Setting up master dark libraries

[Bikerdib]

Both of my cameras are cooled so I want to set up master dark libraries for both with Sharpcap 3.0 (what I use to do NRTV and saving of the images). I know the darks should be taken at the same resolution, color space (RAW 16 is what I use), gain, exposure and of course sensor temp as I will use to capture the actual images at the scope. But what about the color and brightness settings, do they need to be the same? I'm referring to the red, blue, contrast and brightness sliders in the image control panel.

The cameras are a ZWO ASI224MC-Cool and ZWO ASI294MC-Cool

[frankieusn]

I could be wrong but, it's my understanding that the color, contrast, brightness, options only change the way the image looks on the screen and not the source image itself. They do not matter for dark acquisition and are only provided to give the best display of your target on your particular screen. It's more of a compensation for your monitor. The other settings you already mentioned, gain, exposure, and format are critical however.

[Bikerdib]

That is my take too from reading the manual but I want to make sure by getting responses from those in the know. I want to make use of my cloudy days (and nights) by setting up a range of darks. I don't want to later find out that the darks are not correct if the color balance and brightness settings do matter when taking darks.

[turfpit]

I use a ZWO ASI120MC. In the SDK, ZWO's Brightness is the offset http://www.zwoug.org/viewtopic.php?f=17&t=6756. So for me, I always take darks with the same Brightness setting as my lights.

For anyone coming across this for the first time see
https://docs.sharpcap.co.uk/2.9/#Using% ... %20Quality.

The terms Brightness/Offset are used. When the manual gets updated I will change this reference to Brightness/Offset/Black Level.

For deep sky I use this setting to move the histogram from the left hand side and in planetary/lunar set it to zero.

In Altair cameras this setting is know as the Black Level. Nebulosity refers to it as Offset (so I suppose some cameras have an Offset setting which serves the same purpose).


Dave

[Bikerdib]

Thank you for the input. I do use the brightness setting as applicable to ZWO cameras (offset or black level in others) to position the curve in the histogram so it begins just to the right of the left edge and this of course varies by the object.

I was hoping that I could make up the dark library inside while it is cloudy but since I cannot find what the brighness level needs to be without some more time out at the scope, I guess I'll have to wait. I had been using an Infinity with that software so I haven't had much deep space time with my new cameras.

[frankieusn]

With my ASI120MC (native driver) in auto exposure and auto gain mode the camera has a setting to allow a user defined target brightness. Making changes to the brightness slider (not the target brightness) has no effect on the gain or exposure settings when changed. If brightness/contrast etc actually affected the output capture then changing those options in auto mode would cause changes to exposure and gain because the camera would be trying to maintain the target brightness. Since it does not, I still think brightness and contract have no impact on dark image computations and they just change the way the image looks on your screen similar to change the histogram settings. (Happy to be corrected.) We might be talking about different settings. The only settings that should matter for darks are under the "camera settings" section. The items under "image controls" and "display controls" should not affect darks.

[admin]

Ok, so here's my advice on setting up for darks (or setting up in general)

1) The brightness/offset controls do not have to be the same for the darks and the lights - any difference between the two just comes out as a constant offset to the dark subtracted frame which will easily be removed when you stretch the final image. It's best however to make sure that the brightness/offset in the darks is *lower* to avoid getting negative results when subtracting. Set the brightness/offset just high enough to ensure that the histogram is not touching the left hand side of the graph.

2) Don't touch colour balance/gamma/contrast controls - leave them at their defaults. For most cameras these controls perform a digital transform of the image, and it's best not to do that in the individual sub-exposures. Leave all of that sort of thing until after stacking. If you want things to look brighter on screen, use the 'Display Gamma' control (SharpCap 3.0) or the mini histogram level stretch (SharpCap 3.1) - this just affects the display and doesn't do anything to the data being saved to file or stacked.

cheers,

Robin

[frankieusn]

Thanks Robin, I didn't know that "brightness" was code word for "offset". Is that the same offset that I see under the ASCOM properties? That's the only place I've ever seen offset and was wondering why it seemed to be missing from the native driver and the DirectShow driver that ZWO supplies.

[Bikerdib]

Thanks Robin.

Quote: "It's best however to make sure that the brightness/offset in the darks is *lower* to avoid getting negative results when subtracting. Set the brightness/offset just high enough to ensure that the histogram is not touching the left hand side of the graph. "

So from this statement I have to assume that you mean to set the brightness (aka offset) when capturing a light frame of an object and then when doing the dark frames at some future time, I will set the brightness to that same figure. This means that until I know what the brightness setting for the lights needs to be, I can't know what the setting needs to be for a dark. If I were to use the histogram to set the brightness with the camera capped, it would be necessary to crank the brightness way up to get the curve just off the left side would it not?

Sorry for these beginner questions but I've gotten conflicting info on other forums and don't want to waste a bunch of time or get results less than those that are possible.

[admin]

Hi,

yes, so brightness and offset are basically the same control - both just add a constant value to all the pixel values in the image.

A workable technique would be as follows:

Setup to take darks, adjust the brightness to make sure the histogram is a little clear of the left hand edge, note value and take darks. This value will almost certainly be fine for taking the lights too, since they will have sky glow which will lift the histogram even further clear of the left hand side.

If you needed to you could use a *higher* brightness value for taking the lights without any serious ill effects - the only result would be a uniform background to the dark subtracted frames, which isn't a problem as it's easy to remove a uniform offset.

cheers,

Robin