Image Background Dilemma

[PembrokeSteve]

Hi,

How dark should the background of a typical deep sky image be ? 

I notice that many of the experienced imagers on our forum avoid the too dark (black) background, and seem to prefer light to dark grey. However some imagers do seem to prefer the black/ high contrast background in their images., and I frequently see this when browsing images online. 

Is how dark you make the image background, down to personal taste?  It seems what one person might like, will probably not appeal to others.

When reading through telescope reviews, one often reads the comment about “pin sharp stars” against “inky black sky”  which is a sign of a good scope etc.  However the  “inky black” background is not wanted in deep sky images. 

I have often seen images with a darker background and the person who produced the image has been advised that “the black point has been clipped”.

I am now a little bit obsessed with trying to avoid the dark background in my images. However even this is not easy. Very often a different viewing browser can make your image background look darker than what it is.  Is it me just being fussy or what !

Grateful for your views on this.

Regards

Steve

[vlaiv]

First thing to understand is that you can clip the black point even if your background is not black.

This term has more to do with processing of the data than with actual black background. It happens more often in conjunction with black background because of the way it happens in processing (hence the name).

There is a reason why you should not have fully black background. Having just slightly brighter background actually more resembles what one sees thru a telescope when dark adapted. It is only when not fully adapted or having high contrast target that we see pitch black background in our scopes.

It also has to do with very interesting phenomena called Eigengrau.

Here is wiki article on it: https://en.wikipedia.org/wiki/Eigengrau

In short - in absence of light many people describe sensation / perception as not quite black - more grayish like. This image sums it nicely:

Therefore we expect not to see completely black when there is no light. Black is just a contrast thing - it is stuff that we can't see because there is something too bright in the scene (think of night time and car headlights pointing at you - everything else will be pure black in that case).

For above reasons - your background should not be fully black. How much above black level? That sort of depends on your taste and also on the feel you want to impart on the image.

[CloudMagnet]

A lot of it is personal preference and also the type of target you are shooting as well. For galaxies, I tend to have a slightly lighter background to help show up some of the faint outer details. On the other hand, for a star cluster I would have a bit more contrast to show off the stars a bit more.

I just tend to make sure that the histogram doesn't touch the left as past this point you just risk losing data wthout any real gain. Do what "looks" right and you wont be far off.

 

[ollypenrice]

There's a difference between clipping object signal and clipping skyglow. I think we can all agree that clipping object signal is never the right thing to do.  However, skyglow is also genuine and natural. At a very dark site on a clear night you will, when dark adapted, be able to find your way around quite easily. I live in such a place and only need a head torch when I've been imaging and looking at a PC screen. If it's a visual-only night a dark site is not dark. However, if it goes cloudy I can see nothing at all - which shows you that starlight is light. This argues for a non-black background. Leaving the background reasonably high is a sign of confidence in an imager, in my view. For this reason I aim for somewhere between 20 and 23 in Photoshop. Where, in that range, I end up depends partly on the camera: my Sony chip makes it almost impossible to get beyond 20 while my Kodak chips are happy to produce a lighter sky. I don't know why. The other factor concerns the object itself. It's probably a matter of which level of contrast looks best because , in any image, there is a visual interaction between one part and an another, as endless illusory images demonstrate.

Over enthusiastic clipping will also remove all noise from the background. This simply looks wrong. The sky does not look like a shiny, flat, jet black surface. Note surface.  A clipped sky in an image does look like a surface but the sky itself is not a surface, it's a depth. This is related to a point in visual perception theory: a high gloss without grain prevents a picture from creating the desired illusion of three dimensionality because it is surface-like. A little grain breaks that 2D surface up, allowing the 3D illusion to work.

Finally, once you've looked at lots of astrophotos, you can usually guess why a sky is jet black. Usually it's because an inexperienced imager has used clipping to remove gradients. They can, however, be removed without clipping.

Olly

Edited April 29, 2020 by ollypenrice

[ollypenrice]

Another thought. (Sorry but this is a great question!)

Here's what a background of 23 looks like with zero-value text on it.

We must also leave the general background light enough to allow dark patches of obscuring dust to show themselves against it. Here some of the dust is only at a value of 9.

Olly

 

[Magnum]

6 hours ago, ollypenrice said:

Where, in that range, I end up depends partly on the camera: my Sony chip makes it almost impossible to get beyond 20 while my Kodak chips are happy to produce a lighter sky. I don't know why. 

 

Thats interesting Olly, do you mean the noise characteristics of the Sony chips are not appealing when left too bright compared to the Kodak sensor data.

I think ive noticed that with my 2 Atik cameras, although the Kodak chip in the 383 looks worse in 1 sub, but once stacked seems to show a finer more even grain. I think Sara found the same with her QSI CCD when she had the sensor changed from Sony to Kodak.

Lee

[Magnum]

Another thing I noticed to do with the human eye and perception. If I post an image on FB and have the browser using the full screen, im seeing my image as postcard size preview surrounded by a screen of white, this makes my image look totally black, but once I click the image to get the full size version then it looks grey as it should do.

Obviously human perception of contrast at play. but how many people dont click the image to make it full size LOL

Lee

[ollypenrice]

4 minutes ago, Magnum said:

Thats interesting Olly, do you mean the noise characteristics of the Sony chips are not appealing when left too bright compared to the Kodak sensor data.

I think ive noticed that with my 2 Atik cameras, although the Kodak chip in the 383 looks worse in 1 sub, but once stacked seems to show a finer more even grain. I think Sara found the same with her QSI CCD when she had the sensor changed from Sony to Kodak.

Lee

Yes, something like that for me too. If I use a standard log stretch on my Sony data and keep going till the background reaches 23, the fainter object signal is stretched beyond its noise floor and the background is also noisy. (I end up with some background pixels at 23 but a great many at all sorts of values below that.) So I log stretch till only about background 20 and stop. Two steps follow. In Curves pin the background at 20 and fix it above that, then lift the curve a little where it's below 20. This lightens the overly dark pixels for a smoother sky. Then I pin the background at 20 and fix it below that so I can stretch a little more of the object signal.

With the Kodak chips none of this is necessary. I get a consistently better background (and stars) from the Kodaks.

Olly

[PembrokeSteve]

Hi,

Just a BIG thanks to Vlaiv, CloudMagnet, and Ollypenrice, for their time and efforts in answering my question, (which has been bugging me for a while).
I am sure that this valuable advice will also be absorbed by other forum members who are new to Astro Imaging.
Again many thanks,

Steve