Some fun software tricks in image processing

This is a little off topic and I'm thinking that the technology has moved on and up from where it was 14 years ago when I did this kind of thing but I thought this (below) was a pretty neat trick at the time.  "At the time" was the time when I thought I was going to go big in astro-photography, somewhere around 2003.  The combination of high quality optics, digital cameras (yes they were new technology at one point), and image processing software was opening up opportunities for amateurs that would not have been very easy, or even possible (?), a few years earlier. 

In the old days one needed giant telescopes that gathered a ton of light, near zero light pollution, and an extremely good mount so that one could take very long low-light exposures of very faint objects with no tracking error and no shaking.  The new technology, in simple terms, meant it was now (at the time) possible to do something similar by taking many short digital exposures, stacking them electronically, collapsing them together and then subtracting out the noise.  That approach reduces mount tracking errors, the impact of light pollution, and also means smaller less expensive and easier-to-handle telescopes can be in play. 



Here is how it works. You get a good telescope (I had a TMB130 f6 refractor) and mount (Losmandy G11).  Align the mount with the north pole rotation point (harder than it sounds but not too bad) and let the motor run. Find the object. Attach a camera. Get the light to a perfect focus on the camera sensor (this is harder than it sounds; here you can't use the camera viewfinder, you have to play around with special instruments and the physics of light for a little bit). Take 5 or 10 or 30 exposures for a few seconds or maybe up to 30 seconds or even two minutes or more depending on how good you are. Upload the raw images.  Align each image on top of another with reference to at least 2 or 3 reference points common to all images (star points). Collapse "the stack" into one image. Use some fancy software to then subtract out what the software can figure out to be noise (having 10 or 20 images with the same common stars helps amp up the differences between signal and noise for the algorithm).  Then step back and behold the glory of your work (after a few tweaks).

Here is an example of the process in action.  This image below is one of six separate two minute ISO400 exposures of something in the sky. You'll see what it really is in a minute. Six was probably too few and two minutes seems like an eternity when a passing car can shake the ground and a flight overhead or an asteroid or a meteor can rip a streak through your images.  You can see the marginal quality of the raw image and the heavy dose of light pollution that the 2-minute exposure sucks up. This is not a fun image:

single raw un-processed image with light pollution

And here, just for reference, is what the light pollution over St. Paul MN in the winter looks like. This is where the image above and below were taken.  I'm sure it is worse now than it was then (note: this image was processed like I am describing so this is the "after" version). This was about 11 pm so to the naked eye it was pretty dark. The camera is sensitive to the sky glow even at low levels.  This is looking over the Mississippi river valley more or less towards the old Schmidt Brewery near downtown St. Paul.  Yes that is Orion.

St Paul Light Pollution

After 5 more exposures I stopped.  I think it was 18 degrees F and about 10 PM or so.  That's pretty cold even at 2-min per exposure.  At that point you have to bring the camera inside in a plastic bag so that the heat differences and condensation don't mess up your camera. This means you cool your jets for a while, maybe have a glass of wine.  Then you get the files out and up and do the stack/collapse/subtract procedure.  After messing around for a while, this is what comes out of the murk we saw in the first image above....

M31 - Andromeda after stacking images and subtracting out the noise

If you know this stuff then you also know that this is not a very good image of M31...but for me, I thought this was pretty cool for an amateur attempt especially given the raw images. In case you're curious M31 is the Andromeda galaxy. 

As long as we are on this topic, here are some other examples of work I did except that I didn't keep the "before" images.

The Pleiades

NGC 869, 884 Double Cluster

M37 - can't really see those colors with the naked eye

You know this one...

Hyades wide-field with camera only no scope, hence the distortions

This is the equipment I generally used. This scope in particular was at one point one of the better Apochromatic 5 inch refractors on the planet. Three objective lenses in a custom designed near-perfectly-color-corrected-air-spaced optical stack.

In the end I gave all this up because after one has kids, especially when they are infants, staying up from midnight to 4am in -20 winter weather (best clear stable air) becomes unappealing when contrasted with the idea of a TV remote control, a glass of wine, a soft chair, and an early bed time.