RAW8 or RAW16?

[Moonstruck]

Hi,
For planetary imaging, is it best to use RAW8 or RAW16? I heard that RAW8 was better for planetary because it allowed for a higher FPS. Is that true? I also heard it is best to use the "high speed" setting when planetary imaging. Please also let me know if that is correct.

Also, if RAW8 is best for planetary, is RAW 16 better for deep sky?

[Menno555]

This is correct on all counts

Menno

[Moonstruck]

Ok, thanks for confirming.

[Maxchess]

I just tested my ASI462MC using both RAW8 and RAW16. My FPS results tell me I should use RAW16 to capture more detail:

Fps 10ms exposure 5ms exposure
Capture Area RAW8 RAW16 RAW8 RAW16
1024x768 100 90 190 90
968x548 100 100 200 125
For exposures of 15ms & above RAW16 fps = RAW8 at the maximum.

USB3 connection to an SSC disk.

You can see that with a 10ms exposure the RAW16 fps was only a bit down on the maximum theoretical (90 vs 100) for the 1024x768 capture area. For 5ms exposures it dropped off more.
However I am likely to capture 968 x 548 at 10ms so there is no penalty. The file size is of course twice as big
According to the Sharpcap web site: "Higher bit depth may give more information and more dynamic range if images are low noise."

So space allowing, I assume I should capture at RAW16, which should allow a higher gain for any give exposure length given the greater bit depth. (actually RAW10 or RAW12 for this camera).
Views

Max

[admin]

Hi Max,

for high speed imaging where you are using high gain (to enable short exposures) there is no benefit to using 16 bit captures.

The noise in the image caused by the high gain is so large that the variation in brightness from frame to frame at a particular point is *many* 8 bit ADU levels, probably many 10s of 8 bit ADU levels. If you switch to 16 bit captures, the noise will be many 100s to many 1000s of ADU levels in size. You end up doubling your file sizes, but all you are doing is recording the noise in more detail.

cheers,

Robin

[Maxchess]

Robin,
Thanks, very helpful clarification. RAW8 it is.
Regards
Max

[donstim]

What would be considered high gain i in this discussion? I can easily get high frame rates with a small ROI with a gain around 200 with the ZWO 224 camera. Is this considered a high enough gain to cause the noise levels to be high enough to recommend using 8 bit over 16 bit captures? How about 100 gain?

[admin]

Hi,

the simple test is to use the 'Pixel Value Readout' tool (in the tools menu).

Put the crosshair on your target on an area that is all roughly a single brightness, leave it on one spot and watch the values. The variation you are seeing is going to largely be caused by the random variations in numbers of photons captured each frame. If the numbers are nearly constant (varying by only 2 or 3 or maybe 4 between minimum and maximum) then you might get some benefit from RAW16. If they are almost entirely constant - same every frame or only varying by +/-1 then do use RAW16. If they are varying by 5 or 6 or more then I would say there is no point.

I just tried with a ZWO camera at 200 gain in the mid range brightness area of an image, and the variation was at leats 15, so that would be a definite RAW8 situation.

cheers,

Robin