Creating star trails with automatic sky glow removal

Creating star trail images is one of the easier astrophotography tasks there is, since it doesn’t require any messing about with tracking devices. You just set a camera to point skywards, with programmable remote control set to snap a few 100 images over a hour or more. There are then software programs which can merge the individual images together to create the final star trail. On Linux, I use a startrails plugin for GIMP to perform the merging of individual frames which is simple to use and works pretty well.

The basic star trail image with no sky glow adjustments applied

The basic star trail image with no sky glow adjustments applied

This is all very straighforward if you’re capturing images under ideal conditions, but life doesn’t always work out that way. The first thing people do is to add in dark frames, which are images captured with the lens cap on. These frames will record any hot pixels or general sensor noise that may be present. The startrail application will merge all the dark frames and then subtract the result from the light frames, removing the hot pixels from the final image.

When taking images in London though, there is a major problem with sky glow from the ever present light pollution. There are number of techniques for removing sky glow, with varying pluses and minuses. A simple way is play with the curves/levels to reduce the intensity of the background glow and/or use colour balance corrections to try and make it less noticeable. The problem with this is that the corrections will apply to the background sky, the stars themselves and any foreground objects uniformly. All too often the sky glow is not uniform, but in fact a gradient from top to bottom of the frame, meaning the results of curve adjustment show up the gradient

The basic star trail image with curve adjustments to reduce intensity of the sky glow. Due to the gradient, it is only possible to remove part of the sky glow

The basic star trail image with curve adjustments to reduce intensity of the sky glow. Due to the gradient, it is only possible to remove part of the sky glow

The overall goal is to produce a background that is completely black. This could be achieved if we are able to extract the background component from the main image and then subtract it, leaving just the stars behind. The layers feature in any photo editor can be used as mechanism for accomplishing this task.

If we make an assumption that sky glow is approximately the same in each frame, we can follow a simple series of steps to extract a reasonable approximation of the sky glow background. Start by opening one of the individual frames. Then select the menu option Filters -> Blur -> Gaussian Blur. The radius setting should be set to a large value such as 150px. The resulting image should now be a very smooth colour gradient just representing the sky glow background.

A single image frame blurred to leave just the sky glow background

A single image frame blurred to leave just the sky glow background

Copy this blurred image, and switch to the star trail image previously produced by the plugin. Create a new layer, paste the blurred image into it and anchor the floating selection. Finally change the layer mode to “Subtract”.

The final star trail image with the gaussian blurred sky glow subtracted

The final star trail image with the gaussian blurred sky glow subtracted

The final result above doesn’t have a completely black background because the sky glow we extracted is merely an approximation, but it should have a less significant colour cast, and lower overall intensity. A small adjustment of curves can further reduce the remaining glow:

The final star trail image with the gaussian blurred sky glow subtracted

The final star trail image with the gaussian blurred sky glow subtracted

While this is much improved, there are still some limitations with this technique. If the sky conditions were changing during the course of the image capture session, any single frame will not be so close to the average glow of the final image, so will either be over or under correcting the sky glow. Weather often plays a part, sending clouds through the scene during capture. Ideally one would simply wait for a clear day before capturing images, but not everyone has such a luxury of time. When some images contain clouds, the final image (as seen above) will have a quite uneven, patchy sky glow which is not easily removed with a simple subtraction layer.

Ideally one would subtract the sky glow from each individual frame before merging them to produce the trail. With 100’s of frames this is quite a tedious process, but since the GIMP startrails plugin is open source GPLv3+ licensed python code, we have the freedom to modify it. To that end I have created a fork which has the ability to perform automatic sky glow removal by applying the gaussian blur technique to each individual frame. This more than doubles the amount of time required to produce a star trail image, but the results are very satisfying indeed.

The star trail when sky glow has been subtracted from each individual frame before merging

The star trail when sky glow has been subtracted from each individual frame before merging

The only downside is that when there are foreground images present, they tend to get a halo around their edges. On the plus side, subtracting the sky glow from each individual frame has totally removed the clouds from the image. The background is a pretty dark gray, but not completely black. This can be tweaked with a little use of curves to produce the final image

The star trail when sky glow has been subtracted from each individual frame before merging, with curve adjustments at the edge

The star trail when sky glow has been subtracted from each individual frame before merging, with curve adjustments at the edge

One final tip is to run a guassian blur with a radius of 1.2px on the final image before saving. This will smooth out any jagged edges on the trails caused by the tiny gaps between successive frames.

