Category Archives: Noise Reduction

Dark Frames And Your Night Photography

In an upcoming webinar (Down with the Noise) I explain a lot about noise: causes, contravention and cures. This is a bit of a prelude and addresses the questions:

  1. What is a dark frame?
  2. What do I do with my dark frame?
  3. What do I do if I don’t have dark frame(s)?

What Is a Dark Frame?

A dark frame is one or more images taken at the same exposure length, ISO and ambient temperature as the light (normal) frames but with the lens or body cap on the camera to prevent any light from reaching the sensor. When doing many kinds of night and low light photography dark frames can be quite helpful. And when doing star trails or other night imagery dark frames may save your bacon. A dark frame is what your camera does after a long exposure when long exposure noise reduction is turned on. But you’ll be far more efficient if you take those frames yourself. If you’re taking 100 light frames, e.g. for a star trail, you can take 3 or four dark frames and waste 50% less time (and not have gaps!)

Contrary to popular belief dark frames and long exposure noise reduction do little to reduce the random noise that is present in every exposure. That random noise is most pernicious in dark photos and shadow areas.  Dark frames, however are good for the following things:

  1. Reducing or eliminating hot pixels and amp glow
  2. Removing any “bias” in your image – that is bringing the black back.
    Want me to translate that: an unexposed area on your sensor should read as “0,0,0”  for Red, Green and Blue but I will bet you you don’t get zero!

This would probably be a good place to show you what a dark frame looks like. But you’ll be disappointed. Dark frames are usually quite black.  So instead of showing you JUST the dark frame, here is the dark frame boosted to show the speckles from hell – though they may not be obvious. Here I have made the speckles more obvious by boosting the darks using Curves in Photoshop. At this level of detail there are not any obvious hot pixels.

Dark Frame Overview – Boosted to show details. Note where the markers are – they are shown in the next frames.

And next is the same dark frame zoomed to 3200% unaltered. Hover your cursor over the image to see the same area boosted using curves.

Single Dark Frame (Linear Mode) Unmodified – or cursor over to see boosted version.

When inspected carefully, and with the dark level significantly increased it is possible to notice the hot pixels and possibly banding in a dark frame.  While there were plenty of red speckles and obviously green and blue as well in my stable of dark frames, the “hot pixels” didn’t leap out at me.  If you look carefully at the image you’ll notice I also used the color sampler tool to provide RGB values for 3 different locations on the image.  Of note is location 1 where the R (red) value is 14.  What is particularly worrisome about that value is that even after the entire frame has been boosted to the equivalent of 1.6 stops, you’ll notice that a value of 14 is still larger than all three colors at spot 3. After boosting, that red pixel really stands out with a value of 44.  Our first take away is that boosting the brightness boosts the noise.  The second thing to notice is that the red spot is NOT a hot pixel.  How do we know? Compare 4 dark frames (all boosted)

Boosted Dark Frame                             Click these–>   Frame 1 ~ Frame 2 ~ Frame 3 ~ Frame 4

Takeaway 2: There really is randomness!

Now that we have noticed the randomness, we realize that if we average enough of these frames together we can get the average “bias” – that is the amount of offset above zero in the image.  And if there are hot pixels, the good news is they will be in there too.

But How Do I Use a Dark Frame?

The simplest answer is to feed your dark frame(s) to a program that already knows what to do with them like StarStax, StarTrails or Image Stacker.  But you can do it yourself, and perhaps more elegantly using Photoshop.  How?  Place the dark frame as a layer over the image you want to correct and change the blend mode to Subtract (or difference). Adjust the opacity of the blend until it looks just right.

But I Did Not Take a Dark Frame, Now What?

All is not lost. If you have enough frames you can create a unique kind of dark frame. I took over 300 28-second exposure for a star trail along Lake Gaston in North Carolina.

In the image below I created the top frame using the Brighten mode in StarStax. I could just as easily have created the top frame using the StarCircleAcademy Stacking Action.

I used Darken mode to create the middle frame by feeding it my 100 darkest images. Using Darken mode as the stacking option means that hot or stuck pixels that are in every image as well as the lowest value of sky glow will be collected into a single result.

I then loaded the light (Brighten Mode) and the dark (Darken mode) frames into Photoshop. I placed the dark image over the brighten stack and changed the blend mode to Subtract.

What Happened Here?
Dark Frame Substitute process

Several interesting things happened:

  1. The hot pixels were almost completely annihilated
  2. The sky gradient caused by lights glowing in the distance was also almost eliminated.
  3. The contrast in the sky and elsewhere was improved
  4. The red bias on the railing was mostly removed.

A few less desirable things happened, too. The bright red glow on the railing once subtracted caused some of the railing to turn green. And the subtraction created some “holes” and “halos” in the image – especially where the brightest lights are found.  With some minor touch up, most of those issues can easily be fixed.

Is this the end? By no means! There are a LOT more interesting techniques to follow. Stay tuned.

SHARE this on Facebook or Google+ You know you want to help your friend out anyway, right!?

The Elusive Milky Way – How to Find It!

