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.

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

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.

Comet 45P/Honda-Mrkos-Pajdusakova

It wasn’t easy but on Friday the weather cooperated and I was able to capture a glimpse of comet 45P/Honda-Mrkos-Pajdusakova.  That’s if you consider -10°C outside temperature to set-up a telescope and operate a laptop cooperation from Mother Nature.

In my previous post I gave myself a 2-day challenge to capture this comet as it was essentially the last few days at a decent magnitude 7 brightness before becoming non-observable as it swings around the sun over the coming weeks.  And when it returns to the northern latitude sky in mid-to-late February it will be dimer at magnitude 10.  In the image below, I labeled some of the brighter stars with their visual magnitude as reported by the Tycho-2 catalog.

Comet 45P/Honda-Mrkos-Pajdusakova

Comet 45P/Honda-Mrkos-Pajdusakova – around magnitude 7 on January 6th, 2017

I had a very small window of about 30 minutes to make any observation and photograph it.  The challenge started with setting up without polar alignment; the sky was still too bright to locate Polaris,  and instead relied on the position of Venus to align the mount.  As it was still twilight, I was limited to short exposures to keep the histogram on the left half on the camera and to make out a star from the background sky.  I actually started at ISO 400 with only 1 second exposure while adjusting the focus around Theta Cap (magnitude 4).  And as the minutes ticked by I was able to slowly increase my exposure as the twilight darkness permitted.  With neighboring trees, and rooftops coming into view I had to grab as many frames as possible. In the end I got 14 images with 6 seconds exposure at ISO 800 before calling it quits.

With such short exposures no chance of capturing any comet tail, but the green halo is unmistakable comet.

I hope to capture a few more comets this year.

Skywatcher 80ED
Canon XTi (450D)
14 x 6sec (ISO 800)
Registered and stacked with DeepSkyStacker.  Post-processing with GIMP.

The Moon, Mars and Venus

It’s not often that the Moon finds itself between two planets nicely lined up and within a 12 degree field of view.  Just yesterday at around the same time, the Moon was located below Venus.  The image below is a two second exposure at ISO 800 with 53mm lens at f/5.6 on a tripod. I cropped the image to remove a street lamp and light pillars from other light sources further in the distance.

02-Jan-2017: The crescent Moon between Mars and Venus

02-Jan-2017: The crescent Moon between Mars and Venus

The toughest part was actually finding a spot around my block where there was less glare from street lights or annoying power and utility lines in view.  Luckily I found a spot with two extinguished street lamps and setup in between.

A little earlier I quickly snapped the image below from the bedroom window on the 2nd floor.  The sky wasn’t dark yet, and as the camera was hand-held, 1/4sec was the lowest I could go.  Nevertheless with the rooftops in the foreground, it provides for a sense of scale and location in the sky.  The orange-red horizon from the setting sun is a nice touch.

Moon with Venus and Mars in the evening sky

Moon with Venus and Mars in the evening sky


Venus and Crescent Moon

After three days of cloud cover and a good 20cm of snow, it was nice to see a clear and crisp sky throughout the day and into the evening.  With the sun set and the sky still dark blue a crescent Moon and Venus made for a fine pair in the south-west sky for the first evening of 2017.

New Years 2017 Moon and Venus

New Years 2017 Moon and Venus

The Moon will continue to travel towards Mars, located higher up and to the left (East) with a good photo opportunity on the 2nd (tomorrow) with the Moon between both planets.  For the rest of January, Venus will gently move closer to Mars to within 6 degrees at the end of the month.

Photo Details:
Canon XTi (450D)
17mm F4 (1/10sec ISO400)
inset: 85mm F5.6 (1/10sec ISO800)