The Crescent Nebula

The brightest part of this image is the Crescent Nebula (NGC 6888), around 5000 light-years from Earth. It was formed when stellar winds collided with gas ejected from a dying star, sending shockwaves into space. The Crescent is surrounded by huge sweeping clouds of hydrogen gas, seen here in red.

Although the Crescent Nebula itself is a bit small for my wide-angle 400mm Askar FRA400 telescope, it’s surrounded by nebulosity I figured would look good! My ZWO ASI2600MC Pro camera has a fairly large sensor, and combined with the FRA400 gives a good field of view.

To get the framing just right, I used my ASIAIR Plus‘s planetarium function. This is a relatively new feature, and it’s absolutely brilliant. It plate solves to your desired location, and draws a box to show your exact field of view. I could then rotate the camera to change the angle of the box, then take a short exposure, allowing the ASIAIR Plus to update its prediction. After five minutes I had the framing and rotation just how I wanted.

I then used the “current position of the mount” to create a Custom Object, allowing me to easily return to these exact coordinates night after night.

Although there’s essentially no proper darkness at this time of year, I’ve found that it’s still possible to get good data. For more examples, see my recent Lobster Claw Nebula and Shark Nebula, and last year’s Elephant’s Trunk Nebula, along with Controversial Opinion #7.

The nebulosity surrounding the Crescent is quite faint so I knew I’d need a long integration time, which wouldn’t be a problem as that’s what I always aim for. Despite the summer skies, and being in the cloudy UK, this project took just three weeks. I obtained 31 hours of data, and then used PixInsight’s excellent SubframeSelector tool to whittle it down to the best 25 hours (750 subframes, each being 120-seconds long). If you’re currently obtaining integration times of just a few hours, you may find this article useful.

Here are screenshots showing the final selection of subframes. I use two criteria to seperate wheat from chaff: FWHM and Stars.

Then it was onto integration, using WeightedBatchPreProcessor. Here’s the final Execution Monitor, showing the whole process took almost 21 hours. This is despite me using a powerful PC with 48GB of RAM!

The next step was processing. This was relatively straightforward, and I largely followed the steps outlined in my example processing workflow. I did deviate slightly: specifically, using Generalised Hyperbolic Stretch to take the data to non-linear; and NoiseXTerminator for noise reduction. Also, I liked the strong red colours in the image, so didn’t split the channels to create any kind of pseudo-Mono style picture.

It took a bit of teasing to get the faint nebulosity out. Use the slider below to compare a single sub-frame to the final, edited image. These slider comparisons make the whole process seem like magic!

Although I knew that the Crescent itself is really too small to do justice in my telescope, I did have a punt on making an extreme crop to see it better. Lightroom’s Enhance function allowed me to upscale the resolution quite effectively (and of a higher quality than I got with Topaz Gigapixel AI). The result is better than I expected. The outer shell of Oxygen, shown in blue, is quite evident:

When editing the full image, I noticed a strange tiny circular object. At first I thought it was an artifact left over from the star removal stage during processing, but it didn’t look quite right for that. I did some research and found that it’s actually the Soap Bubble Nebula, a planetary nebula. It was discovered only about 15 years ago, by an amateur astronomer using a 160mm refractor. It’s quite amazing that with modern camera sensors and processing techniques it can be imaged from the centre of a city using a small aperture (72mm) refractor. Click on the full image at the top of this page and see if you can spot the Soap Bubble. Clue if you need it: look a little below the Crescent Nebula.

Imaging details

* July 2022
* Bristol, UK (Bortle 8)
* Telescope: Askar FRA400 f/5.6 Quintuplet APO Astrograph
* Camera: ZWO ASI 2600MC-PRO
* Filter: Optolong L-eXtreme
* Mount: Orion Sirius EQ-G
* Guide: William Optics 32mm; ZWO ASI 120MM Mini
* Control: ASIAIR Plus, ZWO EAF
* Software: PixInsight, Photoshop, Lightroom
* 750 x 120 seconds

Total integration time: 25 hours

By Lee Pullen

Example source data

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