I recently obtained a second hand 17.5 inch f/4.5 Coulter mirror which was tested to be 74% corrected from sphere to parabaloid. Star test does show pronounced undercorrection. Images are quite noticeably inferior to my Sky Designs 18-inch f/4.4 mirror which has an excellent star test and has long been a proven performer. Soon I will work with friend and colleague Dan Joyce to make the mirror excellent and have a back up mirror. What a nice thought. Anyways I've been wondering if the undercorrection is evenly spread does anyone know what the wave front rating would be? I'm guessing around 1/2 wave or slightly better based on the star test. Thanks
How good is this mirror?
Started by ddrake, 03/10/2005 03:30AM
Posted 03/10/2005 03:30AM
| Edited 03/10/2005 03:32AM
Opening Post
Posted 03/10/2005 12:19PM
#1
Darren Drake said:
I recently obtained a second hand 17.5 inch f/4.5 Coulter mirror which was tested to be 74% corrected from sphere to parabaloid... I've been wondering if the undercorrection is evenly spread does anyone know what the wave front rating would be?
The 74% corrected should mean the appropriate conic K=-0.74. The max surface error vs. parabola is given by 45D(K+1)/F^3, with "F" being the mirror F# and D the aperture in inches. This gives the max surface error of 2.25 waves.
This error doubles in the wavefront at the focus of either paraxial or marginal rays. However, at the best (diffraction) focus in between (0.707 zone focus), the error is only half of it, or ~1.1 wave.
Vlad
Funding Member
