This image shows one of the toughest objects I have ever imaged. This is PuWe (Purgathofer-Weinberger) 1 (also known as PGN 158.9+17.8), a very weak planetary nebula in the constellation of lynx.
The nebula was discovered in 1980 (original discovery paper: http://goo.gl/fW6TQB) by Purgathofer and Weinberger on the Palomar Deep Sky Survey prints. The nebula is one of the largest planetary nebulae visible in our skies, with a diameter of 20’ (almost equal to the full moon!).
PuWe is also one of the nearest known planetaries with a distance of only about 1200 lightyears (0.365 kpc).
It’s located at the coordinates RA 06:19:36 DEC +55:37:00 and shines very weakly in the H-alpha and OIII wavelengths. In H-a it has an estimated surface brightness of only 23.7 mag/arcsec2 and in OIII only 26.3 mag/arcsec2. The integrated magnitudes are 8.6 and 11.2 respectively. This means that you can imagine the nebula as a star of mag. 8.6 which is defocused to the size of the moon. This gives an indication of the incredible weakness of the nebula.
The nebula has an estimated diameter of about 4 ly, and therefore is probably a very old remainder of a planetary nebula that has expanded that far that the remaining gases are only weakly energized by the central star.
In order to make the nebula visible 1800s (30min) exposures were taken with my f5.5 TMB92 equipped with a QSI 583ws and a 5nm Astrodon H-alpha filter. In total 10 images were used, totalling 5h of exposure time. For more detail a lot more exposure time is necessary, but luckily this object is circumpolar, allowing me to image this nebula during the whole year.
J.P. Metsavainio (http://astroanarchy.blogspot.nl/2011/03/puwe1-planetary-nebula-project.html) has shown this nebula even has a very faint halo, which is a really nice challenge to see if it’s possible to confirm this.
All together this is because of it’s very faint signal and large diameter a very tough target to catch, but a very rewarding challenge 🙂