Last revised: July 5, 2018
Original Publish Date:  June 26, 2012

This is Part 1 of a multi-part series on finding and photographing the Milky Way.  From November through February it is impossible to spot the densest part of the Milky Way because the sun is hovering there. Read on for more information.

What IS the Milky Way?

Path of the Milky Way West-to-East

The Milky Way in Summer from Horizon to Horizon

We are located in a corner of the heavens of a galaxy we call “The Milky Way.” The Milky Way stretches all the way across the sky and some part of the Milky Way is present every night – indeed EVERY star you see in the sky is located within our Milky Way.   Most people, however, think of the Milky Way as the cloud-like stretch of stars from the constellation Scorpius (aka Scorpio) to the constellation Cygnus – particularly the part nearest to Sagittarius.  I’ll try not to be too poetic, but when you have clearly seen the Milky Way, it is hard to describe how awesome it is without breaking into song. In ideal conditions the diffuse light of the Milky Way can cast a shadow on the ground!  Unfortunately there is little chance that you will ever see that shadow because most accessible places in the world are mildly to HORRIBLY light polluted. Constellations that are found in the Milky Way include:  Perseus (off the bottom), Cassiopeia (near the bottom in the picture above-left), Lacerta, Cygnus (near the center), Aquila, Sagittarius, Ophiuchus and Scorpius (very top).  Those in the Southern Hemisphere will also find Norma, Circinus, Crux, and Carina.  There is a faint portion of the Milky Way visible in Puppis, Canis Major and the bow of Orion. Look carefully at the image above and you’ll see a bright “smudge” in the center of the bottom fourth of the image. That is one of our sister galaxies known as Andromeda. The galaxy gets its name from the constellation in which it is found.  With an unaided eye it is readily possible to spot Andromeda in a dark sky. With binoculars Andromeda is observable even in a suburban area. In the southern hemisphere two additional sister galaxies called the Large and Small Magellanic clouds are easily seen. On a dark clear night it is easy to observe the lack of stars in the broad band of wispiness that forms the Milky Way. But the dark void is not due to the absence of stars. The void is due to immense inky dust lanes that obscure the stars!

When to See the Milky Way

The sun is in the constellation Sagittarius in December so during November, December and January it is impossible to view the richest part of the Milky Way.  October and February are generally impossible, too. The optimum viewing time in the Northern Hemisphere is in the summer when the sun is on the opposite side of the sky. Unfortunately summer in the Northern Hemisphere is also when hot, stormy, cloudy weather is doing its worst and also when the nights are the shortest. Those in the Southern Hemisphere have an advantage – longer and cooler nights during winter mean the air is clearer. Using a simple tool called a planisphere it is easy to predict when and where to look for the dense part of the Milky Way. But what must also be factored in is the location and phase of the moon. The time of year and the direction of the least light pollution also frame the parameters for getting the best view of the Milky Way. Generally the dense part of the Milky Way is best viewed when it is as high as possible in the Southern sky. Facing south during April and May the pre-dawn hours are best. From June to early August the best time is near midnight, though the Milky Way will be visible almost all night. From Mid August through September the best time is soon after the sun has set and the sky has grown dark. Below is an illustration that may help you. It was created for 38 degrees North latitude, but will serve the middle of the United States, Southern Europe, Northern China, Japan, and any location at a similar latitude very closely.  The farther North you go, the lower in the southern sky the Milky Way will be.  If you live above 65 degrees north, you will never see the Milky Way core because it never rises above your local horizon. The circle in the image below indicates the star Alnasl in the constellation Sagittarius. Above Alnasl (the spout of the teapot) is the center of our Milky Way Galaxy.

The Milky Way as seen from Mid-Northern Latitudes

The Milky Way as seen from Mid-Northern Latitudes Facing South. Click for a larger illustration

Best Times To Spot the Milky Way

Month Best Time (Local Time) Moon Phase*
  January IMPOSSIBLE
  February Difficult. Before sunrise (late February only) 3Q to New
  March Difficult. Before sunrise New to 1Q
  April 4 AM to Sunrise New to 1Q
  May 3 AM to 6 AM New to 1Q
  June 10 PM to 2 AM New
  July Sunset to Midnight 3Q to New
  August Sunset until 10 PM 3Q to New
  September Sunset until 9 PM 3Q to New
  October Difficult: Sunset (early October only) 3Q to New
  November IMPOSSIBLE
  December IMPOSSIBLE

*1Q means first quarter moon (half full). In its first quarter the moon rises around noon and sets near midnight. 2Q is a Full moon. Nearly full is called a Gibbous. It is nearly impossible to see the Milky Way when the moon is near full. 3Q is the third quarter (also half full) moon which rises near midnight and sets near noon. New means the moon rises and sets very near the sun. Includes a slender crescent phase, too.

