Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

BRIGHTNESS vs PEDESTAL vs GAIN

CMOS imaging
  • Please log in to reply
4 replies to this topic

#1 eglad

eglad

    Lift Off

  • -----
  • topic starter
  • Posts: 6
  • Joined: 22 Nov 2020

Posted 27 January 2021 - 06:23 PM

Hello, 

I'm performing several dark current measurements to characterize my ZWO ASI1600MM Pro. Taking all images in RAW16/.FIT format.

 

The results are very inconclusive from my first run comparing various temps and exposure times. I'm keeping GAIN and BRIGHTNESS at 0, as I didn't want to add any bias. 

 

Although, now I'm convinced the data looks so strange because the 0 offset is causing the low-signal images to be "clipped" at 16DN (but divide by 16 because it's a FIT image, so this = 1 DN), and there is supposedly data that is below this floor. 

 

I believe I need to modify BRIGHTNESS or PEDESTAL to avoid this clipping. From my research, these seem the exact same to me. Does anyone have experience with the affects of either?

 

Thanks, 

Ellen


Edited by eglad, 27 January 2021 - 06:25 PM.


#2 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 23,477
  • Joined: 27 Oct 2014

Posted 27 January 2021 - 06:49 PM

On Sharpcap, with the astro cameras I'm familiar with (ZWO, my Atik has a fixed offset) brightness _is_ offset. 

 

You _always_ need an offset so noise does not drive the signal negative, which just does not work.  About 50 ADU (12bit) or 800 ADU (16bit) is generally sufficient.  On a ZWO camera that's a brightness setting of 10.

 

I've never used a Pedestal in Sharpcap.


Edited by bobzeq25, 27 January 2021 - 07:00 PM.


#3 Der_Pit

Der_Pit

    Vanguard

  • *****
  • Posts: 2,057
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 27 January 2021 - 08:01 PM

I'm not sure about 'pedestal' - it is not a setting of the camera firmware, AFAICT.  To me it suggests it might be something done after the analog-to-digital conversion in the camera, and thus would not help against clipping.

 

The gain is an amplification factor, i.e., it is multiplicative, and therefore doesn't shift things, it stretches.

 

So what you want to change is offset, or brightness as ZWO calls it in its settings.  And indeed you want to have that larger than zero to prevent clipping.  Check it yourself: Take darks with increasing offset/brightness, and watch the histogram.  You'll see the peak 'grow out from the left edge'.  Continue increasing brightness until the histogram clearly drops to zero before the left edge.  At gain 0, that should be at an offset setting somewhere between 8 or 10.

Then you can start increasing the gain, and you can see the peak get broader (the "stretching") with the maximum staying at the same place.  This might make the left slope of the peak reach the left edge again.  Then you'd have to further increase the brightness again to detach it from the edge.



#4 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 24,778
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 27 January 2021 - 09:44 PM

Hello, 

I'm performing several dark current measurements to characterize my ZWO ASI1600MM Pro. Taking all images in RAW16/.FIT format.

 

The results are very inconclusive from my first run comparing various temps and exposure times. I'm keeping GAIN and BRIGHTNESS at 0, as I didn't want to add any bias. 

 

Although, now I'm convinced the data looks so strange because the 0 offset is causing the low-signal images to be "clipped" at 16DN (but divide by 16 because it's a FIT image, so this = 1 DN), and there is supposedly data that is below this floor. 

 

I believe I need to modify BRIGHTNESS or PEDESTAL to avoid this clipping. From my research, these seem the exact same to me. Does anyone have experience with the affects of either?

 

Thanks, 

Ellen

An offset is required. Otherwise you WILL clip to black. The bias offset is the BRIGHTNESS setting, and that MUST be greater than zero. For the ASI1600, I recommend setting it to 50 and forgetting about it. 

 

Why is the offset required? Because read noise causes random deviations from the "baseline mean" of the camera, which IS the offset. In other words, if your offset is zero:

 

 10|

...|     |       |

  1|  |  | |  |  |  , | ,   |  |

  0+----------------------------

 

You might have "noise" like the bars above the zero line in this little graph. Those bars represent the random oscillations causing the signal to DEVIATE from that "mean" of zero. Note, however, that noise is random, and will deviate above and below the mean. Here, there is nowhere for negative deviations from the mean to go...they clip to zero. If we instead have a non-zero bias offset:

 

 

100|

...|     |       |

 11|  |  | |  |  |  , | ,   |  |

 10|----------------------------

  9|`|  |   ||  |  | | `  |  | |

...| |       |       |         |

  1|                 |

  0+----------------------------

 

You lave room for all the deviations from the mean (the offset) both positive and negative to occur without clipping. A bias offset is essential. Not only is does clipping increase the noise in your result, it prevents proper calibration from ever being possible. Because in addition to the random oscillations due to read noise, the bias offset itself, as well as any dark offset, is actually non-uniform itself across the sensor, and due to the differences in pixel response, the DFPN itself may cause pixels to clip.

 

A PEDESTAL is similar to a BIAS OFFSET, however they are not the same thing. A Pedestal is added during pre-processing to ensure that ONCE the bias offset is subtracted (either by subtracting a master bias, or by subtracting a master dark), you ensure that you still won't clip any of the data. Calibration of each pixel must be performed as a single operation, with real numbers that allow negatives to temporarily exist. So usually, during calibration, the bias or dark will be subtracted, leaving negative numbers, then the flat will be divided, potentially lifting some of those negatives back up (potentially), then finally the output pedestal is added to shift all remaining negative values positive. A pedestal should be large enough to ensure that any pixels that would have clipped once the bias offset is removed will remain positive after the pedestal is added. The easiest way to ensure that, is to make your pedestal the same size as your bias offset. So, if the bias offset is 50 ADU, then the pedestal should be the equivalent of 50 ADU. For a 12-bit camera like the ASI1600, you usually need to convert the bias offset in ADU, to a 16-bit number...so 800 DN. An output pedestal of 800 16-bit DN will guarantee that you will never clip anything during calibration. 


Edited by Jon Rista, 27 January 2021 - 09:48 PM.

  • lambermo and bobzeq25 like this

#5 eglad

eglad

    Lift Off

  • -----
  • topic starter
  • Posts: 6
  • Joined: 22 Nov 2020

Posted 28 January 2021 - 08:00 PM

Thank you all! I've been performing some tests the past few days to see how stepping through increments of increasing Brightness affects the data and have come to the same conclusions.

 

Jon, thank you for the helpful insight about Pedestal bias, that's really appreciated.

 

-Ellen


  • bobzeq25 likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics





Also tagged with one or more of these keywords: CMOS, imaging



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics