ASTROPHOTOGRAPHY – How to tell a planet from a star

Astronomers studying variable stars have compiled catalogues listing about 10’000 stars visible with the naked eye (Mag 6.5 and less). This includes both hemispheres, so an observer at mid-Northern latitudes would technically be able to see about 6’800 stars on any given night (this takes into account the Southern stars visible in the North). Out of those 6’800 dots that one can see, five of them are planets. So how to tell the difference?

Stars emit their own light, whereas planets reflect the light of the Sun. Stars are point sources of light, meaning they have no apparent diameter – no matter how high the magnification, it’s impossible to see the actual sphere that makes up the star. Planets are significantly closer and one can see their actual shape, be it small (arc-seconds of a degree). As a result, star light is more prone to distorsions from the atmosphere and this is why stars twinkle, whereas planets do not.

How to photograph the twinkling of stars:

Expose for 2-3s and gently tap on your camera to induce movement and get a line on the photo, rather than a dot. Light from planets will be monochrome (Mars is a good target, because it’s nice and red), whereas light from a star will pass through all the colours of the rainbow as it’s distorted by the atmosphere.

Mars shows a constant red colour whereas a star will vary both in intensity and colour.

OPTICS – Glory

A 20° Glory sporting several rings, indicating constant droplet size distribution on the cloudcover below. This particular Glory was centered on seat 9C, behind the left wing of the Avrojet 100 aircraft I was flying in – passengers seated more to the front saw the Glory centered on their position (the aircraft shadow would be more to the left).

Interesting to notice that as the plane descends for landing its shadow gets bigger but the glory diameter remains the same.

More info and the related math of this phenomenon here.

OPTICS – Drink for free with physics

So I’ll bet you a good Bordeaux I can make the arrows point different ways without touching them.

The physics:

The water in the glass creates a cylindrical shape that resembles a magnifying glass which flips the image on the vertical axis due to refraction: light converges somewhere in the middle of the glass and is projected – flipped – through the front.

More info and original video here.

ASTROPHOTOGRAPHY – The Sun / sunspots 2797, 2798 and 2799

Below, the small sunspots designated AR2797, AR2798 and AR2799. The latter has the particularity of being the first in the Northern hemisphere of the Sun for the last 2 months. This is a one-shot exposure of 1/3200s at ISO 100 on an APS-C CMOS sensor through a 85mm f/5.3 refractor (Takahashi FSQ85) with a visible light sun filter (transmission 0.01%).

The sun filter is a Mylar sheet (a form of dimensionally stable thin plastic) powdered (probably) with aluminium to block 99.99% of the light of the Sun and allow observation of sunspots.

MICROSCOPY – The Swamp III / six months

The Swamp III sample has been collected at the end of summer, in early September. Now, after having spent the winter months inside at a comfy 22-24°C (rather warm, really) I am happy to report there is still life inside the jar: both plants – well, algae to be more precise – and protozoa. Anything moving at this point is alive because it reproduced.

The LED lights that were initially installed eventually died, and were replaced after four months or so. The cap was re-sealed on this occasion (hot glue in between the cork and the sides of the jar, with electric tape on top). I kept the lights on for months at a time to make sure the algae were producing sufficient oxygen and food for the protozoa. After the lights went dim and then died I added a pinch of stark to compensate the loss of light and feed the creatures inside. For the last couple of months I switched the light pattern to the normal day / night alternance and significantly reduced its intensity (placed under a lamp rather than the LEDs inside).


The walls of the jar have became almost opaque with the algae growing on them. Some are long filaments that seem to be growing on the walls, in the body of water and above the water line. There is also a much smaller variety, that sticks to the walls and makes a fine, green film. The substrate is black mud and pebbles; it seems to be partially colonised by a lighter coloured algae-type organism. Will have to check in more detail.

There is no sight of the snail or the bubble bugs. There is no movement that can be seen with the unaided eye. Using a 25mm eyepiece for astrophotography as a loupe allowed me to see the larger protozoa swimming around the jar, so they managed to survive and reproduce. It would be interesting to know how many generations have spawned since I collected the sample.


I collected samples from the body of water, from the side of the walls and from the substrate. First inspection shows algae and protozoa – the jar is alive and I can now reasonably say that I have a self-sustaining ecosystem.

Rotifers are alive and well, they seem to feed happily and there are many of them. I could see different sizes, but there seems to be only one species (Bdelloids). They have really multiplied and are present in great numbers in any sample I take from the jar. It seems they reproduce asexually and all are females.

Rotifers after six months in the jar

More elusive but equally abundant paramecium. They are particularly active and move very fast through, and out of the field of view. here’s a calmer one:

Not many, but rather large nematode worms. I didn’t see them in all samples I collected from the jar but the ones I did manage to catch were huge (several cm) compared to the ones I saw in the beginning (max 1cm).

Large nematode worm

MICROSCOPY – The Swamp III / two months

The water is quite clear, as are most of the walls. There is one (barely) surviving plant and some green algae. The snail and the bubble bugs are still alive and happy. Saw a new, large insect cca 1cm.

I drilled a hole in the cork and installed an aluminium tube with an airtight screw-cap. I re-sealed the cork to the side of the jar with hot glue, then covered it with tape. The ecosystem is, for all intents and purposes, sealed, but allows access to take water and other samples.

I use a large-ish syringe on which i put a shrink tube and a plastic cylinder at the end to get the samples through the hole I put in the cork.

Most of the surviving plant life consists of algae. The one plant that seems to be alive has no more green leaves above the water line. The roots growing out of the stem do seem healthy and alive.

The algae are a deep shade of green and grow attached to the stick on the bottom of the jar, on the decaying leaves and on the side of the jar (above the water line).

I switched the light off for one night and there was a new bug floating upside down near the surface. It was still alive but obviously not ok.

There are more and more eggs, with obvious fresh ones appearing on the sides and on the bottom of the jar. Also new batches on the stick as well. The older ones seem to have been colonised by algae and have become green (picture on the left). The “nucleus” of the egg seems to be an elongated, twisted ribbon that looks pretty much like an intestine. It has not changed since the eggs have been laid. I still suspect the snail.

I found the large bug i mentioned before. It died. it’s big, and ugly, and am sure it could crawl out of the jar if it chose to do so. Seal your jars quickly.

Many nematode worms are visibile with the naked eye, about 1-2mm in length, swirling and jerking around the jar.

MICROSCOPY – The Swamp III / one month


There still is life in the jar; very few plants left. Water is surprisingly clear and the algae have all but disappeared from the walls. The twig has sunk well below the water level.

Only one plant seems to have survived. It’s a long thin blade of grass that produced a very nice root that eventually found the bottom. this seems to be surviving and has risen well beyond the surface – it’s flat against the top.

The snail is large 1cm+ and happily wondering around, as are the bubble bugs. Eggs on the stick underwater and on the side of the jar; I suspect the large snail laid them. The algae have all but disappeared from the sides, probably eaten by the snail, as I’ve seen it munch away at them several times. The water is surprisingly clear – I suspect there is not enough light for algae to develop in large numbers. I think the jar *cycled* in the aquarium sense of the term.

MICROSCOPY – The Swamp III / one week

The swamp is most insulting to the nostrils. Became blurry with algae both on the jar walls and within the mass of the water. Large 8-10mm larvae jerking around on the surface. Am expecting mosquitoes which will be killed.


Many, very small organisms quickly moving about. Larger ciliates twirling and swimming about.

A forest of what look like long, green algae but they move on their own, waving around as if in flowing water. I believe they may not be plants.