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ASI 1600 PRO - Bias recommendation

astrophotography CMOS equipment
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#1 AstroCatinfo

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Posted 13 January 2018 - 04:17 AM

Good morning,

 

I recently upgraded from my old ASI1600MM-C v2 to the new PRO version (you can read the review here: https://www.cloudyni...ading-version2/ ).

 

The only odd thing I found was that at high Gain (bin1) the Bias showed a banding patern. For that Bias I used 0" exposures and it resulted with a random banding difficult to calibrate. Following Jon's recommendations I repeated my Bias library, now using 0,1" exposures. Problem has been completely solved. Thanks again! 

 

I add an image that shows the problem and how it has been fixed:

 

300Gain bin1 Bias comparision January 2018 redu.jpg


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#2 Jon Rista

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Posted 13 January 2018 - 12:25 PM

Glad the longer exposures solved it. I am curious about those bands though. Even when I take 0 second exposures, I haven't seen bands with my ASI1600MM Cool. I get variable gradients, but not the bands. You might want to contact ZWO about it, even though you have resolved it with 0.1 second exposures, and see if there may be a firmware fix. 


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#3 AstroCatinfo

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Posted 13 January 2018 - 01:41 PM

Glad the longer exposures solved it. I am curious about those bands though. Even when I take 0 second exposures, I haven't seen bands with my ASI1600MM Cool. I get variable gradients, but not the bands. You might want to contact ZWO about it, even though you have resolved it with 0.1 second exposures, and see if there may be a firmware fix. 

Yes, I have done that and, even for me it's a "solved problem" (only appears at high Gain and I already got the solution), they are working on it to see if a firmware change can fix it. As always, fast and responsive. Very happy with ZWO service. 


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#4 Jon Rista

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Posted 13 January 2018 - 02:19 PM

 

Glad the longer exposures solved it. I am curious about those bands though. Even when I take 0 second exposures, I haven't seen bands with my ASI1600MM Cool. I get variable gradients, but not the bands. You might want to contact ZWO about it, even though you have resolved it with 0.1 second exposures, and see if there may be a firmware fix. 

Yes, I have done that and, even for me it's a "solved problem" (only appears at high Gain and I already got the solution), they are working on it to see if a firmware change can fix it. As always, fast and responsive. Very happy with ZWO service. 

 

Yeah, they are quite responsive and very eager. :p I think maybe a little too eager these days, maybe. :lol: Now that they are established, I think they could afford to throttle back a bit and crank up the Q.C. It would be good for their reputation.



#5 thompeters65

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Posted 13 January 2018 - 02:44 PM

Glad the longer exposures solved it. I am curious about those bands though. Even when I take 0 second exposures, I haven't seen bands with my ASI1600MM Cool. I get variable gradients, but not the bands. You might want to contact ZWO about it, even though you have resolved it with 0.1 second exposures, and see if there may be a firmware fix. 

Jon, I've read other posts of yours and I'm hoping you can help me. I have a ASI1600MC an OSC.  I've read in other threads that it's unnecessary to use bias frames with these low read noise CMOS astro cameras. Just darks and flats with the lights. 

 

What is your opinion on this?



#6 Jon Rista

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Posted 13 January 2018 - 03:01 PM

 

Glad the longer exposures solved it. I am curious about those bands though. Even when I take 0 second exposures, I haven't seen bands with my ASI1600MM Cool. I get variable gradients, but not the bands. You might want to contact ZWO about it, even though you have resolved it with 0.1 second exposures, and see if there may be a firmware fix. 

Jon, I've read other posts of yours and I'm hoping you can help me. I have a ASI1600MC an OSC.  I've read in other threads that it's unnecessary to use bias frames with these low read noise CMOS astro cameras. Just darks and flats with the lights. 

 

What is your opinion on this?

 

It isn't that the cameras are low noise. The amount of noise isn't really why you don't need (and often don't want) to use biases. It is due to the amp glows. Amp glow is a different "dark signal" that grows at a different rate than dark current (another dark signal), and further amp glow is non-uniform across the sensor area. Most dark scaling is based on a noise evaluation of the data, and with amp glows, it is usually not possible to calculate the correct scale factor. Even if you could calculate a scale factor for dark current, that scale factor will usually be incorrect for the glows. And vice versa, calculate a scale factor based on the amp glows, and it will be incorrect for the dark current.

