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

Solar Eclipse – Post Processing

With the eclipse behind us, and all the gear put away it’s time to transfer and process the images to create something memorable.  I decided to make a mosaic with some of the photos of the eclipse, as well as the visible sun spots. Click on the image below for a high-resolution version.

August 21, 2017 Solar Eclipse

August 21, 2017 Solar Eclipse

The weather cooperated and I had the right gear to get some decent photos. Before the start of the eclipse, the sun presented two observable active sun spot regions: 2671 and 2672. This helped in achieving a proper focus and gave something to observe prior to the start of the eclipse.

Sunspot Region 2671 (right) and 2672 (left)

Sunspot Region 2671 (right) and 2672 (left)

As I had installed and aligned my Vixen equatorial mount the night before, once I had proper focus with the camera, it was child’s play to start an automatic sequence of images every 60 seconds. Hence for the entire solar eclipse, it was hands-off and automated. I could simply glance once in a while at the screen or grab one of the hand-held solar viewers to look up.

58% Cover from the Montreal, Canada Location.

58% Cover from the Montreal, Canada Location.

While the effect was nowhere near that of those in the path of totality, the light level and heat did drop at the peak of the eclipse. The brightness was lower, not like when there are high altitude clouds as the shadows were still sharp and well-defined. And the sun’s rays did feel cooler, a welcomed relief from standing under the sun for the last hour.

In the end, it was a fun experience, especially with the kids. And with over 150 images taken I decided to compile them into two formats. A time-lapse video and a mosaic as seen above.

The video was actually the quickest thing done. With Microsoft Movie Maker, it takes the Canon CR2 RAW files directly and stitches them together into a video. It actually took me longer to find a suitable soundtrack to the clip.

With that experience under my belt, I’m looking forward to April 8th 2024 total solar eclipse that will pass close to home.

Telescope: Skywatcher 80ED with Thousand Oaks R-G solar film
Camera: Canon Rebel XTi (450D)
Setting: 1/1000s at ISO 100

Animation – Movement of Comet 41P

The word “planet” comes from the Greek work “planan” which means to wander. Early star gazers noticed that some bright stars moved with respect to other fixed stars.  Those bright stars are our closest planets: Mercury, Venus, Mars, Jupiter and Saturn. Comets also move a fair bit across the sky, but the origin of the word has more to do about stars “with long hair” than it’s traveling behavior.

Last weekend I managed to photograph comet 41P//Tuttle–Giacobini–Kresák, and I identified in my blog that it’s movement was visible frame to frame. Well I’ve finally gotten around to create a small animation of that movement. For those wondering what’s the comet’s velocity, it’s currently travelling at 37.4 km/s.

Animation of comet 41P/Tuttle–Giacobini–Kresák (45 minutes)

Animation of comet 41P/Tuttle–Giacobini–Kresák (41 minutes)

The above is composed of 32 frames, each a 1 minute exposure spanning a time of 41 minutes. You are probably thinking “it should be 32 minutes, not 41!”. That is because I have a delay between each frame to allow the camera to send the photo to the computer. Hence between the first and last frame, 41 minutes have elapsed.

Jupiter and Three Moons

Started processing some of the images taken on April 8th, the only evening with a clear night. I spend a good hour in the near freezing air to capture Jupiter with various settings. The one below was taken with a 2X barlow and a simple webcam. This is a mosaic of two frames as not all moons fit into the rather narrow 640×480 CCD sensor. Unfortunately the fourth moon, Callisto, is just out of the frame to the right.

Jupiter - 2017 opposition - SW80ED and 2x barlow

Jupiter – 2017 opposition – SW80ED and 2x barlow

Telescope: Skywatcher 80ED with 2x barlow lens
Sensor: Philips Vesta webcam with IR-UR cut filter
Processing: Registax and GIMP

Took 40 seconds of video at 20 images/sec which produced a 351MB AVI file. The video is then analysed, registered and stacked with Registax.  Color saturation and light levels where then adjusted in GIMP.

I also took many more video with a 3x barlow, but getting the focus right was a challenge. And I’m afraid the end result is just a “bigger” Jupiter, no additional details. I will need a few nights to process those and see which one turned out well. I will also try using the drizzle algorithm on the image above to see if I can get a larger and better image.

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.


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.

Processing RAW Cassini Spacecraft Images

Did you know that you can get access to the latest RAW images from the Cassini spacecraft directly from the NASA and JPL website?  Not only will you have first look at some stunning images of Saturn, the rings and the Moons like this one below from January 16th.  Click the image below for more information from NASA/JPL on that specific photo.

Daphnis making waves - Cassini spacecraft Jan. 16, 2017 - JPL/NASA

Daphnis making waves – Cassini spacecraft Jan. 16, 2017 – JPL/NASA

But you can also download raw images to try your luck at processing.  For this exercise I selected these series of pictures of the strangely perfect hexagonal-shaped storm on Saturn’s north pole.

Downloaded raw image set

Downloaded raw image set

These are images taken with different filters by the wide field camera, and I noted in an Excel file some information on each image, most importantly which filter was used.  Both the narrow and wide CCD on Cassini operate with two filter wheels, hence each image will always list two filters.  For those surprised at the rather “small” 1 mega-pixel camera, keep in mind the spacecraft was launched nearly 20 years ago, and development started in the 1980s.

There is a very detailed document on how to use, calibrate and process the images found at the following link.  But for what I wanted (quick processing) I only needed to find out which filters were the closest to an RGB setup.

Cassini ISS Broadband Filters

Cassini ISS Broadband Filters

Luckily this is well documented, and found them with the BL1, RED and GRN filters.

The image below is a quick addition of those 3 respective images assigned to red, green and blue channels.  The resulting image would be somewhat near the real colours, but I did not take any time to calibrate, hence they are probably a little off…

Saturn with normal RGB assignment (close to real colours)
Saturn with normal RGB assignment (close to real colours)

I also decided to try something that would provide a little more contrast and dive a little into the atmosphere and went with a IR-Red-Blue for RGB assignment by using a one of the narrow-band filters.

Cassini ISS Narrow Band Filters

Cassini ISS Narrow Band Filters

Saturn with IR, Red and Blue for RGB assignment

Saturn with IR, Red and Blue for RGB assignment

Both images above have not be calibrated, stretch or adjusted other than combine the raw images from Cassini.

The NASA/JPL site even has a section for amateurs to submit their photos and host a gallery to see what others have done.

Cassini NASA/JPL site
Cassini Imaging Science Subsystem (ISS) Data User Guide