Reference of timestamp

Using SharpCap for other Astro Imaging such as all sky cameras and meteor detection
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michastro
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Joined: Wed Jan 22, 2020 12:10 pm

Reference of timestamp

#1

Post by michastro »

Hello,
I use SharpCap for occultations, and I have to make quite long exposure time for faint stars. I use the inserted timestamp, but I would like to know if the timestamp inserted is the beginning, middle or end of exposures?? Of course logicaly it should be middle of exposure, but...
Thanks
Michel Meunier
http://team-janus.astrosurf.com/index.p ... janus-sud/
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admin
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Re: Reference of timestamp

#2

Post by admin »

Hi Michel,

Sadly the answer to this question is 'it depends'. Assuming that you are using a normal camera (not a QHY GPS camera), then the timestamp is usually the time at which SharpCap received the frame from the camera, which will be a point in time after the end of the frame.

Unfortunately SharpCap has no idea how long it takes for the image to be transferred from the camera to the computer and then handed over to SharpCap from the camera manufacturers code. Typically four USB3 CMOS cameras, the delay is small (much less than a second). For CCD cameras or USB2 CMOS cameras being run at long exposures the download time to the computer may be several seconds.

The exception to the rule about the timestamp being after the end of the frame is if you are capturing to FITS format in a recent version of SharpCap 3.2. In those circumstances, SharpCap will attempt to estimate the start time of the frame by subtracting the exposure duration from the time that it receives the frame and putting that value in the DATE-OBS header of the FITS files. This change can be found in SharpCap version 3.2.6131 and newer versions.

Hope this helps, Robin
Ddaniel84
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Joined: Sun Apr 26, 2020 1:31 pm

Re: Reference of timestamp

#3

Post by Ddaniel84 »

Hi Robin
thanks for the precisions brought 2 years ago to Michel. I also use Sharpcap for occultations, but I use .ser files. We have to bring the more corrections as possible (some msec) to obtain the very precise time where the occultation occurs. So we have to evaluate each "systemics errors" generating a difference between the exact time of occultation and the time written in the file, for each frame. To evaluate the acquisition delay, we use GPS system with a LED blinking at very precise intervals. With ShapCap, we generate a .SER file. This file is analyse and the difference between GPS time and frame time is mesured. So it's very usefull to know what correspond to the written time. Is there the same reference as for .FITS (time where the data from camera arrive to SharpCap)? What is usefull else to know?
Thanks for your help, and bravo for SharpCap which is exceptionnal!
Cheers, Dominique
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Re: Reference of timestamp

#4

Post by admin »

Hi Dominique,

I just checked the code for SER file writing...

If you have a camera with onboard GPS that is active then the GPS start time for the frame becomes the SER frame timestamp.
For all other cameras, the time that SharpCap receives the frame is written as the SER frame timestap, which is after transfer to the PC and any processing in the camera SDK.

As long as you are on Windows 8 or newer, SharpCap uses the high resolution time API (GetSystemTimePreciseAsFileTime), which should give a stable precision timer as far as is possible - see https://docs.microsoft.com/en-us/window ... asfiletime

cheers,

Robin
procyon12
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Joined: Tue Jan 14, 2020 11:32 am

Re: Reference of timestamp

#5

Post by procyon12 »

Hi,

Have a look: http://www.iota-es.de/SC-Timestamps_01.pdf

Cheers

Christian
Ddaniel84
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Re: Reference of timestamp

#6

Post by Ddaniel84 »

A lot of thanks to both of you, Robin and Christian for your reactivity and yours so accurate informations!
I have now all the answers needed. I'll share these informations with 2 of my friends, also invested in occultations (Pierre et Jean-Baptiste).
Cheers, Dominique
From France (84)
Skyvision T250 Newton on CGX with PrimaLuce Sesto Senso motor focus - ZWO motor filter wheel
C8 XLT Evolution with Celestron Motor Focus
ZWO ASI 2600 MC pro - ZWO ASI 224 MCS
CPWI -PHD2 - Sharpcap - Siril - Stellarium - NINA
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