Obsolete and Discontinued: a collective photographic project

The project background

This post is talking about my involvement in the Obsolete & Discontinued project which has just had its first exhibition at the Releva-T analogue photography festival in Spain. The seeds of the project were sown back in March 2015, when London based photographer & expert printer, Mike Crawford, was given a huge quantity of photographic paper (and a few rolls of film) by a client, David Yates. The paper had come from darkroom supplies left by David’s late uncle, Bret Sampson. There was quite a variety of paper, including Agfa Record Rapid, Kodak Bromesko, Agfa Brovira, Kodak Royal, Kentmere Bromide, Ilford Ilfomar and more besides. Most of the paper was so old – at least 20 years, perhaps as much as 40 years for some – that common opinion would suggest it to be mostly worthless and fit only for the rubbish. Thankfully Milke did some test prints to investigate the condition of the paper, finding some in fine condition, while others had heavy fog. Even those which had heavy fog turned out to be quite amenable to lith developer which was not nearly as badly affected.

After seeing these results, Mike unveiled a proposal to a meeting of the London Alternative Photography Collective (LAPC) meeting. He intended to give out batches of paper (~10-20 sheets per person depending on number of participants) to a group of photographic artists with an open brief to produce any type of work they wished. The only rule was for the supplied paper to be used in some manner, whether for the final work or just in an intermediate step. The idea was immediately appealing and many attendees of that meeting signed up to participate in the project straight away, myself included. After getting around 60 people signed up Mike sent out details on what paper was available (and its response with test prints) to all participants asking them to provide a preference list.

Developing the techniques & idea

At the time, having a 3 month old baby, I wasn’t really going into the darkroom at all, so using any kind of traditional black & white process was out of the question, never mind learning lith printing techniques. I decided to focus on alternative photographic printing techniques that could be done without any use of a darkroom, specifically the lumen and chemigram processes which I had recently started experimenting with in the back garden. I had trays suitable for 8×10 paper and wanted FB paper rather than RC. Beyond that I was not fussed on the choice, whether it was fogged or not, since this is pretty much irrelevant to the chemigram process which takes place in full light. Essentially any photographic paper will work for chemigrams no matter how old or badly looked after it is. Eventually I was given approx 15 sheets of Agfa Brovira G3 as the official paper to use for the project work.

Rather than risk wasting this precious paper, I bid on a couple of ebay auctions acquiring some ancient kodak and ilford papers for exploration of the techniques I wanted to use in the project. With the ilford paper I learnt how to combine the lumen and chemigram processes to create very pleasing hybrid works. Showing one of the works to some friends at another LAPC meeting, a remark was made that one of the prints (shown below) gave the impression of a river running through a city:

Resulting print after combining the chemigram and lumen processes.

Resulting print after combining the chemigram and lumen processes.

I myself felt that the dark brown / black texture of the print felt like a desolate landscape, devastated by industrial development or some natural disaster – quite representative of the state (reckless) human development can leave land in. At the same time I was feeling that too many chemigram images I’ve seen focused on the totally abstract and so wanted to explore how to control the chemigram process to bring in more recognisable forms.

Melding these thoughts together, my intent for the Obsolete & Discontinued project was to try to produce semi-abstract, but still recognisable, images of coastal cities around the world using the chemigram and lumen processes.

Behind the scenes

The starting point was to identify some coastal cities whose border between land and sea would give rise to reasonably recognisable outline and a good balance. In other words I spent a while browsing around Google maps looking at coastlines for random cities I’ve visited in the past. This identified a number of candidates including London (Isle of Dogs), Southampton (Isle of Wight), Plymouth, New York, Boston, San Francisco, Rio de Janeiro and Tokyo. With these identified I printed out the maps on plain paper and using a scalpel cut around the coastline to remove the land

Printed map outline after cutting away the land mass

Printed map outline after cutting away the land mass

The paper outlines were then traced onto pieces of 2mm thick card, then cut out with a scalpel again, to create the master heavyweight stencils

Cardboard stencil after being traced and cut from the paper outline

Cardboard stencil after being traced and cut from the paper outline

The purpose of the stencil is to assist in applying the resist for the chemigram process. The chosen resist was a thick paste of flour and water, the same that was used in earlier chemigram experiments. The stencil was placed on a sheet of the photographic paper and the resist spread on in a thin layer. The ultimate result was that flour paste adhered to the paper in the regions that would represent the land mass, while the sea remained clear.

