NGC 1499: The California Nebula...

NGC 1499 (Sh2-220): The "California" Nebula...

Objects: NGC 1499 (Sharpless 2-220)
Type: Emission nebula
Constellation: Perseus
Distance: 1,450 light years

Equipment: Atik 460/EFW 2, Samyang 135mm lens@ F2, Vixen GPDX mount, guiding with Lodestar X2/PHD
Date: January 5th. 2022

Subframes: 24 x 300s for Ha, 9 x 300s for SII, 12 x 600s (2x2 binned) for OIII each, flats, no darks (hot pixel removal in Astroart). 

Discovered on November 3, 1885 by Edward Barnard, the California Nebula (NGC 1499) is a large emission nebula and star-forming cloud approximately 100 light-years in length, located in the Orion spiral arm of the Milky Way galaxy (where our Solar System is also located) in the constellation of Perseus. It is believed to be one of the nearest H II emission regions to Earth. It is so named because it allegedly resembles the outline of the US State of California. It has a very low surface brightness and it is very difficult to observe visually.

Stellarium map showing the location of NGC 1499

The nebula can be found on winter evenings high in the northern hemisphere sky in the “tail” of the constellation of Perseus, just above the Pleiades and right next to the bright star Xi Persei.

This hot blue-white star is the brightest star in the above image, just right of centre. Xi Persei is indeed one of the hottest stars visible in the night sky; its surface temperature is about 37,000 Kelvin (about 66,000 degrees Fahrenheit, or more than six times hotter than the Sun). Because of its high temperature, it appears blue-white to the human eye. It has about 40 times the mass of the Sun and gives off 330,000 times the amount of light. Xi Persei is a runaway star, and the fast stellar wind it blows is piling up in front of it to create a shock wave. This shock wave is heating up dust.

Xi Persei, which is receding from us at about 20 kilometres per second, is a member of an association of very hot stars that were born within the California nebula only a few million years ago. These massive and luminous stars are lighting up the nebula, as well as heating and ionizing it. In visible light, the ionized gas glows red, while in infrared light we see the heated dust. 

Conditions were quite good, with a clear, dark, moonless sky for once, although the Ha frames were interrupted by some transient clouds for an hour or so, which put the sequence back a bit.  As a result, I only got nine SII frames before the target went behind my house. 

24 x 300s H-alpha stack, starless

12 x 600s 2x2 binned stack OIII stack, starless...

9 x 300s SII stack, starless...

The OIII data was quite faint and noisy (hence the 600s binned frames) and a good flat field was an absolute must in pulling out what faint signal there was from the background. Even then I was a bit sceptical that what I had wasn’t just a result of vignetting, but it seemed to match the OIII area in other people’s images. 

I removed the stars from the stacked and stretched channel images with Starnet++ which allowed some selective stretching and sharpening.  I then RGB combined the starless images in PaintShop Pro to give a colour starless image. I added the Ha luminance frame back over the colour image at around a 25% blend level, which helped to sharpen the whole thing up a bit without washing out the colour too much. Subtraction of the starless images from their respective original frames gave RGB channels for a star field.  Once combined, I pasted them back into the starless RGB frame in “screen” mode.  

I had not realised just how much this nebula tails off to the south-east and so it wasn’t particularly well-centred in the frame, hence a fairly heavy crop to give the final image above.

Reference: 

http://annesastronomynews.com/photo-gallery-ii/nebulae-clouds/the-california-nebula-ngc-1499/