Signal and Noise

What makes it possible to be able to generate a photo of the Milkyway from what appears to be just a faint trace in the original shot?

The final image (left) and a single frame as obtained from the camera (right)

It all comes down to the signal vs noise. Whenever we record something, sound, motion, photons, etc… there is always the information you WANT to record (the signal) and various sources of noise.

Noise can have many sources:

  • background noise (light polution, a bright moon, sky glow, etc…)
  • electronic noise (sensor readout, amp glow, hot pixels)
  • sampling noise (quantization, randomized errors)

This noise can be random or steady/periodic in nature. A steady or periodic noise is easy to filter out as it can be identified and isolated because it will be the same in all the photos. However a random noise is more difficult to eliminate due to the random nature. This is where he signal to noise ratio becomes important.

In astrophotography we take not only the photos of the sky, but also bias, darks and flat frames: this is to isolate the various sources of noise. A bias shot is a short exposure to capture the electronic read-out noise of the sensor and electronics. The darks is a long exposure at the same setting as the astronomy photo to capture noise that appears during long exposures due to the sensor characteristics such as hot pixels and amplifier glow. Cooling the sensor is one way to reduce this noise, but that is not always possible. Finally the flat photo is taken to identify the optical noise caused by the characteristics of the lens or mirror as well as any dust that happens to be in the way.

But what can be done about random noise? That is where increasing the number of samples has a large impact. For a random noise, increasing the number of sample points improves the signal to noise ratio by the square root of the number of samples. Hence averaging 4 images will be 2 times improvement than a single photo. Going to 9 will be 3 times better. Etc…

You might be thinking: “Yeah but you are averaging, so the signal is still the same strength.” That is correct, however because my signal to noise ratio is improved I can be much more aggressive on how the image is processed. I can boost the levels that much more before the noise becomes a distraction.

But can’t I just simply duplicate my image and add them together? No that won’t work because we want the noise to be random, and if you duplicate your image, the noise is identical in both.

So even if you are limited to just taking 30-second, even 5-second shots of the night sky and can barely make out what you want to photogram, don’t despair, just take LOTS of them and you’ll be surprised what can come out of your photos.

Milkyway from a stacking of 8 x 20 second photos.

Nucleus of Comet C/2020 F3 NEOWISE

When observing a comet, what we see is the outer coma; the dust and vapor outgassing from the nucleus as it gets heated from the Sun.

So I decided to take one of my photos taken with my Skywatcher 80ED telescope (600mm focal length) and see if I could process the image to spot where the nucleus is located.

This can be achieved by using the MODULO command in IRIS and viewing the result in false color. The results are better if you do a logarithmic stretch of the image before the MODULO command. It took some trial-and-error to get the right parameters, but the end results isn’t so bad.

Studying the internal structure of comet C/2020 F3 NEOWISE (Benoit Guertin)

For the fun of it I tried to see if I could calculate the size of the comet nucleus using the image. At the most narrow the nucleus on the photo spans 5 pixels. Based on a previous plate-solve result I know that my setup (Canon 80D and Skywatcher 80ED telescope) results in scale of 1.278 pixels per arc-second. Then I used Stellarium to get the Earth-coment distance on July 23rd (103.278 M km)

When I plugged in all the numbers I get a comet nucleus size of approximately 2000 km, which to me seamed a little on the BIG size.

Sure enough a little research revealed that measurements made by Hubble points to a 4.8 km ball of ice. So yeah, I’m quite far from that… but it was fun to give it a try.

C/2020 F3 (NEOWISE) Thanks for Swinging By

I live in a heavily light polluted city, therefore unless it’s bright, I won’t see it. But boy was I ever happy with the outcome of this comet! In my books C/2020 F3 (NEOWISE) falls in the “Great Comet” category, and it’s by far the most photographed comet in history because it was visible for so long to folks on both sides of the globe.

My last encounter with a bright comet was in 2007 with periodic 17P/Holmes when it brightened by a factor half a million in 42 hours with this spectacular outburst to become visible to the naked eye. It was the largest outburst ever observed with the corona becoming temporarily the biggest visible object in the solar system. Even bigger than the Sun!.

Comet 17P/Holmes November 2, 2007 (Benoit Guertin)

So when the community was feverishly sharing pictures of the “NEOWISE” I had to try my luck; I wasn’t about to miss out on this chance of a lifetime.

I have to say that my first attempt was a complete failure. Reading up when it was the best time to try to photograph this comet most indicated one hour before sunrise was the right time. So I checked on Google Maps where I could setup for an un-obstructed view of the eastern horizon (my house was no good) and in the early morning with my gear ready at 4am I set off. To my disappointment and the “get-back-to-bed-you-idiot” voice in me, it didn’t work out. By the time I got to the spot and had the camera ready, the sky was already too bright. No comet in sight, and try as I might with the DSRL, nothing.

