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Western Veil & Pickering’s Triangle

Back in 2018 I was incredible excited to take my first image of the veil nebula. It wasn’t very good though I was pretty excited about it at the time. The two major problems were insufficient data and too aggressive processing. I knew I was going to come back to it again in the future.

It turns out the future is now. That’s probably related to the idea that no matter where you go, there you are.

There were a few differences from the last attempt. First, I have a lot more experience with image processing. Second, I was using narrowband filters. Narrowband filters are special filters that let in only very specific wavelengths of light. These filters are designed to only let in light that is produced in emission nebulae, planetary nebulae and supernova remnants. The most common filters are hydrogen alpha, oxygen III and sulphur II. In this image I only used the hydrogen alpha and oxygen III filters. There is actually sulphur in the veil and I captured some but need more and for this image was more interested in the approximately realistic representation that using just hydrogen and oxygen filters can produce.

This is an HOO image. The “H” is for hydrogen alpha and the “O” is for oxygen III. The hydrogen is mapped to the red channel in the image and oxygen is mapped to blue and green. This has the happy coincidence of approximating how this looks in reality since hydrogen alpha light is red and oxygen III light is a teal color. So the nebula looks approximately as it would if our eyes could see it directly but the star colors are not correct.

The first thing that’s obvious is that the narrowband filters capture a lot more fine detail than the broadband image did in my Bortle 7 sky. The filters block most of the light pollution and allow the detail to come through. The amount of wispy gas that was visible here surprised me. I wasn’t expecting so much for relatively little total integration time.

I did notice that the eccentricity on my individual frames is higher than normal. My mount is just over a year old now and it may be time for some maintenance. Fortunately, the errors averaged out in integration and the final image had good round stars.

As to the nebula itself, the Western Veil is the top part of the nebula. It’s also known as the Witch’s Broom in some parts of the world. It was discovered by William Herschel in 1784. Pickering’s Triangle is in the lower left corner.

At their time of discovery these, along with the Eastern Veil, were all given different designations and believed to be separate objects but they all are part of a supernova remnant called the Cygnus Loop. The Cygnus Loop is quite large taking up more than four degrees of the sky. If it were bright enough to see with the naked eye, it would be spectacular. The Western Veil alone is more than a degree in length making it four times larger than the full moon.

The supernova that created the veil is estimated to have occurred about 8,000 years ago and the nebula is about 2,600 light years distant. When the star exploded it was probably noticed by our distant ancestors. I can’t help wondering what they thought of it.

Overall, I’m pretty happy with this version. There is a lot of faint detail and, if I can ever get another clear night to acquire some more SII data, I will try producing a tricolor version.

You can find the image along with the technical details on astrobin.

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