FILM for Astrophotography
Posted 23 September 2006 - 09:05 AM
Film is created in layers, and each layer is sensitive to range of the light spectrum. By comparing the sensitivities of these layers you can avoid a film that will be difficult to capture good astro photos.
First you want a film that has pretty even response among all the color layers.
Second, you want a film that has strong sensitivity in the very faint, but all-important Hydrogen Alpha line, which is at 653nm. If the film's sensitivity drops off at that point, you will notice that the film cannot record most of the red emission nebula in the Cosmos. If you trying to capture an object that has a lot of blue, such as the blue reflection nebulas in the Pleiades (m45) or the Trifid nebula (M20), then you may want a film with slightly more blue sensitivity.
Here is a chart that the spectral sensitivity of 4 films. Note how the curves on the top two films almost peak at the hydrogen Alpha (Ha) line. These films will record faint red emission nebula very well. Also, note how these two films have a slightly higher blue response, another plus when capturing blue reflection nebula such as the Trifid nebula or the Orion nebula complex.
These charts also show why the popular over-the-counter film Kodak Max 400 and Fuji Superia 400 are poor for recording red nebula. Their sensitivity near the 656nm line is low, and the Kodak Max 400 drops off almost completely!
While sensitivity will help you choose a film, you also must be aware of how much RECIPROCITY FAILURE a film has. As discussed earlier in this thread, reciprocity failure is where a film's sensitivity drops dramatically during an exposure. Worse yet, many films have different reciprocity failure for each color level. This means that while the sensitivity to red may go down a lot, the yellow and blue may not drop as fast. This mans you end up with an overall green image. Not good! The only way to know how well a film does in this regard to test them. There are no charts produced by the film companies to show this well. Some astrophotographers have made some charts, but they are now outdated. If you want a project to make a lot of your peers happy - build a light-box to do a controlled study and post it here!
Posted 23 September 2006 - 09:16 AM
Posted 21 October 2006 - 10:18 PM
Posted 23 October 2006 - 09:06 AM
Posted 14 March 2007 - 06:39 AM
The law of RECIPROCITY states that if you DOUBLE the brightness on a given subject, you should HALF the length of the exposure. And the reverse is also be true. If you half the brightness, you can simply double the exposure to get the same result.
In everyday common daylight use, film works well. A photograph taken at 1/500th of a second at f/4 and can be also be done at 1/1000th of a second when you open the aperture up one more stop to f/5.2 (doubling the f/stop to let more light in). But film image formation is a complex chemical process, and does not follow this rule very well in very dim settings – like deep sky astrophotography. This process breaks down to create a failure of reciprocity – or better known as Reciprocity Failure.
While I will not get too technical, here’s about why this happens.
In order to create an image with film you first need silver crystals in the film emulsion. These silver crystals combine to create a film grain – what we see when you magnify the film – these are the equivalent of digital pixels. The initial silver crystals are created when silver atoms react to single photons from your subject. It takes a minimum of two photons to start this chemical process – and if the second photon does not arrive within a few seconds, the reaction fails and must be restarted with two new photons. The exact time is based on chemical instability and is somewhat random. Also the photons arrive at a random time – so you have a lot of variability. In 2 seconds you may get the two photons needed, while 2 seconds later you do not. If it arrives too late, the opportunity is lost.
Then, once you have the reaction (from the two photons) the silver molecule is not yet completely stable. The new reaction can disintegrates after a few minutes from interactions with chemicals from the emulsion dyes, oxygen and water molecules. After a few minutes you start to loose reactive silver atoms. If you have millions of atoms, then you can afford to loose a few. If you have only a few then the odds of them surviving film development is low.
It takes many silver atoms to clump together to create one silver crystal – which in turn creates the film grain. And if that is not bad enough – in color film you have 3 or 4 layers of this mess to deal with. Each color reacts differently to the low light level, and while the yellow and blue may hold on to reactive atoms, the red may not, and you end up with a green image that shows none of that beautiful red Lagoon Nebula!
As you can see, the fainter the subject (less photons) the more time you need to make up for wasted reactions that keep reverting back to a neutral state. It’s an uphill battle… like carrying water in a bucket with holes in the bottom. To get to the top with any water left you’d better have a lot of extra water (photons)!
Note: The film’s rated speed (ASA or ISO) are not calculated for extended exposures. Those ratings are for short exposures of a second or less. Don’t be fooled into thinking ASA 200 film is slower than ASA 800. When dealing with long time exposures, these numbers are meaningless and often the opposite is true. Kodak E200 (200 ASA) is MUCH faster than Kodak Royal Gold 800 after 10 minutes of exposure. The Royal Gold leaks like sieve!
So to help in this matter – you can do several things.
1. Grab more photons per second. This is done with shorter focal ratios. This is why most film astrophotographers stay at f/5 or faster.
2. Choose a film who’s chemistry was designed to hold on to those initial reactions and thus preserve the silver crystals over a long period of time. Most commercials films have little interest in this – and they are lousy for astrophotography. Others, like Kodak E200 and Fujichrome 400F Provia are very good at it.
3. Remove the very reactive oxygen from the film emulsion. Oxygen is usually in the form of water, so if you remove the water molecules from the film you will slow the reciprocity failure. One way to do this is to bake the film (low temperatures – this is not cake) in a pressurized container filled with s small amount of hydrogen and the rest some non-reactive gas like nitrogen. The hydrogen will combine with excess oxygen in the film, and then be baked out of the film. When you vent the chamber, you end up with very dry, oxygen starved film. A lot of work, and with some films yields little difference. Other films are helped tremendously. This technique is called Gas Hypersensitation and was popular in the days before Kodak E200 and Fuji Provia. It is almost never done these days.. but it makes for a good history lesson.
4. Speaking of history – here’s another old way to stem the loss from Reciprocity Failure: Slow the loss reaction by freezing the film far below zero degrees F. Astrophotographers used to use dry ice or nitrogen to freeze the film in the camera. A tiny piece of optical glass was between the frozen film and outside of the camera, and a vacuum pump was used to remove moister to prevent frost from fogging up the glass or collecting on the film. Most of these cameras where hand made from used camera parts, although a couple of commercial units were built. The main disadvantage is that you could only load one frame at a time in the camera.
5. Develop the film as soon as possible after you expose it.
Posted 22 March 2007 - 05:44 PM