Monday, 1 January 2018

M1 - The Crab Nebula...

Click on image to enlarge
Object: Messier 1 (NGC 1952)
Type: Supernova remnant
Constellation: Taurus
Distance: 6500 light years
Date: 27&28 December 2017
Equipment: SXV-H9, Vixen 114mm f5.3 ED refractor, guiding with Lodestar X2/PHD
Subframes: 30 x 300s H-alpha, 20 x 300s OIII, 20 flats for each channel, no darks

This object is clearly visible in my (200mm aperture) VC200L.  With averted vision some structure can be seen, even from my light-polluted sky. Visually, a dark notch on the eastern side of the nebula does give the object a passing resemblance to a crab's claw, although this is much less apparent on filtered CCD exposures than it is at the eyepiece.  I previously imaged this object in "white light" and the feature shows up quite well.

Subframes for this effort were acquired and preprocessed in Astroart.  I have started to use the hot pixel removal function rather than muck about with dark frames.  My ancient SXV-H9 is not actively cooled so the camera temperature (and hence dark noise) probably varies during an imaging session.  This seems to lead to odd artifacts over long sequences of exposures when using conventional dark frames.  Just hitting any hot pixels with the filter as part of preprocessing seems to a better job and is a lot less effort than making master darks.

For imaging M1, I decided to try colour imaging just based on H-alpha (as red channel) and OIII (as blue channel) data.  The first hurdle came when I looked at my OIII images.  I had not used this filter for CCD imaging before, and the images showed what seemed to be horrendous reflection rings around the stars (section enlarged below).

I contacted Astronomix, the filter manufacturer. They replied that the filter I had was for visual use only, and that the rings were due to it not being IR/UV blocked.  I am not so sure, but will try again sometime with a blocking filter in place.  For the purposes of this image however, I found that the situation was not irretrievable.

When I layered the bloated OIII image stack over the much sharper Ha data and blended in "darken" mode in PSP, this effectively subtracted out the halation, as below, giving a "clean" OIII channel.

To try and avoid the odd pink and blue stars that two colour-only channels seem to give in some instances, I decided to make an additional artificial green channel by layering the above image over the Ha stack and selecting the "multiply" option in PSP.  This allowed me to generate an RGB colour image, using the fake green channel, plus the above OIII image as blue and the Ha stack as red.  I then added the Ha stack back in as the luminance channel to give the final LRGB image.  A bit of tweaking in PSP (curves, cropping) gave the final image as shown at the top.

Quite how "valid" any of this is in terms of true colour is questionable of course, but I think it looks OK and not too unrealistic compared to what else is out there on the web.

The Crab Nebula is perhaps the most intriguing object in the deep sky realm.  Information about it can be found here.

No comments:

Post a Comment