Astrophotography in the City – Part 1

Most people don’t try astrophotography, shooting the stars and constellations, because they think it requires specialized equipment and dark skies.  While nothing beats getting away from the city and light pollution, anyone with a camera with a MANUAL setting and capability to save RAW files can create nice photos of starry skies even if you live in the city. Below is a quick run-down of a fool-proof recipe: Part 1 – taking pictures.

Astrophotography is heavily dependent on post-processing the images as we are trying to get a desired signal from noise. That noise can be electronics (the camera and sensor) and it can be the light pollution. Like the old saying: garbage in = garbage out. If you can find the right camera settings to reduce noise on your photos, you’ll get fantastic results with much less processing and effort.

original-processed2

 

Setting up the Camera

DSLR are the best camera to use, but any camera that can set to manual will work. First thing is to set the file to be saved in RAW. Astrophotography is a heavy user of post-processing, so you want to work with as much unaltered data as possible. We want the image as the sensor captured it, and leave the processing to powerful algorithms on a computer.raw-format

Next is to set the camera to full MANUAL mode such that you can control lens aperture, exposure and ISO setting. If you are going to use a remote device to take the pictures, you may need to set it to B or BULB, but for my Canon 80D connected via WiFi to the smart phone, below 30 second exposure time M will work.

manual-modet

Next you want to set the lens opening as big as possible.  For most variable focal zoom lens, that is F4.0, but you may have opted for a fixed lens which can open up to F1.2.  Note however that large openings with consumer photo lens tends to cause either chromatic aberration (colors will “leak” around bright stars) or distorted stars the further towards the edge of the frame.  If you notice this, simply stomp-down to a slower opening by 2 or 3 settings.  Yes that means you get less light, but it’s a trade-off. You can also simply crop the final image at the very end.

Next set the ISO to about 6400.  Can’t go that high? No problem, as long as you can reach ISO 400, you are good. I know, high ISO is very noisy, but the next step is simply to get the right focus, so we don’t care about the noise and with the camera live view, the exposure is not very long and we want to see the stars.

Mount your camera on a tripod as the exposure length will be between 2 and 10 seconds. Hand-holding is OK for the Moon, but not to get nice round stars at those long exposures.  If you don’t have a tripod, setting the camera on a bag of beans or rice, even a bunched-up towel will work. Find a spot where you don’t have glaring lights entering the lens, and aim you camera at the desired spot in the sky.  This is also the time when you set you focus to manual and crank it to infinity.  If there is no marking on the lens for focus at infinity and you don’t know which way to turn, simply pick a distant object like a far away house or light post and manual focus on it. The Moon will also do the trick.

If you have live view mode on the camera, enable it and manually adjust your focus to get nice sharp stars. Some cameras will even allow you to zoom on the preview screen, if so zoom as much as possible and fine-tune the focus. If you don’t see stars: 1) increase the ISO setting, 2) increase the exposure duration, 3) verify that you are at F5 or lower.

If you don’t have live view, simply take a picture and then review it (don’t forget to zoom in on a star). Make a small focus adjustment one way and take another picture.  If the stars are smaller and brighter, you are adjusting the focus in the right direction and keep going until you passed the best setting.  Then simply back-it a small amount.

Getting the Right Exposure

Once the focus is right, the next step is to balance the ISO and exposure length.  The longer the exposure the more the stars will become trails instead of pin-points.  However longer exposures gather more light to capture more stars and faint objects. If you are shooting with a 15mm focal length, you can probably go as high as 20 seconds before it becomes too much of a blur. However at higher focal length the stars will “move” faster, so choose wisely.  Aim for about 5 to 10 seconds of exposure.

