Sunday, May 13, 2018

Heart Nebula (IC 1805)

I took the data for this image October 2016 (!!!) It's a mosaic of 4 images and I always had problems stitching them together. Especially the OIII data which had very different noise levels in the 4 panels. But I finally figured out a way to minimize the effect! As always an awesome tutorial from Light Vortex Astronomy helped me!

(click here for a full-resolution view)

The Heart Nebula is at a distance of 7,500 light years from earth in the Perseus arm of our galaxy. It spans 200 lightyears! The nebula is ionized by the relatively young stars at the center of the nebula (their open star cluster is known as Melotte 15). The nebula is 150 arcminutes in size (the moon is 30 arcminutes!) in the constellation Casiopeia.
It was discovered by William Herschel on November 3rd 1787 (he first discovered the brightest part in the lower left - NGC 896).

Being a mosaic there is SO much interesting detail here:

1. The bright NGC 896

2. The open star cluster Melotte 15 at the core


3. Beautiful dust pillars (created by the energetic light from the young, hot stars at the center)

Each of the four panels has and integration time of 10 hours (10xHa, 20xOIII, 40xSII - each 10 minutes). I think this is the longest integration time I ever had.

Saturday, May 12, 2018

Mosaic with different background levels

Taking the images for my mosaic of the Heart Nebula took a long time (several weeks). And as a result the background levels of the different panels were quite different (mostly because of the moon). This was most noticeable in the OIII channel:

Here is mosaic of them without modification:

The first advice that I found was to remove backgrounds as much as possible using ABE or DBE. That made it better, but the differences were still very noticeable.

On one thread in the Pixinsight forum, somebody recommended to use LinearFit to equalize the levels - but that did not work at all for me:

The histogram got completely squished...

On another thread, somebody mentioned AutoHistogram:

The usage is fairly straight forward: select the image that you want to use as a reference image (preferably the image with the largest range) and click on "Set As Active Image". This reads out the median pixel value of the selected image to be used for the adjustment.

Then you just apply this process to all the other images:

Doing this to all images and combining them with GradientMergeMosaic results in:
Yei! That looks much better! I had to play with the Feather Radius to avoid a pinched star, but apart from that it was now straight forward.