M13 Globular Cluster in Hercules

Testing out the barn door tracker

Looking to test the newly improved ‘barn door’ tracker to its limit, I dusted off my old Sigma 75-200mm/f3.8 with 2x tele-converter. But with a hefty 600mm 35mm equivalent focal length, I was prepared for the worst.

barn door tracker image of M13 Globular ClusterAfter less than a minute of polar alignment, and unable to frame the shot through the viewfinder (due to an awkward angle), I pointed the camera vaguely in the direction of the Great Globular Cluster in Hercules, and opened the shutter for a 30 second shot. Without a watch to count the seconds, I manually moved the dial round, counting the seconds off in my head.

Checking the screen, to my amazement, there it was! Not only had I centred the cluster first time, but there were virtually no trails. I know the shot is pathetic compared to most, but for a home-made, hand guided barn door tracker, cobbled together for under £10, I was blown away by the result.

Shown here is a single 30 second exposure (iso 2000) without the didymium filter, giving it much more colour than the final stacked image.

Details: Nikon D7000, Sigma 75-200mm/f3.8 (at 400mm with 2x converter). 5 minute exposure (10 x 30 seconds) at iso 3200 with didymium filter. Stacked with Deep Sky Stacker and tracked with barn door tracker.


  1. Jim Gage Reply

    Nick, I forgot to ask, what is the timing rate for a 6mm screw? I woudl like to experiment with some other helping devices. It is slightly faster than 5 sec between pulses, but how to calculate that? Thanks!
    Jim in Sweden

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