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Target exposure levels for today’s printers?

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#1 bigjy989

bigjy989

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Posted 14 January 2020 - 09:36 PM

Hello Cloudynights,

I have seen many topics on ideal sub exposure to swamp read noise but not so many considering the final printed image.

For example: Monitor contrast ratio 3000:1 (high end); typical 1000:1. High end printer maybe 200:1 level max, 100:1 is typical.

Typical stacked and stretched object histogram (Astrobin IOTD/top picks) occupies 5%-60% with sky fog peak at the 5% level.

If my non linear stretch has large stars at the upper end, sky fog at 10% and the object highlights at 90%. I am really looking to set maximum 3000 levels in the middle 80%.

Therefore prior to stretching...
In the case of 12 bit cameras. (All number converted to 16bit range of 0 to 65536)

12bit —> 16bit Bias offset 50x16 = 640
Read noise swamping (ASI1600 at unity) = 10*1.7^2 = 29X16 = 464
Target left edge histogram 640+464 (I use the edge not average) = 1104 (bortle 5 can do this in 8s L, 32s RGB or 480s narrowband).

For a single image if I want the best visible delineation on a 12 bit camera.
3000:1 ratio implies right edge of histogram =1104 + 3000*16 = 49000/65536 (way over exposed for single sub).

If 16 subs (average) then 1104+3000*4 = 13404 is target RH histogram. Most stars blown out, galaxy cores on 1 min L subs, 4 min RGB, can’t do narrowband without 30min subs and a much bigger FWC at unity.

If 64 subs (average) then 1104+3000*2 = 7404 is target RH histogram exposure. (Can be done easily for RGB, need 15 min narrowband subs (but not enough FWC >15k-20k needed at unity.))

If 256 subs (average stack) then 1104 + 3000 = 4004 for right edge (this is high end for narrowband). The challenge is 256 exposures x 8 minutes implies 30 or more hours of total integration is needed to complete the project.

The above is for monitor display... however a typical printer is much more forgiving with only 100 levels possible but the noise gradient will be much more prominent as a percentage of the image. If my typical total noise per sub is in an sqrt(std Dev) 8-9e range I would still want around 64 subs to reduce the stacked noise to about 1e (16). The exposure should then be 16x100 + 1104 = 2744 for the right edge of the histogram to allow for reasonable printing if stretched to the print histogram of 10-90%. This would be accomplished by 64 ~4-5min narrowband exposures or 5-6 hrs.

All of this is just a thought but wondering what others have found on their finished products. Guidelines for print or nice internet display at the cost of bloated stars.

Cheers,
Jonathan

Edited by bigjy989, 14 January 2020 - 10:12 PM.



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