GCSE Astronomy Controlled Assessment B4: Constellation Photography

This is a picture of a sky chart of what would be seen in Britain in winter taken from the ‘Astronomy now’ website:

Aim:

I plan to take my photographs in my garden in my home which is slightly on the outskirts of Cheltenham, Gloucestershire. Even though this is not the most ideal site (due to street lamps and other houses around me hindering accurate observations of the sky), it is better than somewhere like the inner city where there are relatively more streetlamps, thus worse sky glow.  An ideal site would be somewhere in the middle of a field in the countryside as the skies would be darkest here but it is not possible to travel to such a location as I do not have a house there and these observations would be done in school time.

I have chosen the December period in which to do my coursework because this is when the skies will be darkest for longest and there are some interesting winter constellations available to be observed. I will have to find out the Moon phases (optimal when the Moon is new as then there is not light from it overpowering the stars) and weather so I can take my photos without being stopped by the seeing conditions.  I will use a star chart to choose constellations which have bright and interesting stars which will be visible enough to allow me to determine their colour. When actually observing I will need to following equipment:

• Red filter torch (so my eyes stay dark adapted)
• Camera (I will use my Sony DSLR-A230 on a long exposure setting of 30 seconds as this is the optimum. A longer setting can show star trails which would cause the image to look blurred but a shorter setting would not allow in enough light so the stars would not be visible.)
• Tripod (on which to place the camera on so it has a steady place to rest while the image is being taken to ensure the maximum optical quality of the photo)
• Warm clothes
• Gloves
• Flask of coffee (to keep me warm and refreshed as I carry out my coursework)
• A map of the sky (obtained from a reliable source giving me an image of the sky in the
• December time in Britain.)

Once I have photographed my images I will use a noise-reduction software called Neat Image which will enhance the quality of my photos.

Here is an image of the sky chart I will be using to identify constellations when I am out in the garden, photographing them.  

Detailed Planning

I have found out the dates at which the moon is new or close to new using the Time and Date website and I have also found out the weather conditions for each date using the Met Office website.

After looking at the acquired data I have chosen to take my photographs on the 13th of December as this is when the moon is new and on the 12th and 15th of December as these dates are closest observable dates near the new moon date so the moon will be very slight in the sky; not having many adverse effects on my photography.

From the star chart which I have obtained; I have compiled a list of possible constellations I might want to observe:

-Orion
-Taurus
-Aries
-Ursa Major (Plough)
-Ursa Minor
-Cygnus
-Cassiopeia
-Lynx

From these I have chosen to observe:

• Orion: this is a very interesting winter constellation with many intriguing features such as Orion’s Nebula (which I will try to observe) and it also has pointer stars which lead the way to Sirius (brightest star in the sky), Aldebaran and Pleiades (an open cluster). This has a very bright star Betelgeuse which will allow me to determine the colour more easily.  I will use a labelled image of Orion to help me identify individual stars with which to state their magnitude and colour:

• Ursa Major (Plough): even though this is not strictly a whole constellation, it is a very interesting asterism aswell also called the ‘The Big Dipper’. This has pointer stars: Merak and Dubhe which point to Polaris (used to determine North) aswell as the ‘arc’ of the plough pointing to Arcturus. The stars in Plough are relatively bright allowing me to work out colours. I will use a labelled image of it to help me identify individual stars with which to state their magnitude and colour:

• Cassiopeia: this is a prominent constellation seen in the whole year and is easily seen in the sky as a ‘W’ shape; this is often on or near to the zenith position. I will use a labelled image of Cassiopeia to help me identify individual stars with which to state their magnitude and colour:

Observations 

I took photos of each constellation on each of the days I planned so I had 3 photos of each constellation. I then chose the best one of each:

Analysis

The weather was clear and there was little or no light pollution in my photos so I managed to take some clear photos of each constellation. The stars were high enough for me to estimate magnitudes (to the nearest whole number) from my pictures in comparison with reference stars in similar/same parts of the sky. My reference stars are:

The tables below show my estimated magnitude and colour and the official colour and magnitude for the stars in each constellation. I have obtained official figures from Collins Stars and Planets Guide.

ORION

PLOUGH

CASSIOPEIA

Evaluation

Looking at my three tables in the ‘Analysis’ section, mostly my magnitude estimates were correct to the nearest whole number and the colour was generally correct. Those that weren’t correct were due to the fact that I was only doing the magnitude to the nearest number and this was by my own judgement not by a measured technique so my numbers couldn’t be that accurate anyway. This could have been fixed by using a telescope to produce images as that would have magnified the constellations more and made their relative appearances more accurate; allowing for better analysis.

The colours were sometimes incorrect due to ‘skyglow’ from light from streetlamps and other houses overlaying my pictures and making the colours unclear. This ‘skyglow’, together with the frequent appearance of houses/trees/ power lines, also caused my image quality to decline dramatically. This problem could have been solved by travelling to a rural area with minimal light pollution, houses, trees and power lines thus causing the image quality to be much better. It could have been reduced by just going out very late when the lights in other houses would be turned off. This ‘skyglow’ was made worse by the long exposure as it lets in more light so the orange haze of the sky is significantly worsened and made more prominent.

Another way that the image quality would have been increased dramatically, if I had the budget, would be by buying a camera mount. This camera mount allows the exposure time to be increased significantly without causing star trails by rotating in sync with the rotation of the Earth so it collects more light and it is focused in one place rather than spreading like in star trails. This produces an image like this (much better than my feeble attempt!):

Another way this project could have been improved was if I went out on more than just three dates; increasing my chances for a better photograph. I should have also experimented with the Camera settings (such as exposure time and ISO) and written each down each setting for each picture I took so I could see which setting provided me with the best photos; this extra knowledge would have enabled me to produce much better images. ISO is how sensitive the roll is to light, aperture is the size of the objective lens and the shutter speed is how long the shutter stays open.  A higher ISO (and thus a smaller aperture and higher shutter speed) would produce a better image but would have much more noise (a more grainy image).

Despite all this, I think my project was well designed considering the location and price limitations.

Bibliography

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