Here is where we adjust the ISO.  High ISO setting will generate a noisy image.  In astrophotography we “stack” multiple images to improve the Signal to Noise Ratio (SNR). Hence a noisy high ISO image isn’t so bad, but you still need to keep the noise to a minimum. When we focused with the live view, the ISO was cranked quite high, but this will result in an image with the background sky way too bright. In the image below, the “hump” in the histogram is entirely past the half-way mark in the over-exposure region, this is not good for astronomy post-processing, where we want to have as much dynamic range as possible. As a general rule in astrophotography, you are better off under exposing.

overexposedIn the photo above, I was at ISO 6400 with a 5 second exposure. You can barely make out the constellation Orion in the sky. After reducing both the ISO and the exposure to ISO 3200 and 2 seconds the sky darkens, and pin-point stars start to appear.

iso3200_2sec

Once you’ve got the right settings, take a series of pictures.  If you can trigger the shutter from your smart phone, tablet, remote or laptop then it’s best as you avoid nudging the camera and smearing the stars. If not, well… go gently. Take about 20 images. These will be your “LIGHT” frames as they are the images you captured light photons.

Once this is done, you need to two other sets of images that will be used in post-processing.

Dark and Offset Frames

With all cameras, the longer the exposure, the more noise and “hot pixels” appear. This noise needs to be removed from the image. Some camera have settings to automatically do this for night shots, but it will do so with every image, doubling the time it takes every image, and the result is not optimal.  Software on your computer is much more powerful than the camera to process and remove the hot pixels, so you are best to take a series of DARK frames yourself.

Hot pixels are essentially pixels “firing off” during a long exposure causing it to create a bright pixel in your image.  Two factors increases the number of hot pixels in an image: 1) exposure length; 2) temperature. Most of your photos with your camera are daytime, short exposures, hence hot-pixels are either non-existent, or not visible. However with a dark sky and exposure in the 5 to 10 seconds range, they will be present.  Temperature will also play a factor, it’s why specialized astro-cameras are Peltier cooled to 40deg C below ambient. Yes, you will get more hot pixels in a summer night shot, then in winter.

Furthermore, all digital cameras uses an electronic circuit with an amplifier to read the sensor.  This amplifier generates heat, which often shows up on the sensor by making one corner brighter than the rest of the image. The longer the exposure, the greater the effect.

DARK frames are REALLY easy to take.  After you are done taking your LIGHT frames, simply put on the lens cap and take another 10 photos with the lens cap on. You are essentially capturing the noise of the sensor when no photons enter the camera. The reason to take a high number like 10 is to generate a MASTER DARK, which will be an average of those 10 dark images, this gets rid of any random elements to the noise.

Last you will also need to take OFFSET frames. These are like the DARK frames explained above, but this time with a short exposure setting like 1/250s.  Here we want to capture the electronic read noise of the sensor.  With such a short exposure, there are no hot pixels or amplifier glow. Yes, still with the lens cap on, so it’s a nearly black image, but there is a bit of signal, a bit of noise registered within it, and this is what we want to isolate. So like the DARK, take another 10 images.

IMPORTANT: Every-time you will do astrophotography, you will need to take DARK frames to match the camera settings and temperature.  However for OFFSET frames, you only need one set per ISO setting. So OFFSETs can be kept for use another day if you took photos with the same ISO setting.

To conclude if you followed the above steps you now have:
– 20 light frames of the night sky
– 10 dark frames
– 10 offset frames

I’ve purposely kept FLAT frames out of this process as they are a pain to take, and if done incorrectly cause more trouble than good, FLAT frames are images of a uniformly lit white surface with no texture or details. The purpose is to capture the shadows on the sensor caused by dust as well as to correct to brightness uniformity and optical imperfections. Lets just keep that out for the time being…

Now head back indoors, it’s time to process these images

Star Trails, Plane, Meteor and Cosmic Ray

Simply setting up a camera to take a series of images of the night sky can pick up a lot more than a few stars.

trails_secondaries_smallIf you have a wide-angle lens, and live near a large city there is a good chance that some aircraft will fly into the field of view.  The linear streak and alternating lights are a dead give-away of a plane having crossed the camera’s field of view.  If you don’t have the alternating lights, it’s mostlikely an orbiting satellite reflecting sunlight.

Meteors are also somewhat of a common occurrence.  These are easily recognized by their characteristic increasing than decreasing brightness as they burn up in the upper atmosphere. The meteor in the image above is from the Geminid shower.

