Astro-Imaging: Stretching the Reality – Sky & Telescope

February 15, 2019 - Comment

There’s multiple strategy to stretch your knowledge . . . and the reality. What does it imply to stretch your picture? Merely put, stretching means scaling your knowledge. Final month I talked about how knowledge from a 12- or 14-bit digital camera could be scaled to fill the 16-bits of knowledge vary out there for

There’s multiple strategy to stretch your knowledge . . . and the reality.

What does it imply to stretch your picture? Merely put, stretching means scaling your knowledge. Final month I talked about how knowledge from a 12- or 14-bit digital camera could be scaled to fill the 16-bits of knowledge vary out there for manipulation by image-processing software program. This scaling is linear. For instance, photos from a DSLR digital camera vary from zero to 16,384 (14-bit), and that may be scaled by multiplying by four to cowl the 16-bit vary (zero to 65,535) for picture processing, or it’s divided by 64 to make it match the Eight-bit vary (zero to 255) that may be represented by your laptop show.

Now, there are those that say that when you’ve stretched the information, you’ve destroyed it. Or, if you manipulate the information from the digital camera, you’re not getting a real illustration of the thing. As a pc science skilled and graphics professional, I simply roll my eyes once I hear this. In any other case, I would begin an argument, which is very like the one I had with my daughter when she was six and insisted that the Moon adopted us house from grandmas, as a result of she noticed it along with her personal eyes.

In actual fact, it is surprisingly laborious to show a RAW picture from a DSLR digital camera with none changes. Virtually all software program stretches knowledge simply to point out it on a display screen. (Let’s skip the truth that the un-manipulated knowledge isn’t in colour — we’ll speak about that another time.) Right here’s a colorized picture from a well-exposed daytime shot proper off my Canon DSLR:

raw data

Uncooked 14-bit knowledge unscaled is fairly darkish, even for a daylight picture!
Richard S. Wright Jr.

Fairly darkish, isn’t it? That’s as a result of it’s 14-bit knowledge displayed in 16-bit containers like I talked about final month. When mapped to your laptop’s show it has roughly one-quarter of the brightness vary that it ought to have. Even when we scale it up by an element of four, you should still not be all that impressed.

First Stretch

The identical knowledge scaled up (stretched by 4x).
Richard S. Wright Jr.

It’s nonetheless fairly darkish although the information vary covers the total 16 bits (and continues to be linear). That’s proper, the brightness values go up into the 65,000s. When mapped to your show, the pixel values go all the best way as much as 255, a minimum of within the inexperienced channel. Did you discover that the picture appears a bit inexperienced, to not point out flat and considerably featureless? “Images needs to be about what comes out of the digital camera, not the processing” . . . yeah, proper. A contemporary DSLR does an amazing quantity of processing for you. In case you are utilizing a scientific digital camera for astrophotography, it’s important to right for all types of issues your self, or else.

What does the above picture appear like if we open it in a typical DSLR RAW conversion program, after which do, say, 15 seconds value of labor on it? Ta-da:

Finished Image

What we count on from a DSLR, and fairly near what I really noticed.
Richard S. Wright Jr.

This picture appears much more like what I noticed on the facet of the highway someplace in Kansas. It’s not even remotely linear now. Neither, by the best way, is the human visible system. I’m going to let that work on you for a minute or two. If it sinks in appropriately, then you can also roll your eyes the subsequent time you hear some “professional” declare that astrophotography is all simply fantasy. Such consultants are in love with  knowledge, to the purpose that they do not grasp what “pictures” is about. I’m unsure a greater instance of “lacking the forest for the bushes” may be discovered.

Raw image of M31

Uncooked astronomical knowledge. Additionally, not that spectacular to start out with.
Richard S. Wright Jr.

So, why am I utilizing a daily terrestrial picture to reveal this after we are speaking about astrophotography? As a result of if I used an astrophoto, somebody would in all probability nonetheless attempt to argue with me about what’s subsequent, which is that astrophotography is not particular, (aside from how we really feel about it in fact!). It’s simply actually, actually, REALLY low-light pictures. We now have to pay much more consideration to issues we take as a right when there’s an abundance of sunshine. And the low-light situations means  the method of stretching our knowledge turns into a bit extra fascinating.

Aside from that, yep, we fear about the identical stuff as in common pictures: We nonetheless have to paint stability, stuff typically appears too inexperienced proper off a colour digital camera, and we have now to stretch the dwelling daylights out of our knowledge to get end result. And in case your picture is under-exposed (which suggests it doesn’t have signal-to-noise ratio), it’s going to look horrible if you begin stretching it.

Let’s have a look at a typical monochrome astrophoto and its histogram. That is M31, the Andromeda Galaxy. The core reveals up with none stretching, however many of the picture may be very darkish. This end result isn’t nearly bit depth, both; it is as a result of the may be very darkish. As you possibly can see within the histogram beneath it, many of the pixel values are down on the backside of the brightness vary.

Linear Stretch

This knowledge is stretched, however nonetheless fairly linear. Sadly, we have scaled the intense areas up past what we will signify, even with 16-bits. Richard S. Wright Jr.

So, how a few good linear stretch? Seems we have now to multiply the complete picture by 256 to have the ability to see many of the galaxy effectively. Ugh, however look what occurred to the center a part of the galaxy! It’s all stable white and saturated, and the histogram reveals a tall, brilliant line all the best way to the precise. We’ve misplaced all the small print within the middle of the picture as a result of the center is a lot brighter than the outer a part of the galaxy.

If you wish to correctly signify this object, you possibly can’t simply do a linear stretch of the information. Maybe someday, laptop shows may have extra than simply 256 brightness ranges; till then, we have now to make use of a nonlinear stretch to compress the brightness vary. This scaling helps us brighten the darkish areas (the shadows) however not the already-bright areas (the highlights).

We normally carry out this stretch with a “curve” software, which applies a brightening issue to the picture data in a nonlinear approach. The tip result’s that we brighten up the shadows and kinda-sorta go away the highlights alone, or a minimum of brighten them a lot lower than the shadows.

Non Linear

Successive non-linear stretches are typical for any form of pictures, reality be instructed.
Richard S. Wright Jr.

If it’s essential keep the linearity of your knowledge for scientific causes, then a linear stretch can nonetheless make the information extra fascinating to have a look at, whereas nonetheless sustaining its scientific integrity. Nonetheless, in the case of non-linear stretches, I would like you to think about a number of issues: The human visible system is non-linear (the truth is, it is exponential), and we’re able to seeing a far wider dynamic vary than any laptop show can reproduce with out some fudging of the information. We carry out nonlinear stretches on photographs of birds, sunsets, and bridal showers on a regular basis, and no curmudgeons get on cleaning soap bins about how the pictures are ineffective and deceptive.

Maybe we must always simply not argue anymore about what issues we will’t really see ought to appear like!

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