Setting exposure time

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nickw
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Setting exposure time

Post by nickw » Fri Nov 24, 2017 8:38 pm

I am a new user of SharpCap and the ZWO camera. If I select an exposure time of 60 seconds in the Camera control using the Quick Pick dropdown menu, the Frame Timer in the lower right corner begins counting and cycling through 60 seconds of time. If I then go to Capture/Start Capture and select a capture limit of one frame and start that frame when the Frame Counter in the lower right corner is at 50 seconds the result is a captured frame of only 10 seconds duration. How do I correct this? It seems I need to turn off the Frame Counter.

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admin
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Re: Setting exposure time

Post by admin » Fri Nov 24, 2017 8:41 pm

Actually, you are capturing the 60 second frame that had already started and only had 10 seconds left to run when you pressed the 'capture' button. SharpCap works a bit like a video camera in that it is taking frames all the time. When you choose to capture then those frames get displayed *and* saved to disk. When you are not capturing they just get displayed. Pressing the start capture or snapshot buttons doesn't disturb the underlying rhythm of frames (1 every 60s in your case).

hope this helps,

Robin

waruna
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Re: Setting exposure time

Post by waruna » Tue Oct 16, 2018 5:58 pm

Hi. Sorry if I digging old topic but I think its better than creating the same topic.

So, I’m just like TS about this concern. If I want to have 20 frames (60s/frame) but at the time the first frame is already pass 50s, do I need to set to capture 21 frames so that I can get 20 full frames with same exp?

Thank you.

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admin
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Re: Setting exposure time

Post by admin » Tue Oct 16, 2018 6:26 pm

If your aim is to capture 20 frames of 60s exposure each and the exposure is already set to 60s then you do not need to worry.

If your aim is to capture those 20 frames starting *NOW* - ie capture for the next 1200s from the point in time when you press Start Capture, then you could pick 21 frames to allow for the frame already in progress being captured.

cheers,

Robin

waruna
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Re: Setting exposure time

Post by waruna » Wed Oct 17, 2018 12:30 am

Ah okay. Understood :D

Thank you Sir

Candieman
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Re: Setting exposure time

Post by Candieman » Thu Nov 29, 2018 6:45 pm

I'm new to SharpCap and it took me a while to figure out that SharpCap was always capturing an image whether I asked for it to start a capture or not. I've seen that once I change the exposure time, the buffer gets cleared and SharpCap starts capturing a new image, which makes perfect sense. The issue I have is that if I decide to change the gain after setting the exposure time, the buffer does not get cleared, and the same image is still being captured. Because of this, I'm thinking that if I change the gain, I then should change the exposure time to something else, then immediately change it back to what I want, then a new image starts getting captured with my intended gain and exposure settings, and all is well. So my questions are: (1) Why doesn't SharpCap clear the buffer and start a new capture when the gain is changed like it does when the exposure time is changed? and (2) If I change the gain during a capture, will the resultant image be some combination of an image using the original gain and an image using the new value for the gain? It appears to me that changing the gain after setting the exposure time may be an egregious sin that ought to be avoided.

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Re: Setting exposure time

Post by admin » Sat Dec 01, 2018 1:31 pm

Hi,

this is largely a case of optimism on the part of SharpCap – a number of controls on many models of camera will apply the value that is active at the end of the frame to the frame being captured, so adjusting the value of some of these controls (for instance frequently colour balance controls) during the frame will have the expected effect of the adjusted value being used for the frame currently being captured. Unfortunately not all controls and work this way and in particular the exposure control doesn't work this way, which is why SharpCap treats exposure specially and will reset to start a new frame if you change the exposure when the existing exposure as above about one second.

Given the large number of models and makes of camera that SharpCap supports, especially when you include Ascom and direct show cameras, it's impossible to SharpCap to be sure which controls take immediate effect and which only take effect at the beginning of the next captured frame (this sort of thing is rarely, if ever, documented by camera manufacturers). As such it comes down to the user working out for their particular camera whether adjustments would take effect instantly or not.

Cheers, Robin

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turfpit
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Re: Setting exposure time

Post by turfpit » Sat Dec 01, 2018 2:03 pm

A good example of this is when I switch from capturing lights to darks. I have learnt from experience that once the cap is put on the end of the telescope I wait until after the completion of a frame before I start to capture a darks set. This is because the exposure is remaining the same. Failing to do this left me with a tainted first dark frame.

Dave

Candieman
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Re: Setting exposure time

Post by Candieman » Sat Dec 01, 2018 9:21 pm

Thanks for the info. It looks like SharpCap acts like I thought it did. I've started to try to build into my DNA the thought process that the last thing I do before starting a capture is to set the exposure time to some arbitrary value, then set it to what I want it to be. I guess it's because I'm one of those impatient type of people. If I put the cap on for darks and I see there's still a couple minutes to go in the current capture, I'll reset the exposure to some other time, then reset it back to what I want it to be, and then I'm good to go without having to wait the full two minutes.

Rich

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