Astrophotography Part 1 - By Nick Glenister


Copyright © 2004 Nick Glenister
home Introduction The Trouble with Telescopes

Part 1) Taking photos of the stars with an film based SLR

I would firstly like to recommend a superb book called Astrophotography for the Amateur by Michael A. Covington. This book goes into greater detail than I will in this journal and I strongly urge you to purchase it if you are at all interested in astrophotography (see references at the bottom of the page).

I have been taking photos of the stars now for just over a year so in this first journal entry Iíd like to explain the workings of astrophotography without a telescope as I have found them. This is really intended for those who havenít taken photos of the stars before and reflects a very personal approach that has worked for me. Iím not a professional photographer but I like to understand why things work so I have gone into a reasonable amount of depth but I have summarised the main points at the end of each section.

Throughout this journal Iíll refer to two common types of camera; the SLR and the compact. SLR stands for Single Lens Reflex and this is a camera where the same lens is used for both view finding and picture taking. In comparison a compact camera has a separate lens for view finding. If you want more detailed information on the equipment use please take a look at my equipment list

Light pollution

One thing we are really blessed with here in Norfolk is dark skies. Being miles away from the nearest cities means that we get to see large numbers of stars that our poor city cousins can only dream of. Even for me though light pollution from neighbours security lights and the occasional badly designed street light can cause havoc with astrophotography as illustrated in the photos below.

Orion and Sirius showing of lot of light polution

Orion and Sirius. Timed exposure of 20 seconds on Fuji Superia 400 film with light pollution coming from a street light and house back lit from neighbours security light
[photographer: Nick Glenister]

Ursa Major. The constellation is almost obscured by local backyard security light

Ursa Major. Timed exposure of 20 seconds on Fuji Superia 400 film with light pollution coming from neighbours security light
[photographer: Nick Glenister]


Even at a short exposure time of 20 seconds you can see that light pollution can become a very great nuisance even in a small village in Norfolk. For me the simple solution is a five minute trip out of the village to a local field but not everyone is that fortunate.

Shutter Speed

For the most part compact cameras are extremely limited when it comes to astrophotography and this is down to the inability to select the shutter speed which determines the length of time the film is exposed to the light. Most SLR cameras however have a shutter speed setting of ďBĒ or ďBulbĒ. The bulb mode of the camera gives you manual control over how long the shutter is held open for. This means the longer you hold the button down the longer the shutter stays open. This is perfect for astrophotography as it gives plenty of time for the tiny amount of light coming from the stars to react with the film.

But there is a balance. If you donít hold the shutter open for long enough (too short an exposure), you wonít get enough light onto the film so you wonít get a picture of the stars. If you hold the shutter open for too long you will get the phenomenon know as star trails which you can see in the meteor photo (Pic 1). This occurs when exposures of over 20 seconds are made and is caused by the Earths rotation which makes the stars drift across the sky. To ours eyes this movement is imperceptible over short times but the camera picks it up easily. The book Astrophotography for the amateur will explain this in more detail. All you need to know is exposures of 20 seconds is about the maximum length of time you should hold the shutter open for and should give you nice sharp photos of the stars.

Because every camera and lens is different I have always ďbracketedĒ my photos into intervals as suggested to me by a professional photographer. This allows you to see what exposures best suit your equipment. For each subject I first take a 10 second exposure then a 15 second exposure and finally a 20 second exposure. I personally find 15 seconds to give the best results for constellations as the lower magnitude stars do not show up leaving just the higher magnitude stars that make up the constellation visible. As I progress to different equipment and new subjects like the moon I will vary these brackets to see which is the best.

Aperture

At all times when taking photos of the stars the aperture should be wide open to allow as much light in as possible. This will be dependant on your camera lens. A good quality lens with an f-number of f1 Ė f2 (a fast lens) will let more light in than a slower lens of f4 - f5. The F-number system means that every lens set to the same number and focused on the same subject will give the same image brightness regardless of focal length or camera. This means you can change lenses or camera but as long as you have the same f-number selected the image brightness with remain constant. For you the reader to achieve the same results as seen in my photos you would need to select an f-number of f2.2 and hold the shutter open for 15 seconds.

