Potential Error Live Stack Save as RAW (32bit)

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garyhawkins
Posts: 54
Joined: Sun Dec 22, 2019 6:59 pm

Potential Error Live Stack Save as RAW (32bit)

#1

Post by garyhawkins »

I've been using SharpCap Pro Version 4 (beta) for photometry in recent weeks.

Today, I tested Livestacking on a sample data set. I was stacking five frames and then doing a 'Save as' from the Live Stack. The 'Save as 16 Bit Stack' did what I was expecting when inspecting the saved image using AIJ (AstroImageJ). Maximum pixel values were the average of the pixels over the five frames stored as 16bit - approx 19,000. However, when I tried 'Save as RAW (32bit) Stack, the results were not as expected (the sum of all stack frames). For my five stacked frames, the max pixel should have been around 90,000, however, it was being reported as 637,045,888.000, and further this appeared to be a saturated value as it did not change across multiple pixels.
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Re: Potential Error Live Stack Save as RAW (32bit)

#2

Post by admin »

Hi,

My first thought was that you should end up with a maximum of 90,000 too, but then I went and checked the code. It looks like the code is set up to expand the range when saving in this format to use the maximum possible range of four byte integer data. For instance, if the maximum raw data value was 20,000, to scale up SharpCap would multiply all data by 65536 (which is the largest power of two that you can multiply by and still keep the range inside the -2^31 to +2^31 range of a four byte integer).

I suspect that I added this to avoid people reporting that their 'images were black' when saved.

cheers,

Robin
garyhawkins
Posts: 54
Joined: Sun Dec 22, 2019 6:59 pm

Re: Potential Error Live Stack Save as RAW (32bit)

#3

Post by garyhawkins »

Hi Robin,

I checked the math, it looks like you're multiplying by 2 power 15 (32768) rather than 2 power 16 (65536). However, knowing this makes the data useful.

I think more people will start considering SharpCap for photometric measurements, particularly now the sequencer is there :).

Best regards,

Gary
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