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experimenting camera settings


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Hi folks......pretty hazy skies here lately so I haven't been able to try for some Jupiter videos.

I have the ZWO ASI120MC camera and using the  supplied all sky lens have been experimenting with different settings indoors.  On a 155 second capture of 8000 frames I used 2x binning and got 51.5 fps. I had set to a 50 percent histogram I wonder if this will be helpful   when I can have another Jupiter session .

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Hi Roger,

I replied to another post concerning fps and settings using Firecapture and I've repeated the relevant points below and added others applicable to colour cameras. It applies to any capture software.

Don't use binning as the image of Jupiter will be quite small and you want to get as much detail as you can.

As Jupiter rotates relatively quickly limit your videos to 30 to 45 secs duration hence the need to get your fps as high as possible. 

Frame rate is set automatically to be as high as possible. Frame rate will be limited by the processing power of the computer running Firecapture and the camera itself, but is initially  determined by the exposure duration. How many exposures will fit in 1 second. It's the reciprocal of the exposure. An exposure of say 10mS or 0.01 seconds gives a maximum frame rate of 1/0.01 fps which is 100 fps. A 5mS exposure gives a maximum of 200fps.

You won't achieve these maximum framerates if the full sensor is used as the camera will not be able to process the full frame data fast enough. A smaller ROI big enough in which to fit  Jupiter will enable higher framerates to be achieved, up to the exposure determined maximum, though the camera may max out at 250fps or similar where an even smaller ROI doesn't help.

It's only the height of the ROI that determines the frame rate available, so the ROI width can be left at maximum if you wish, for no loss in fps. This means you can have a wider image to get some of Jupiter's moons in thepicture, if your camera is orientated correctly, without causing a reduction in fps.

Don't enable gamma when capturing the video, leave it unticked or at 50 which is off anyway. Gamma processing each frame takes considerable processing time so your frame rate may suffer. You can easily gamma adjust the final image later in PS or whatever you use.

Aim for an exposure around 5mS if possible, and adjust the camera gain to get the histogram maximum around 70% of full. Having a very high camera gain is not a problem and will benefit from lower read noise. Although the preview may look very noisy, once several thousand frames are stacked in post processing this noise will disappear.

Check that the histogram peak corresponding to the 'black' sky background is not butted up against the left edge of the histogram and that the left side of the peak is just visible. Set the camera gain first, and then the camera offset to set the 'black' background peak position on the histogram.

Don't enable debayering while capturing the video as that will drop the framerate quite a bit. Your stacking/processing software will do the debayering. Firecapture lets you debayer the image for previewing but automatically disable it while capturing the video.

Also, you may as well use 8-bit capture and enable the 'high speed' option in the Firecapture camera setup panel if it's available. This will get the highest framerate. 'High speed' uses less bits for conversion like 9 or 10 bits which you might as well use if you're saving in 8-bit.

Stacking at least 100 8-bit frames will gain you around 4 extra bits of image depth so will be equivalent to a 12-bit image. This is only if noise is present in the individual frames, which there will be. Here's the theory behind this by Craig Stark The Effect of Stacking on Bit Depth, which is an interesting read.

If you have the USB2 ASI120 then you may struggle to get high frame rates, compared to the USB3 version.

Hope this helps. 😀

Edit: I forgot to mention about using the optimal focal ratio to maximize your potential image resolution. The optimal focal ratio is 6 x the pixel diameter in microns for a colour camera and 3 x the pixel diameter for a mono camera. For your ASI120MC this means optimal focal ratio is 3.75 microns x 6 = f22.5

So use a barlow or powermate on your setup to get somewhere close to this number. 

The maths behind this is explained clearly in this article posted by a member of the forum. It's in Dutch but Google translate works very well. 😀

For focusing I focus on a nearby star with a bahtinov mask though others have focused using a mask on Jupiter's moons.

Alan

Edited by symmetal
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All good advice above, I’d just reiterate the importance of not aiming for a pretty preview image. During the capture the image you will see on the laptop screen will look awful ( dim, b&w, noisy, jittery) but it doesn’t really matter what it looks like, as long as you’re in focus. 

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