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I'm having a problem with Canon T1i + Canon 70-300mm USM IS lens at 300mm and wide open at f/5.6.

Shutting night sky with a tripod I started noticing that the stars and planets are forming a "drop shape", angled 45 degrees. Speeds between 0,5s and 1s at ISO 1600 (grainy)

I am using tripod, mirror lockup, remote shutter release and 10 seconds delay.

Started noticing it when trying to capture the Neowise comet back in July/August(?).

Yesterday night I pointed at a red light in a cell tower (a stationary light) to check if the problem occurs. And yes, the tower light is blurred at 45 degrees angle. So, I've tried tilting the camera sideways, at a 90 degree angle. The image resulted in drop shape stars angled +45 degrees relative to the previous images. Mind blowing.

I am trying my best to pinpoint focus at infinity, both with AF and Live View. I don't know what to do, what kind of tests I should perform. My camera doesn't have micro focus adjustment.

Any help is highly appreciated.

Thanks in advance, Ivan

Example image: 300mm f/5.6 2.5s @ ISO 1600 HERE 1 enter image description here

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    \$\begingroup\$ I'd guess the lens is astigmatic - but I'll let the ones here smarter than me confirm or refute that. \$\endgroup\$
    – Tetsujin
    Jan 13, 2021 at 20:03
  • \$\begingroup\$ Can you post a link to an image of this? It is hard to understand what you are talking about. \$\endgroup\$
    – emmit
    Jan 13, 2021 at 21:02
  • \$\begingroup\$ @emmit Example image link at the end of the post. I'm new here, I dont know if I can embed it. \$\endgroup\$
    – Ivan
    Jan 13, 2021 at 21:09
  • \$\begingroup\$ Just to make sure - when you are using Live View you are focusing manually and at high magnification? I believe that is the recommended technique - along with super sturdy tripod. I'm not a astrophotographer, but I've read a little, back for the Saturn/Jupiter thing. \$\endgroup\$ Jan 13, 2021 at 21:26
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    \$\begingroup\$ @Tetsujin I think you solve it \$\endgroup\$
    – Ivan
    Jan 13, 2021 at 21:53

1 Answer 1

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I believe what you are experiencing is the result of one or more issues.

First, I would say that tails are indicative of trailing which is caused by the speed setting. Depending on your focal length, speed affects the shot differently. 10s on a short focal length won't produce long trails as it can on a long focal length. The way to mitigate this is following the rule of 500 that can help determine the maximum speed you should use when shooting at a given focal length. Note that you stated your camera is a Canon T1i, that is a APSC crop factor of 1.6., while a full frame would be crop factor 1.

Second, the teardrop nature makes me think this can be a result of coma, where stars in the corners appear tear drop, but point towards the center. This seems to be caused by the nature of most lens optics being curved. The other common possibility is a problem with collimation, where the lens elements are not aligned well, but I don't think that is happening here.

This video on youtube appears to be using a similar lens to yours, and it also seems to suffer from teardrop stars - this makes me think that it is not a collimation problem with your lens and instead is the lens element curvature causing coma. In the comments of another video by the same creator, he attributes the problem to coma. He seems to try to manage it by shooting an area, and then trying to crop out the sides with more severe teardrop stars.

This may not be the best lens for astrophotography. If you delve more into astrophotography, I recommend the dpreview astrophotography forum section for some discussion on technique and lens/telescope selections. They may be some astrophotography subreddits as well that can offer discussion on this.

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  • \$\begingroup\$ I'm aware of the trailing due to long exposure. What is concerning me is this tear drop shape that I am getting. \$\endgroup\$
    – Ivan
    Jan 13, 2021 at 22:32
  • \$\begingroup\$ My explanation covers that. This appears to be caused by coma, or a limitation of the lens. \$\endgroup\$
    – emmit
    Jan 13, 2021 at 22:35
  • \$\begingroup\$ This doesn't explain a similar result on the cell tower light. No trails there. I think there may be an issue with the lens... Also make sure IS is turned off when on a tripod. It can cause issues. \$\endgroup\$
    – BobT
    Jan 13, 2021 at 22:39
  • \$\begingroup\$ @emmit I'm aware of the trailing due to long exposure time. What is concerning me is this tear drop shape that I am getting now. Thanks for the response and the videos. I've had that 75-300mm lens, sold it because of the poor design of that lens, and I'm using the 70-300mm USM IS, since 2012/2013, wich has better construction and optics. I'm not a astrophotographer, but recently I've been unhappy with the overall quality of the images I'm getting with it, from distant birds to the moon, and now stars... \$\endgroup\$
    – Ivan
    Jan 13, 2021 at 22:41
  • \$\begingroup\$ @Ivan You will get better results from an optically superior Canon L series lens, than the 70-300 USM IS. Just understand that they were not designed for astrophotography and that is why coma is an issue. I've used that lens before. It is a decent lens. \$\endgroup\$
    – emmit
    Jan 13, 2021 at 23:11

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