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I've recently developed an improved focusing mask for CCD imaging.
 After working with diffraction focusers and Hartman masks, they were still time consuming.
Basically a focusing mask is an opaque material, placed in front of the telescope's objective lens.  Ususaly 2 holes are placed on opposite sides of the mask to let light through. When the 2 images merge as one, as seen in an eyepiece or  CCD camera's image, the telescope is focused.
    I have determined that placing a triangular hole on an standard 2 hole mask, simplifies the process. The third hole is used as a guide to inform the user if the focus has gone beyond the desired focus.  If the focus has gone beyond the focus point, the triangle will move to the opposite side of the focus point.
    First, the triangular hole is placed at the North (top) of the telescope's objective lens.  The 2 round holes represent East & West. The telescope should be pointed at a fairly bright star. The first images are at a low resolution to speed download time.  Even with the telescope way out of focus, the mask will automatically guide you as which direction you need to turn the focuser. The triangle will move either north or south, depending which direction the focuser is turned.

 The set of images makes this plain to see.

(For simplicity's sake, we will assume that the triangle being north, means focus in.
And thus the triangle in the south will mean focus out.)
On Image 1 the focus needs to be focused in dramatically.
On image 2 focus in needs to continue.
On image 3 we find we need to focus out.  Without that triangle, which way would we go?
On image 4 we find we need to focus  in.  Without that triangle, which way would we go?
By halving the distance the focuser travels, we should be getting close.
On image 5 almost there. Now it's time to increase the resolution of the CCD camera.
 
 

On image 6 we see image 5 in higher resolution
On image 7 we are getting closer.
On image 8 we have increased the resolution again, and decreased the time exposure.
On images 9, 10, & 11 the CCD chip has bloomed and time exposures are decreasing.
These next few images can best be seen with magnification.  Feel free to copy the image and magnify.
On image 12 with low enough exposure, we see the focus still needs to be in more.
On image 13 we are fine tuning the focus in.
On image 14, 15 & 16, any improvement is difficult to determine. Although I like 16 best.
   Now even a very out of focus telescope can be brought into focus quickly.
There may be a time when you don't have a parfocal eyepiece to get you near focus.
And if you have one, do you need to focus in or out afterwards?
The addition of a 3rd triangluar hole on a 2 hole mask, makes it all that much simpler.

12/30/97 Paul Gitto



 

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