abell 33, the diamond ring nebula

Here's a spherical planetary nebula with a bright foreground star conveniently superimposed on the edge, providing the "diamond" for the ring:

Abell 33, the diamond ring nebula OIII RGB
click on image for full size

The small blue star in the center is the white dwarf creating the nebula.  A close up of the RGB version shows a hint of a second star to the lower right.  This is likely another superimposed star or a "visual double", as a double central star should (in theory with rare exception) create a bipolar, rather than a spherical nebula.  

central star of Abell 33
RGB, upsampled 2x

apparently an open issue in astronomy is whether the percentage of planetary nebulae double central stars matches the percentage of doubles in the general stellar population.  there are several recent surveys which address this, and may confirm my supposition, but i don't have access to them.  

The binary fraction of planetary nebula central stars - II. A larger sample and improved technique for the infrared excess search

The binary fraction of planetary nebula central stars: the promise of VPHAS+



The RGB image shows two other things:
1. how faint the nebula is, as it was barely detectable on the RGB image, requiring narrow band filters to bring it out.
2. how bright the "diamond" star is as it created a "bloom" (bright white artifact to the right of the brightest star).  
should have shot the RGB unbinned to better resolve the central star and minimize blooming.  

I did take a few trial Ha shots, but got only extremely faint signal in a circle, matching the OIII, but without detail.  

Image details:
8" LX200R, SX Trius 694 binned x2 to 0.8"/px,
astrodon 3nm OIII, RGB E SERIES GEN-II
ASA DDM60
R 18x4 minutes, G 16x4 minutes, B 8x4 minutes
OIII 24x20 minutes.
4/14/17-5/1/17, bortle white skies
eastbluff, CA