Wrong display of exposure time

[MikeB838]

Hi Robin

last night with SC 12572 I observed a small discrepancy:

I take frames with exact 30 seconds length. The shown total exposure time (under live stacking and in the status bar) increases with each frame:
0m 30s - 1m 00s - 1m 30s - 2m 00s - but then: 2m29s

Mike_Exp-Time.jpg (11.47 KiB) Viewed 632 times

But the files all have the name xxx_150s_xxx, the same in the log.

But in the fits header of the stack and the single frames I find:

SUBEXP = 29.999999

So it seems that the inherited exposure time of 30.0s ist modified slightly.

I first thought it has something to do with plate solving (I used it for the annotation), but a new stack with no plate solving also has this effect.

It is no show stopper, I only want to report it.

Thanks for looking!

cheers,
Mike

[admin]

Hi Mike,

this sort of thing is pretty common, especially if you have been adjusting the exposure using the slider... Even if you click left/right of the slider position which tries to halve/double the exposure, little rounding errors can creep into the calculation.

In most places where SharpCap displays a value, it will round to 2 or 3 digits, which means that 29.999999 will show as 30.00, but FITS file headers store the full value of the number rather than rounding, so you see all the gory details. In practice, when taking long exposures, I doubt that the camera exposure is 100% accurate to the value it is set to - you might ask for 30.000s, but the camera might end up taking 29.995 or 30.015 or similar and it would be almost impossible to tell. Some cameras are only accurate to about 0.1s once the exposure goes above about 2s!

cheers,

Robin

[MikeB838]

Hi Robin,

you surely are right with the true exposure time of the camera, but:
I did not use the slider, I entered 30 directly and had not seen this in all the older versions.
It is at least a little bit puzzling, when the displayed exposure time of the stack differs from the expected one. The value in the fits file is not so obvious at the first look.
Did you change the rounding or does this happen by accident?

Cheers,
Mike

[admin]

Hi,

what camera are you using? The value you enter in the box is converted to camera units to set the exposure on the camera and then converted back, so that could lead to it not being exactly as entered (although I would expect it to be fine for a whole number of seconds to be honest). It's also possible that the slider itself may move the slider position to the nearest whole pixel, but I'm not sure about that.

cheers,

Robin

[MikeB838]

Hi Robin,

I have a PlayerOne Neptune-C II and it is the first time for over a year now that I use SC with all the SC versions that I see this.

When I start a session I enter 30s manually into the Exposure entry field of the Camera Controls. After this I only click on the slider (left or right of the picker) to sometimes change the exposure time (eg. going to be faster for focusing etc. and back to 30s for imaging).

Due to your tip (How do I => Restarting Camera (19. Sep. 2024)) I now use F1/F2 to restart the camera. Maybe this is the reason to not going back to 30s exactly.
But when I used it the first nights, I did not see this rounding effect. I see it at first with SC 12572 but that may be by chance.

Thanks for your help!

cheers,
Mike

[admin]

Hi Mike,

I think that it is likely the F1/F2 trick that is causing the exposure to be a bit out - when you click left and right of the slider, it halves (or doubles) the exposure, which is usually exact. When you use F1/F2, it requires four presses to halve/double, so each press adjusts by a factor of sqrt(sqrt(2)) - roughly 1.189207115. That calculation will lead to an exposure value that the camera cannot represent exactly (the exposure is measured in microseconds on the Player One camera) so it gets rounded and there is no guarantee that the opposite change will exactly put things back to the starting value.

I still couldn't quite work out why the live stacking total time was going to 2m29s, so I checked the code - it looks like it was rounding the number of seconds down rather than rounding to the nearest whole number, so even if the total time was 149.999999s it would display 2m29s not 2m30s. I will fix that!

cheers,

Robin

[MikeB838]

Hi Robin,

ok, thanks for the explanation.
If F1/F2 would use a factor of 2 (as the click on the slider does), that would not happen, but I fear that is not intended (factor 2)?
But I can live with it.
And thank you for the coming fix!

cheers
Mike

[admin]

Hi Mike,

yes, you're right - a factor of 2 is not going to show the same problem (at least not until you get down to the point where the number of microseconds needed to halve the exposure stops being a whole number). However, the keyboard shortcuts are deliberately set to a finer step so that you can get pretty good control with the keyboard if needed. Click left/right of the slider gives you a factor of 2 because you can always make finer adjustments by sliding the slider with the mouse if you are using the mouse anyway.


cheers,

Robin

[MikeB838]

Hi Robin,

I see and that would speak for an other way to restart the camera
Or to jump by 2 with F1/F2 together with Ctrl oder Shift? But this is only a workaround.

It saves me a lot of time during the evening, when I can restart my camera (that was, why I now used F1/F2). The best way may be when Clear (live stack) is pressed, that if Ctrl or Shift is pressed together, not only the stack but also the camera is restarted (as it is with F1 or F2)

I hope for less hectic on your side!

Have a nice weekeend

cheers
Mike