M42 – Orion Nebula – one year later

 Update! I’ve reprocessed the image:


The steps were mostly the same as before, but the HDR stage was tweaked a bit:

  • RGBWorkingSpace changed to 1 for each channel
  • DynamicPSF to generate PSF for Deconvolution
  • Luminance mask created from RGB image
  • Deconvolution used to bring out some more details
  • Multiple HistrogramTransformation runs to stretch the image carefully, and align the channels
  • HDRMultiscaleTransform with the brightest stars masked, once on the larger scale (~10) and once on a smaller scale (~7)
  • SCNR to reduce green
  • ACDNR to reduce noise
  • HistogramTransformation to stretch the image a little further
  • RangeMask+StarMask used to protect the background & bright stars
  • LocalHistrogramEqualisation used on the nebula to brighten things up
  • CurvesTransformation to increase contrast
  • UnsharpMask to sharpen some details slightly
  • Slight Convolution to reduce a bit of the harshness from UnsharpMask
  • ColourSaturation three times, one with a mask protecting everything but the brightest stars, one protecting the background and bright stars to increase saturation on the nebula, and once with the previous mask inverted to reveal some colour in the dusty background.
Original post:


I’ve shot this target before!

I haven’t done a HDR shot before, most targets don’t really need it in my experience. There is one huge exception – M42! The core of M42 is super bright compared to the rest, and then there’s the dust which usually goes unseen until you take much longer exposures.

M42 is easy to capture, hard to master.

General Details:
  • Date taken: 14/12/2014
  • Location: Adelaide, South Australia
  • Camera: Canon 450d w/ IR filter removed, GSO coma corrector
  • ISO800
  • Mount: HEQ5PRO
  • Scope: GSO 8″ f/5 Newtonian
  • Autoguider: Orion Starshoot AG
  • Imaging: BackyardEOS w/ PHD dithering
  • Guiding: PHD2
  • 50 bias frames, 20 dark frames (for the 300 second lights only!), 50 flat frames
  • 15 x 10 second exposures
  • 15 x 60 second exposures
  • 12 x 300 second exposures
  • Total integration time: 4650 seconds
Pre-Processing details (performed on each exposure length individually):
  • BatchPreProcessing, calibration only
  • BatchDebayer
  • Star Alignment with drizzle (took a lot of tweaking to get the 10 second exposures to align correctly!)
  • Blink to check for and remove any bad frames
  • Image integration with drizzle (using Winsorized Sigma Clipping)
  • Automatic background extraction on the three integrated RGB images to remove the light pollution
Processing the resulting RGB Images:
  • BackgroundNeutralization
  • ColourCalibration
  • SCNR to remove the green tinge
Prep the images for HDR:
  • StarAlignment of the three different integrations so they’ll line up
  • Blink to make sure they are lined up OK
  • DynamicCrop to cut out any bad edge bits
HDR Time:
  • HDRComposition to combine the three different exposure length integrations
  • RGBWorkingSpace
  • ACDNR to reduce noise
  • HistogramTransformation x2 to bring out the background dust
  • HDRMultiscaleTransform to reveal the core of M42
  • ColorSaturation to enhance the colours a bit
  • CurvesTransformation to enhance contrast and luminance
  • LocalHistogramEqualisation to brighten the image a bit

First Orion Nebula (M42) shot with my gear

Orion is a super easy target for beginners, and it is instantly recognisable in the night sky. This shot was taken without any guiding so I threw away quite a few exposures before I got something I was happy with.

This is a stack of nine 30 second exposures between ISO 400 and 800. The processing was done using DeepSkyStacker.


After stretching the image quite a bit, much more detail is revealed!


This is how my gear was set up taking these images – no autoguiding at this point!

Fun with coma correctors…

One thing I started to notice in photographs taken through my GSO 8″ f/5 newtonian was the stars towards the edge of the field were stretched out a reasonable amount. This is called comatic aberration or coma for short. I knew this would happen, but I decided I would try imaging without any corrective optics to begin with.

Eventually I decided that it was time to fix the comatic aberration in my images by using a coma corrector. I don’t like to spend money when it isn’t needed, so I figured that buying a cheap and cheerful coma corrector from Bintel would be the best idea. It says in the product description that it has a 2″ barrel, and it can be used with a DSLR. Perfect I thought! When I received the corrector, and tried to use it, I discovered things weren’t quite that easy.

Bintel Coma Corrector

Now, it’s really easy to make this mistake. If you look at the product description, it tells you that you can use a T-ring by unscrewing the 2″ barrel and screwing on a “wide” T-ring. Now here’s what tripped me up – I don’t have a “wide” T-ring. This appears to be some sort of “special” T-ring from Orion (and maybe others?) that is 48mm, versus the standard 42mm for a T-ring. I think the idea behind this size increase is full-size sensors in much more expensive DSLRs tend to experience an amount of vignetting when you use a 42mm T-ring. I’m running a 450d which is a crop sensor camera, and I don’t see any significant vignetting.

I was playing around with my current T-ring to 2″ barrel adaptor, and discovered that the barrel has a 48mm thread inside it for filters. It turns out this is the exact same thread that the coma corrector uses, and it screwed right in! I enthusiastically set up my telescope and tried to get it to focus. I failed, until I moved the combination of camera, tube and coma corrector out fairly far and racked the focus right out. Not good. I thought maybe it was because the distance between the lens and sensor was wrong (which it is), so I thought I could just get some spacers to correct this.

Back to Bintel – I found this product:
Orion T-Thread Spacer Ring Kit

This looks like it’ll do the job, but it’s for T-thread (42mm) which just isn’t going to help. The lens and sensor are already too far apart so this will just make the problem worse! I checked the Bintel site (and several other Australian telescope suppliers) but I couldn’t find a product to adapt a 42mm t-ring to the 48mm thread. I contacted Bintel about this problem, and it turns out they have a solution. They have a M48 to M42 step down ring for this exact problem. This, combined with the expensive T-thread spacers should allow the camera to mate up with the coma corrector at the right distance.

Fast forward a couple of days, the parts arrived. $50 for a few bits of metal that would hopefully make life easier. I tried several combinations of the spacers with the M42-M48 adaptor connected to the coma corrector, and I have found a combination that gives me roughly 70mm spacing. According to several sources on the Internet, this is the right amount of spacing to correct any comatic aberration in my f/5 newtonian.

As is always the case with these things, there hasn’t been a clear night since I received the spacers and adaptor so I haven’t had the chance to test this gear out. It’s looking likely that in the next couple of days there might be a few hours of clear skies during the night. I will attempt to get some before and after shots as soon as I can.