The last artifact comes for outside our solar system, it is cosmic rays.  The CCD or CMOS sensor of your camera works by performing an electric read-out of photons captured by the lens.  Cosmic rays are high-energy sub-atomic particles that have traveled through space and managed to make it through the atmosphere down to us.  The one in the photo above just happens to hit my camera sensor.  As the near light-speed sub-atomic particle smashes into atoms on the sensor it looses energy, freeing up electrons which register as “light” by the CCD.  Most of the time the cosmic ray will hit the sensor straight on,  but sometimes it impacts at a shallow angle and causes a series of pixels to “light” up, as in the photo above.

Take time to examine your photos, you never know what surprises you may find.

Open Cluster NGC 6633

Open star clusters are the galaxy’s youngest stars. They are created from the collapse of giant molecular gas clouds, often forming large and very hot stars shinning brightly in the blue-white part of the spectrum.  As they are rapidly consuming their fuel, they are also short-lived.  By ending as a super nova, they create the heavier elements beyond carbon that exists all around us.

Below is open star cluster NGC 6633, estimated to be 660 million years old (our solar system is 4.6 billion years old). The cluster is of a decent size covering just about the size of a full Moon in the night sky.  The brighter and whitish stars stand out against older and further stars in the background.

Open Star Cluster NGC 6633

Open Star Cluster NGC 6633

Younger star clusters such as the Pleiades (Messier 45) have yet to burn away their molecular gas clouds.  However there is no hint of glowing gas (nebula) with NGC 6633.

Skywatcher 80ED
Canon Rebel XTi
51x30sec (25.5 minutes) ISO 400

Cassiopeia – the W in the sky

Some constellations are easier to spot than others.  Cassiopeia with its distinctive W is visible year round in the northern hemisphere above the 34th parallel. In the image below it easily stands out from the fainter background stars.

Cassiopeia above the three line - Benoit Guertin

Cassiopeia above the three line – Benoit Guertin

The five stars drawing a W in the sky are all naked eye magnitude 3 and brighter stars, and in the image above I used a layering technique to increase the color and brightness of those stars to really make them stand out.

  1. Duplicate your base image, and set this layer to lighten only
  2. Apply a blur to the top layer(about 8-12 pixels)
  3. Increase the color saturation and brightness.  Play with the curves to brighten the bright stars, but not the background sky.
  4. Use a mask as required to filter out the bright foreground elements, such as light reflecting off a building roof-line in my image above.

Canon Rebel XTi
17mm f/4
4 x 20sec ISO800

 

The Milky Way (Sagittarius to Aquila)

The summer is ideal time to view our galaxy.  Because of Earth’s position with respect to the Milky Way, it runs north-south across the sky.  Anyone with a camera and tripod can easily capture the Milky Way if you are located in a dark area, away for city lights.  We were up north in the Malbaie, Québec area for vacation, so I took some time in the early night to observe and photograph the sky.  Unfortunately, a full Moon was present in early August and the sky would actually brighten past midnight.  The best time was around 11pm for any good viewing and astrophoto. Click on the photo for a high-resolution version.

Milky Way - Sagittarius (just above the trees) to Altair (bright star upper left)

Milky Way – Sagittarius (just above the trees) to Altair (bright star upper left)

Here is a quick run-down of a quick setup if you want to give it a try:

  1. Use as short a focal length as you can, 15mm to 25mm is good.
  2. Set the camera to MANUAL for everything, including the focus and disable any image stabilization. Due to the low light level the camera’s electronic won’t be able to automatically focus or stabilize, so disable them.  It’ll just seek and ruin your setup and photos.
  3. Set the ISO to a high value; 800 on older cameras and 3200 on newer models. Higher ISO will give you a brighter image, but with more noise.  You can test various ISO settings to see which one you are comfortable with.  If you are planning on taking many images and stacking them, you can run with a higher ISO as the stacking process will increase your signal-to-noise ratio.
  4. Set the aperture opening as large as possible. Larger openings bring in more light, but depending on the quality of the optics will distort the stars around the edges of the frame.  If you see that the stars stretch near the edges, simply stomp it down one or two stops. Trial and error is best to find the right setup.  If you’re not sure simply go with a large opening and you can later crop the image if the results isn’t pleasing.
  5. Set to capture in RAW, this is best for post-processing.
  6. Look on your lens and set the focus to infinity; this is where you’ll start. If you don’t know where infinity is, look at a faraway object and manually focus on it.
  7. Mount the camera on a tripod and aim at the desired part of the sky.
  8. If you have live preview, use it to fine-tune the focus to get the stars as small as possible. Don’t forget that you can often ZOOM in on the live preview screen.  If you don’t have live preview (like mine) simply take 3 short test photos (5 seconds each) adjusting the focus in the same direction between each photo. Review the three shots to see which one has the smallest stars and repeat this until you’ve achieved what you believe to be the best image.
  9. Set the exposure time to 20 seconds. With focal lengths in the 15-25mm range the stars will remain relatively round.
  10. Take as many photos as you wish.

