I too found out that shooting the max (5 minutes) you will get trailing around the edges of the frame. I shoot at 18mm a lot and have learned to not exceed 120 seconds (and I am considering going to just 100 seconds, for a bit of extra margin).
I also use the 200 rule (rather than the 500 rule) when not using the AstroTracker (200/focal length = max tracking time) in order to keep the stars from streaking out of about a pixel. So,the difference is essentially about a factor of 10 (i.e., even using the 100 second max, it's 10 longer than what I can shoot with out it).
I ran across this the other day.... It is based on a Canon t5i body (18MP). The author is RM Clark who has an excellent website on all things astro...
Quote: First, the 500 rule is an idea for low quality lenses, web sized images, or for people who don't car about short star trails. Here is the real story:
Your t5i has 4.3 micron pixels. With an 18 mm lens, the 500 rule is 500/18 (or 500/(18*1.6)) giving 28 or 17 seconds.
Stars on the celestial equator move 15 arc-seconds per time second. The pixel spacing in arc-seconds is:
- plate scale = 206265 * pixel size in mm / focal length in mm
- so plate scale for your camera with an 18-mm lens is:
- plate scale = 206265 * 0.0043 / 18 = 49.3 arc-seconds.
- A star will cross one pixel every 49.3 / 15 = 3.29 seconds.
The 500 rule above (28 or 17 seconds) would give star trailing of 28 / 3.29 = 8.5 pixels or 17 / 3.29 = 5.2 pixels.
A couple of pixel trail will be noticeable with a sharp lens. So a 200 rule is needed for actual round images with good lenses.
The key to astrophotography is collecting light. The f/ratio tells light density, but not total light gathered from the subject. Larger physical aperture diameters collect more light. Recommendations like 11 mm f/2.8 means an aperture diameter of only 3.9 mm, smaller than dark adapted human eyes.
An 18 mm f/1.8 lens (as in the Sigma 18-35 f/1.8 art lens) has an aperture of 10 mm, and collects ( 10 / 3.9 )2 = 6.6 times more light than an 11 mm f/2.8 lens from the subject. A 24 mm f/1.4 lens has an aperture of 17 mm; a 35 mm f/1.4 has an aperture of 25 mm and would collect ( 25 / 3.9 )2 = 41 times more light than an 11 mm f/2.8.
So get the largest aperture diameter lens that fits your subject in your field of view (or do a mosaic). The other way to get more light is longer exposure times. For night sky, that means a tracker. Something like an iOptron Skytracker (about $400) can track lenses 100 mm and shorter for long periods.
I would recommend the Sigma 18-35 f/1.8 Art lens for crop cameras, or Sigma art 24 f/1.4. If you use a tracker and will consider mosaics, the Sigma art 35 f/1.4 is in my opinion the best lens for nightscapes under $1000.
I thought a fair amount about the problem. Decided 1) the O-GPS was more than enough for me, and 2) a better lens and 18mm was reasonable for what I had been shooting - Astro Landscapes. So, splurged on a Sigma 18-35/f1.8, about a year ago, and the GPS unit 2 years ago. I had designed a star tracker for a little telescope down in Texas (
HET). Between using the GPS for longer exposures and the substantially shorter exposure time with out GPS tends to provide all that I really want to do. I am to the point that I would rather go out shooting, than sit down figuring out all the math again. Also, I did not want to haul around a tracking unit and set it up and align it all the time.
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The reason for the streaking around the edges, is that the GPS tracks the center, in an X - Y space (discounting any movement about the z axis - i.e., rotation). The longer the duration of the track, an angular component starts to creep in, that is really not taken into account by the GPS tracker. Thus, the longer duration tracks will start to record this angular movement around the center.