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[Sticky] How to correct a stack/integration for vignetting  

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(@mabula-admin)
Quasar Admin
Joined: 3 years ago
Posts: 2160
November 15, 2017 13:25  

Okay, that's great information then. You now know were you can directly gain improvement with your data calibration 😉 and thus your end integration !

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@thompeters)
Main Sequence Star Customer
Joined: 2 years ago
Posts: 39
November 15, 2017 13:43  

On another topic do you have or plan to have a tutorial on how YOU feel flats should be created??


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(@mabula-admin)
Quasar Admin
Joined: 3 years ago
Posts: 2160
November 16, 2017 16:12  

Hi Thom,

No I am afraid not, currently, I have no specific tutorial in mind regarding the actual creation of flats.

To do that properly, one needs to have examples of all sorts of optical configurations, since different optics tend to need a different approach. This is due to

  • focal ratios and
  • focal lengths of the optics and
  • the optics themselves, (a SCT is different than a APO for example)

But I can definitely give you some pointers for the histogram, iso/gain and exposure values :

  • try to illuminate the flats for as long as possible, while
  • not clipping the histogram on the right side and while
  • staying in the linear domain of your sensor's response. (I know this is not trivial knowledge for any sensor)

 

Practically, this means,

  • set the gain or iso of the camera as low as possible, that enables you to make longer flat exposures. (for isos, try to not go lower than iso 100)
  • try to get the peak of the linear histogram halfway or a bit past halfway, that should prevent you from having the peak in the non-linear domain of most sensors.

If you can illuminate for more than 10 seconds while not filling the histogram enough, you can increase gain/iso. The main point is that flats with exposures of fractions of a second, tend to give all sorts of problems. So make sure that the flats have exposures of at least 1 second, preferably more. You can accomplishe this by damping the light source.

Last, always make sure that the light source for the camera's focal lenght is very diffuse. (So making good flats for a wide-angle objective, focal lenght less than 50mm, is not very easy in my own experience).

I hope this will help you for now.

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@thompeters)
Main Sequence Star Customer
Joined: 2 years ago
Posts: 39
November 17, 2017 21:25  

OK those are good guidelines!! Thanks. I'd always read the flats needed to be shot at the same ISO as the lights. I'll try this way and see what I come up with!!


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(@mabula-admin)
Quasar Admin
Joined: 3 years ago
Posts: 2160
November 18, 2017 12:18  

Hi Thom,

You're welcome,

You can make the flats at a different iso/gain than the lights, but then you must take into account that the masterbias or masterdark that you use to calibrate the flats (to subtract the bias pedestal for correct flat calibration) must have this same iso/gain value 😉

It's explained in detail in this post:

https://www.astropixelprocessor.com/community/tutorials-workflows/astronomical-data-calibration-priciples-must-read/

An example:

Let's suppose that we have light frames of iso 800 and we will use a masterbias /  masterdark, masterflat and BPM for the light frame calibration.

Now let's assume we shoot the flats at iso 100.

In this case the light frames need a masterbias/masterdark of iso 800.The flat frames however need a masterbias/masterdark (so called masterflatdark) of iso 100.,  to get good calibration.

Cheers,

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@thompeters)
Main Sequence Star Customer
Joined: 2 years ago
Posts: 39
November 19, 2017 16:59  

If I understand you correctly: Prior to my creating my stack of lights ( integration) I need to do 2 calibrations first.

1) I need to do a calibration of Bias, Dark and Flat frames at ISO 100. From that I will get a master flat(iso 100).

2) I will also need to use create master bias, and a master dark at ISO 800.

When I do my calibration and integration I need to add my lights(iso800), my master flat(iso100), my master bias(iso800), my master dark(iso800) and my BPM(iso800)

 

Is this correct?

Thom


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(@mabula-admin)
Quasar Admin
Joined: 3 years ago
Posts: 2160
November 23, 2017 18:26  

Hi Thom,

Yes that should be correct 😉

The BPM can be made of any ISO value, that won't influence the results when using the BPM in calibration.

Kind regrads,

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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