Rule of 500, Ioptron SkyTracker and Deep Sky Stacker

To understand why/when you need to use a tracker you need to understand the “Rule of 500”. There’s a rule of thumb in astrophotography that to avoid visible star trails (or elongated stars) you need to keep your shutter speed below 500/focal length. For example, if I were using a 50mm lens then maximum shutter speed before getting star trails is 500/50 which equals 10 seconds.

Of course, this needs to be adjusted if you’re using a crop sensor camera like a Canon Rebel, or the Canon xxD series. Because a 50mm on a crop sensor Canon becomes a 1.6 x 50 = 80mm. And then using the rule of 500 you only get 6 (500/80) seconds. 

A tracker will move/rotate at the same rate of the stars thus eliminating elongation or trails of stars. This will allow you to expose an image longer than the rule of 500 allows.

I was out a couple of nights ago with a full frame Canon 6D and a Canon 70-200 f2.8. Unfortunately is was a weekday and I couldn’t get out of the city into darker skies, but I tried anyway.

This time of year, Orion is the obvious target. It’s easy to find, there’s lots of interest, and to be honest, relatively easy to photograph.

The first image below is a single exposure, straight from camera with no post processing. The details of the image are: 

  • 148 mm
  • F/3.2
  • 15 seconds
  • ISO 320

I recognize that this is not an impressive image, but I think it’s important to share the what the entire process looks like not just the final result.

Upon review of the image, I’m super happy with the focus, the stars look pin-sharp. I find this amazing for 2 reasons. One, getting sharp focus on the stars is really challenging. Two, the image is taken at 150mm meaning I should have only been able to expose for 500/150 = 3 seconds before seeing trails, but I exposed for 5 times longer!

I could have zoomed in an extra 50mm to the maximum 200mm focal length, but I chose to take a wider image hoping to find the Horsehead nebula. I think it’s fair to say I failed.

Happy with the image, I took another 12 consecutively with the exact same specs. 

This brings us to Deep Sky Stacker (DSS). For those of you that don’t know DSS is a free Astrophotography image stacking software. Used by amatuers and professionals alike, it is very powerful and very popular. I’ve tried to use DSS in the past, and after hours and hours of stacking and editing I never saw an improvement from a single exposure. Clearly this is a user problem, and my understanding of how these images work is not where it needs to be.

But I figured since I had 13 clean, sharp images maybe this is a good time to try to use DSS again. Without changing any of the default settings I loaded the 13 images and stacked them. Then following the steps from this website: http://flintstonestargazing.com/2009/06/26/my-quick-deepskystacker-tutorial/ 

I was able to produce an image that was a noticeable improvement. Here’s a screen shot from DSS showing the number of images and total exposure time in the top left corner. The vignetting is REALLY bad. I’m not sure what the cause of that is, but it definitely had to do with the post processing steps.

After cropping the image, and a couple of final touches in Photoshop, below is my final result. Considering this is in the heart of Toronto and I was using a 150mm lens I’m very happy.

Coming back to the Ioptron SkyTracker, I’m disappointed that I didn’t take one image with the tracking on and one with it off. But considering I was able to take 15 second exposures with a 150mm lens with a simple alignment (i.e. I did not use the App to polar align - see my other post) and see no trails, I’m happy.