I took my longest ever exposure this weekend, lasting a total of 5.5 hours in an attempt to capture the NGC1499 (aka California Nebula) under bright moonlight.
The secret to this image is the use of a Hydrogen-Alpha filter to cut out all wavelengths bar a narrow band 12 nanometers (FWHM) in the region of 656nm wavelength. This effectively cuts out all emission from mercury and sodium lights and general sky (and moon) glow. The downside is that although it does transmit 97% of the h-alpha light, prolonged exposures are required due to a dimming of the image.
NGC 1499, California Nebula (central potion)
Canon EOS300D modified
EQ6 autoguided with a Toucam Pro webcam & PHD
20 x 15 minute H-Alpha subs
5 x 5 minute colour subs (in bright moonlight)
Processing of the image in ImagesPlus included flatframe, bias and dark noise reduction and a simple average stack, followed by Digital Development Processing (DDP algorithm).
Further image adjustment including H-Alpha/RGB combination to produce a "colour" image. Noise reduction, nebula enhancement, star reduction and diffraction spikes were applied using Noel Carboni's Astronomy Tools. Further contrast enhancement was applied with a high-pass filter of several iterations.