Off-axis Hα-band image quality

January 20, 2014

This post shows results of on-axis and off-axis image quality tests of my Takahashi FSQ-106EDX 530 mm f/5 lens using my implementation of the Roddier & Roddier test. See my prior post for more information on the method. The results with a 3 nm bandwidth Astrodon H-alpha filter are shown below for the lens. Strehl ratios between 0.87 and 0.90 are estimated. Since the wavefronts were estimated from images that were affected by seeing, the delivered Strehl from the lens may be larger than estimated.

 

The intra-focal (left) and inverted extra-focal (right) images obtain on-axis and off-axis at the four corners are shown below with linear stretch. Each image is the mean of sixteen 20.0 second, binned 2 x 2, bias-subtracted and co-centered integrations of Capella. Defocusing results in a ~11 mm lower bound on the size of the resolved wavefront corrugation period. Spherical aberration is evident in all positions. The pupil images in the off-axis positions have a eccentric shape with a slightly flattened edge. This eccentricity is due to vignetting.

 

blog Ha, on-axisblog Ha, on-axis

 

blog Ha, lower-leftblog Ha, lower-left

 

blog Ha, lower-rightblog Ha, lower-right

 

blog Ha, upper-leftblog Ha, upper-left

 

blog Ha, upper-rightblog Ha, upper-right

 

The tables below show Zernike polynomial least-squares fits to the estimated wavefronts. The most significant aberrations present in all positions are primary and secondary spherical. Primary astigmatism is also present in the off-axis positions.

 

blog Ha, on-axisblog Ha, on-axis

 

blog Ha, lower-leftblog Ha, lower-left

 

blog Ha, lower-rightblog Ha, lower-right

 

blog Ha, upper-leftblog Ha, upper-left

 

blog Ha, upper-rightblog Ha, upper-right

 

 

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