Where to See the Milky Way

Central Nevada, Eastern Utah. Montana. In short, remote areas far from city light pollution afford the best view. But if you know what to look for and when and where to look you can spot the Milky Way from many places throughout the world. Or you can wait for a massive regional blackout. I have seen the Milky Way very clearly from the top of Mission Peak in Fremont, California – an area with over 8 million people in literally every direction. However that glimpse required that the entire Bay Area be blotted out by low, heavy fog. My perch was above the darkness blanket that the fog provided. Yosemite National Park is still mostly dark despite cities like Fresno that are doing their best to ruin the darkness. As Numerous at the Grains of Sand Anywhere along a remote area of the coast far from cities there is a chance to see the Milky Way. For example, I spotted a washed out Milky Way just 8 miles north of Santa Cruz, California.  A long exposure and some photo editing improved the view.  If you get farther away from civilization the results can be much more spectacular as you see here. Finding somewhere in the country where it is truly dark, like the Black Rock Desert in Nevada, or White Mountain in Central, Eastern California the Milky Way reaches its most inspiring awesomeness. If you live outside the United States, do not despair, you have a good chance of seeing the core of the galaxy from anywhere south of 55 degrees northern latitude. Above that latitude the core of the Milky Way will never rise above the horizon.

How do I See the Milky Way

Visor View [C_033780]I know what you’re thinking: don’t I just “look” in the right direction? The answer is no! It takes your eyes 15 to 20 minutes to see their best in the dark.  Any bright light source in the direction you look will diminish the view. Running out of a well-lit house, or jumping out of a car where you’ve spent the last 15 minute driving with the headlights on will make the Milky Way far less awesome.  Avoiding ALL light and shielding your eyes from anything you can’t avoid will help a lot.  Do you see the Milky Way in this photo from the top of Clouds Rest in Yosemite? I promise you it is there. It juts out above the Yosemite Valley near the center of the image. Here is a view from the wilderness in Yosemite.

Lost in Yosemite [C_033706]

Can You Help Me Find the Best Time Where I Live/Work/Travel?

In short, no. Please read through the comments for many such questions and answers. It’s impossible to cover information for everywhere on earth, but our notes here cover all the general concepts. Moreover, the moon phase is different – the moon doesn’t behave like the sun, so even though the Milky Way may be glorious this year on say August 8th, next year the moon may be full and obscure your view. Also, weather conditions may affect your view, so do not neglect that variable!

How Do I Photograph The Milky Way?

Cameras are getting better all the time, and there are some nifty tricks you can use to make a compelling photograph of the Milky Way even if your camera is not the top heavyweight performer in the gear smack down.  We’ll cover cameras and techniques in installment 2 of this series!

Famous III [C_035478]

Simple Astro Processing Technique to Conquer Noise

Published: Jun 20, 2012
Last Updated: March 5, 2017

Is one of these your scenario?

  • It’s really dark. The ISO is bumped up, the noise is screaming at you, but you REALLY want the shot.
  • The Milky Way looks SO gorgeous, you want to take it home with you like a trophy, but when you shoot short enough exposures to prevent smears, mostly what you get is noise.
  • You are surprised that you can faintly make out the Milky Way. You know your buddies will be jealous if you can show them a photo of the Milky Way that you took from IN TOWN. They won’t believe you!
  • You have a great star trail, but your foreground is not lit. The photo would sing if you could tease out that foreground – minus the noise, of course.

In the Star Circle Academy’s “Astrophotography 101: Getting Started without Getting Soaked” webinar we cover all the theory and equipment you need to take gorgeous photos of deep sky objects (nebula, galaxies):

Colorful Neighbor

But absent the fancy equipment, all you need is a wee bit of Photoshop skill to get a pretty compelling image. Less than 10 miles away from Palo Alto, California, with over 8 million households in a 50 mile radius I got the image you see below. I understand why you might not believe me.  Is it the most compelling Milky Way you’ll ever see – definitely not.

Urban Milky Way [C_036919-23PSavg]

Here is the best I could do with a single image from the same location:

Milky Skyline [5_006550]

After much processing it’s still noisy (grainy) and contrast poor.

We covered the processing technique in our Night Photography 150: Photo Manipulation I Webinar – among many other topics. Below is a 7 minute video describing how to do that simple astro photography processing.

If you think a webinar on photo processing would be of interest, join our Interest List for this or other topics and you’ll be notified when we schedule the next webinar. You can influence the topics we choose to cover by making your comments here.

Simple Astro Photo Processing in Photoshop CS5 from Steven Christenson on Vimeo.

NOTE: If the above says password required, enter scanp150 In the video, you’ll also learn how to constrain the healing tool, use curves, layers, and the history tool to undo inadvertant changes.

ALSO NOTE: Advanced StackerPLUS has a built in averaging operation. You just feed it the images. It does NOT do auto alignment, however.

In our next installment, we will talk about how to get the Milky Way shots in the first place. Camera considerations, settings, tradeoffs.

By the way, this image consists of a single sky shot and a multi-processed foreground using the technique described above. Click the image for further details.

South Side Truckin' [C_009842]

 

We have another video tutorial that uses some similar processing techniques:

12 Minute Star Trail using Advanced Stacker PLUS version14D from Steven Christenson on Vimeo.