Home Made Hartmann Mask
Focus Pocus
Using a Hartmann
Mask with a Digital Camera as a Focusing Aid.
Home Made Hartmann Mask |
One of the things that I found different about the Nikon Coolpix is the dark LCD viewer as compared to my Kodak DC290. Luckily I had just finished making a Hartmann Mask and was able to use it during my Nikon's first light. A Hartmann Mask is made to fit the end of your telescope like its dust cap with holes cut into it. When out of focus you will see multiple images and as you approach focus the multiple images will merge into one. I made my mask out foamcore board available at Staples or art supply stores. Depending on the size of your aperture you may have three or two holes and others have even tried small arcs along the edges. Since this was my first attempt I just cut three holes, using an Xacto blade, aligned with the collimation screws on my 8" SCT. I also cut a center hole to fit over the central obstruction providing a tighter fit. I kept the three holes for later use in fine focusing. See picture at left. |
| On my first night out I shot a couple extra frames of the focusing process while shooting Mars until it really got dark. Below you will see the focusing process in action in the three *raw* images. | |
| In this picture you can see that Mars is shown in triplicate and this demonstrates an out of focus image. There is one image for each hole in the mask. | |
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In this picture you can see that the three images have come closer together. The bottom image is almost invisible but I could see three in the LCD viewer. |
| In the final image the three images have become one and the image would be in focus or at least closer than you could see with the LCD viewer. | |
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The homemade Hartmann mask cost $3.47US for a three pack of foamcore boards that will supply at least 6 masks. I tried focusing the best that I could by eye and then adding the mask and I could always get a little closer with the mask. For other digital camera astrophotographers with dark or small LCD viewers and about $4.00US the hartmann mask may prove a valuable tool in the quest for the perfect image. Gregory A. Pruden |
Links:
Focus Technique
by Will McLaughlin
http://willmclaughlin.astrodigitals.com/focus1.htm
Focusing for Astrophotography
by Jerry Lodriguss
http://www.astropix.com/HTML/I_ASTROP/FOCUS/FOCUS09.HTM
How to Focus a
CCD camera by Paul Laughton
http://www.laughton.com/paul/rfo/focus/focus.html
The Art of CCD
Imaging by Steven Lee
http://www.aao.gov.au/local/www/sl/sl-iproc.html#focus
A Commercial Version
of the Hartman Mask
The "Kwik Focus"
http://www.kendrick-ai.com/kwikfocus.html