So,… here is M42 the Orion Nebula, that big bright center part to Orion’s sword. This is a true colour image which is the result of only 50 minutes of total exposure, 30 minutes for red, green and blue and 20 minutes luminance.

The image above is taken with a second telescope and in the picture below you can see the two telescopes, one larger than the other.

Yer-good-old-camera probably had a zoom lens and with it you could adjust it to get that up close personal snap or that grand wide picture of that fabulous sunset. In astronomy photography, we have no such luxury. Telescopes are fixed focal length things. In telescopes, a main goal is to keep the optics, lens and mirrors, as simple as possible, adding complexity compromises lost light and distortions.

Which one

Which scope do you think produced which picture? Did the smaller one take the wider (bigger) view?

…

Yes it did. Just like holding a ring up to your eye, the further away it is (i.e. the longer), the less you see thru it. This explains the desire to have both scopes present in the observatory. Below is a representation that shows both ‘fields of view’. The larger telescope cannot see more than the smaller rectangle, and the smaller telescope can see only as much as the larger rectangle.

Why such long exposures?

It used to be the case that taking a picture with yer-good-old-camera was always accompanied with that snap-slap sound of the mirror and shutter and a typical exposure was as little as 1/400^th of a second on a bright day, or as long as 1/30^th of a second indoor in the evening perhaps.

An old 40W bulb produces about 400 lumens of light [ref], and each lumen is about 10¹⁶ photons per second [ref], so your 1/30^th of a second exposure collects about 10¹³ photons. (I have taken a 1 part in 10,000 to account for lens aperture). A typical new camera will have as many as 12 million pixels, each pixel gathers only the light it sees, so we can guess each pixel gathers 10¹³/(12×10⁶), about one million photons per pixel.

1000000 photons per pixel typical indoor picture

Now, Lets guess how many photons we get in a 1/30^th second exposure of the Orion Nebula.

From the nebula the telescope will gather 90 million photons per second and these photons are spread out over almost all the pixels in the camera so, only about (90×10⁶)/(12×10⁶), 7 photons fall on each pixel per second. Finally, if the exposure is 1/30^th of a second then we would expect not even 1 photon to be gathered per pixel. The image would be basically black.

Extending the exposure to 5 minutes allows us to gather more photons in each pixel.

2000 photons per pixel if 5 minute exposure

Even with long exposures, in astrophotography we are working with about 1000 times less light than usual photography.

Adrien

Let’s have a bit of fun, I hope :). We can look at what a star chart is. I can show you what the sky will look like and how the chart fits. I’d like you to go outside and see if you can see the Orion Nebula and I will publish a picture of it in the coming days.

Chart

The star chart below is of the Orion Constellation. It only takes a bit of imagination to see Orion’s torso, his belt, and his knee-length tunic. What is not drawn in this diagram is the slayed lion in his outstretched arm to the right and his weapon, with a raised arm to the left. I find it interesting how many civilizations, from all over the globe have embraced this constellation in their own ways. https://en.wikipedia.org/wiki/Orion_(constellation)

Charts are usually presented such that, as drawn, when held up in front of you, up-is-up. What I mean to say is, you don’t have to flip or rotate it to compare to the sky you see.

Freestarcharts https://freestarcharts.com/messier-42

There is one more part to the constellation to imagine. His sword is directly below the center star in his belt and what we are looking for is the Orion Nebula, which is the central object in the compact line of three objects of the sword.

Night Sky

Below, is the view of the sky you should see in Ottawa, 19^th January, at or about 10PM. This is a south facing view. The position of the moon will be very different after the 19^th, be careful not to get misguided. On progressing days the moon will retreat off to the left quite quickly.

Orion should be nicely present, front and center stage.

In Stockholm, below, on the 19^th, at or around 9PM a similar southern sky happens. Orion will be lower to the horizon because Stockholm is a bit more north than Ottawa.

Finally, from Auckland New Zealand, things look quite different. Instead of looking south, Orion is in the north, and is upside down compared to our view in the Northern hemisphere. It is easy to think that the way things appear are the same for all, but it is not so.

Seeing it

The Orion Nebula is that center area of light in what should appear as three bright areas of the sword. Unlike many objects, it can be seen without visual aids (aside from a clear dark night). It is a very bright object and it is BIG, almost 1degree across, twice the size of a full moon.

A pair of binoculars will net you a view similar to what is shown below, and the red circle below is probably similar to your binoculars. I suspect you will be able to make out the shape of the nebula.

Colour, where is the colour,…

This is one of the disappointing things you learn soon after you buy your first telescope. Our eyes are not that good at seeing colour at such low light levels. It has something to do with rods and cones in our eyes, and the colour seeing cones are not as sensitive as the rods that perceive black and white. Seeing colour is almost exclusively the work of the cameras we put on our telescopes.

Another interesting thing is, the center vision area of our eyes is dominated by these less sensitive cones compared to the periphery where rods dominate. I guess, as a species we care more about the colour of things we directly look at. Or, if something is creeping in from the side to eat us in the dark, we really don’t care what colour it is! What all this means is, if you look away from the nebula you may be able to see more of it. It takes a bit of practice to do this but its worth a try.

