Target Fun Facts
NGC6888 Crescent Nebula is an emission nebula in the constellation Cygnus, about 5000 light-years away from Earth. It was discovered by William Herschel in 1792. It is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 400,000 years ago. The result of the collision is a shell and two shock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to X-ray-emitting temperatures.
NGC6888 Crescent Nebula is a rather small object located about 2 degrees SW of Sadr. While considered bright by astronomical imaging standards, visually it is relatively faint. For most telescopes it requires a UHC or OIII filter to see. Under favorable circumstances a telescope as small as 8 cm (with filter) can see its nebulosity. Larger telescopes (20 cm or more) reveal the crescent or a Euro sign shape which makes some call it the “Euro sign nebula”.
Distance: 4700 light years
Type: Emission Nebula
Apparent dimensions: 20’x10’
Constellation: Cygnus
Designations: Crescent Nebula, Ear Nebula, NGC6888, C27, SH2-105, LBN204, Ced179, DWB19
{From https://en.wikipedia.org/wiki/Crescent_Nebula and Stellarium}
Capture & Processing Notes
This image represents first light for Big Bertha’s new TS 2.5” Rack and Pinion Focuser, replacing the Orion Crayford focuser she came with. That is the short story of the night’s capture – if you’re interested in the rest of the story, read on. If not, please feel free to skip the next three paragraphs!
I’d been struggling with the SGPro EAF algorithm when I used autofocus during a sequence. Most times it would fail – repeatedly attempting the focus routine at the designated time (I had it set for once an hour), never coming to a greater than 90% confidence solution, and ultimately returning the focuser to its starting point – taking up imaging time without changing the focus. So, I decided to read the SGPro instructions (YES, really!). There I found an exercise to determine the algorithm’s appropriate step size for your equipment. I’d done this investigation while we were camping at Powderhorn and decided I didn’t want to waste valuable dark skies time on it (especially since the instructions start off by saying “you’ll need approximately 20 minutes and a lot of patience”). So, I turned off the autofocus routine and deferred the exercise until I had a clear night at home.
I had a clear skies opportunity at home on 28 July 2024 – it failed miserably! In the course of troubleshooting the issue with SGPro support, I remembered a conversation I’d had with MikeinWI, who I’d met through Thingiverse while searching for a 3D digital print for the autofocuser’s bracket. At that time, he’d said he had changed from his telescope’s Crayford (friction-based, https://en.wikipedia.org/wiki/Crayford_focuser ) focuser to a rack-and-pinion focuser. I reached out to him with my current issue and asked about the rack and pinion focuser. He sent me the link to the Teleskop Systems focuser he purchased and said he was very happy with it. I passed that link on to Paul, telling him I had one more experiment to do but then was considering buying one for Big Bertha. While I was in the midst of the secondary experiment (meant to determine if the autofocuser was moving the focuser the same distance in and out against gravity with the same number of steps), Paul ordered the new rack and pinion focuser! It arrived a few days later. BTW – the experiment revealed that the Crayford focuser was being moved nearly the same distance in and out, but I had the “reverse direction” check mark selected in the SGPro autofocus settings – so the algorithm (and my selected settings) were to blame, not the focuser. But – I have a new toy (er, I mean piece of equipment) to integrate into my set-up!
So, fast forward (okay two paragraphs later is maybe not so fast!) to the night of 16 August – our first clear night since the 28 July exercise, the experiment, and the receipt of the TS 2.5” Rack and Pinion focuser. I started the step size experiment at dark (approximately 2100MDT) and worked on it (patiently?) until almost midnight. It was better than the Crayford’s (with the reverse direction checkmark) result, but still not good enough for the autofocuser to run reliably. So, again, I turned off the autofocus routine and turned my attention to capturing the NGC6888 Crescent Nebula. Then, during my “sleeping” time that night, I came up with other ideas of what I need to try the next time that I have a clear night!