Two evenings later and another cloudless overnight sky I decided to try again, but this time I would make it happen by setting the alarm one hour earlier: 3am. That is all that it took! I was able to set-up before the sky could brighten, and then CLICK! I had this great comet recorded on my Canon SD memory card.

Comet C/2020 F3 (NEOWISE) in the dawn sky on July 9th. (Benoit Guertin)

I didn’t need any specialized gear. All it took was a DSLR, a lens set to manual focus, a tripod and 5 seconds of exposure and there was the comet. I snapped a bunch of frames at different settings and then headed back home to catch the last hour of sleep before starting another day of work. Lying in bed I felt like I had accomplished something important.

As the comet swung around our Sun and flipped from a dawn to a dusk object I decided I should try to photograph it once again, but this time with the Skywatcher 80ED telescope. At that point, the comet was dimming so every day that passed would be more difficult. It was only visible in the North-West horizon at sunset, which meant setting up in the front the the house, fully exposed to street lights. Not ideal, but I had nothing to loose trying.

Setup in front of the house, fully exposed to street lights to catch the comet.

I used our tree in the front yard to act as a screen and was able to locate and photograph this great comet. Polar alignment wasn’t easy, and when I had the comet finally centered and focused with the camera, overhead power lines were in the field of view. I decided to wait out 30 minutes and let the sky rotate to the lines out of the view. Besides, it will get darker anyways which should help which the photo. But I also realized that my “window” of opportunity was small before houses would start obscuring the view as the comet would dip to a lower angle with the horizon.

C/2020 F3 (NEOWISE) July 23, 2020 – Skywatcher 80ED (Benoit Guertin)

I’m sure in the years to come people will debate if this was a “Great Comet”, but it my books it’s definitely one to remember. It cemented with me the concept that comets are chucks of “dirty ice” that swing around the sun. Flipping from a dawn to dusk observable object after a pass around the Sun is a great demonstration of the elliptical nature of objects moving in our solar system.

Now waiting for the next one…

The Great Comet of 2020 That Never Was

Back in March, the astronomy crowd was buzzing about a possible”naked-eye” comet expected in late May 2020.  Comet C/2019 Y4 (ATLAS) was first detected at the tail end of December as a very dim magnitude 19.6 object and by mid-March it had brighten to an easy telescope target magnitude of 8. Those not familiar with the magnitude scale, going from 19.6 to 8 is not a doubling in brightness, but around a 4000 times increase!

That dramatic increase in brightness help fuel the hype for the Great Comet of 2020, and there were two other factors that got people excited:

  1. It would be visible at dusk from the Norther Hemisphere, hence within easy viewing to much of the world population.
  2. It was following a similar orbital path as the “Great Comet of 1843“, suggesting that it was from the same original body and could potentially provide the same viewing spectacle. That 1843 comet was visible in daytime!

Well all that went south when the comet’s breakup was observed in late March after peaking momentarily at magnitude 7. It began to dim, along with any hopes of a Great Comet repeat. Below is a graph showing the the original (grey line) and revised (red) comet brightness forecast (dots being observed measurements) on this chart created by Seiichi Yoshida (comet@aerith.net)

Comet C/2019 Y4 (ATLAS) Brightness - Copyright(C) Seiichi Yoshida

Comet C/2019 Y4 (ATLAS) Brightness – Copyright(C) Seiichi Yoshida

Comet C/2019 Y4 is expected to make its closest approach to the sun on May 31st, however most experts believe it will disappear (disintegrate) before that date.  Seeing that I had a small window of opportunity to capture the comet I decided to try my luck last Saturday evening.

Below is an extremely processed (and ugly) image that I got by combining 25 photos (15 seconds each at ISO 3200) using my Skywatcher 80ED scope. The photo just about makes out the distinctive blue-green hue and elongated shape of a comet. It is around magnitude 10, very diffuse and about 147 million km away from us the day this photo was taken.

Comet C/2019 Y4 (ATLAS) on April 18, 2020 - Very faint at about magnitude 10. Imaged with 80ED telescope 25 x 15sec

Comet C/2019 Y4 (ATLAS) on April 18, 2020 – Very faint at about magnitude 10. Imaged with 80ED telescope 25 x 15sec

I pushed the image processing so hard that I was able to pick up faint magnitude 13 galaxies!

On to the next comet!

Telescope: Skywatcher 80ED
Camera: Canon 80D
Image: 25 x 15sec at ISO3200 (6 minutes)

A Crater Named Tycho

10 Days old Moon (April 04, 2020) - Benoit Guertin

10 Days old Moon (April 04, 2020) – Benoit Guertin

The photo above is of a 10-day old Moon taken a few days ago. After the darker “seas” of old lava flow, one particularly bright crater in the southern hemisphere stands out, especially with the rays that appear to emanate from it. That is Tycho, a 85km wide and 5km deep crater and one of the more “recent” ones if you consider 109 million years the not-to-distant past. The Moon is 4.5 billion years old after all… having formed just 60 million years after the solar system. On the Moon, “fresh” material have a higher albedo and hence appear brighter, whiter.

