Author Topic: BPP and Calibration  (Read 401 times)

Offline Farzad_k

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BPP and Calibration
« on: 2018 December 04 12:06:24 »
Hello,

I am just wondering if BPP internally calibrates Flats with Biases and Flat-Darks, and Darks with Biases and Calibrated Flats when default values are used. I imagine that it does, and if so, then the resulting masters are all pure such that the master flat has flat-darks and biases subtracted from it and master dark has the clean master flat and bias subtracted from it.

Thanks a lot.

Offline aworonow

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Re: BPP and Calibration
« Reply #1 on: 2018 December 04 13:31:25 »

Offline Niall Saunders

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Re: BPP and Calibration
« Reply #2 on: 2018 December 04 18:02:17 »
Quote
I am just wondering if BPP internally calibrates Flats with Biases and Flat-Darks, and Darks with Biases and Calibrated Flats when default values are used. I imagine that it does, and if so, then the resulting masters are all pure such that the master flat has flat-darks and biases subtracted from it and master dark has the clean master flat and bias subtracted from it.

A CalibratedFlat is a Flat with the MasterFlatDark subtracted from it. It is better (IMHO) to acquire a batch of dedicated FlatDarks for the purpose of creating the MasterFlatDark than it is to bother scaling NormalDarks using any kind of MasterBias or SuperBias - so I would never bother doing this.

A calibrated Light starts off by having the MasterDarks being SUBTRACTED from it. This now leaves part-calibrated Lights that have no trace if the Bias data (because Bias data is identically present (statistically speaking) in both types of frames. And, further, the Dark data has also been (statistically) eliminated from the part-calibrated Lights. This is why the process has to be SUBTRACTIVE.

The same applies to the Flats - they have to be calibrated as well - which is why you again have to SUBTRACT the MasterFlatDark from the Flats. No further calibration is then needed, and they can be averaged (Intehrated) to give the MasterFlat

You now have to complete the calibration of your part-calibrated Lights - by DIVIDING each by the Master Flat (where neither the MasterFlat nor your part-calibrated Lights have any Bias or Dark information present). After this DIVISION step, you will have fully-calibrated Lights ready for further StarAlignment and ImageIntegration (possibly preceded by aDeBayer step if using an OSC imager.

I hope this makes things clearer for you - I am concerned that you might be miunderstanding what some of the stages do. I don't use BPP myself - I like to get my 'hands dirty' and control the different aspects of all the stages myself.
Cheers,
Niall Saunders
Clinterty Observatories
Aberdeen, UK

Altair Astro GSO 10" f/8 Ritchey Chr├ętien CF OTA on EQ8 mount with homebrew 3D Balance and Pier
Moonfish ED80 APO & Celestron Omni XLT 120
QHY10 CCD & QHY5L-II Colour
9mm TS-OAG and Meade DSI-IIC

Offline Farzad_k

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Re: BPP and Calibration
« Reply #3 on: 2018 December 05 07:25:44 »
Thanks for the explanation.

I share your concern about me not quite understanding what goes on under the hood. I happen to watch what BPP does and it creates the Bias first, then Dark, then Flat, and then Light. Traces of Bias are present in the Dark, Flat and Light and for purity should be eliminated from all three.

My question might not have a straightforward answer. But:

Does Bias get calibrated out of darks?
Do biases and flat-darks get calibrated out of flats?

And aren't Biases, cleaned darks, and cleaned flats calibrated out of lights before lights get integrated?

Offline Niall Saunders

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Re: BPP and Calibration
« Reply #4 on: 2018 December 05 09:42:33 »
Hi Farzad,

You asked:
Quote
Does Bias get calibrated out of darks?
Yes, MasterBias gets subtracted from individual Darks - but this action is only necessary if you cannot control the temperature of the imager, or if you choose to work with a set of Darks that have a different exposure time to your set of Lights.

Quote
Do biases and flat-darks get calibrated out of flats?
The same argument as used for your first question also applies here - after all, Flats are really just Lights taken of a different target object.

Quote
And aren't Biases, cleaned darks, and cleaned flats calibrated out of lights before lights get integrated?
If you have taken a set of matching Flats and FlatDarks, then you will have exactly the same Bias data present in both, and and this is also the case for matching Lights and Darks.
When you subtract your MasterDark from your individual Lights, you also eliminate all traces of the 'common' Bias signal present in your Lights - without needing to have taken any Bias images at all.
Finally, you can thendivide each of your individual, pre-calibrated,  Lights (which will have neither Bias nor Dark signal present) with your MasterFlat (again, with neither Bias nor Dark signal present) - giving you your desired CalibratedLights, ready for further processing (such as ImageIntegration)

So, simplistically:
01) Take Lights
02) Take Darks
03) Take Flats
04) Take FlatDarks
[the above sequence can be in any order]

05) ImageIntegrate FlatDarks to make a MasterFlatDark
06) Calibrate Flats using the MasterFlatDark to make CalFlats
07) ImageIntegrate the CalFlats to make a MasterFlat

08) ImageIntegrate Darks to make a MasterDark
09) Calibrate Lights using the MasterDark and MasterFlat, to make CalLights

10) DeBayer the CalLights to make RGB_CalLights (if required)
11) StarAlign the CalLights (or the RGB_CalLights if required)
12) ImageIntegrate the CalLights to make a MasterLight

Perform post-processing to suit the image.

Please remember - this is how I tackle the pre-processing stages. It is the way I understand (or, at least, remember) it. There may be errors and omissions. Your challenge is to try and create a method that suits you. (And, if I have made any errors, to point them out to me  :police: )
Cheers,
Niall Saunders
Clinterty Observatories
Aberdeen, UK

Altair Astro GSO 10" f/8 Ritchey Chr├ętien CF OTA on EQ8 mount with homebrew 3D Balance and Pier
Moonfish ED80 APO & Celestron Omni XLT 120
QHY10 CCD & QHY5L-II Colour
9mm TS-OAG and Meade DSI-IIC

Offline Farzad_k

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Re: BPP and Calibration
« Reply #5 on: 2018 December 05 10:56:05 »
Thanks a lot.