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QHY600 - Yet another thread!

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#151 anismo

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Posted 30 September 2020 - 10:08 AM

Well, once I checked the unclipped box, I got a bunch of 0 pixels. I can see how to add a pedestal if I calibrate using the Calibration Process but there doesn't appear to be an option to add a pedestal using WBPP. Is that correct? 

yes. I dont think there is an output pedestal option in BPP.  



#152 anismo

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Posted 30 September 2020 - 10:14 AM

<Snipped>

 

btw, do you think this CTU is any good - https://www.teleskop...l-Adapters.html

it has shorter optical path (9 to 12mm)

That looks interesting. This looks like the a push-pull screw type. (the gerd neuman is not using that method). The only issue may how precise the adjustment could be made. Last night I was adjusting the Gerd Neuman CTU and its adjustment is very precise (A whole turn is 0.2mm ) and I found that a whole turn makes my stars in edge go from pin point to a full donut..(and with a reducer, it will be even worse). 



#153 ezwheels

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Posted 30 September 2020 - 11:05 AM

Yes, the GN CTU is very precise. Also one should consider the physical location and orientation of the push pull screws on the TS CTU part. Having them in this location means you will need to have clear access to the screws with the allan wrench. This may actually require needing more back spacing that you may not have. 



#154 Peter in Reno

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Posted 30 September 2020 - 11:10 AM

Would increasing offset during capture help avoid dealing with pedestal during calibration? 

 

Peter 

Can anyone answer this?

 

Thanks, 

Peter 



#155 jdupton

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Posted 30 September 2020 - 11:35 AM

Anis,

 

Thanks John. Here they are . Seems like definite clipping going on. Would the increased mean in uncalibrated due to the hot pixels?  

   No, the higher mean in the uncalibrated sub is simply due to the fact that it still contains the bias offset. When you calibrate, the dark subtraction removes both the dark current and the offset that is contained in both the light and dark subs.

 

   Adding an Output Pedestal in Image_Calibration will help keep the amount of clipping to a minimum. The Output Pedestal is limited in PI to a value of 1000 ADU. Jon Rista recommends using a value of at least 800 for many cameras. Sometimes, even the maximum value cannot prevent all clipping.

 

   If there are any clipped pixels in the raw frames themselves, the use of an Output Pedestal in Image_Calibration may not be fully effective. In those cases, you just need to increase the camera's Offset higher so that data isn't starting out clipped. Check a few of your raw uncalibrated light frame subs to see if clipping is happening during acquisition. If you see more than a few clipped pixels in each frame, it is a good idea to raise the camera's Offset.

 

   I regularly use a "Clipping_Checker" saved process icon in PI. To create one, you can do the following:

  • Open PixelMath and enter the following equation
    iif($T == 0, 0.999, 0.00)
    Set PixelMath to create a new file (in the Destination Section of PM)
  • Save the PixelMath process for later use. Call it Clipping_0_Checker or something similar.

   To use the checker, just drag the saved process icon to the image frame to check. A new image is created. It will look black. Open up the Statistics Process and look at the statistics of the new frame. In this case, Uncheck the "Unclipped" option box. (I wrote that wrong before because I was thinking of my use of this Clipping_Checker. You want the Unclipped Option checked when looking for the minimum value in the frame. You want it unchecked when examining the output of the clipping check.) The total number of pixels that were clipped to black will be shown in the "Count (px)" field of the statistics.

 

 

John


Edited by jdupton, 30 September 2020 - 11:39 AM.

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#156 lucam

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Posted 30 September 2020 - 11:45 AM

Can anyone answer this?

 

Thanks, 

Peter 

No, it does not. If you apply the same offset to lights and darks, it will cancel out. Offset is just what it says. Offset helps if the uncalibrated images (be it lights or darks) have black spots. If the calibrated ones have black spots: 1) they shouldn't, so it's good to understand why they appear; 2) an output pedestal offsets the light frame but not the dark and gets rid of the black clipped pixels.


Edited by lucam, 30 September 2020 - 11:46 AM.

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#157 Peter in Reno

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Posted 30 September 2020 - 12:00 PM

No, it does not. If you apply the same offset to lights and darks, it will cancel out. Offset is just what it says. Offset helps if the uncalibrated images (be it lights or darks) have black spots. If the calibrated ones have black spots: 1) they shouldn't, so it's good to understand why they appear; 2) an output pedestal offsets the light frame but not the dark and gets rid of the black clipped pixels.

I was afraid of that. It's not difficult to add pedestal offsets. It's that there can be lots of guessing to determine proper pedestal because the unit is DIN when using PixInsight. 

 

Thanks for the response. 

 

Peter 


Edited by Peter in Reno, 30 September 2020 - 12:20 PM.


#158 lucam

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Posted 30 September 2020 - 12:24 PM

Gain is 26 and offset is 10 all images are binned 2x2. I checked one of the dark spots and it corresponds to a hot pixel in the uncalibrated frame. The dark spots are "0" there around .03 but once stretched they become more obvious. This seems to have started when I changed over from -10 to 0. So, I took a fresh set of darks/bias/flats last night and will redo everything this morning. 

 

Here's an earlier calibrated sub I took back in May that doesn't have them. 

 

attachicon.gifno_black_flecks.JPG

 

 

Rgrds-Ross

If the black spots correspond to hot pixels in the uncalibrated frame, an output pedestal may not be the best way to handle this. If we assume that the black clipped pixel is because the hot pixel is perfectly canceled by the master dark, an output pedestal will just give a finite value to the clipped pixel, which will only make it harder to get rid of in the averaging process of calibrated frame integration. 

 

A better way to handle it is to: 1) dither so the black pixel is not always in the same place, 2) in the ImageIntegration process click the Clip Low Range option with a Range Low value 0.000. Alternatively, Cosmetic Correction with Cold Pixel Threshold enabled (both Master Dark and Use Auto detect) and tuned to remove the cold pixels immediately followin image calibration, before integration. 


Edited by lucam, 30 September 2020 - 12:26 PM.

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