The bright rays surrounding Tycho are made of material ejected (up to 1500km away) from the impact of a 8-10km wide body. In time these rays will disappear as the Moon continues to be bombarded by micro meteorites, which stirs the material on the surface. The rays are more present on the eastern side, as would be expected from a oblique impact.

Tycho is names after the Danish astronomer Tycho Brahe.

The Surveyor 7 space craft landed about 25km north of the crater on January 10, 1968.

Ever wondered how mosaic space photos were done before the invention of powerful software algorithm to stitch them together?  Take a look at the series of Surveyor 7 mosaic photos.  Someone had to painfully print each photo and lay them on a grid in a specific pattern matching optical field and geometry.

Ending the Year with Betelgeuse

A few days prior to the holiday break there was news of Betelgeuse dimming to an all-time low, potentially signaling the start of the process that will transform this star into a Supernova. What? Wait a minute… A star in our own galaxy exploding? But that hasn’t been observed since 1604!

Remnant of SN1604 (NASA)

Remnant of SN1604 – last galactic nova (NASA)

There are plenty of novas at any point in time, they just happen to be in galaxies far away (cue Star Wars intro). During those few days or weeks of otherworldly explosions these stars become the brightest object in their host galaxies.

SN2018ivc in galaxy NGC 1068 (Credit: Bostroem et al., 2019.)

SN2018ivc in galaxy NGC 1068 (Credit: Bostroem et al., 2019.)

So if we can see them when they are millions of light years away, what would an exploding star just 700 light years away, like Betelgeuse, look like?

Well if we base ourselves on SN1604 it will be visible to the naked in eye for three weeks, including during daytime. SN1604 was 20,000 light years away, while Betelgeuse is at a fraction of that, so most experts anticipates that it would be as bright as a full Moon.

Now before we go crazy anticipating when Betelgeuse, a red super-giant, will explode, let me present some information to put everything in perspective.

Betelgeuse is a red super-giant of class M1-2 in the constellation Orion, 2nd in brightness just after Rigel. Betelgeuse is one of the largest start we can see when glancing up at the night sky. If Betelgeuse was our Sun, it would engulfed all planets up to Jupiter. Stars of that size aren’t like the nice Smith Ball of fire we imagine our Sun to be. They are more like a loose ball of foam, constantly bubbling and bloating from the incredible heat created in the inner core. If you are starting to think unstable, you are partly right.

Betelgeuse is also a well documented variable star, meaning it periodically varies in brightness.

Recorded Brightness of Betelgeuse Over the Years (credit: AAVSO)

Recorded Brightness of Betelgeuse Over the Years (credit: AAVSO)

So while it is at an all-time low compared to its known ~425 day cycle, it also has a ~5.9 year cycle, and this episode just happens to be a combination of both lows. So no need to panic… for now.

Betelgeuse will one day end as a type II supernovae, probably not for another 100,000 years. Until then we can all glance up during these cold winter nights at how easily the Orion constellation can be spotted and enjoyed. The three bright stars marking the belt and the hour-glass figure is easy to find. Take a few moments to look at Betelgeuse as on a galactic scale it will be gone tomorrow.

Betelgeuse Red Super Giant in Orion (Benoit Guertin)

Betelgeuse Red Super Giant in Orion (Benoit Guertin)

Messier 3 and a Fast Moving Star

A few weeks ago after taking some photos of Jupiter, I changed my setup to do some long exposures on an easy target: a globular cluster. Unfortunately I forgot to note down the name of what I had photographed!  So a few weeks later when I found the time to process the images I was at a loss to identify what Messier object it was. However, after an evening of matching up stars surrounding the cluster and I was able to correctly identify it as Messier 3.

Globular Cluster - Messier 3 (Benoit Guertin)

Globular Cluster – Messier 3 (Benoit Guertin)

The above was taken with my Skywatcher 80ED and Canon 80D. It is a stack of 27 x 10sec exposures at ISO3200 on an unguided and roughly aligned mount.

Looking at my archives I found that I had imaged M3 about 10 years ago with the same telescope, so I decided to align both old and new image and see if anything would stand out. And to my surprise, spotted one star that appeared to have shifted. To help identify the star I colorized one of the photos and subtracted from the other (done in GIMP).  All the stars within the field of view lined up except this one; the two colored spots are not aligned!

High PM Star BD+29 34256

High PM Star BD+29 34256

To be sure this wasn’t on an error on my part I did a bit of research and found it to be a know high proper-motion star BD+29 34256.

