The Shark Nebula (LDN 1235) is a cloud of interstellar gas and dust spanning 15 light-years head-to-tail. It’s incredibly faint, and very challenging to image from a city.
Version 2, reprocessed December 2022
I was originally happy to get anything at all with this target. Fast forward six months, and new processing tools — in particular BlurXTerminator — have allowed me to get a much more refined end result.
Version 1, June 2022
This was definitely the toughest target I’ve imaged yet! Astrophotographers wanting to capture the Shark Nebula tend to travel to the darkest skies possible, but my approach is always to try from a city and test what’s possible. For this, I used my faithful Askar FRA400 telescope and ZWO ASI2600MC PRO camera. Being a broadband target, I didn’t use any filters, instead relying on a long total integration time. Well, I can now safely conclude the Shark can be imaged from Bortle 8 skies, but I wouldn’t necessarily recommend it!
It’s so incredibly faint that it didn’t show up at all in single 2-minute subs, and can’t really be seen even in a freshly integrated 20-hour image.
Still, once I began processing the data it was possible to draw out some details. The signal was still very weak though, and I ran into many problems.
First, noise. Normally I use Topaz DeNoise AI, but I’d heard good things about the recently released NoiseXTerminator plug-in for PixInsight. I gave it a go and wow, it’s impressive. See below for a before / after slider comparison. NoiseXTerminator will be my go-to noise reduction method from now on.
The next problem was controlling star sizes. There are countless pin-pricks of light in the image, and quite a few larger stars too. I find that Generalised Hyperbolic Stretch does an excellent job of stretching to non-linear while controlling stars, so used that to good effect. I then used StarNet2 to remove the stars, and did a bit of work to reduce their number before adding them back in.
The third big problem was with colour. To get any detail I was mangling the data quite hard, and as a result was producing lurid colours. I wrestled with this for a while, and in the end did a fair amount of desaturation. This helped, but unfortunately removed some nice blue nebulosity. Oh well.
In all, this was a really tough target to image and process. I can see that dark skies would really help to produce a much better end result, but hey, at least it’s possible for city astrophotographers to glimpse the Shark too.
Imaging details
* April to May 2022
* Bristol, UK (Bortle 8)
* Telescope: Askar FRA400 f/5.6 Quintuplet APO Astrograph
* Camera: ZWO ASI 2600MC-PRO
* Filter: none
* Mount: Orion Sirius EQ-G
* Guide: William Optics 32mm; ZWO ASI 120MM Mini
* Control: ASIAIR Plus, ZWO EAF
* Software: PixInsight, Lightroom
* 600 x 120 seconds
Total integration time: 20 hours
By Lee Pullen
Example source data
Donations gratefully received!
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