Exposure vs FPS

[brisguy]

I am still quite new to using SC and I am getting confused about proper exposure settings for planetary imaging work. My impression after reading postings is that faster FPS is better, but I don't understand the relationship between FPS and exposure. I would expect that if there were no limitations in the camera or the storage device, FPS would always be the inverse of the exposure. However, in my recent experience it is obviously being affected by other things. I know some cameras have maximum FPS and that smaller image sizes cause smaller files resulting in faster file writing. Recently however, I have found that my FPS varies but up to 2:1 even though my exposure, ROI, and bit-depth settings are the same. E.g. one time, I get 40 fps, another 75 fps. So my questions:

1. What can cause the FPS performance to change even though exposure, ROI, and bit-depth are the same?
2. If the FPS is significantly slower than 1/exposure, are the exposure rates still the same or do they slow to 1/fps?
3. If the answer to (2) is that exposure time is always maintained, why does FPS matter, other than taking longer to capture a sequence?

Thanks!

[admin]

Hi,

In general, FPS is limited to the lower of

1) inverse of exposure
2) a limit set by the rate that image data can be transferred to the PC (depends on resolution, bit depth, USB speed parameters, etc)
3) the rate at which data can be written to disk if you are capturing
4) (rarely) the rate that your PC can process the data - only relevant on low performance PCs.

Note that 2 can also be affected if you are sharing the USB path back to the PC with other devices (for example via a USB hub).

A change in FPS will *NOT* influence the exposure time of the images, but a change in exposure may influence the FPS.

High FPS and short exposures for planetary imaging are recommended because short exposures freeze the seeing, allowing the lucky imaging frames to work well when they occur in times of steady seeing. High FPS allows data to be captured quickly - this is particularly valuable for Jupiter with its fast rotation rate meaning that if you capture a long video you can get blurring due to rotation in the final stacked image.

Hope this helps,

Robin

[brisguy]

Very helpful! Thanks for confirming my understanding about exposure. Since lucky imaging is based on exposure time, I could never understand why FPS was so important (for getting the best images). I also had not considered how little time it would take for Jupiter's rotation to smear an image, but just worked out the number for my sensor and lens setup and realized that it could happen in as little as two minutes! I guess I'll have to look into Winjupos as well. Saturn's rotation rate is just as fast, but I guess that is not an issue since there is no variation in the bands?

Also, I did some experiments yesterday and found that the issue with the varying FPS rates was due to the USB speed control! I had never intentionally set this control before and the main camera control manual page does not mention it, so I had ignored it. Found that setting it to 2 for my SV305 got my rates close to the published limits. So I will watch that control closely from now on - it got bumped at some point.

Thanks for making SharpCap and supporting it so well!

[turfpit]

Brisguy

Read this viewtopic.php?f=35&t=2590 to assist with systematically understanding your cameras exposure and USB controls and the impact on frame rate.

https://www.thelondonastronomer.com/it- ... ry-imaging for some info on planetary rotation and maximum capture time. Plus a get you started with WinJupos.

Jupiter capture/process doc viewtopic.php?f=35&t=1156

Dave