M42 - The Great Orion Nebula

M42 is always the first object a budding astroimager most wants to capture, and I was no exception. The shot above was the first and last attempt I ever made at capturing deep sky images with emulsion flim, back in January 2002. It was a 10 minute prime focus exposure on Kodak Gold ISO200 film using an Olympus OM1 mounted on the back of my VMC200L. Maybe some folk out there are capable of accurate manual guiding, but I reckon that the only way to really make film photography work for you at long focal lengths and long exposures is with an autoguider, which I don't have.

When I finally bought a digital camera, this gave me the option of digitally processing much shorter exposures (up 60 seconds) at higher sensitivities (ISO400 or even 800), avoiding to a certain extent the guiding problems. Even so, maybe half of the frames I took in my early sessions still showed signs of trailing. It was then that I learned the importance of polar alignment!

Once I got the hang on my GPDX's alignment scope, then there was the ever-present problem of sky-glow and camera noise to deal with. Five years ago before the advent of low-noise chips, decent internal camera software and CMOS devices, digital cameras were REAL pixel monsters. Here's one frame out of a useable batch of around eight that I acquired from a session in December 2002.


Click on the image (or any other for that matter) for a larger view, that shows the noise and vignetting. The vignetting (the dark corners of the frame) arises from the afocal mounting of the camera, which was fixed to peer down a 25mm eyepiece on my VMC200L by means of a home-made bracket. I never tried making flat frames as the field of view was so wide that I simply cropped out the edges in Paint Shop Pro. Similarly, the noise vanishes with a click of the "Despeckle" option.

Removing the sky-glow and beating down background noise is a bit more challenging. I've reworked many of my old digital camera astro photos with AIP4Win since I've had it, as to be honest, stacking multiple images in PSP7 was a pain and never too successful in my hands. Importing a batch of despeckled frames into AIP4Win brakes the colour images down in to individual LRGB FITs elements, each of which can then easily be stacked to give quite smooth L, R, G and B frames.


The image above is a stack of 7 frames, all disassembled and stacked as individual LRGB frames, and then exported back into PSP as TIFs (I don't think the colour stacking part of AIP4Win v1 is much good, so I use PSP). I merge these the black and white TIFs representing the red, green and blue frames with red, green and blue overlay frames to restore colours and give R,G and B TIFs.

Each colour TIF is loaded into a new frame as a layer (with the luminance one as the bottom layer), which you can then move around to align and then adjust transparencies using the "layers" function in PSP. I guess the colours are subjective - I always think M42 looks slightly greenish through the eyepiece, so I just chose a combination of layer intensities that I thought was the most astheticially pleasing.

With the purchase of a Starlight Xpress SXV-H9, the Orion Nebula was again one of the first objects I wanted to try out. The image below was taken with the SXV-H9 and 4.5 inch refractor, and is an average of 10x15s, 10x30s and 6x60s exposures all at full camera resolution, stacked and processed in AIP4Win.


I wanted to avoid the "burnt out" look you see in a lot of images of M42, where the bright Trapezium area gets overexposed in the attempt to capture the fainter outer nebulosity. Using a stack of shorter exposures in with longer ones has achieved this.

The SXV-H9 is a monochrome device, so if you want colour images you can shoot additional frames through red, green and blue filters, or you can do what I usually do for extended objects like nebulae, and cheat. I have a colour filter wheel, and although you can use 2x2 and even 3x3 binned exposures on your RGB frames to cut down times, I find that using artificial colour masks on monochrome frames gives results that are almost as pleasing in practice.

Below is a monochrome frame blended with a red frame in PSP7. Basically, I make a new file in PSP7 that's the same size as the monochrome, select a nice red colour from the colour pallet, flood fill the new frame and then copy and paste the monochrome one over the top. Using the "Layers > Arrange > Move down" option, I send the monochrome layer to the bottom, and in the layer pallet select "colour" as the blend mode. Somewhere around the 50% blend mark gives you a nice red nebula without red stars.


I then repeated the process for a frame consisting of an average of the 15 second exposures alone, to give a green frame as below:


Blending the two, again using the layer pallet, with the red layer as the bottom one and the green layer as the top and set to "color" in the blend mode, gives you a frame that has the central Trapezium region showing good detail and that hint of green you see visually. Final adjustment in "Curves" to tweak back the RGB and red channel brightness levels gives you a pleasing picture, I think..


Colours in deep sky objects are, of course, largely subjective. Only the very brightest, such as M42, show any colour at all through moderate aperture telescopes, and then that tends to be just green. I'm not a fan of the garish images you see in a lot of the astromony magazines, especially those in "mapped" colour (where Ha is often assigned to the blue channel, making everything look plain weird).

It's all in the eye of the beholder, and often, monochrome images often look the best.