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.

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.

December 25th – No Sunspots

The sun has been without sunspots for two days, but that is expected as we are heading to a minimum in the 11-year cycle.

Cycle 24 Sunspot Number

Cycle 24 Sunspot Number (NASA)

Nevertheless as it was a nice afternoon grabbed the scope and did some observation of the sun.  A little of a challenge to focus when there is no contrasting details to base yourself on.

December 25th 2016 - No Sunspots

December 25th 2016 – No Sunspots

Skywatcher 80ED
Canon XTi (450D) ISO 100 – 1/800sec
Thousand Oaks R-G Solar Film

DeepSkyStacker – Faster and Better Results (updated)

Tried DeepSkyStacker and I think I’ve found a better and faster way of processing my images.

I had been using IRIS for the better part of the last 6 years, and I remember how impress I was at the results compared to the early versions of Registax for deep sky images.  While  IRIS is quite manual and command-line based, it nevertheless got the job done and allowed me to experiment with different methods.  But now, I decided it was time to move on to something a little modern.  I looked at what others were using, and came across DeepSkyStacker.

DeepSkyStacker

While IRIS offers a complete package, from image acquisition, pre/post-processing, and analysis tools; DeepSkyStacker only performs the registration and stacking.  But it does so in a faster and more efficient way.  DeepSkyStacker can fully utilise RAM and multi-core processing; hence what took 30 minutes in IRIS is now down to 5 minutes in DeepSkyStacker.

It also automates many steps, and you can even save the process and create batches.  So it’s down to load all your files, and then one click to register and stack.

DeepSkyStacker - Processing Files

DeepSkyStacker – Processing Files

I tried the with some wide field of views I had taken back in September.  And the resulting image appeared to be better.  Now I still have to use IRIS as I like how it can remove the sky background gradient and adjust the colors.  And GIMP is still required for the final adjustments.  So here are the main steps that gave me good results:

  1. Load the light, dark, offsets and flat frames (I had no flats or bias/offsets in my trial run, but that didn’t appear to cause an issue)
  2. Ensure that all pictures are checked and select to Register the checked pictures
  3. For the stacking, I found that selecting RGB Channels Background Calibration provided good color, and used the Kappa-Sigma clipping to remove noise.
  4. After stacking DSS will create an Autosave.tif (32-bit TIFF file).  I need to convert this into another format, but without loose the dynamic range.  My current solution is to use Microsoft Photo Gallery to open and save another copy as JPEG.  Finally did a quick stretching of the RGB levels to ensure better dynamic range when saving to 16-bit TIFF.  16-bit TIFF appears to be the only one that will open correctly in IRIS.
  5. Once in the image loaded in IRIS to remove the background sky gradient.  And then save it in BMP format for import into GIMP.  Yes I know I another file format, so far it’s what I find works best.  GIMP converts FITS and TIFF to 8-bit, causing incorrect color depth.
  6. Final adjustments with levels, light curves, saturation, noise filtering, etc.. is done in GIMP.

Now for a little more playing around, and trying it on some on my older pictures.

UPDATE:
DeepSkyStacker saves files in 32-bit TIFF by default.  After stacking many images the dynamic range is quite large, and this is not data we want to loose.  But the problem was finding a program that was able to correctly handle the 32-bit file format.  The next release of GIMP (version 2.10) will handle 32-bit files, but GIMP 2.8 was limited to 16-bit and even there it would convert the image to 8-bit for manipulation (GIMP 2.9.2 and up might work, but needs to be compiles on your computer – development package).  Not good…  Before downloading yet another photo imaging software I tried some of my current programs and found that the  Microsoft Photo Gallery software for Windows 10 does a great job of handling the 32-bit TIFF files.  Once the image opened, under File – Make a Copy I save a version in JPEG.  Yes I know not ideal, but I avoid a lot of the quantization conversion error and I’m able to continue my processing in IRIS and GIMP.

 

Novembre 14th SuperMoon

Status

Lots of talk in the last week about the upcoming SuperMoon on November 14th.  While it will be the closest and largest Full Moon since 1948, the differences won’t be that significant.  It’ll look just like any other Full Moon, one that happens every month.  Nothing special will happen to the solar system or the Earth.

However it’s a good opportunity to experiment with your camera and composition as the Moon is rising over the Horizon.  Play around with different settings and different lens.  You can even spend the day before scouting a good spot with view to the East.

Don’t need dark skies.  Don’t need a special mount or stand. This is astro-photography accessible to all.

How Many Stars?

Image

Looking at the sky at night from your backyard you’ll probably be able to see about 50 stars, more if you are away from the city.  So how many do you think is in the frame below?

Portion of the Milky Way near Vulpecula.

Portion of the Milky Way near Vulpecula.

Taking a 200 x 200 pixel sample in the middle I counted 155 stars.  Hence extrapolated to the entire picture comes to 38,000 stars for this 18 x 10 degree portion of the sky.  OK I cheated in taking a picture of a portion of the Milky Way… Nevertheless that is a rather small fraction of the 300 billion stars estimated within our own Milky Way.

Bonus if you can spot the meteor!  Showed up in a single 30sec frame, which I added separately in post processing, else it would have been eliminated from the final image as it’s a random event and I always use a sigma distribution for my stacking.   Hint: it’s located just above open cluster CR399, also known as Brocchi’s Cluster.