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Shooting with ATP, Sony A7Riv, should I used RAW or FITS (or are they the same)  

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(@linwood)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 5
July 23, 2020 02:00  

My first attempts were just with the intervalometer on the camera, and I loaded raw, and it worked great. 

However, I'm about to try (if it stops raining) using APT to shoot a session.  It will output FITS, or I can save the files (without all the nice APT added info) on the SD cards and use raw. 

While on the raw subject, Sony uses a compression algorithm that is lossy, not lossless (one of the few things I miss from Nikon).  Compressed raw is 60 MB, uncompressed about 120MB, so it's a substantial difference, and in any terrestrial subject I cannot tell the difference.  But for astrophotography I do not know how to assess.  Does anyone know if it matters, or is that little extra data lost in the noise (literally)?

Final and related question, some people convert to DNG to compress, as that's lossless.  I've never understood what the conversion to DNG does internally with the RGGB data itself, how much it is pre-processed, and whether that's a bad thing.  I know you don't want to provide TIF's to APP, I saw that said clearly.  But how about DNG's, do they matter, or are they just as good (but half the size if the answer is I really shouldn't do lossy compression)?

Sorry for all the questions, unfortunately they are all kind of tied together.

Linwood


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3955
July 25, 2020 20:51  

I would never (personally) use anything that compresses or changes the RAW data in any way, loading the untreated RAWS directly into APP will always make sure you can let APP use its best algorithms for noise removal etc using the RAW bias, darks etc. Regarding APT I'm not sure what it produces myself, I have no experience with it, fits is fine usually, but you're less sure if the data is kept exactly the same (you'd have to check with APT for that). At least always try to use the same capture program.

 


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(@linwood)
Molecular Cloud Customer
Joined: 1 year ago
Posts: 5
July 25, 2020 21:01  

Thank you.  Since posting this I found that (contrary to all experience in Nikon raw), the Sony camera also changes the raw data when you have lens compensation on, strongly so for compressed raw, weakly so for uncompressed.  Someone on CloudyNights pointed me to a discussion on DPReview about this discover, so I need both off.  Here's the DPReview (very long) thread.  

https://www.dpreview.com/forums/post/62199570

Just as an aside, I didn't discover this in APP, because it's so automated I never looked at a stretched dark or bias frame.  I discovered it as I am also doing a trial of Pixinsight, and there you have to manually go through each step (and least I haven't learned otherwise). 

So the wonderful set of defaults and automation in APP is a mixed blessing.  😀 

I've read about the ASCOM drivers for APT for Sony, and SUPPOSEDLY they preserve the raw data.  But I think I need to go through and see if they show any of these artifacts.  However, for an unrelated reason, I may just skip APT for most use for now (namely that I can't use silent shutter with it; so long as I'm stacking 30sec or less, I get a LOT less vibration in the first 5 seconds or so by using the intervalometer in the camera and silent shutter. 

Thanks again for the followup.


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3955
July 25, 2020 21:07  

Ah yes, it can be a mixed blessing in that sense. You can start to "forget" to actually look at separate frames at some point (usually this is not a big issue when you have the setup working as-is), in APP you can always look at noise graphs and other parameters for your dataset after having done the normalization steps, right-clicking on the list below gives you that option. Not sure if that would've given you the same answers though.

And you're welcome! 😉


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