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.