Lenses that have f-numbers of greater that f3 are not well suited to astrophotography as not enough light can reach the film in the 20 seconds of exposure time. I have tried using my Pentax SLR and F3.7 lens many times and the results have always been very disappointing compared to the Fujica SLR with the F2.2 Lens. Lenses with lower f-numbers of course come at a higher price but all of my successful photos have been taken with a lens of f2.2.

Stability

The reason you need a tripod and a cable release is that if you hold the button down on the camera the vibrations of your hand will make the pictures of your stars look all wobbly. The tripod gives the camera a solid base and the cable release puts a flexible cable between you and the camera allowing you to hold the shutter open without the vibrations. For those of you who donít own a cable release or you cannot get one for your camera, you can still minimise vibrations if your camera has a self timer and allows to set the shutter speed to 20 seconds (most modern cameras should allow this). The benefit of the self timer is that it allows any vibrations to disappear.

Film type and speed

Films sensitivity to light is indicated by its speed or ISO number. The higher the ISO number the faster the film. Most standard films (Kodak Gold etc.) are usually ISO 200 or ISO 400. Logic states that the faster the film the better it would be for astrophotography but I have yet to try this.

The film I use most often is Ilfordís black and white. This is only because I can process this myself and control the contrast and level of detail in the prints in my darkroom. The film speed is always 400 as this is a fast film that reacts quickly to the light that falls onto it. I have used colour print films before but Iíve had very mixed results in the prints I get back from the labs. This ranges from dark grey skies with bright colourful stars two unprocessed rolls as the lab technician/computer assumed the roll of film was blank. The latter was a mix of mine and their fault. In the beginning I was not aware that you need to notify the lab that the photos are very dark and will need printing appropriately. More recently I have dealt with a lab that seemed to disregard my instructions for processing. As it stands I have only found one lab that has actually given me a set of prints back and they were superb but also very expensive so I have reverted back to black and white and manual processing. My recommendation to you is if you have a love of photography and like the idea of processing and printing your own photos start with black and white and invest in a dark room. If not then go for colour and visit a local processing lab in person and explain to them what you are trying to achieve. All labs should be open to processing the photos as you require and visiting them in person should mean you avoid the problems Iíve encountered in the past. The book Astrophotography for the amateur gives great advice on what to tell the lab so you get the best results.

Itís quickly worth noting that you can do you own colour processing but this is really quite expensive and time consuming and really becomes a labour of love.

I would always stick with ISO 400 film speed to start with (for both colour and black and white) as slower films (ISO 100-200) might not pick up enough light. I do have one roll of Ilfordís ISO 3200 black and white film which is a considerably fast and very grainy film mostly used for artistic portrait photography but Iím keen to try out this film for astrophotography purposes and when I do I shall let you know the results. Also on my list to try is a colour slide film as the author of the Astrophotography for the amateur book recommends this over print film. As always once I have tried this I shall let you know my findings.

Last minute notes

Before we wrap up this journal entry Iíd just like to give the beginners to astrophotography one important tip I have learnt the hard way. In the winter months when the nights are cold and frosty, your breath forms great clouds of mist which can have quite disastrous effects on your photo taking. Not only will the your breath block out the light from the stars if you breathe in front of the lens but worse still if it condenses onto the lens youíll have a hard job picking up anything and guaranteed it will not be until several photos later that you notice.

Taking photos of the stars with just your camera is rewarding but once mastered is very short-lived. After all once you have photos of all the major constellations what is left to photograph? In answer to this in the next journal entry I will focus on photographing the moon using telephoto lenses and in a future entry photographing meteor showers. Also I hope to borrow a digital SLR and will compare film vs. digital in a head to head battle.

Until then happy star gazing and good luck with your photos.

Nick
Additional (2007)

Since the time of writing I have learnt a great deal more. In hindsight my initial choice of telescope proved insufficient to my needs. While its fully capably of doing what I want I found it to be too much of a chore to set up and therefore the enjoyment reduced dramatically. While I certainly don't regret buying it (as i have had much use and enjoyment from it) the effort required in taking the longer exposure outways the gain.

To highlight some of the problems I had and still face I have added another page which can be found here: The Trouble with Telecsopes

References

Astrophotography for the amateur
Michael A Covington
Basic Photography
Michael Langford