Which camera settings are relevant for darks/bias cali frames?

[Revs]

Hi. After some saving I've taken the plunge and purchased a cold CMOS camera, the ZWO ASI294MC Pro. I've got the day off and I'm using this time to collect some dark/bias calibration frames. But I have some questions. I understand that the temperature and gain have to be the same for all frames and that exposure times of the darks and lights needs to match, but...

Which of following camera settings used to take darks/bias frames have to match the settings used for light frames...

USB speed - My understanding is slower means more amp glow, so will have to be the same, at least on the darks, right..?

Offset - do the positions of the peaks of the darks/bias frame have to be in the same place on the histogram as the lights?

Any input appreciated 🙂 also any other tips for a newby more than welcome!

[Astrosharkey]

I keep everything the same for all settings, and use -10c for all my imaging.  When I make a library i take 50 frames with the cap on in a dark place, all the same cables in the same ports and then when i process my first image with the darks let DSS stack them into a master frame.  I have a set for 350 and 500 gain (but that is my camera) and a range of exposure times that I might use.

[Revs]

4 minutes ago, Astrosharkey said:

I keep everything the same for all settings, and use -10c for all my imaging.  When I make a library i take 50 frames with the cap on in a dark place, all the same cables in the same ports and then when i process my first image with the darks let DSS stack them into a master frame.  I have a set for 350 and 500 gain (but that is my camera) and a range of exposure times that I might use.

Cheers. Did you spend much time finding a good offset setting for each gain setting or just go for a 'safe' amount for both?

[kirkster501]

Why should the USB speed cause AMP glow?  USB is a digital medium to connect two devices together.  It should have no effect at all on the media being transferred between the two devices.

[Revs]

4 minutes ago, kirkster501 said:

Why should the USB speed cause AMP glow?  USB is a digital medium to connect two devices together.  It should have no effect at all on the media being transferred between the two devices.

I'm probably wrong but I'm sure a read that amp glow was caused during the transfer of data, so the faster you can get that done the less amp glow.

[symmetal]

By USB Speed do you mean using USB2 vs USB3 or the USB Speed setting in the driver.

The latest cameras use memory in the camera to read out and store the images quickly before transferring them to your PC via USB. Earlier cameras without this memory had issues with excessive amp glow using USB2, mainly with short exposure imaging. So now USB2 just means it takes longer to download the image and doesn't affect the quality. The USB speed setting in the driver is to avoid sending the data too fast that the PC can't cope. If set too high you may get dropped frames or no image at all.

As many CMOS camera have 'amp-glow', (the later more expensive ones don't) bias frames are not very useful as you need to use darks to calibrate your lights and flats. The darks already contain the bias signal so using darks takes care of bias and amp-glow.

The position of the peaks in the histogram don't need to match the lights. The light peaks will always be higher than the darks as extra light is entering the camera causing the peak to move to the right with increasing exposure.

The offset needs to be the same setting for all your images, lights, darks, flats, and bias (if used). The offset value used is to avoid getting any image values at zero which means clipped data. If different offset values are used then you can't calibrate the data correctly.

Alan 

[Revs]

13 minutes ago, kirkster501 said:

Why should the USB speed cause AMP glow?  USB is a digital medium to connect two devices together.  It should have no effect at all on the media being transferred between the two devices.

OK just confirmed it makes no difference. Tried speed of 40 compared to 80 and they're just the same. So USB speed is sumply to get the transfer done faster. Must have mis-read or mis-understood what I was reading.

[symmetal]

19 minutes ago, Revs said:

Cheers. Did you spend much time finding a good offset setting for each gain setting or just go for a 'safe' amount for both?

Varying offset values for different gain settings is more trouble than it's worth. Just use one offset value that avoids black clipping at any of the gain values you may use. The gain in dynamic range by using a smaller offset value is negligible.

Alan

[Revs]

3 minutes ago, symmetal said:

By USB Speed do you mean using USB2 vs USB3 or the USB Speed setting in the driver.

The latest cameras use memory in the camera to read out and store the images quickly before transferring them to your PC via USB. Earlier cameras without this memory had issues with excessive amp glow using USB2, mainly with short exposure imaging. So now USB2 just means it takes longer to download the image and doesn't affect the quality. The USB speed setting in the driver is to avoid sending the data too fast that the PC can't cope. If set too high you may get dropped frames or no image at all.

As many CMOS camera have 'amp-glow', (the later more expensive ones don't) bias frames are not very useful as you need to use darks to calibrate your lights and flats. The darks already contain the bias signal so using darks takes care of bias and amp-glow.

The position of the peaks in the histogram don't need to match the lights. The light peaks will always be higher than the darks as extra light is entering the camera causing the peak to move to the right with increasing exposure.

The offset needs to be the same setting for all your images, lights, darks, flats, and bias (if used). The offset value used is to avoid getting any image values at zero which means clipped data. If different offset values are used then you can't calibrate the data correctly.

Alan 

Thanks for that Alan, much appreciated

[Astrosharkey]

I use an offset of 50 for all my imaging, tried 30 but it clipped so now just stick to one setting for all