 

So darks are usually not scalable with any camera that has amp glow. Doesn't necessarily even matter if it is CMOS or CCD. The only reason you really need biases is if you intend to scale your darks. In order to properly scale a dark, the bias must first be removed. Since you can't scale darks when there is amp glow, then you simply don't need biases. You can just create a master dark, subtract that from each light, and that will remove both the dark current and bias offset all at once.

 

Flats may be different, it depends on the camera. Flat frames are often short enough that you don't get any meaningful amp glow. In those cases, rather than take a bunch of different sets of flat darks that match each flat, you could just use a master bias to remove the bias from the flat frames before integrating them. Removing the offset from the flats is important because the flat frames must be scaled in order to be integrated properly. If your flats are a few seconds long or longer (may be possible with narrow band) or if you are using a slow readout (i.e. USB2 with a non-Pro ASI camera), then you might need dark flats instead of a bias to correct amp glow in the flats as well.


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#7 thompeters65

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Posted 14 January 2018 - 01:59 PM

Thanks for your explanation!!  Being new to this it helps my understanding to have such a clear manner!!

 

One more question if you wouldn't mind. I understand how gain is related to ISO in a DSLR. But what is the Offset? Currently I'm using the settings in the driver for the camera. But there are just 3 settings, High Dynamic range, Unity Gain, and Low Read Noise. As the gain gets higher the offset also gets higher. I'm trying to understand the WHY behind these settings, and how they affect the images!!

 

Thom

 

 

 

Glad the longer exposures solved it. I am curious about those bands though. Even when I take 0 second exposures, I haven't seen bands with my ASI1600MM Cool. I get variable gradients, but not the bands. You might want to contact ZWO about it, even though you have resolved it with 0.1 second exposures, and see if there may be a firmware fix. 

Jon, I've read other posts of yours and I'm hoping you can help me. I have a ASI1600MC an OSC.  I've read in other threads that it's unnecessary to use bias frames with these low read noise CMOS astro cameras. Just darks and flats with the lights. 

 

What is your opinion on this?

 

It isn't that the cameras are low noise. The amount of noise isn't really why you don't need (and often don't want) to use biases. It is due to the amp glows. Amp glow is a different "dark signal" that grows at a different rate than dark current (another dark signal), and further amp glow is non-uniform across the sensor area. Most dark scaling is based on a noise evaluation of the data, and with amp glows, it is usually not possible to calculate the correct scale factor. Even if you could calculate a scale factor for dark current, that scale factor will usually be incorrect for the glows. And vice versa, calculate a scale factor based on the amp glows, and it will be incorrect for the dark current.

 

So darks are usually not scalable with any camera that has amp glow. Doesn't necessarily even matter if it is CMOS or CCD. The only reason you really need biases is if you intend to scale your darks. In order to properly scale a dark, the bias must first be removed. Since you can't scale darks when there is amp glow, then you simply don't need biases. You can just create a master dark, subtract that from each light, and that will remove both the dark current and bias offset all at once.

 

Flats may be different, it depends on the camera. Flat frames are often short enough that you don't get any meaningful amp glow. In those cases, rather than take a bunch of different sets of flat darks that match each flat, you could just use a master bias to remove the bias from the flat frames before integrating them. Removing the offset from the flats is important because the flat frames must be scaled in order to be integrated properly. If your flats are a few seconds long or longer (may be possible with narrow band) or if you are using a slow readout (i.e. USB2 with a non-Pro ASI camera), then you might need dark flats instead of a bias to correct amp glow in the flats as well.

 

Thanks for your explanation!!  Being new to this it helps my understanding to have such a clear manner!!

 

One more question if you wouldn't mind. I understand how gain is related to ISO in a DSLR. But what is the Offset? Currently I'm using the settings in the driver for the camera. But there are just 3 settings, High Dynamic range, Unity Gain, and Low Read Noise. As the gain gets higher the offset also gets higher. I'm trying to understand the WHY behind these settings, and how they affect the images!!

 

Thom




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