It’s not everyday someone with amateur backyard astronomy gear can show how a star has moved in 10 years.

Coming this March – Reentry of Chinese Space Station

Its inevitable, what goes up must come down. On average there is one large piece of equipment that re-enters our atmosphere every week. Some are controlled and planned decommissioning of satellites after their useful life. They are purposely commanded for re-entry and burn-up in the atmosphere to avoid adding debris to our already crowded space orbits or worse, cause a collision with another satellite creating an enormous field of debris. Other objects that re-enter are left to fall on their own such as discarded rocket bodies and old satellite that ceased to operate long ago or malfunctioned and can no longer be controlled.

Tiangong-1 : First Chinese space station launched in 2011

Tiangong-1 : First Chinese space station launched in 2011

This coming March the 8,500kg (18,700lbs) Tiangong-1 Chinese space station is coming back to Earth. Launched in September 2011 and used for two manned missions, it suffered a malfunction and the Chinese have not been in control of it since 2016. The space station has been in a decaying orbit ever since, and now below the 300km altitude  where Earth’s atmosphere is causing the space station to slow down due to aerodynamic drag it will soon make its re-entry.

Delta 2 rocket fuel tank surviving re-entry near Georgetown, TX, on 22 January 1997

Delta 2 rocket fuel tank surviving re-entry near Georgetown, TX, on 22 January 1997

Now there is no need to panic. Most of Earth is ocean, and we’ll probably not see anything let alone have a piece of it land in a city. However as this is a fairly large body, there is a good chance  not all pieces will burn up and some may make it to the surface.

This isn’t the first time a space station makes a re-entry.  The American Skylab at 77 tons re-entered in 1979, and Russian Mir (120 tons) made its re-entry in 2001.
For the Mir re-entry, Taco Bell even got it onto the re-entry buzz by anchoring a large

Taco Bell target for Mir re-entry (2001)
Taco Bell target for Mir re-entry (2001)

target off the Australian coast along the planned re-entry track, and should Mir crash into it there would be free tacos for all Americans. The fast food chain even took out an insurance policy just in case it would happen.

In early January 2018, Tiangong-1 is orbiting at an altitude of around 270-290km (to put that into perspective, ISS is at a 400km orbit) and in a 45 deg orbit, hence the re-entry will be within those latitudes.  The green area in the map below is where Tiangong-1 could make a re-entry, and also marks where the re-entry could be observed.

Tiangong-1 ground coverage - http://www.aerospace.org

Tiangong-1 ground coverage – http://www.aerospace.org

It’s still too early to determine the time and location of potentially crash site, as Earth’s atmosphere is influenced by space weather and swells based on our Sun’s moods, which alters the drag force on the space station.  However various space centers and organizations will continue to track the space station the coming weeks to improve the prediction.

You can follow everything at Aerospace.org for up to date information and predictions.

What could the re-entry look like?  Below is a video shot by NASA of the Japanese Hayabusa spacecraft during a controlled re-entry on June 13, 2010

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.

Does Earth Influence the Sun?

I recently came across an article in the french Science & vie magazine, where a reader asked if Earth influences the Sun. I found it rather interesting, and while I had my doubts I still wanted to know more about it.

sun-earth

The reader wasn’t the first to wonder if there was any interaction, various models and observations have been put forward since the late 1800s. We often read about two bodies interacting in space. The first exoplanet was discovered due to its gravitational influence on its star causing it to wobble. This type of gravitational influence works when two bodies have a mass within one or two orders of magnitude of each other.  But in the case of our Sun, it is 99.86% of the solar system’s mass, and most of the remaining is taken up by Jupiter and Saturn.  Therefore from a gravitational perspective Earth has no effect on the Sun.

But could the 11 year period in solar activity, characterized by the rise and fall of number of observed sun spots be caused by the planets? The exact source of that periodicity has yet to be clarified.  Well a team of researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) put out a paper in 2016 after demonstrating that every 11.07 years the planets Venus, Earth and Jupiter are aligned.  Coincidence?

They explained that while the effects are rather small, the repeated nudging could be enough to tip the Sun’s magnetic field instabilities one way or the other causing this 11 year solar cycle that we observe, much like an object entering into resonance.  In this case it’s the Sun’s magnetic field acting like a dynamo that would resonate due to the planet’s alignment every 11 years.

However many are skeptical about any real effect pointing that the source of the Sun’s magnetism comes from deep within, while the planet’s effect, if ever, would be limited to the Sun’s surface. But the crushing blow is when you look at fact that the solar cycle varies between 7 and 14 years, the number 11 just happens to be the average over the last 24 observed cycles.  Unfortunately the three planet’s alignment don’t vary by that amount.

In the end, the Sun is still king and does what it wants in this solar system, regardless what the planets say or do.