Star Elongation on One Side of Image

[cwine]

Hi! I've been struggling to get rid of slightly spiked/elongated stars that appear on the left hand side of my images. The stars have small spikes coming out the bottom left hand side of them.

I use an unmodded Canon 600D with a 70-200mm Canon f4 lens (which I have recently cleaned to try and solve this issue). I use the star adventurer to track, but this doesn't look like the type of trail you get with bad alignment.

I've attached some images as an example of what I mean - Example 1 is the bottom left of an image of the North American Nebula, and example 2 is the top right of the same image. Any ideas?

[alacant]

Hi

The LHS is astigmatism. Assuming the lens sits tightly on the camera and this doesn't happen with other lenses, I don't think there's much you can do. It's gonna cost a small fortune to tear the lens apart and align it. Perhaps better to get a fixed lens. At 200mm, old Takumar and Zeiss examples are available cheaply and have the advantage of being corrected for full frame and so perform well corner to corner over aps-c.

HTH

[cwine]

13 minutes ago, alacant said:

Hi

The LHS is astigmatism. Assuming the lens sits tightly on the camera and this doesn't happen with other lenses, I don't think there's much you can do. It's gonna cost a small fortune to tear the lens apart and align it. Perhaps better to get a fixed lens. At 200mm, old Takumar and Zeiss examples are available cheaply and have the advantage of being corrected for full frame and so perform well corner to corner over aps-c.

HTH

Ah so frustrating. Just bought this lens second hand (and I certainly don't have budget to be buying more lenses as a student). Maybe there is something I can do in post processing? Would be very frustrating if I couldn't solve this at all.

[ollypenrice]

20 minutes ago, cwine said:

Ah so frustrating. Just bought this lens second hand (and I certainly don't have budget to be buying more lenses as a student). Maybe there is something I can do in post processing?

If you have Photoshop or a program with similar functions there is something you could try.

1 Concentrate on a zone in the image where the elongations are all going the same way. Make a copy layer.

2 Rotate the image so that these elongations are now horizontal or vertical on your screen.

3) Set the blend mode in Layers to Darken.

4) Go into Filters - Other - Offset. You'll be able to nudge one layer (either vertically or horizontally, depending on how you rotated your image) relative to the other to mask the elongations. Use a large, well feathered eraser to remove those parts of the image damaged by this modification, keeping only those zones in which it's an advantage.

5) Flatten and repeat with a different rotation for other parts of the image.

6) Restore the original orientation of the image.

Make no mistake, this is essentially a bodge, but done with care it might considerably enhance your image.

Olly

Edited September 25, 2020 by ollypenrice

[cwine]

12 minutes ago, ollypenrice said:

If you have Photoshop or a program with similar functions there is something you could try.

1 Concentrate on a zone in the image where the elongations are all going the same way. Make a copy layer.

2 Rotate the image so that these elongations are now horizontal or vertical on your screen.

3) Set the blend mode in Layers to Darken.

4) Go into Filters - Other - Offset. You'll be able to nudge one layer (either vertically or horizontally, depending on how you rotated your image) relative to the other to mask the elongations. Use a large, well feathered eraser to remove those parts of the image damaged by this modification, keeping only those zones in which it's an advantage.

5) Flatten and repeat with a different rotation for other parts of the image.

6) Restore the original orientation of the image.

Make no mistake, this is essentially a bodge, but done with care it might considerably enhance your image.

Olly

Thanks for your help! Ill try that out but otherwise just deal with it for now. It doesn't stop me from shooting so that is certainly a positive!