Rosette Nebula

Well, after a long time trying, I finally took an image of the the Rosette Nebula which I can be pleased about, instead of embarrassed.

We’ve had a little bit of luck with the weather, managing three Society field nights at The Oaks in January. This was a hot night with no need to wear any more than shorts and sleeveless shirt – quite a novelty at our field location, where it normally cools quickly after sunset.

Not much time was available to us, with sunset at 8.07, twilight ending at 9.41 and the Moon rising 11.12

I’ve had the ZWO camera for twelve months and have been very unsure how to set exposure and gain levels, so I’ve been reading up on the capabilities of the SharpCap software.

Lo and behold!

It has a smart feature which advises the best exposure and gain levels for the sky that night, to eliminate all the guesswork. After a few seconds work, during which I pointed the scope towards a relatively dark section of the sky, it advised the optimum settings (for that location; on that night; for my telescope; and for my camera) were an exposure of 85.760591 seconds with a gain of 210 and if I would just like to hit the “Apply” button it would kindly set it for me.


Off I went with SharpCap, live stacking 22 frames – of which it discarded four, probably due to the number of satellites passing across – although a couple more of the pesky objects still avoided the cut.

The early Moon rise put an end to image aquisition and we all had an early night!

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Rosette Nebula:

The Rosette Nebula (Caldwell 49) is a large spherical H II region. The open cluster stars at its centre were formed from the nebula’s matter.

According to Wikipedia, the complex has the following NGC designations:

NGC 2237 – Part of the nebulous region (Also used to denote whole nebula)
NGC 2238 – Part of the nebulous region
NGC 2239 – Part of the nebulous region
NGC 2244 – The open cluster within the nebula (Caldwell 50)
NGC 2246 – Part of the nebulous region

Object Details:

Constellation: Monoceros.
Visual magnitude: +5.5
Apparent diameter: 80′ x 60′ (over 2 Lunar Diameters).
Actual diameter: 128 light years.
Distance: 5,500 light years.
Altitude: 50° above northern horizon.


Exposure: 25.7 min: 18  @ 85.76 sec
Stacked: live 18 of 22
Gain: 210
Date: 2019-01-25
Location: The Oaks, NSW.
Sky: semi-dark rural.
Cloud: clear.
Moonrise: 11.10 pm at which imaging ceased.
Image acquisition and live stacking software: SharpCap.
Image post-processing: GIMP.
Cropping: minor trim.
Trailing: only at high image zoom.

Imaging telescope: Skywatcher Esprit 120ED Super APO triplet refractor.
Focal length: 840 mm, focal ratio: f/7.
Imaging camera: ZWO ASI 071 MC Pro
Guiding: not essential.
Telescope mount: SkyWatcher EQ6-R.
Polar aligning method: QHYCCD PoleMaster.
Polar alignment error: ShatpCap estimated 1′ 57″
Field flattener: yes; filter: no.

Observing Notes:

I’m still having problems aligning my EQ6 mount, despite returning it to the manufacturer three times and now getting a replacement mount.

The Rosette “aligned” outside my telescope field of view but was visible in my finder and I was able to centre it.

Due to the very short optimal exposures recommended for CMOS cameras, it is no longer essential for me to use a guide camera. So I took the opportunity of available space to piggy back my unmodded 60D DSLR camera and take simultaneous wider angle shots of the same object.

The resulting image was: a different orientation; lots of stars; and a Nebula with different colour characterisics:

Picture saved with settings embedded.
2019-01-25 NGC2238 Rosette Nebula 135mm 60x1min f3.5 ISO800. FOV: 4.82° x 3.11°

The bright object (far left) is the nebula IC 448.

Also visible are Collinder objects 97, 106 and 107.

Also Relevant:

My Nebula Portfolio

Geek log:

[ZWO ASI071MC Pro]
Debayer Preview=On
Output Format=PNG files (*.png)
Capture Area=4944×3284
Colour Space=RAW16
Hardware Binning=Off
Turbo USB=100
Frame Rate Limit=Maximum
Timestamp Frames=Off
White Bal (B)=50
White Bal (R)=50
Cooler Power=100
Target Temperature=-15
Auto Exp Max Gain=300
Auto Exp Max Exp M S=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Anti Dew Heater=Off
Banding Threshold=35
Banding Suppression=0
Apply Flat=None
Subtract Dark=None
#Black Point
Display Black Point=0
#MidTone Point
Display MidTone Point=0.5
#White Point
Display White Point=1

Images © R.Powell



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