Diffraction Focusing method

The most difficult thing in the CCD imaging, against the astrophotography with classic films, is the focusing. Because, it is the CCD chip who observes directly and not our eye. Our eyes see the picture on the screen of a laptop computer with a time of download.

But we can use several techniques to achieve a very good focusing. The first techniques i used were the Hartman disk from Jim Kendrick (the KwickFocus) and after that the FWHM method with the "Focus Inspector" of the MaxIm DL CCD software.

Unfortunately, i found these techniques give a good focusing but not an accurate focusing. I discovered, on the Bill McLaughlin web site, the diffraction focusing method. I decided to test it.

I made a cross with a perpendicular section, in the middle, with one aluminum stick of 0.6mm of diameter. I install it in front of my Kendrick dew cap. I use sticky strips from accessories school to fix it.

My aluminum cross (the small is for my APO refractor), the yellow elements are the sticky strips.

To realize my focusing, i use the SBIG CCDOPS software, with the "Planet" mode in the Focus mode. I can select an area around the star to have faster rate. For the star i use the LX200 HPP star. In the High Precision mode, the LX200 searches a bright star close to the object to have an accurate pointing. I use it. My time of exposure is 2 or 3 seconds according to the transparency of the sky.

Used with a Crayford focuser (to me it is the JMI NGF-S) this method is very very accurate and very efficient. With the Hartman disk and FWHM techniques, my final focusing had the same results like in the "Step n°3" picture . We can see, between the "Step n°3" and the "Step n°4", we have a significant error.

The next pictures show the different steps to obtain an accurate focusing with the diffraction method. This method is, to me, the best. Cheaper, faster and very accurate.

Follow the steps (click on the picture)