You can experience with different setups (F-stop, ISO, focal and exposure lengths) and you’ll be able to review and compare later to see which gives you the best image.  That way the next time you’ll have your GO-TO setup for great shots.

The above was a stack of 4 images taken 17mm F/4, 20 seconds at ISO 800.

I also identified the constellations and some interesting objects in the above shot.

Objects in the Milky Way

Objects in the Milky Way

Comet 41P/Tuttle-Giacobini-Kresak

Periodic comet 41P/Tuttle-Giacobini-Kresak is currently a magnitude 8 object for telescopes and unlike many other current bright comets like C/2015 ER61 (PANSTARRS) and C/2017 E4 (Lovejoy) it is visible for a good portion of the night while the other two are only visible in the morning twilight for those like me in the northern hemisphere.

On April 13th comet 41P was in the constellation Drago, which is where I managed to photograph it.

Comet 41P/Tuttle-Giacobini-Kresak (13-Apr-2017) - Benoit Guertin

Comet 41P/Tuttle-Giacobini-Kresak (13-Apr-2017) – Benoit Guertin

Not much of a tail on this comet, and I’ve checked other photos taken with larger scopes and the result is also just a coma around the nucleus.

Because it is passing near Earth, its movement in the sky is quite noticeable frame-to-frame in the captured images. For the registration and stacking with comets, this is done by alignment on the comet and not the stars, hence the star trails in the above image. I performed another stacking, this time using the stars to align, and the comet’s movement becomes obvious. The displacement measures 2.6 arc-minutes in the 41 minutes that elapsed between first to last exposure.

UPDATE: Created a short video showing the comet’s movement

Distance traveled by the comet in 41 minutes

Distance traveled by the comet in 41 minutes

My setup was less than ideal, as the constellation was only visible from the front of my house.  Yes that is a lovely street-light shining right across the street.  Luckily the telescope was pointing a little to the right, and a rolled piece of cardboard help act as an dew-shield extension to block the glare.  But on the good side I had a nice solid concrete surface and got a very good polar alignment with 1 minutes exposures giving me nice round stars.  Hmmm, might explore this setup a little more often…

Setup in the garage to image comet in constellation Drago

Setup in the garage to image comet in constellation Drago

Telescope: SW80ED
Camera: Canon XTi (450D)
Exposure: 32 x 60sec ISO 800
DeepSkyStacker, IRIS, GIMP

Other comets of interest for 2017

Lower Orion Constellation

Just when you think you have a good “recipe” to process astronomy images taken with your gear, things don’t quite work out and you end up spending three evenings trying different settings, techniques and steps because you know there’s a better image waiting to be teased out.

M72 and Lower Orion Constellation

M72 and Lower Orion Constellation – Benoit Guertin

The image above (click for a full frame) is as much as I can stretch out from the lower half  of the Orion constellation and nebula with a 20 seconds ISO 800 exposure on 85mm F5.6 Canon lens from my light polluted backyard.

Below is the sky chart of the same area showing the famous Orion Nebula (blue and red box) and the Orion belt with the three bright stars Alnitak, Alnilam and Mintaka.  What is unfortunate is there are lots of interesting deep space nebula structures that glow in the hydrogen-alpha spectral lines of near infra-red, but all photographic cameras have IR filters to cut on the sensor those out.  That is why many modify the cameras to remove the filter, or get dedicated astro-imaging cameras.

