applying slight tension with more loops of string threaded through the wire grid at the ring.  The ring should be sized so it fits safely and snuggly over the front of the telescope tube without the wire being anywhere near the glass at the front.   For added safety use some sort of padding, like the cheap concertina electrical hose that I used around the edge of mine.    A long flexible strip of foam or plastic would also do.

Adjust focus until the diffraction spikes are brightest.  You will quickly learn to recognise the differences in the diffraction pattern to tell when you are in focus.  This is when the spikes are thinnest, and brightest, and when the dark gaps that are spaced along the spikes are well defined, widest and darkest.

The resulting focus won't be exact.  Like the Hartmann Mask you still have to make a subjective decision on brightness and sharpness - of the diffraction pattern.  When you are near focus you will be able to move the telescope's focus wheel slightly without knowing whether this has improved or worsened the focus.

In my opinion focus with the wire mesh focuser appears more precise on average than with a Hartman Mask when the seeing (air turbulence) is bad or average.  I've demonstrated this on video by recording the dots from a Hartman Mask dancing about, going in and out of focus as the mask's two light rays took slightly different paths through the turbulent atmosphere.  A moment later when I held up the chicken wire it was obvious from the sharp diffraction pattern, that I was still in focus.  It does seem that the diffraction pattern is not as susceptible to atmospheric turbulence.

Also check out my other equipment reviews:

    The Bahtinov Mask which is a sensational new variation of the Hartmann Mask.  Excellent !!!

    A home-made Hartmann Mask which is another primitive device for judging focus.

     The Telrad which all newbies absolutely must have.

    A Barn Door Tracker for taking long exposure wide-field photos of the sky.


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