
A method for improving Milky Way exposures in light pollution - sndean
https://www.lonelyspeck.com/the-milky-way-in-los-angeles-light-pollution/
======
icanhackit
> _Shouldn’t the image taken at ISO 6400 show more noise than the image taken
> at ISO 1600? If the exposures were the same brightness, yes. But these
> images were not taken with the same exposure. The first image started with
> twice as much shutter open time (30 seconds versus 15 seconds) and two stops
> more gain on the sensor (ISO 6400 vs ISO 1600). The result is that the first
> image has significantly more light data. This makes the final signal-to-
> noise ratio of the processed images higher on the first image. The higher
> the signal to noise ratio, the less noisy the resulting photo. So in this
> case it was actually better to overexpose and compensate later in post
> processing, despite the initial unprocessed images appearing unusable._

This is really neat. I've got a full frame camera with a sensor that has very
good dynamic range and I'd learned to under-expose and then pull detail from
the shadows in post processing as information can't be saved from blown
highlights. This sort of flips that on its head, except the featured
overexposed shot didn't have blown highlights, it just looked like it did.
Instead it had a wealth of low-light data.

~~~
michrassena
What I've learned over the years with digital cameras -- you can't trust the
preview image. The histogram tells much more of the story (if not the whole
truth). My photographs have become much better (in a technical sense) after I
learned to expose almost to the point of blowing highlights, making sure there
is little blank space at the right (where the highlights are displayed) of the
histogram as possible (hence the term Expose To The Right). As someone
downthread stated, this improves the signal-to-noise ratio of the captured
data. There's an unwritten assumption that the camera's built-in auto-exposure
capability isn't sophisticated enough to expose the scene properly, and the
photographer has to take control. This is often the case.

And speaking of SNR, there are a few tricks astro-photographers have been
using for years that occasionally find their way into mainstream photography.
One is image stacking, where you combine multiple photographs of the same
scene (useful for landscapes if a little blur isn't a problem for you) into a
single image. This pushes the shot noise (the random distribution of noise)
down even further. I've gotten clean images out of cheap cameras, and even
from cheap cell phone cameras (given enough images).

The other technique is dark frame subtraction, where you take an photo with
the lens covered at the same ISO/shutter/aperture as the rest of the set.
Digital sensors have readout noise and heat noise which tends to be very
predicable. Subtracting this noise gives you a cleaner shot as well.

On a side note, I do wish when talking about ETTR and even digital photography
in general that the term "overexpose" wasn't used when explaining the
technique of overriding the cameras default metering with exposure
compensation. Overexposure would be blowing the highlights, or at least the
highlights of details that the photographer considers important (e.g. blowing
the highlights of street lights is usually okay). The "over" portion makes it
sound like the photographer is doing too much of something, when in reality
ETTR is a technique of giving enough exposure, but not too much.

~~~
vanderZwan
> One is image stacking

Since you seem to have experimented with this, I'd like to ask: don't you need
a certain "minimum" time to capture the faintest of stars, because their
signal otherwise wouldn't rise above the noise floor? I've been curious about
this with stacking in atrophotography for a while.

> The other technique is dark frame subtraction, where you take an photo with
> the lens covered at the same ISO/shutter/aperture as the rest of the set.
> Digital sensors have readout noise and heat noise which tends to be very
> predicable. Subtracting this noise gives you a cleaner shot as well.

For long exposures this heat noise becomes more problematic: long exposures
heat up the sensor quite a bit. Have you tried building your own customised
electric coolbox to keep temperatures down? It can be a fun project!

I actually used this in a different context: during long Skype sessions at
home with my people abroad, my phone heats up _a lot_ and the video would
start stutter. Putting it on a cooling block actually fixes that. This had
more to do with the system not having to scale back CPU performance to let the
phone cool down, but a side-effect according to people on the other end of the
line, was that the image quality improved too

~~~
jxcl
> don't you need a certain "minimum" time to capture the faintest of stars,
> because their signal otherwise wouldn't rise above the noise floor?

I'm not an expert or even very knowledgeable on image stacking, but I'll share
my experience.

I've taken a stacked picture of the Orion constellation. 15 exposures at 5
seconds each, and combined them to get this photo:

[https://www.flickr.com/photos/abliskovsky/31756623694/](https://www.flickr.com/photos/abliskovsky/31756623694/)

In none of the individual exposures is the Orion Nebula visible, but in this
photo it's pretty clear. So in the end I'm fairly sure that your statement is
true, but the signal required is very, very faint.

~~~
craftyguy
Do you know of any good resources for "getting into" image stacking for a
complete newbie? 2 years ago I tried stacking images I took of Jupiter in my
16" dob (taken with a Canon DSLR), but it was terrible and I'm not confident I
used the app I found (I _think_ it was AstroStack..) for image stacking
correctly.

~~~
jpk
RegiStax is popular for solar and planetary imaging. I've used it several
times on H-alpha solar photography. It has a fairly well defined workflow and
there are lots of video tutorials on how to use it.

