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A longer read - Imaging the Rosette Nebula - February 2018


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Imaging with a 106mm telescope with pulse guiding

I have been posting a few images since I started using the ASI1600MM Cool  about a year ago, but they all used my 135mm Samyang lens. In the new year we had a few clear nights and although I wanted to carry on with the mosaic of Barnards loop in Ha7nm, the trees in the hedgerow behind my house became part of the frame – not a good thing. I looked at Star Gazers Lounge for inspiration and many images in January were being posted of the Rosette Nebula, so I slewed over to that, with the 135mm lens and started imaging. The result was ‘ok’ but I felt that the nebula got lost in the stars in such a wide field image. So I decided to put my Astro Tech 106mm EDT APO on the mount and set up for pulse guiding and get up close and personal with the Rosette Nebula. The 106mm scope/guiding wasn’t completely new  to me as this was the setup I had used for my initial DSLR imaging. The full run down of the set up is below.

Capture configuration

Telescope

AT106EDT

Riccardi 0.75x reducer/flattener

 

Camera

ASI1600 MM cool

Baader LRGB Ha(7nm) filters

Guiding

AT72ED

Lodestar X2

 

Mount

Skywatcher

AZ EQ6-GT

 

I run everything from a Microsoft Surface Pro3 with a single USB3 connection to a USB3 powered hub. The capture software I use is Astro Photography Tool alongside Carte Du Ceil and PHD2. Within APP I also have setup the plate solving and linked that to CdC, I can then use Goto++ to get on target very quickly. I also use the framing mask tool so I can fine tune the aim of the FoV to get repeatable results. This all works quite well, but I always sync the mount and software, so for this sequence of captures I used Zeta Orionis to sync everything up, then I can use Goto++ effectively.

Guiding was ok, I need to understand it a lot more, but I have managed to set it up and get guiding accuracy RA 1.4” RMS and DA 0.72” RMS. What I have to try and improve is that the correction pulses are quite large and occasionally says I have to increase the maximum correction pulse duration. PHD2 reports that I have a polar alignment error of 1.5’ arcmin.

Image Details

The imaging was carried out over several nights between 9th – 17th February 2018, sky quality was at best variable so the subs were quite a mixed bag. However I captured quite a large number of subs and had to cope with the Rosette transiting the meridian and being obscured by a large bamboo hedge, so I had to meridian flip to keep target in FOV, but kept camera in same position (I don’t have a rotator, or wanted to create a completely different set of flats for flipped images) with the result that the captured subs were a mix of pre and post meridian flipped subs, but stacking software can take care of the inversion.

Dataset

Baader 1.25” filters

L – 60subs @60s

R – 88 subs @60s

G – 122 subs @60s

B – 84 subs @60s

Ha7nm – 74 subs @90s

Flats - 30

Darks - 30

Bias -100

 

The ASI1600 was set to -20degC and unity gain.  over a period of  4 nights captured a total amount of data for the image of 7.75 hours, however the sharp eyed among you will spot the weakness in this data set – Luminance – I did not capture enough, why, because I was obsessed in getting colour captured , but due to the issues with meridian flip and guiding after meridian flip I ended up with incomplete data sets, FOV offsets and inversions, so I ran my RGB capture plan on two separate capture runs. However, once I got all the RGB, the clouds rolled in and by mid-Feb the moon was too bright for luminance capture and as each day goes by the Rosette headed westwards and into the light pollution bubble from the big logistics warehousing area on edge of town -sigh….

Stacking

Up to now I have been using Deep Sky Stacker, DSS, however with this Rosette project I found that the DSS stacked images had a lot of noise. I had been reading about Astro Pixel Processor, which Sara Wagner has been using and posted several video tutorials. I decided to use this and have no regrets, yes it has a lot more processing steps and configuration options, but they all make sense and for my subs gave a significantly better final stacked image to take into the next processing stage.

Registration

I use RegiStar, I came across this application when reading posts by Olly Penrice, it’s a one trick pony, but it is a brilliant trick and getting all the base stacked images precisely aligned before you start more detailed processing.

Misc

I also ‘found’ another very useful piece of software, called Straton, it removes stars from an image and you can then process the nebula without over stretching the stars and losing colour. I played with this quite a bit as another great use is when blending Ha into red channel (and sneaking some into luminance as well) it helps avoid getting holes in stars as the Ha stars are so much smaller than RGB.

Photoshop

There are so many ways to process an astro image it is not my intent to detail workflows here. I use bits and pieces from many sources, including processes from Steve Richard’s Dark Art or Magic Bullet. I have Noels actions, Google NIK plugin, Hasta La Vista Green and Astroflat Pro. My biggest effort goes into reducing the crazy gradients I get on my images due to sports field floodlights, logistics warehouse lighting, and the usual street lighting issues.

NGC2237 Rosette Nebula

So after many, many evenings of processing, reprocessing, trying different blends and masking techniques I leave you with my ‘final’ NGC2237 Rosette Nebula

thanks for reading and getting here, this is the JPEG version with a very light crop to remove stacking artifacts, the PNG was nearly 70Mb so I decided not to post that

Bryan

5aa274fdb11ec_NGC2237RosetteNebulaLRGBHa7nmReprocessStarenhanceForPublication08032018v8.thumb.jpg.f73f5c5b222196169fcaf2b8aee822e2.jpg

 

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@stevewanstall

Thank you, very pleased that you found my write up useful - I did seriously consider the ASI071, however after a lot of research I decided that mono plus filters gave much more options regards LRGB, adding Ha into the mix and the possibility of narrow band imaging in the future. I assume you can get into Exmoor and take advantage of significantly darker skies that would help imaging with OSC.

Bryan

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