I got the sequence going right after its meridian flip and planned to collect until the end of astronomical twilight (at 0515). All went well other than the last subframe, during which the telescope and autoguider scope had gone below the roofline elevation. So the sequence ended a bit prematurely when the autoguider couldn’t find its guide star, while looking through the house! The moon was 90% full throughout almost the entire imaging sequence, so I used the Optolong LeXtreme light pollution filter. Although the result doesn’t have the color dynamics of a dark skies capture (see the image below), I was pleased that I was able to bring out a little bit of color other than red during post-processing.
- Sequence plan: Gain: 158, Temp: -0°C, offset=30. 59x5min. Total exposure time: 295min, 4:55hrs
- Captured: 16 August 2024 (16Aug2024, 2357MDT – 17Aug2024, 0508MDT)
- Shooting location: HCH, Colorado Springs, Colorado
Processing summary: Captured in SGP, stacked in APP (HaOIII Color), star removal with Starnet++, processing with LR/PS
Equipment
All equipment controlled by HP Probook running Sequence Generator Pro v4.4.0.1339.
- Imaging (ASI2400-BB-LeX): ZWO ASI2400MC imaging camera on (Big Bertha) Orion 8″ f/8 Ritchey-Chretien Astrograph Telescope, TS 2.5” Rack and Pinion Focuser (first light), Teleskop Service Flattener 1.0x for RC Telescopes (TS-RCFLAT2), Optolong L-Extreme LP filter (LeX)
- Autofocuser: ZWO EAF Electronic Automatic Focuser – Standard (New 5V Version) (EAF-5V-STD)
- Mount: Rainbow Astro RST-300 (controlled by iHubo ASCOM driver)
- Polar alignment: QHYCCD camera (controlled by Polemaster for polar alignment)
- Autoguiding: Orion 60mm Multi-Use Guide Scope with Orion StarShoot AutoGuider Pro Mono Astrophotography Camera (controlled by PHD2)
Other images of NGC6888 Crescent Nebula
This image is of NGC6888 in its “neighborhood” in the Sadr Region captured with a smaller focal length (Southern Cross’ 600mm vs. Big Bertha’s 1600mm). The image has its own gallery page – since I couldn’t decide to place it in NGC6888 Crescent Nebula’s or IC1318 Gamma Cygni Nebula’s pages. That gallery page is: https://beersastrophotography.com/gallery/ngc6888-crescent-nebula-floating-through-ic1318-gamma-cygni-and-the-sadr-region/
Capture & Processing Notes
This image was captured during a one-night trip to CSASTRO’s Starry Meadows property near Gardner, Colorado. It was the first target of the night because the Cygnus constellation at this time of the year is rising early in the day and setting shortly after midnight. By nightfall, when I could start the imaging (~1900MDT), it was already past the meridian so the collection went flawlessly. I cut it short (ending at 2235MDT) to have more time to spend on the night’s second target, NGC1909 Witch Head.
- Sequence plan: Gain: 158, Temp: -0°C, offset=30; 40x5min; Total exposure time: 3:20hrs.
- Captured: 4 November 2023 (4Nov2023, 1907 – 2235 MDT)
- Shooting location: Starry Meadows, Gardner, Colorado
Processing summary: Captured with SGP. Stacked in APP. Star removal with Starnet++. Processed in LR/PS
Equipment
All equipment controlled by HP Probook (DSO CTRL 1) Windows 10 laptop running Sequence Generator Pro v3.2.0.660.
- Polar alignment: QHYCCD camera (controlled by Polemaster for polar alignment)
- Imaging: (Southern Cross) Askar FRA600 on Rainbow Astro RC-135E, ZWO ASI2400MC#1 camera
- Mount: Rainbow Astro RC-135E (controlled by iHubo ASCOM driver)
- Autoguiding: Orion 60mm Multi-Use Guide Scope with Orion StarShoot AutoGuider Pro Mono Astrophotography Camera (controlled by PHD2)