Live View sometimes not honoring the exposure time, counting down well past "0"

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BobHehmann
Posts: 17
Joined: Thu Sep 16, 2021 11:17 pm
Location: Moorpark, California, USA

Live View sometimes not honoring the exposure time, counting down well past "0"

#1

Post by BobHehmann »

I've run into a minor anomaly - it occurs frequently for me, but not always. Live View will sometimes keep an exposure going well beyond the current SharpCap exposure setting - the exposure progress counter will keep going up, while the remaining-time counter will go negative. Once doing this, it seems to consistently repeat the error, until the errant Live View is stopped. Cycling through Still Mode, and then restarting Live View, usually (always?)fixes the exposure time.

It seems to be associated with reducing the exposure time while in Still Mode. For example, Live View @8 second exposure, shift to Still Mode, reduce exposure to 4 seconds, initiate Live View - exposures are still running at 8 seconds. The "Time remaining" counter will start each exposure at 4.0 seconds, but will run until ~ -4 seconds. Revert to Still Mode, then back to Live View, exposures are now the correct value. Anecdotally, when the initial longer exposure time is large, the excess exposure seems to cap at around 10 or so seconds.

For example, upon completing an imaging sequence run by the Sequencer, with exposures of 10 minutes, the camera is left in Still Mode. I then reduce exposure time to 2-4 seconds with increased gain, perhaps to support plate-solving on a new target, or an auto-focus tweak. I then start Live View - exposures are often 12-14 seconds long, with the time-remaining counter going down to -10 seconds. Still Mode, then Live View: all is good.

Is this a known issue?

I've seen this with multiple versions of SharpCap, including the current 4.0.8949 and at least several month's worth of prior releases. OS in always fully current (patches up to date) Win10 Pro. Camera in a ZWO ASI071MC, running the current ZWO camera driver (not an ASCOM camera driver).
Cheers, Bob
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Re: Live View sometimes not honoring the exposure time, counting down well past "0"

#2

Post by admin »

Hi Bob,

the exposure time remaining goes negative when the frame is overdue - this can be normal for some devices where the image readout takes time... For instance if the exposure is set to 10s and the download takes 4s then the countdown will go to -4 for each frame. Another cause of this can be if the camera is dropping frames. Again with an exposure at 10s, if the camera skips a frame for one reason or another then it may count to -10s while waiting for the next frame.

I would expect this to possibily happen on the frame in progress when the exposure is changed under some circumstances, but not repeatedly for subsequent frames. If it is doing that, let me know and I will investigate further to see if I can work out why.

cheers,

Robin
BobHehmann
Posts: 17
Joined: Thu Sep 16, 2021 11:17 pm
Location: Moorpark, California, USA

Re: Live View sometimes not honoring the exposure time, counting down well past "0"

#3

Post by BobHehmann »

Thanks sir! When I'm next at the scope, I'll see how short an exposure time I can request before drops/delays are introduced, as an estimator of frame transfer rate. But in real life, unless I am intentionally forcing things to a situation that will drop every frame, a random drop is an exceptional rarity. Camera, cabling, and active powered hub on-mount are all USB3 compliant.

Fortunately, this is highly repeatable for me, and in fact recurred at the first opportunity during my imaging session last night. A Sequencer-managed imaging run taking 90-second exposures had just completed. I manually reduced exposure time to 4 seconds in preparation for the next target's SC plate-solve/refine/synch operation, and I then engaged Live View. The 4s exposures ran 14.x seconds i.e., -10.x "overrun". There might have been a little fractional-second variability, but never a -9.x or -11.x overtime. After 5 or 6 repetitions, I stopped it by returning ti Still Mode, then reengaged Live View, and all was well. That is quite typical.

The overruns seem to be the lessor of ~10 seconds and the difference between the two exposure times: 8s reducing to 4s will expose for ~8s (~4s over); 12s to 4s runs for 12s (8s over); >=14s to 4s for 14s (10s over)...

This is at most a minor infelicity, as the work-around is quick, easy, and reliable. Let me know if there are any particular tests you'd like me to run.

Cheers, Bob
Cheers, Bob
BobHehmann
Posts: 17
Joined: Thu Sep 16, 2021 11:17 pm
Location: Moorpark, California, USA

Re: Live View sometimes not honoring the exposure time, counting down well past "0"

#4

Post by BobHehmann »

I did a quick check on camera throughput - I can sustain ~8FPS for the ASI071MC Pro. At about 2FPS, SC will use 2, sometimes three buffer frames if writing to disk. Faster than that, then disk-write speed and SC frame buffer size become the limiting factor. In practice - never an issue.

Cheers, Bob
Cheers, Bob
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