~~~
craftyguy
Awesome, I will check it out. Thanks! Also looks like it runs under Wine on
Linux, which is good news for me!

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teraflop
While idly thinking about light pollution a while ago, I thought of a
ridiculously impractical solution:

Mandate the replacement of all outdoor night-time illumination with LEDs that
are pulse-width modulated at a low duty cycle. Synchronize them all to an
accurate global clock (e.g. from a GPS receiver), so that for instance, all of
the lights are simultaneously turned on for the first tenth of each UTC
millisecond. Then an image sensor with a sufficiently fast global shutter
could disable itself during every brief pulse of light, so that it picks up
90% of the incoming starlight, but almost none of the light pollution.

~~~
ssalazar
Fix the duty cycle to a particular slice of the phase of AC voltage (e.g. the
first 0-pi/2) and you accomplish the same thing much more practically, at
least within a single power network.

~~~
NickNameNick
It's going to be a bit trickier than that, because power is usually
distributed as 3-phase, not single-phase.

So that's 3 different offsets, depending on which phase any given light is
wired into.

Plus that's probably going to be slow enough to cause visible flicker.

~~~
hxtk
Have it trigger on a voltage threshold. 3 phase means you're hitting 0V 6x per
cycle so that'd bring the pwm up to 720Hz.

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Florin_Andrei
If you're into deep space astrophotography, this is not really news.

It's all about exposing for the stuff you care about. When imaging DSOs (deep
space objects), you often don't care about the relatively bright stars nearby.
But you do care about that faint little object in between. So you adjust
exposure so the stars are usually blown out (overexposed), but the DSO along
with the light pollution background have peeled off the left hand edge of the
histogram.

So now your DSO is sitting comfortably in the middle of the histogram, where
you can process and extract it easily.

In practical terms, for DSOs you tweak exposure until the big light pollution
hump on the histogram becomes disconnected from the left hand edge of the
graph. Turn exposure up gradually until there's a gap at the left hand wall.
That usually does it. Overexposure is bad, but underexposure is super-duper-
evil, so you must avoid it. OTOH, don't push things too far to the right - you
still want some chroma info in the star halos.

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A_No_Name_Mouse
Would it be possible to filter out the pollution by subtracting an out of
focus image from the in focus one? The pollution is already spread out while
the stars are point light sources. So wouldn't that leave the stars untouched
while greatly reducing the light pollution?

~~~
vlasev
All you need to do is make a copy of your image, blur that copy enough to get
rid of the stars, and then subtract it from the other image. It's going to
affect larger structures like the milky way, though.

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avenoir
Overexposing and underexposing (formally known as Bracketing) is typically a
great technique in landscape photography too because it captures as much of
the dynamic range as possible. Most DSLRs can bracket right out of the box.
Set it up to take 3 exposures (1 underexposed, 1 normal and 1 overexposed)
with about 2 or 3 stops in between. Then merge all 3 exposures in Photoshop
and you'll get an image with a TON of information embedded in it for you to
tweak in post-processing.

Also, while I have't tried this, I'd think that using an ND filter in light-
polluted areas like this could help a little bit with astrophotography.

~~~
js2
There are light pollution reduction filters made specifically for
astrophotography. I was shocked at how good these photos were w/o using such a
filter. I don't think you ever could have recovered so much of milky way using
film w/o a filter in such a polluted sky. (I was certainly never able to.)

Googling for an example, amusingly enough, the first link is to the same site:

[https://www.lonelyspeck.com/hoya-intensifier-review-an-
affor...](https://www.lonelyspeck.com/hoya-intensifier-review-an-affordable-
light-pollution-filter-for-astrophotography/)

That filter is not specifically for astrophotography, just one that happens to
work well. Here's a discussion of other options:

[https://www.cloudynights.com/topic/494162-light-pollution-
fi...](https://www.cloudynights.com/topic/494162-light-pollution-filter/)

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StavrosK
Here's something I've wondered about a lot, but haven't been able to find a
definitive answer: Isn't ISO pretty much software gain control? Why ever
increase the ISO on the camera instead of just taking photos at the native ISO
and then post-processing the photo to a higher exposure? That way, you don't
store the blown-out pixels (if you do overexpose).

~~~
hug
ISO is not just software gain. On a lot of higher-end cameras (read: just
about any ILC) there's a definite analog gain stage prior to ADC.

Since there's a fixed level of sensor noise, when you increase the analog gain
you decrease the level of noise in the shadows relative to the sensor noise,
essentially increasing SNR. This isn't true for all cameras, however.

