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Green / Magenta vignetting - artifact on bright starts after Sequator

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#1 tschopp

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Posted 22 September 2020 - 11:00 PM

Hi first time posting,

 

I heard astro is pretty demanding of lenses so I took some astro photo to compare my canon 135mm f/2 with my new sony 135mm f/1.8.  Centered on polaris because I wanted to minimize star movement. Set exposure length to 10s based on some french site that calculates max exposure time to prevent blurring across pixels.

 

Results looked pretty good with both lenses. Saw some guy on youtube take photos of Andromeda with a 200mm and thought, hey I should try that.  Yesterday with the help of an iphone app I was able to find Andromeda and by a slight miracle point the camera in the right direction.  Took 300 x 1 sec photos (again the french site said 1 sec max untracked).

 

Equipment:

Sony A7iii

iso 640

300 x 1s

sony 135mm @ f/1.8

B&W MRC UV filter

 

took 60 x 1 sec dark frames

today took 40 frames of a paper in sunlight for vignetting

 

Entered the above into Sequator and the results look really good (for not knowing what I am doing).

 

1st question: The bright stars in the Andromeda photo hava an artifact, they have a horizontal dark band next to the stars. Specifically look at HIP 3293 about 2 o'clock from M32.  For some reason my images of the polaris area don't show this.  Only difference was those are 10 sec vs 1 sec images.  I also took photos of pleiades and other things they all show the horizontal band.  Only other thing I can think of is star movement.  The other sites had 1 sec photos because the stars were moving faster.  Any idea what caused this and how to get rid of it?

 

2nd Question: On all my stacked photos I see green / magenta alternating rings that look like vignetting.  I am guessing this might be an artifact from the anti-reflective coating on my UV filter, but haven't had a chance to try photos without it (cloudy tonight).  Any other thoughts, might be able to remove this in photoshop (don't have - and I don't like doing a lot of post processing?)

 

thanks

Steve

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  • andromeda_crop.jpg

Edited by tschopp, 22 September 2020 - 11:01 PM.


#2 sharkmelley

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Posted 22 September 2020 - 11:25 PM

Using 1sec exposures at ISO 640 will result in images whose background is horrendously underexposed.  This will make Sony's in-camera processing artefacts more obvious.

 

Switch off file compression to remove those horizontal dark lines and bright lines around stars.

 

Switch off all lens corrections to reduce the effect of coloured polygonal banding in the background.

 

Mark


Edited by sharkmelley, 22 September 2020 - 11:26 PM.

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