Settings FPS vs. exposure time

[PaoloFini]

Hi and good night.
I have utilized the version 4.14310 PRO version for .SER recording on occultations event with QHYGPS camera. For many years actually so far without issues.
My last utilization I was setting 75mS exposure time (expected about 13.3FPS). However what happened during .SER post processing I discovered that exposure time was 138mS (about 7.2FPS) or at least each frame was every 138mS.
Question:
- timestamp is start, middle, end for this GPS special setting?
- first time this is happening and first time I used banding suppression=30.. is this related? How banding can "modify" exposure time without operator awareness?
- also USB utuilization was accidentaly set to 98% rather 90% even this should not affect in this way, isn't it?

Attached is the camera setting file.. inconsistency between FPS vs. Exposure tIme (shoudl they match?). Thanks for a clarification
Thanks.
Paolo F

[admin]

Hi Paolo,

the FPS you get comes from the higher of two times:

1) The frame exposure time (presumably plus some very small amount of time needed between one frame finishing and the next starting)

2) The time taken to transfer a frame from the camera to the PC via USB

Whichever is bigger of those will determine the frame rate by the usual formula of frame rate = 1.0 / <time>.

For the frame rate drop that you were seeing, it seems most likely that the second time was unusually long for some reason - a high setting on the USB Traffic setting perhaps (remember that for QHY cameras, lower USB traffic settings give faster frame rates and higher ones give lower frame rates). The other alternative would mean that exposure was nearly twice what was expected, and that doesn't seem likely.

The timestamps in SER files written by SharpCap are selected as follows (the first available data is the one chosen for the timestamp)

1) The GPS derived time for the frame start for GPS cameras with lock
2) The estimated time for the frame start based on the time SharpCap received the frame less the known exposure
3) The time SharpCap received the frame (in the case that the exposure is unknown)

So, your SER file will have GPS timestamps for the frame start times. Unfortunately SER only has space for one timestamp per frame - if you save some frames to FITS if you see this issue again, SharpCap can write multiple timestamps into the FITS headers for start, mid, end frame (and FITS comments explaining the data source for each value).

Hope this helps,

Robin

[PaoloFini]

Thanks Rob;
so there is no effect on this timestamp and frame duration topic whatever is the band suppression setting.
All the best.
regards

[admin]

Hi,

sorry, forgot to mention that - no, the banding suppression has no effect. Banding suppression is a digital processing step applied inside SharpCap after the frame has arrived from the camera. All of the timestamps are set up as the frame is arriving, so those will not be affected by having processing active or not.

I should point out though that processing may slow down the captured frame rate - if the CPU can only process 7 frames per second, but the camera is providing 15, 8 frames per second will be dropped. I don't think the banding suppression is likely to cause this though, since it is fairly fast to execute. Some processing steps (most notably satellite trail removal) can take a significant amount of time and are most suited to longer exposures for deep sky. You would see any effect by watching the frame rate shown at the bottom left as you turn on/off various processing options - if the frame rate drops when you turn something on then the effect described above is happening.

cheers,

Robin