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[Sticky] Combining subs with multiple exposure time  

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(@marc_theunissen)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 26
June 15, 2017 17:39  

How can I best combine subs with multiple exposure times?

Say I have 50x30s with a Master calibration files and 20x180s subs (again with master calibration files). What would be best strategy to combine?


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(@mabula-admin)
Quasar Admin
Joined: 3 years ago
Posts: 2160
June 15, 2017 19:25  

Excellent question Marc,

There are several ways that you can do this and it all depends on what you are trying to achieve.

If you still need to calibrate the data, then I would advise to calibrate both sets of data and save them in 2). If you have RGB data with chromatic aberration save them with the "align channels" mode, which reduces the amount of chromatic aberration significantly in most cases.

Then proceed with your calibrated frames.

You can play with the weights in 6) INTEGRATE

  • integrate with weights: equal, if you want all frames to be weighted equal, this will give you a result which will be subtoptimal in SNR, and suboptimal in sharpness
  • integrate with weights: exposure, if you want the longer exposure frames, which should have higher SNR, to have more weight. The integration should have better SNR but less sharpness
  • integrate with weights: quality, if you want to use APP's quality calculation. The quality parameter is based on noise, stardensity, star size and shape. Usually this gives the best integration for noise and sharpness combined.
  • integrate with weights: SNR, if you want to use the SNR of the frames. This is a really dangerous method. Any deviating gradients between the frames will make the SNR metric totally unreliable. From all the settings, this is the least attractive one. I wouldn't advise to use SNR for weights. Also bad frames with some clouds or bad transparancy will give higher SNR strongly reducing the integration result.
  • integrate with weights: noise, if you aim to have the lowest noise in the end result.
  • integrate with weights: star density, if you want to give more weight to the frames with the highest star density, this usually is a very good parameter to indicate good transparancy.
  • integrate with weights: star shape, if you want the frames with the smallest and roundest stars to have the most weight. This will give you the integration with the sharpest results. This works really well. Frames that have stars that are NOT round are punished a lot, so these frames will have little weight. Star shape means both roundness and size. The smaller and rounder the stars are, the higher the weight of that frame. This is a very nice integration setting if you have some frames without perfect guiding but still want those frames to help reduce the noise in the data.
  • Last option, you can make 2  integrations of the two datasets and combine them using the RGB Combine tool in which you have full control over how the images are combined. You will need to register both integrations first, before loading them into the RGB Combine tool.

 

Scale Independent quality parameters in APP

APP has the parameters star density & relative FWHM which are very helpfull if you combine data of different scales. APP calculates the star size/shape and star density relative to the scale differences between the frames. The scale differences are calculated using the homographies (projective transformations) between the frames.

 

 

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@marc_theunissen)
Main Sequence Star Customer
Joined: 3 years ago
Posts: 26
June 16, 2017 14:14  

Thanks for this explanation!


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(@chrisjuh)
Hydrogen Atom Customer
Joined: 2 years ago
Posts: 1
August 13, 2017 14:41  

Hi Mabula,

How to combine dslr images with multiple exposure time? Can I also use the RGB Combine tool?


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(@mabula-admin)
Quasar Admin
Joined: 3 years ago
Posts: 2160
August 13, 2017 15:43  

Hi Chrisjuh,

Yes, you can use the RGB combine tool for combining 2 RGB images/stacks/integrations of different exposures, but you need to make sure that these images are registered to each other. Ther RGB combine tool won't do this automatically yet for you. (it's on my todo list).

If you have more than 2 frames, you should really use the normal procedure and play with the integration weights.

A real HDR combination will become possible in the near future using the RGB combine tool as well.

Let me know if this answers your questions,

Kind regards,

Mabula

Main developer of Astro Pixel Processor and owner of Aries Productions


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(@bengourben)
Molecular Cloud Customer
Joined: 2 years ago
Posts: 3
November 9, 2018 00:45  

Hello Mabula.

Can I ask you what if I'm combining Dslr subs of different length, but I'm shooting with a h-alpha clip filter? I know how to achieve the desired h-alpha finished stack with subs of equal length, but do I need to do something different if the lengths vary? E.g. I've 20x420 30x600 and 20x540.

Thanks

Jason


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