Sep 282021

Here, on the south coast of England, the nights get very short indeed for a couple of months around the Summer Solstice. In fact, there are several weeks where theoretical ‘astronomical twilight’ never ends and the Sun never drops below 18 degrees below the horizon. I normally abandon deep-sky imaging but, this year, I was testing out a new system and decided to have a go at a few easy and classic Summer deep-sky targets.

The main thing that helped my productivity during these short nights was a new dual-band narrowband filter from Optolong called the L-Extreme. These multi-band filters are becoming very popular with deep-sky imagers these days. The pass-band spectrum graph is shown below, and you can see that there are two peaks – one centred on H-Alpha and the other on OIII and both are 7nm wide.

I’ve been imaging with Ha, OIII and SII narrowband filters for many years, but so often in the past I have been unable to capture a full set of sub images due to poor weather or lack of time, and this filter brings the possibility of acquiring more finished images as these three testify. By the way, the SII band is not included with this filter but, so often, the SII signal is so weak it rarely adds much to an image. However, since I have this filter in my filter wheel (so that I can use a Luminance filter for RGB imaging) I still have the option of adding my SII filter into the mix if I so desire.

I should mention that these narrowband filters are generally used with one-shot colour cameras. The Ha signal ends up in the red channel and the OIII signal is often mixed between green and blue. My new system includes the amazing QHY268C one-shot colour cooled CMOS camera which is very sensitive and has 16-bit resolution.

I will add a separate article showing the new setup, but it includes the superb Takahashi FSQ-85EDX APO refractor working at f/5.4 riding on a Skywatcher AZ-EQ6 Pro mount. The field of view is 179′ x 120′ which is 3 x 2 degrees (1.72 arc-seconds per pixel).

All three images consist of just over 2 hours of exposures – that’s all the darkness I had on each night! I took 600 second exposures throughout and calibrated with dark, flat and flat-dark frames.

Please click on each image below to see the full size of the images (which are only 50% of the originals).

The first image is the North America Nebula in Cygnus (NGC7000)

The second is IC1396 in Cepheus which contains the Elephant Trunk Nebula near the middle.

Lastly, NGC6888, The Crescent Nebula in Cygnus which is sometime referred to a van Gogh’s Ear!

Hopefully, my next article will not be too long coming. Thanks for reading.

 Posted by at 11:00 am

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