Phil's Astronomy Amateur Page

The tunnel I'm talking about, being a tad metaphorical, is the camera lens and over the last couple of evenings I've been revisiting astro imaging with digital cameras. Having recently aquired an Olypus E1 I thought it was time I actually read the instruction book and took it through its paces! (I'm an 'engineer' - engineers don't read instruction books....)

Some years back, I've lost count...OK, about 8, I played around with a Olympus OM20 SLR film camera with which I experimented with direct viewing through telephoto lenses, screwing it onto variable telescope lenses and using it in prime focus mode. (Using the camera as the telescope eyepiece). To be honest my enthusiasm was cooled quite quickly due to the limitations - not seeing the results until the film had been developed mainly - and I regrettably let it slip.

With a digital camera able to recall the results immediately, my enthusiasm was reignited, motivated mainly by the Perseids meteors. Why I didn't seriously look into it earlier, I don't know - too lazy, too busy doing other stuff... I don't know. For goodness sake, I had access to my wife's Canon EOS 300 so there was no excuse really. To cut a long story short, I missed photographing the Perseids and was annoyed at that, given that over the course of three days I saw so many beautiful displays, so the next targets will be the Orionids in October... And I WILL have a camera ready.

So what are the main difficulties imaging the night sky? Well, cameras work by letting light fall onto a light-reactive sensor, be that a film or a semiconductor chip. The obvious difficulty with the night sky is that there isn't much light and what there is, you don't necessarily want to see (neighbours' bathroom lights, security lights, street lights etc.) . Consequently you may need to work around problems, gathering as much of the RIGHT light that you can. Of course, the best thing to do is get yourself out into the countryside if possible. But that's not always possible.

So here was I in my back garden with lights going on and off all around me, the sky glowing orange from the north west to the eastern horizons, trying to aquaint myself with the settings on two different cameras.

First, the Canon. I'll be honest, the trickiest thing is setting up the camera the way you want it. I couldn't find the instruction book...

First I found that the EOS won't do Manual mode unless the focuser is also set to manual - makes sense, I suppose, but an automatic override would be nice, instead of it just not working at all. I focussed on Jupiter - note that twisting the focus ring all the way round, takes it too far (can you go past infinity?? Obviously...) so rely on getting the sharpest image here, that you can, then don't touch the ring again!

I set the ISO to 1600 and the exposure to 10" (seconds) and aimed the 50mm lens at the Plough. (What was the aperture? I have no idea, I couldn't work it out.... I know that when the chipped adaptor ring is used, it defaults to F2 - I'll have to follow this question up...)

Anyway, I was moderately surprised by the result - a bit orange and grainy (noise) but at least I had the whole asterism in shot.
I then put the lens cap on and did another 10 second exposure. Why? Ah, the magic of digital photography. Read on, this is important.

When you take exposures of more than a second or so, you tend to get 'noise' building up on the image. It looks like little red dots scattered all over, sometimes you'll get a haze in one corner. Sometimes you'll see some blue dots and occasionally, some green too.
What you are doing with the cap on, is creating a blank image with the same imperfections. These imperfections should be for the most part, virtually identical to the imperfections on your picture. What you will then do when you get indoors after your session, is upload the pictures then edit them.
You'll probably open Paint Shop Pro or some photo editing software like that, then choose an image and a corresponding dark-field image. By a clever bit of manipulation using the Image/Arithmetic/Subtract function, you can effectively deduct the imperfections from your required picture, leaving a very much improved image. Simples :o)