ATM Re: Pringle Wavefront

[Atmj1@aol.com]

Mr. Ferrick:

I have not studied your problem thoroughly. However, at first glance, it 
would appear that you are being introduced - in a rather hard hitting way - 
to astigmatism. 

1) Don't worry about the Pringle at this point.
2) Also don't worry about the on axis image. A scope has to be pretty bad to 
screw this up.
3) Try to get the rays to fall inside the Airy disk, for green or 
yellow-green light, a bit off-axis.
4) You may want to start experimenting with all spherical surfaces at the 
start knowing you are going to have to aspherize a bit down the road. Trial 
and error is not taboo in the realm of lens design. Designers who would tell 
you that they don't do so, should stop drinkin' the cheap stuff.
5) With this you will start improving off-axis imagery.
6) When all the rays fit into the Airy disk, some designers would say that 
you are done. The fact is you will probably have a design better than hands 
can make. Still, you want to learn. So, don't stop yet.
7) Now you may want to switch you optimization routine from being based on 
spots to OPD (the method used by the professionals because the smallest spot 
is not always found at the best focus - Smith's Modern Optical 
Engineering...page 306, I think)
8 When you have the OPD to an acceptable limit, you will have a desire to 
recheck it with the old tried-and-true spot method. Don't be worried that it 
now looks worse. That's life. Remember, you're now using OPD as your standard.
9 If your wavefront looks like a Pringle, a doughnut, or a saddle with a golf 
club stuck through it...it's OK...as long as the OPD is within tolerance. 

Just some thoughts. I hope I didn't foul you up too badly.

Kindest Regards,

Bill Cook
editor / publisher, Amateur Telescope Making Journal


4) If you can produce multiple spots at the same time, see where the image 
goes sour.