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Uneven exposure across solar image

Astrophotography Solar
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#1 lostinthesky

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Posted 25 February 2024 - 04:14 PM

Hello, 

Recently I started taking pictures of sunspots with my Sony alpha 5100 on a Celestron C5 with Baader ASTF 120 AstroSolar Telescope Filter. Images are consistently darker at the northern pole of the sun. To troubleshoot, I rotated the camera 180 degrees so the camera is upside down in the visual back. Using the camera upside down resulted in an image darker at the southern pole. This seems indicate an issue with the camera, not the telescope.

 

I repeated using a second Sony alpha 5100 (I have more than one of this camera smile.gif). Results were the same with the second camera (darker at the northern pole, then after rotating the camera 180 degrees the image was darker at the southern pole).

 

Darkening is more obvious as lower ISO. Attached are representative images taken at ISO 800 and 1/4000 s.

 

1.) Any thoughts on why the solar images show uneven exposure between the top and bottom of the image?

 

2.) Any thoughts on how to correct the issue?  

 

Regards,

Mike

Attached Thumbnails

  • 2024 0225 camera comparison.jpg


#2 SpaceMax

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Posted 25 February 2024 - 04:19 PM

Hi Mike,

can you shoot a flat frame and check if the issue persists? If so, I would look at a collimation related issue, I.e., tilted mirror in respect of the optical axis.
It could also be Tilt in the imaging train diverting the light cone.
Let’s us know what you find.

Cheers, Max
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#3 BQ Octantis

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Posted 25 February 2024 - 04:56 PM

I've had a similar problem with lunar imaging with the mirror (I assume) casting a shadow. Try 1/1000sec@ISO100 and see if it persists. Or try capturing a frame from LiveView (assuming the Sony allows it).

 

med_gallery_273658_12412_736857.png

 

BQ


Edited by BQ Octantis, 25 February 2024 - 05:02 PM.


#4 Matt78

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Posted 25 February 2024 - 05:28 PM

I deleted it due to having about 100 gb of this footage, but yesterday the sun went through a tree sooner than I had anticipated. I figured I had maybe 20-30 mins left, but you know the sun gets so bright you just don't (and shouldn't) be looking at it much at midday. In my case the laptop screen was also washed out and I couldn't see that it was getting uneven exposures. Once I went in, I could make out the shape of the twigs. Could something more uniform, like a building have gotten in your way?

 

I solved my washed out laptop screen today by building an advanced solar data capture facility (I put it in a cardboard box). Maybe you could rig up a small box to see your camera's screen better (ignore this if it has a viewfinder, I guess).



#5 KLWalsh

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Posted 26 February 2024 - 01:37 PM

Your images are labeled ‘Upright’ and ‘Upside down’, but I’m wondering if the camera was actually sitting 90 degrees to the positions shown.
Because… At 1/4000th of a second, even a very slight delay in how the shutter moves might cause the leading or trailing edge of the image to be dimmer compared to the rest of the image.
Try shooting at other shutter speeds, and alter the ISO settings as needed, to see if one side is still dim.

#6 lostinthesky

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Posted 27 February 2024 - 07:45 PM

Thank you for the feedback, everyone.

I was able to dodge some clouds to take some more photos but this time at various shutter speeds from 1/4000th sec to 1/500th sec (and also at a various ISO). Reviewing the different shutter speeds shows that the darkening at the northern pole of the sun decreases as the shutter speed is slowed down. 

Attached is a comparison at faster and slower shutter speeds and at lower ISO to darken the image, which accentuates the darkening of the northern pole at faster shutter speed. 

I appreciate the help. 

 

Regards,

Mike

Attached Thumbnails

  • 2024 0227 Shutter speed comparison.JPG
  • DSC06574_mod1 resized.jpg


#7 lostinthesky

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Posted 27 February 2024 - 07:51 PM

...and providing a little contrast and color smile.gif

Attached Thumbnails

  • DSC06574_mod2 resized.JPG


#8 BQ Octantis

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Posted 27 February 2024 - 09:00 PM

Well done! applause.gif

 

BQ




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