date and time in camera settings file

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calan
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date and time in camera settings file

#1

Post by calan »

Add the date and time to the camera settings file. File system date and time tags are not reliable or as convenient as having it noted in the file itself.
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admin
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Re: date and time in camera settings file

#2

Post by admin »

Fixed for next build.

Note that for videos the timestamp is the start of capture time, for single frames it is after the frame has been captured.

cheers,

Robin
alx
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Re: date and time in camera settings file

#3

Post by alx »

It was said that " for videos the timestamp is the start of capture time ''.
But in the settings text files which come with data files, I can find two different times : ''TimeStamp" and ''StartCapture''.
What is their exact respective meaning ?
On the other hand, SER files come with a time for each exposure. Does this time correspond to the beginning or the end of the exposure ?
Thanks by advance for answering.
alx.
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Re: date and time in camera settings file

#4

Post by admin »

Hi,

StartCapture is the estimated start time of the first frame that is in the capture file. This is estimated by taking the time that SharpCap receives the frame and subtracting the exposure length from it, so no account is taken of how long it might have taken to transfer the frame from the camera to the computer.

TimeStamp is more basic - it's just the UTC time at the time when the capture file starts being written - this was the original timing entry in the settings file, StartCapture is newer and more accurate.

SER file times are the times that SharpCap receives the frame from the camera, so some point after the end of the frame capture - how much determined by the transfer time.

If you want truly accurate frame times then you need a hardware approach (like the QHY cameras with built in GPS). Note that in the case of the GPS cameras, the SER times are the start of the frames (which seems wrong now I look at the code - I should make it consistent with both being the start time... - maybe for SharpCap 4.1)

cheers,

Robin
alx
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Re: date and time in camera settings file

#5

Post by alx »

Hi,

Thanks for answering so accurately.
I am using ZWO cameras without any hardware link, so I must only rely on software timing.

I observed that SER times in a given video are very well equidistant, in accordance with the likely regular timing of the camera. Therefore the main unknown remains the delay between the actual start of the first exposure and its receiving time at the computer.

An idea might be to rather timestamp the initial computer start command (e.g. the "ASIStartVideoCapture" call in the case of ZWO's software), since the corresponding USB transfer delay should a priori be very short. The camera being already prepared, such a command has also good chance to be executed immediately.

Regards,
alain.
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Re: date and time in camera settings file

#6

Post by admin »

Hi Alain,

based on experience with other cameras that do have the GPS info, I am fairly sure that the download time for the frame getting to the computer will depend on both the exposure length and the setting of the 'Turbo USB' control in a way that is complex enough to be almost impossible to work out without further input. Obviously it will also depend on the resolution/bit depth (amount of data to transfer) and whether you are working on USB2 or USB3.

Essentially there are too many unknowns - without the source of timing at the camera end, any estimate would be speculative and could be fine at one set of settings and horribly wrong at another.

I have heard that for some scientific measurements, people arrange for a set of LEDs to be in view that count the time in binary - these can run from a calibrated time source and potentially have fractional seconds showing.

cheers,

Robin
alx
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Re: date and time in camera settings file

#7

Post by alx »

Dear Robin,

There is indeed some GPS receivers with flashing LED synchronized with the PPS signal that could serve as a calibrator.
I agree with you that solving the problem by software only might be inextricable.

But my guess is that the sending of the video start command (the ''Live view On'' in your nice program) should occur very close in time from the actual beginning of the corresponding first frame. Comparing the starting time of the first frame to the ending (receiving) times of the next ones might be of some help.
Do you agree ?

Cheers,
alain.
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Re: date and time in camera settings file

#8

Post by admin »

Hi Alain,

just did a quick test - under some circumstances the delay for the first frame to arrive seems to be about twice (and a bit more) the delay between subsequent frames, under others the delay for the first frame is **less** than the delay between subsequent frames.

I think that this means that what goes on at video start is dependent on the previous state of the camera - for instance it could have been already in video mode and will just 'start' by sending frames to the PC as they are completed. I did try testing with the camera freshly plugged in and running SharpCap from scratch - that seemed to usually get me a somewhat fast first frame, followed by a second frame about 50% longer than expected, followed by regular frames (to within a couple of percent) after that.

This was testing with a ZWO 120MM-S on USB3.

To be honest, I don't think I'm seeing data here - just noise :(

cheers,

Robin
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