Applying a resist of flour & water to the photographic paper, using the cardboard stencil

Applying a resist of flour & water to the photographic paper, using the cardboard stencil

The coated paper is then left overnight to allow the flour paste to dry out and form a hard crust on the paper. With the areas representing the sea still exposed, it was time to start on the lumen processing. The idea was to try to get an abstract texture to represent the rippling surface of the sea. To this end, several layers of crinkled clingfilm were placed over the paper in a random fashion, which would (in theory) control the amount of UV light affecting the paper.

Paper covered in clingfilm and placed in a photo frame ready for exposure under the sun

Paper covered in clingfilm and placed in a photo frame ready for exposure under the sun

The paper was placed in a budget photo frame obtained from a local pound shop (aka dollar store) and then left out under the autumn sun for approx 4-5 hours. The UV light had quite a nice affect on the paper turning it a fairly intense purple/blue color, and the use of cling film had been partially successful in introducing variation in the colouration.

The paper after it had been exposed to the sun for several hours

The paper after it had been exposed to the sun for several hours

The lumen exposure was immediately preserved by putting the paper through a regular fixer bath. Sadly with the Agfa Brovira paper, almost all of the colouration from the lumen exposure disappeared. This is entirely expected when fixing lumen prints, but the degree of fading varies across different papers and the Brovira seemed particularly badly affected. The final step was to slowly dissolve away the flour resist, moving the paper between developer and fixer baths every 10 minutes or so. Where the developer came in contact with the surface of the paper first, it would go dark gray/black, while the fixer would preserve undeveloped areas in pale brown. In this way the texture was gradually built-up as the flour resist dissolved. The chemigram stage took at least an hour of hard work per print to complete.

In the end six prints were made on the Agfa Brovira paper supplied for the project, and the three most successful ones were selected for presentation to the project.

Final results of the process on Agfa Brovira paper

Final results of the process on Agfa Brovira paper

My time producing prints for the project spanned a couple of months, with me snatching 2-3 hours at a time to work on it when the weather was favourable to lumen printing, in between the usual time consuming duties of parenting. The most time intensive part was the chemigram process where the dried flour paste would very slowly dissolve, but the results obtained were certainly worth the effort. Participating in the project helped focus the mind, allowing for intensive effort to control & master a particular set of techniques, with a clear target in mind. It has been a great learning experience and I’m very pleased to have gotten involved in it.

Creating chemigram images with caffenol ingredients

Over the past year or two I’ve done a bunch of experiments with the Chemigram process and even combined it with the Lumen process. In the work so far I’ve used various different substances as resists to control the action of the developer and fixer on the paper, thus influencing the pattern of the light & dark regions. Meanwhile for film processing at home I have been using my Caffenol-C-H-UK recipe almost exclusively as the developer. Caffenol is not just for film, it can be used for developing paper too and it occurred to me one day that instead of mixing up the caffenol in a jug, it might be interesting to just let the caffenol ingredients mix and react directly on the paper. So began a new series of chemigram experiments without using any kind of resist at all.

Caffenol is made by mixing washing soda crystals, vitamin C, instant coffee and optionally some iodized salt. The coffee and vitamin-C are the developing agents while the washing soda acts as an accelerator. The first step was to make a solution of washing soda and water and thoroughly soak the sheets of paper in it. The granules of instant coffee can be placed individually on the paper where needed, or simply sprinkled in an adhoc manner. The vitamin-C powder can just be poured or sprinkled onto the paper. The theory is that when the coffee/vitamin-c hits the wet paper it reacts with the washing soda to form caffenol on the surface. This is done in normal lighting conditions so the paper is universally and totally exposed and should gradually turn black where the caffenol has formed.