Sky Chart - Lower Orion with nebula and open star clusters

Sky Chart – Lower Orion with nebula and open star clusters

Now, back to the main topic of trying to process this wide field image.  I had various issues with getting the background sky uniform, other times the color just disappeared and I was left with essentially a grey nebula; the distinctive red and greenish hue from the hydrogen and oxygen molecules was gone.  And there was the constant hassle of removing noise from the image as I was stretching it a fair bit.  I also had to be careful as I was using different software tools, and each don’t read/write the image files the same way.  And some formats would cause bad re-sampling or clipping, killing the dynamic range.

Below is a single 20 seconds exposure at ISO 800.  The Orion nebula (M72) is just barely visible over the light pollution.

orion_2017-02-27_original

Original image – high light position for 20 seconds exposure

The sky-flog (light pollution) is already half way into the light levels.  Yes, there are also utility lines in the frame.  As these will slightly “move” with every shot as as the equatorial mount tracked I figured I could make them numerically disappear.  More on that later…

Light levels of a 20 second exposure due to light pollution

Light levels of a 20 second exposure due to “sky fog”

The longer you expose, the more light enters the camera and fainter details can be captured.  However when the background level is already causing a peak mid-way, longer exposures won’t give you fainter details; it will simply give you a brighter light-polluted background.  So I needed to go with quantity of exposures to ideally reach at least 30 minutes of exposure time. Therefore programmed for 100 exposures.

Once the 100 exposures completed, I finished with dark, flat and offset frames to help with the processing.  So what were the final steps to reach the above final result?   As mentioned above, I used three different software tools, each for a specific set of tasks: DSS for registration and stacking, IRIS for color calibration and gradient removal and finally GIMP for levels and noise removal.

  1. Load the light, dark, flats and offset images in Deep Sky Stacker (DSS).
  2. Perform registration and stacking.  To get rid of the utility lines as well as any satellite or airplane tracks, the Median Kappa-Sigma method to stack yields the best results.  Essentially anything that falls out of the norm gets replaced with the norm.  So aircraft navigation lights which show up only on one frame of 100 gets replaced with the average of all the other frames.  That also meant the utility lines, which moved at every frame due to the mount tracking, would vanish in the final result.
  3. As my plan is to use IRIS to calibrate colors, where I can select a specific star for the calibration, I set the no background or RGB color calibration for DSS.
  4. The resulting file from DSS is saved in 16-bit TIF format (by default DSS saves in 32-bit, but that can’t be opened by IRIS).  I didn’t play around with the levels or curves in DSS.  That will be dealt later, a bit in IRIS, but mostly in GIMP.
  5. I use IRIS to perform background sky calibration to black by selecting the darkest part of the image and using the “black” command.  This will offset each RGB channel to read ZERO for the portion of the sky I selected.  The reason for this is the next steps work best when a black is truly ZERO.  While IRIS works in 16-bit, it’s actually -32,768 to + 32,768 for each RGB channel.  If your “black” has an intensity of -3404, the color calibration and scaling won’t be good.
  6. The next step requires you to find a yellow Sun-like star to perform color calibration.  As a white piece of paper under direct sunlight is “white”, finding a star with similar spectral color is best.  Sky chart software can help you with that (Carte du Ciel or C2A is what I use).  Once located and selected the “white” command will scale the RGB channels accordingly.
  7. The final step is to remove the remaining sky gradient, so that the background can be uniform.  Below is the image before using the sky gradient removal tool in IRIS.
  8. Image before removal of sky gradient in IRIS

    Image before removal of sky gradient in IRIS

  9. Once the sky gradient is removed, the tasks in IRIS is complete, save the file in BMP format (will be 16-bit)  for the next software: GIMP
  10. The first step in GIMP is to adjust light curves and levels.  This is done before any of the filers or layer techniques is performed.
  11. Then I played around with the saturation and Gaussian blur for noise reduction.  As you don’t always want the transformations to take place on the entire image, using layers is a must.
  12. For the final image above, I created two duplicate layers, where I could play with color saturation, blurring (to remove the background noise) and levels until I got the desired end result.  Masks are very helpful in selecting what portion of the image should be transparent to the other layers.  An example is I wanted a strong blur to blend away the digital image processing noise, but don’t want a final blurry night sky.