QHY174M-GPS calibration LED timing+width

[Kai Getrost]

I've been playing with QHY174M-GPS calibration for some time now, and trying to understand it as much as I can. At the moment, my questions involve the actual LED pulses: their number, width, and timing. I'm using 3.2.6383 (thanks for fixing the auto-load-from-profile of Start/End Pos BTW). Some background first.

I've noticed the following:

a) (EndPos - StartPos)/Exposure is nearly constant (when calibrated) across all exposure values -- about 75000, as others have observed too
b) When going from LED-off to LED-on, the counter values for when the LED just barely appears, and for when it first reaches full brightness, always differ by about 40 or so (this is somewhat subjective, and depends on histogram etc.), and then it doesn't get any brighter even for large further changes in the counter; again regardless of exposure
c) When looking at the exposed CMOS chip and the blinky LED, it always appears to stay on a very short amount of time (<= ~1ms, from a cell phone super-slow-motion video; best external timing I have)
d) When switching groups (from Start to End Pos or vice versa), we never see an "overlap" of the LED for both groups*. I.e. if we adjust Start Pos to LED-on, then switch to End Pos which is LED-off, we don't still see the LED-on from Start Pos.

From these I draw the following conclusions:

1) The counters are increments of a 75MHz clock (from (a))
2) The pulses are a constant width of ~0.5us (from (b) and (c), with math from (a): 40/75MHz)
3) There are two LED pulses -- one for Start Pos, one for End Pos (from (b))
4) But only one is ever lit at a time* (i.e. whichever group is being adjusted) (from (d))
5) When adjusting Start and End Pos, we are adjusting when the pulses occur, but never their width (from (b) and (c))

Are any of these conclusions incorrect? Thanks for any details you can shed on this.

-Kai

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P.S. *The start and end LED pulses can appear momentarily "at the same time", i.e. overlap, in the same exposure. This can happen for one frame, if we adjust End Pos to be lit during the same exposure that Start Pos was already lit. But the next and following frames, only End Pos will be lit, because both were never lit at the same instant; they were just lit -- one after the other -- during the same one-time exposure.

P.S. 2: There can be some wonky behavior that occasionally differs from the above observations. But that's another rabbit hole for another time.

[admin]

Hi,

yes, that's pretty much correct – in fact SharpCap has control over the width of the pulses and passes the value 40 to the SDK – in theory I could make that configurable by the end user, but I don't think there's any value to it.

As I understand it, essentially you are calibrating the delay between the on-board controller commanding the frame to begin and the frame actually starting and also the delay between the command for the frame to begin and the frame ending.

Cheers, Robin

[Kai Getrost]

Thanks Robin.