Christmas Tree Cluster NGC 2264

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Christmas seems to be coming early in 2020 amid the Covid-19 pandemic as many people have been putting up their Christmas decorations early in November. We haven’t put our Christmas decorations up yet but I nevertheless felt justified in observing NGC 2264, also known as the Christmas Tree Cluster.

This photograph includes four different astronomical objects:

1. The Christmas Tree Cluster, an open cluster of stars embedded in a glowing red star-forming cloud. The rotated horizontal Christmas Tree is to the left of the cone; the very bright star in the centre is the tree trunk; the bright star just to the left of the cone is the tree topper; and probably now you can make out the triangular formation of stars making a Christmas tree shape.
2. The Cone Nebula (on the right), a dark cold cloud of dust and hydrogen backlit against a glowing hydrogen emission nebula.
3. The Fox Fur Nebula, the bright white region to above right of the very bright central star.
4. The Snowflake Cluster, just to the left of the Cone Nebula.

For some reason, occasionally there are nights when little works on a technology front. This was one of those nights. I set up with plenty of time to spare as NGC 2264 is not observable from my back garden at this time of year until about 10pm. I used the early evening time to acclimatise the OTA and take dark frames. The trouble began once I tried to start imaging: I installed my quad-band filter and just after I’d finished orienting the sensor and focusing, the sensor misted up with condensation and I couldn’t see anything (I don’t understand the physics of how adding a warm filter to a cold OTA resulted in condensation, but it happened)! I eventually removed the filter altogether to acclimatise it more quickly, I then I had to refocus and reorient it again.

Once I’d framed my target, I then had to contend with terrible guiding accuracy. Usually I can easily guide at 0.6″ root mean square (RMS) error, which is more than perfect for my setup. For reasons I still do not understand, however, on this night I was guiding at between 1.2″ and 2″ RMS error, which is awful and gives star streaks. I recalibrated PhD2 near the celestial equator, I recalibrated PhD2 near the target, I checked the balance of the OTA, I redid the polar alignment, I checked the clutches were engaged, I checked the mount wasn’t sinking on the wet grass, I switched it all off and started again, I tweaked the PhD2 control gains, and I experimented with different guiding exposures. After all that, the best I could manage was between 1.2″ and 1.3″ RMS error. 1.2″ is the maximum I can tolerate for sub-pixel guiding, and so it was passable, and I accepted it in the interest of actually getting some data.

Imaging finally commenced at about 11pm and came to an end at about 2am as it clouded over. I captured 34× 5-min frames, and amazingly, 32 of them were acceptable for the final integration. I was really pleased to get this image despite the relatively short integration time, the 85%-illuminated moon, and all the technical hitches I experienced. It was worth persevering for!

Frames

• 32× 300-s light frames (Gain 900)
• Full use of calibration frames (darks, flats and dark flats)

Equipment

• Explore Scientific ED 102 mm Apo f/7 refractor
• Sky-Watcher EQ6-R PRO SynScan GOTO equatorial mount
• Altair Hypercam 294C PRO colour fan-cooled camera
• Altair quad-band one-shot colour (OSC) 2″ filter
• Revelation Adjustable Field Flattener
• Altair 60mm guide scope
• Altair GPCAM2 AR0130 mono guide camera

Software

• Sharpcap
• PHD2
• DeepSkyStacker
• Photoshop
• StarNet
• Topaz Labs DeNoise AI