Sunday, November 30, 2014

The Wizard Nebula (NGC 7380)

The Wizard nebula is an emission nebula. It's in a distance of 8,000 light years from earth and lies in the plane of the milky way (hence the myriads of stars). The stars that are formed by the nebula are relatively young (5 million years).

Caroline Herschel discovered the open star cluster in 1787 (her husband William included it in his catalog as H VIII.77). It was many years later that the nebula itself was discovered and finally cataloged as NGC 7380.

(click image to get see a full resolution version)

This image consist of 3h10min Ha, 5h50min OIII and 23h10min SII data. The H694 chip seems to be no too sensitive to SII - with that much exposure time, I expected a much stronger SII signal. I tried boosting it more, but the red quickly overwhelmed the background and stars. This and all the stars made it quite tricky to process this image. After stacking with CCDStack, I did all the processing with Pixinsight:

  1. Cropping all images
  2. DBE for all individual images
  3. TGVDenoise on all individual images
  4. LRGBCombine
  5. Another DBE to remove remaining gradients that weren't visible in the monochrome images
  6. BackgroundNeutralization
  7. ACDNR for further noise reduction
  8. Initial Stretch
    • I also did a stretch with MaskedStretch which resulted in less star bloat. But the image lost a lot of contrast. I transferred the stars from the MaskedStretch to the other image. I learned about this from one of the many great videos on Harry's Astroshed.
  9. Correcting black point
  10. CurvesTransformation to further drop the background
  11. Using the image itself as a mask, I then increased the red and blue signal in the nebula. Using a mask reduced the effect on the background
  12. Inverting the mask and reducing the red and blue in the background
  13. Using a RangeMask+StarMask I used LocalHistogramEqualization
    • First to bring out the fainter, outer areas, and then
    • (after inverting the mask) to increase the inner layer
  14. Using the image itself as a mask, using ATrousWaveletTransform to increase contrast in the nebula
  15. CurvesTransformation to drop the background
  16. ColorSaturation to increase the blue areas
  17. Final black point correction using HistogramTransformation
  18. Removing the magenta from the stars
It took me many, many attempts to come up with this sequence.

Wednesday, November 26, 2014

Loose rotating base of my Mach1 mount

I am still experiencing subs like this one:

I.e. the scope suddenly moves quite a bit and the guider needs several steps to bring it back in. The rate is <10% but it's pretty annoying.

I looked for hanging wires and all other stuff. So far without success. But when I recently worked with the scope, I noticed that the rotating base of the mount has some play. You can see it in this video:


In this video, I am moving the scope without much force from side to side. Everything else on the mount is very stable.

I asked on the Astro-Physics mailing list for advice...


George from Astro-Physics told me that I should slightly and carefully tighten the tension adjustment screws on the side of the base. I did that ... and haven't seen these issues anymore!!!

Sunday, November 9, 2014

Pacman Nebula

It's clear where the pacman nebula gets its name from.
(click to see a full resolution image)

This image consists of 23+ hours of imaging time (22x10min Ha, 59x10min SIII, 58x10min OIII).

It's 9.200 light years away from us. Because it is 1,000 light years above the plane of the milky way, it can be observed very well.

Here is a close-up of the nebula:

The stars in the open cluster in the middle of the nebula have been formed only in the last few million years. Their strong winds form trunks which turns them into birth places of more stars:

Saturday, November 8, 2014

California Nebula

The California Nebula is an emission nebula in only 1500 light years distance (it's in the Orion arm of our milky way - the same arm that we are in).

(click on the image for a full resolution image)

This was my first mosaic - I did it with Pixinsight. Overall exposure of this image is 12.7 hours (3+ hours Ha, 5 hours OIII, 4.5 hours SII).

Unfortunately, I am still fighting with vertical and also horizontals stripes in my images. You can see it in the lower right corner. But apart from that, I quite like it.

Tuesday, November 4, 2014

First Mosaic

I wanted to capture the California Nebula which is huge! So, I tried for the first time a mosaic. SGPro's framing wizard makes capturing super easy.

You just draw a rectangle over the whole area that you want to capture and SGPro fills it in with overlapping images. The most important part is to have the rotation of the camera exact - otherwise the images will end up crooked. Creating a sequence of this mosaic creates two targets - one for each subframe.

I captured the images over several nights. Unfortunately, the vertical stripes were still in some of the images - I would try to get rid of them in post processing.

For processing, I first calibrated, stacked ... all images the same way in CCDStack.

For further processing, I found this tutorial how to do this with Pixinsight. I started to follow this.

First: cropping - nothing special here.

Resulting Ha images:

Next, we have to merge the images. First, using staralignment (Register/Union Mosaic). Strangely enough, Pixinsight could only find 30-40 stars in each image - which wasn't enough to overlap them. I had to play with various parameters in the "Star Detection" tab. Decreasing the "Log(sensitivity)" parameter detected more stars (40-50) but still not enough to match the two images. Increasing "Peak response" to 1 (maximum) lead to more the 100 stars being detected - which gave almost 20 stars in the intersection. Enough to overlap the two images:

The little black bars on the outside are because of cropping the individual images (and because the rotation wasn't 100% set correctly). But if we look closely at the intersection we can see the seam from the two images - this will be tough to remove. That's why Pixinsight has another process to smoothly stitch mosaics: GradientMergeMosaic. It's a little tricky to apply (see tutorial) as it requires a few temporary files to be created. But after doing all that, I could finally apply it and got this:

There is clearly something wrong at the seams. After closely inspecting and trying out a few things, I realized that this is a result of the different background levels of the two images. So, we need to remove the background gradients of the individual images first (DBE). Doing that* and then using GradientMergeMosaic results in this:

Even under close inspection, there are no seams left: 

From here on it's normal processing. Including removing gradients (once more) in the combined images. The reason is that the two images still had slightly different background levels and combining them resulted in a (smooth!!) gradient.

*Also, by removing the background gradients first, I could use the default star detection parameters in StarAlignment!