• Pentax K-1 Mark II Astrophotography Review – Lonely Speck

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Astrotracer loses a little bit of its utility when shooting with very wide angle lenses. The rectilinear distortion of most wide angle lenses causes different parts of the image to appear move at different rates across the sensor, depending on its position, even if actually moving at the same rate in real life. Objects moving at a the edge of the frame appear to move faster than objects in the center. That means that even with the most accurate of Astrotracer shots, there will always be some star trailing in the corners of a wide angle rectilinear image. Less than a non-tracked image, but still some trailing.

That said, Astrotracer still does an excellent job at tracking the center portions of a super wide angle shot and still allow for greatly extended exposures with reduced star trailing. Using the wide angle example shot from before, let’s compare the center and corners of the image:

• It's understood that UWA lenses distorts along the edges in static situations, however an overview of why the rate of movement appears to change when the objects are moving (center vs the edges/corners) - as in the case of astrophotography.

• Lens Aberrations - There is a LonelySpeck article (link below) that provides a good overview of the topic, addressing various types of aberrations - including distortion. However, it does not address the wide angle rectilinear distortion in terms of object movement (with apparent rate of change) with in the frame.

• A Practical Guide to Lens Aberrations and the Lonely Speck Aberration Test – Lonely Speck

• AstroTracer vs equatorial tracker - Touching on the difference between the AstroTracer approach to the tracking problem (moving the sensor in the x,y and xy plane only - and only moving the sensor with the rest of the camera held stationary), vs. an equatorial tracker that moves the entire camera body (as a monolithic unit) in opposition to the earth's rotation. I find it interesting to note that images from an equatorial tracker exhibit the same type of wide angle rectilinear distortion in the corners as illustrated in the following link. So, this phenomena just can't be unique to the AstroTracer's functional design and implementation. It certainly appears to be lens based that is exhibited when there is some type of dynamic tracking involved.

• St. Michael's Mount | Milky Way and Mars over St. Michael's ? | Flickr

• Focal lengths affected - Certainly the ultra wide angle focal lengths, however just where this phenomena starts to dissipate, would be helpful to know.

• Exposure times - This has been pretty well documented in a wide variety of posts here on the forum, but to have the information all in one place for reference, would certainly be helpful. Personally, I have found that 50 to 70 second exposures (@ both 15mm (full frame) and 18mm (crop frame) are very acceptable). Others have reported 90 seconds up to 120 second exposures to be acceptable.

• Approaches to handling the corner trailing - Several come to mind. Cropping the offending area. Also, shooting additional frames to stitch together, so that then cropping can be applied to remove the trailing that has been regulated to areas of the resultant frame that were not really wanted. Shooting with longer focal lengths (potentially coupled with stitching), or with shorter exposures.