https://www.facebook.com/photo.php?fbid=1586866471326904&set=a.269535393060025.83092.100000107007390&type=3&theater 
BY
Fritz Helmut Hemmerich
Comet 2017 E4 Lovejoy from this morning. During only half an hour the morphological changes in the tail are enormous. I was very happy the capture the evolution of a disruption event finally visible in the right most image. Each of these 12 images are a stack of 5 subframes with a total exposure time of 150 sec each. The most important for me is to realize, what happens, when you use total exposure times of 10 or 20 or 30 minutes and stack these subframes. There is a smear effect resulting in 'impressiv' tails which are totally fake as you can see in the bottom image which is a stack of 30 min exposure time. (If you would stack 10 pictures of a jumping person, you would not be able to recognize the person but instead you'ld get a smeared tall something). I myself made this mistake in the past and I saw the same artificial tails in the images of some well known comet photographers in the past days. I think we have to rethink our imaging techniques in regard of the total exposure time at least in the case of comets with such a fast changing morphology. Please follow this link to see the details more clearly from one 2 1/2 minute step to the next one (you'll need use zoom to detect the differences:
https://www.flickr.com/photos/fhhemmerich/33682359082/in/dateposted-public/lightbox/Technique: 12 x 5 x 30 sec. Hyperstar 14" F1.9. Sony A7s (CentralDS), ISO 3200, UV/IR cut filter. FOV 1.7° (horizontal). 2017-04-04 6:26h - 6:56h UT, Tenerife 1180 m altitude.
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