Related to boosting your vision, taking good deep breaths will help your eyes get all the oxygen they need but this is probably best done alone.

It takes about twenty minutes for your eyes to get their night vision and most any light will shut them down rather quickly, but not red light. I do not know the reason why red light is so less disruptive to our night vision. Perhaps that is why lights on the back of cars are red.

Adrien

The common name of IC5146 or Caldwell C19 is the Cocoon Nebula. It is about 4000 light years away and is approximately 8 light years in diameter. It is a collection of a small number of stars along with a nursery that is generating hundreds of new stars.

IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg), CC BY 4.0 via Wikimedia Commons

The Cocoon Nebula is also located in the constellation Cygnus and is high over head in the late evening (~10PM) in late August / early September.

The image is a modest 5 hour exposure, 2 hours Luminance and 1 hour each colour channel (RGB) taken over three nights in early November 2021.

Online references

Universe Guide https://www.universeguide.com/nebula/ic5146

NASA https://apod.nasa.gov/apod/ap090305.html

Wikipedia https://en.wikipedia.org/wiki/IC_5146

Astrophysical Journal https://iopscience.iop.org/article/10.1086/587687

First we have to talk about colour.

Images we consume on our computer screens are made by presenting measures of red, green and blue at every location, or pixel, in the image. All images we consume can be made following this simple recipe.

In the astrophotography presented here the used sensor is not colour selective, it measures, almost equally well, the amount of light that falls on it regardless of colour. If it was used on its own, a black and white image would be the result. To create a colour image three colour filters are used, Red, Green and Blue. In the production of the image we view, the three resulting sensor images are mapped to Red, Green and Blue as you would expect and this produces what some call a true colour image. This is what was done in https://old-photons.blog/2021/10/12/ngc-6888/ .

There are elements, (remember that periodic table?) which are interesting to the physics of what is going on inside objects in our universe because, we can specifically look for them with matching filters.

When Hydrogen, Oxygen, and Sulphur are present in objects in our universe we can look specifically for them if they are being ionized by a nearby energy source. This ionization is somewhat a fancy word for what happens in neon lights and we should think about the characteristic stable colours.

Doubly ionized oxygen (OIII) is very popular (abundant) in nebula such as NGC6888 and produces a blueish-green colour.

Hydrogen-alpha (Ha) also is extremely abundant because it is a early stepping stone in the generation of all other elements and has a very rich deep red colour.

It appears to be a bit harder to track down information on SII. 🙂

The point of the above diagram is to show how the Hydrogen Alpha and Doubly Ionized Oxygen images are mapped into the RGB colour space and the reason for saying the outcome image depends is because we have a choice on how this is done. The resulting image is called a false colour image, but we can understand what elements are where. Hydrogen Alpha is mapped to Red and Doubly Ionized Oxygen is mapped to a equal amount of blue and green.

Below is a comparison of the Hydrogen Alpha – Doubly Ionized Oxygen (above) image and the RGB (below) image.

From the Hydrogen Alpha – Doubly Ionized Oxygen image we can clearly answer that most of the Crescent Nebula is due to the ionization of Hydrogen due to the abundance of red, and there appears to be a preceding envelope of Oxygen.

Discovered in 1792 by William Herschel the Crescent Nebula lies in the constellation Cygnus. Cygnus is the large unique cross shaped constellation aligned with our galaxy’s central plane. For those of us in the eastern time zone here in Ontario it is found high over head in the late evening (~10PM) in late August / early September. Cygnus is the swan, visually represented head to tail from Albireo to Deneb with outstretched wings. The Crescent Nebula is comfortably riding along on the swan’s back.

To help set the scale of things, the entire picture above would comfortably fit inside the small green block in the sky map below. The sky map below is about the size of your outstretched hand held up to the sky.

Freestarcharts https://freestarcharts.com/ngc-6888

From a structural perspective, Crescent Nebula is an emission nebula which means we can see it because of the presence of a nearby energetic source making its gases glow. In this case, the source of this fury is a red supergiant (WR 136), in the image it is the bright star nestled in the centre of the crescent. The gases it is illuminating are its own outer shell it ejected into space as it progresses along its life from super giant to super nova.

The description of NGC6888 at https://chandra.harvard.edu/photo/2003/ngc6888/ gives a good scene of the speeds and times involved in this nebula.

The image scale is 0.57arc-sec/pixel and the crescent is 1870pixels long on its major axis, so NGC6888’s angular size from earth is about 18arc-min. From our perspective this makes NCG6888 is a very small object, it would fit between your fingers spaced to hold 2 coins, edge on, at arm’s length. Most sources indicate NGC6888’s distance from earth is approximately ~5000 light years and so I calculate NGC6888 is ~25ly across on its major axis. Way too far to drive with the kids in the back asking if we are there yet.

The image is a 5 hour exposure, 2 hours Luminance and 1 hour each colour channel (RGB) taken on one night in early October 2021.

Online references

NASA Science https://science.nasa.gov/ngc-6888-crescent-nebula

Wikipedia https://en.wikipedia.org/wiki/Crescent_Nebula

Chandra obs https://chandra.harvard.edu/photo/2003/ngc6888/