Seestar photometry problems?

[Steve Knight]

I have plotted T CrB magnitude since June 2024 using nightly stacks from an S50. You can clearly see a problem on recent data. It’s been reported online than an Aug 2025 update has made the FITS files non linear. Has anyone else seen this?

Attachments:

1. 

   T-CrB-photometry.jpg
   

[Steve Knight]

The effect is even more obvious on my S30 data.

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   IMG_5542.jpeg
   

[Andy Wilson]

Hi Steve,
I will see if I can put you in touch with other BAA members who are using Seestars for photometry. I have taken a quick look at the VSS Photometry Database. Matthew Finch has Seestar observations of T CrB and they not showing the same large scatter as yours, though with less time coverage than your data. The attached plots for the shorter time period from August to November 2025. First showing Matthew’s Seestar data and the second with all the V data in the database as it is better at showing the trend.
Best wishes,
Andy

• This reply was modified 2 weeks, 5 days ago by Andy Wilson.

Attachments:

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   TCrB_MFinch_Seestar_Zoom.png
   

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   TCrB_V_Zoom.png
   

[David Conner]

I have recently started experimenting using a Seestar S30 for observing eclipsing binaries. One issue I found was that the automatically ‘in focus’ images generated larger scatter than certain manually adjusted out of focus images. The attached ‘light curve’ of DO Cas shows one example of this effect. However, in these results the scatter was not as large as in the T CrB data, so possibly not relevant in this case. (All my data points are from single images, not stacks.)

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   S30-focus-test.jpg
   

[Andy Wilson]

I am adding to this thread after hearing from Matthew Finch, whose light curve from the BAA VSS Photometry database I showed above. Matthew has seen a significant increase in scatter of his Seestar photometry since late 2025.

[Robin Leadbeater]

John Graham AKA Airship on Cloudy Nights has also been following T CrB with Seestar(s) for some years. His latest data here shows a trend to low brightness currently but does not seem to show increased scatter
https://www.cloudynights.com/forums/topic/932002-blaze-star-t-coronae-borealis-starting-to-blow/page/14/#findComment-14640801

Cheers
Robin

• This reply was modified 2 weeks, 4 days ago by Robin Leadbeater.

[Steve Knight]

Think I have found the cause of my problem.

Three seeing profiles attached for T CrB with S30.

Stack from February 2025.
Stack from April 2026.
Single sub from April 2026.

Looks to me like ZWO have screwed up their stacking algorithm, the April
2026 stack has saturated, 16 bits so maximum ADU is 65,535.

Note the peak ADU for the stack from Feb 2025 is similar to a single
sub from April 2026.

So the good news is that my data might be recoverable, the bad news is
that I am going to spend a lot of time stacking subs in an external
program.

Stop press, in La Palma, ran S30 on T CrB overnight, 2,134 10 sec subs,
again stack is saturated, individual subs look OK.

Attachments:

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   T-CrB-S30-seeing-profile-April-30th-2026-single-sub.jpg
   

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   T-CrB-S30-seeing-profile-April-30th-2026-stack.jpg
   

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   T-CrB-S30-seeing-profile-Feb-2nd-2025-stack.jpg
   

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