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QHY High precise time measurement

CMOS Equipment
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#1 jlipinski

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Posted 22 January 2025 - 05:41 AM

Hi!

First of all, I'm not entirely sure if this is the right forum for this issue, but a quick search showed that there are several QHY-related threads here. I hope this post isn't considered spammy.

I recently acquired a QHY 268M Pro CMOS camera. I’d like to use it for more scientific purposes, which require precise time measurement. The camera is equipped with a 6-pin GPIO port.

Based on what I’ve found (specifically, the "GPXBOX-QHY600Pro High Precise Time Measurement" section on this page: https://www.qhyccd.c...y600pro-imx455/), it seems I can use the ERIS Framework to switch the GPIO port to Mode 2. In this mode, I am able to use a custom-made circuit board to record the time of the rising edge of the TrigOut pin. If I understand correctly, the rising edge of TrigOut marks the end of the exposure of row 0.

Additionally, I can measure the signal period from the LinePeriod pin using an oscilloscope. This measurement should provide the time offset between two rows due to the rolling shutter.

In theory, this would allow me to determine precise exposure timing as follows:

Use the TrigOut signal to get the time of the end of exposure for row 0.
Subtract the exposure time to calculate the start time of exposure for row 0.
For row N, multiply the LinePeriod by N and add it to the exposure start time of row 0.
Does this approach sound correct?

 

T_end_0 = T_trigout
T_start_0 = T_trigout - ExpTime

T_start_N = T_start_0 + N * LinePeriod

T_end_N = T_end_0 + N * LinePeriod

 

But something seems off. I set the exposure time to 500 ms and started a continuous, unlimited capture in SharpCap (which shows 2.0 FPS), but the period of T_trigout is slightly longer than 500 ms.

Additionally, there’s a hand-drawn diagram in the section I linked above, which I don’t fully understand. However, I have a feeling it might help solving this issue.

Does anyone here have experience with precise exposure time measurement?


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#2 dghent

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Posted 22 January 2025 - 08:25 AM

Does this not seem like a time where one should reach out to QHY directly regarding such technical details?
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#3 Yilia Yu

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Posted 22 January 2025 - 11:45 PM

Hi, please submit a ticket to QHYCCD help center: https://www.qhyccd.com/warranty/


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#4 StellarWanderer8055

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Posted 23 January 2025 - 08:21 AM

Your approach seems solid, but the slight discrepancy in T_trigout could be due to readout overhead or buffering delays. Rolling shutter cameras often have a small delay between exposures, especially in continuous mode.

 

Have you tried measuring the actual LinePeriod instead of relying on theoretical values? The hand-drawn diagram might indicate additional timing offsets—perhaps related to the camera's internal processing. If possible, testing with a hardware timestamping method (e.g., GPS-based trigger) could help validate the timing accuracy.

 

Would love to hear if you find a solution!


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#5 jlipinski

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Posted 23 January 2025 - 09:01 AM

I agree with dghent and I appreciate Yilia Yu's answer. The truth is that I've already asked QHY support about this. Unfortunately, due to (I think) language barriers and vastly different time zones, the conversation with QHY isn't as fruitful as I hoped. I don't want it to be taken as something negative; I have only good things to say about the quality of QHY customer support; it's just the communication layer and getting the point across in such a technical topic that is difficult.
 

As I wrote, I have measured LinePeriod using an oscilloscope; I don't really have any ideas how to measure it from captured frames. I know that this camera has GPIO mode that supports LED calibration, but I have no idea how to use it. 

I have connected a (borrowed) GPS receiver and denoted the exact (up to 100 ns) time when T_trigout happens, then compared them. It seems that the time between each pulse is 500.1997 +/- 0.001 ms. Unfortunately, I had to return the GPS before I could test different exposure times than 500 ms, so I don't know if this 0.2 ms offset is constant or dependent on exposure time.

I agree that there are probably some additional timing offsets, probably stemming from optically black and overscan rows - as long as those offsets are constant, I can later do some timing and astrometry and calibrate those time biases. The most important part is to find every time offset that is not constant and is dependent on things that would change between observations, like region of interest or exposure time.


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