This chart
[[http://www.photonstophotos.net/Charts/PDR_Shadow.htm#Canon%2...](http://www.photonstophotos.net/Charts/PDR_Shadow.htm#Canon%20EOS%206D,Nikon%20D750,Sony%20ILCE-7RM2)
] demonstrates the difference between three cameras, one of which is known to
be largely "ISO invariant" (the Nikon D750), one which is not (the Canon 6D),
and one which looks like it has a 'native' ISO of 800 (the Sony A7R II)

As you can see, the D750 doesn't gain much (heh) from increasing ISO -- the
total image noise in the shadows isn't improved by increasing the ISO through
the range. If you're shooting a 6D in a low-light situation, however, you see
improvements in shadow quality up to about ISO 3200, after which it turns out
that you will be fine if you just raise the shadows in post. If you're
shooting an A7R II you might as well just leave the ISO dial on 800 all the
time.

That's not all, though, since it turns out when you increase ISO you basically
lose 1EV of highlight off the top of your image, as you allude to, and you
also lose around 1EV of DR. (Chart here:
[http://www.photonstophotos.net/Charts/PDR.htm#Canon%20EOS%20...](http://www.photonstophotos.net/Charts/PDR.htm#Canon%20EOS%206D,Nikon%20D750,Sony%20ILCE-7RM2))

It is, like all things in life, a balance.

~~~
StavrosK
Fantastic, that's exactly what I wanted to know, thank you. This is very
useful for setting my max auto ISO.

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jsjohnst
As a photographer who's shot the Milky Way a lot, I can say this is excellent
advice.

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anon1253
Or get an IDAS-LPS, CLS or UHC filter (either on the lens or as a sensor clip
on). Or stack a bunch of images with proper dark substraction. If you stack
enough light sub frames you can efficiently reconstruct dynamic range. Of
course the ETTR advice is good too, but you can go much further even with
consumer equipment.

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bemmu
I have trouble visualizing how the view of the Milky Way as seen from Earth
reconciles with pictures showing whole galaxies. Are we seeing it edge on? Is
there any cool animation showing it first from the outside, then rotated to
match how we see it in the sky from Earth?

~~~
herodotus
Imagine a vinyl record, and that you are sitting on the record, near, but not
at, the outer edge of the record. South of you, but close to the horizon, is
the centre of the record - containing a massive black hole. North of you are
all the stars in the galaxy between you and the edge of the galaxy. Most of
the stars in the galaxy are scattered along the disc, but with a fairly large
variation. The closer you get to the centre, the greater the number of stars -
that is the brightest part of our night sky.

Some ancient clusters of stars have drifted a fair bit away from the disc, but
are still gravitationally bound to it, and its dark matter. If you were on
another galaxy looking from above, our galaxy would look like a spiral with
arms. If you looked edge on, it would look more like a cigar.

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e12e
Interesting. I would've liked to see the result of 1600 iso at 30s too - just
for comparison.

On a note about light pollution, the mention of sodium street lamps
immediately made me think of filters - stars are suns, so should have wide
spectra - why not just filter out the orange bit? Apparently I'm not the first
with the idea (obviously):

[http://www.nezumi.demon.co.uk/nonad/spectra.htm](http://www.nezumi.demon.co.uk/nonad/spectra.htm)

[https://petapixel.com/2016/12/14/purenight-filter-cuts-
light...](https://petapixel.com/2016/12/14/purenight-filter-cuts-light-
pollution-better-night-sky-photos/) (has some nice with/without filter images)

~~~
lobster_johnson
The PureNight filter is coincidentally a crowdfunded product by Lonely Speck,
the people (Ian Norman and Diana Southern) who contributed this story article.

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basicplus2
When i was a child all the street and business lights were turned off at about
11pm.

Now they are on 24/7

Incredibly wasteful, although i suppose it is safer to go about ones business
at night.

It certainly gave the opportunity to view the stars even if one lived in city
areas.

~~~
russdill
I'm curious if in many decades self driving vehicles will eliminate the need
for most night time street lighting.

~~~
dgsdgdsg
Street lighting predates cars.

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johndoe90
I wonder where are the star trails. Wouldn't 30s exposure make earth movement
noticeable?

~~~
hug
The answer is the 500 rule. To avoid star trails, exposures should be less
than 500/focal-length-in-135-equivalent.

In other words, if he shot this on his 'favourite' Rokinon 24mm, an exposure
time of 20 seconds is reasonable to avoid all star trails.

~~~
johndoe90
Didn't know that one. Thanks a lot!

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michrassena
I'm quite surprised any images of the Milky Way are possible in such a light
polluted environment and I'll be using these techniques when I can to see if I
can replicate the results.

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srrge
This articles reminded me the wonders that surround us while we go on living
our boring human lives, so easily forgetting about the marvel of life and
nature. Thanks for sharing this.

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sickrumbear
Resource limit reached, anyone have a cached version?

~~~
lifthrasiir
[https://web.archive.org/web/20170912050030/www.lonelyspeck.c...](https://web.archive.org/web/20170912050030/www.lonelyspeck.com/the-
milky-way-in-los-angeles-light-pollution/)

