The Butterfly Nebula - Urban Astrophotography

Overview

4000 light-years from Earth lies the Butterfly Nebula (IC1318), part of the much larger Sadr Region in central Cygnus. This photo spans 80 light-years, but the area it occupies in the sky is only twice the size of your thumbnail held out at arm’s length.

Background

Cygnus is high overhead as seen from the UK this time of year, and it’s full of intricate nebulae. My Askar 130PHQ telescope has a focal length of 1000mm, which means it’s well suited for tight framing, detail-oriented shots. For this project I chose the Butterfly Nebula (not to be confused by a planetary nebula with the same name) because it looked interesting and would entirely fill the frame edge-to-edge.

As is now my standard practice for these types of nebulae, I used an Optolong L-Ultimate filter for H-alpha and OIII data; an Askar D2 filter for SII and more OIII; and then no filter just for the stars, to give them fairly accurate colours. So it’s an SHO image from an OSC camera. In terms of integration times, the L-Ultimate is 15 hours; D2 is 20 hours; and the no filter RGB stars is just 30 minutes (stars are really bright so you don’t need much time on them).

Imaging details

Date	19 July – 24 August 2023 (five weeks)
Location	Bristol, UK (Bortle 8)
Telescope	Askar 130PHQ Flatfield Astrograph
Camera	ZWO ASI 2600MC-PRO
Mount	Sky-Watcher EQ6-R PRO
Guide	William Optics 50mm Guidescope with 1.25″ RotoLock; ZWO ASI 120MM Mini
Control	ASIAIR Plus
Software	PixInsight, Lightroom
Filters	– Optolong L-Ultimate (Ha / OIII): 450 x 120 seconds (15 hours) – Askar Colour Magic D2 (SII / OIII) : 600 x 120 seconds (20 hours) – No filter (for RGB stars): 15 x 120 seconds (30 mins)

Total exposure time	35.5 hours
Image credit	Lee Pullen

Processing

I’m receiving a lot of requests for a processing guide for this approach, and I plan to make one soon. Meanwhile, I made what I think is an interesting comparison of the data sources. I took just the Optolong L-Ultimate data and made a rough edit, producing a starless image. Then I did the same thing with the Askar D2. You can compare them using the slider:

Left is Optolong L-Ultimate data (Ha/OIII); right is Askar D2 (SII/OIII).

The L-Ultimate data is better quality despite the lower integration time, simply because there’s so my H-alpha in the nebula. But the D2 is clearly bringing a different wavelength into the mix. My final image is the result of mixing L-Ultimate and D2 data, with RGB stars sprinkled in top.

There are methods of creating pseudo-SHO images from just the stronger Ha/OIII data. Perhaps best-known nowadays is the Bill Blanshan Pixinsight script that does this with a single click. For interest, I ran this script on my L-Ultimate data and gave it a very quick edit: