Re: [APML] Horsehead and Pleiades testshots with 6x9 camera/OAG and BRC

[Chris1011@aol.com]

In a message dated 12/19/2001 2:46:20 AM Pacific Standard Time, bobm@koyote.com writes:


> Takahashi claims 2 micron stars on axis.


As an optical designer/engineer, I would argue with this claim. The Airy disc itself is no smaller than 7 microns at this F-ratio, and if you count the first diffraction ring which is pretty substantial at 40% obstruction, that number will double. These claims of 2 micron images arise from using theoretical ray tracing which ignores all diffraction effects. This is an old fashioned way of designing optics and has been superceded in recent years by more advanced programs that give a realistic picture of the actual spot sizes. If you measure the size of your smallest spots, I doubt that you will find any that are anywhere near this 2u claim. By the way, if you ray trace a Newtonian on axis with the old methods, the spot diameters are zero microns - not very realistic.

Here are the numbers for Airy disc size for different F-ratios:

F5  = 7u
F6  = 8u
F8  = 11u
F10  = 13u
F15  = 20u

Here are the numbers for distribution of light in the diffraction pattern assuming perfect optics:

40% obstruction:
Airy disc  (% of total) = 60%
First diffraction ring    = 26%
All other rings           = 14%

25% obstruction:
Airy disc  (% of total) = 73%
First diffraction ring    = 18%
All other rings           = 9%

No obstruction:
Airy disc  (% of total) = 84%
First diffraction ring    = 7%
All other rings           = 9%

A 1/4 wave error on the above will drop the Airy disc numbers down and increase the energy in the first diffraction ring. As an example, the unobstructed aperture will drop down to 64% for the Airy disc with the first diffraction ring rising to 16%. For heavily obstructed scopes, the energy in the diffraction rings will actually surpass the amount in the Airy disc, so the first diffraction ring and Airy disc will merge, and the real spot diameters will double in size.

Roland Christen