The Art of CCD Focus

On January 10, 2001, under a full moon, I pointed the NGT-18 into the double cluster to image to take a quick one second image of NGC 0884 using the ST-9E.  I did this with the following result:

At first clance, the image looks like it is in pretty good focus.

I achieved this level of focus by using an IFocus eyepiece (originally purchased for an ST-6, but re-worked for the deeper back focus of the ST-9.  This gave me "blobby" stars.  I may be able to adjust the retainer ring on the IFocus eyepiece to do better, but for the time being, getting to the ballpark is good enough.  

From here, I used  maximum pixel strength to achieve the level of focus that I got.  One of the biggest problems with this technique is the wildly swinging numbers the closer to focus that is gotten.  Precise focus is difficult to achieve for this reason.

At first glance, the image appears to be in pretty good focus, but a careful look at the bright star in the lower right quadrant shows:

Do you see the double vanes?  This is a sure sign the image is not in perfect focus.   The vanes appear to have a dark pixel line between them.  The vanes are produced by diffraction around the vanes of the spider.  The presence of two vanes indicates that the image is not in perfect focus.  The focus is, however, very close to perfect with the vanes this close.  Perfect focus is achieved when the vanes merge into one.  At this point the star image would show one strong vane for each vane on the spider.