For this first test I used off-cuts of some outdated ~|40 year old) Ilford FB paper approx 5×4 inches in size. It was soaked in warmed (~25 C) washing soda solution then some coffee was sprinkled on, followed by some vitamin-C powder. After a minute or so it is possible to see hints of development along the edges where the paper is going dark gray

Caffenol chemigram initial stateAfter 5 minutes the developed areas didn’t appear to be getting any darker. I figured that since the paper was quite lightweight and moderately glossy, it was probably not able to absorb very much of the washing soda solution thus limiting how much caffenol can form. The washing soda is critical as an accelerator, without it coffee/vitamin-C are far too slow. So to try and intensify things I used a syringe to squirt on some more washing soda, which made the vitamin-C fizz very nicely. This image shows the run-off is developing the paper quite efficiently after just a minute or so:

Caffenol chemigram developing

The surprise came when I decided to wash off the ingredients. It turned out that the instant coffee had formed quite a sticky sludge which had adhered well to the paper. Given the limited water holding capacity of the paper, the areas with great concentrations of coffee granules had ironically developed the least. There is just a slight gray speckling effect where the coffee had a very limited action on the silver halides. The areas of most intense development were along the edges where the coffee and vitamin-C had mixed initially, and then in the broad areas of run-off which had well mixed caffenolCaffenol chemigram result

The process was repeated, but without pouring washing soda over the ingredients, just relying in that initially absorbed by the paper. The results were fairly similar to the first test, but with less development of the surrounding paper, as would be expected due to lack of run-off.

Caffenol chemigram resultThe results obtained were partially aligned with the initial expectations of the process but, as always when experimenting, there were surprises. In particular the inability of the paper to absorb sufficient washing soda solution was a key limiting factor in the results. It was surprising to see how the coffee + vitamin-C alone were fairly weak, but when they combined they became stronger than the sum of the parts. Finally the way the coffee became a sticky mass on the surface of the paper actually caused it to act as a chemigram resist, as well as a developer at the same time!

With the initial experiments successfully completed it was time to try some larger scale work with full sheets of 8×10 paper. The goal was really to just do more of the same but on a larger scale. The first work was fairly light on washing soda, thus forming broadly static patterns showing the texture of the vitamin-c powder and coffee granules, though there were some limited areas of runoff creating dynamic swirling patterns

Caffenol chemigramWith the second print the aim to was make a very dynamic image showing the motion of developing liquid on the surface of the paper, at the expense of any fine detail.

Caffenol chemigramPleased with the results of caffenol in a pure chemigram process, I decided to take it a step further and try to combine chemigram with a traditional B&W development process in the darkroom.

A while ago I took an simple photo of the Moon with a DSLR and teleconvertors which I then used to create a digital negative on acetate for printing as a cyanotype. Astrophotography is an increasingly popular endeavour for many people, but almost without exception the aim is to produce images with the best sharpness and finest detail the equipment will allow. An unfortunate result is that any two images of the moon will look broadly alike, and my own astrophotography images of the moon are no exception. So I decided that this digitally captured moon image would provide a good challenge as source material for creating a truly unique photographic print.

In the darkroom under safe light conditions, I placed a sheet of outdated Ilford FB 8×10 paper under the enlarger. The digital negative went on top of the paper for purpose of contact printing. From previous experience contact printing on this paper I just guesstimated the exposure at 15 seconds, with lens at f/8. The paper now has an invisible latent image ready to be worked on by the caffenol.

I soaked the paper in a washing soda solution, randomly sprinkled instant coffee and vitamin-C onto the paper and then just let it sit for a few minutes to give time for the caffenol to start working. Part way through I also added a very small amount more washing soda in some areas to encourage the development. After approximately 3-4 minutes (I wasn’t really timing this) I could see slight hints of the paper starting to turn gray in places. Washing off the caffenol residue though showed almost no development across most of the paper, which was initially disappointing.

None the less I now put the print through a regular B&W dev, stop & fix process. Rather than leaving it in the developer for the full 1 minute though, I noticed it was developing quite fast and choose to just move it to the stop bath when it “looked about right” – about 35 seconds. What was happening was that although not really visible yet, the caffenol had indeed kickstarted the development across the paper and the normal developer was just needing to finish off the process. If I had let the image site in the developer for the full 1 minute it would have been over developed and lost some of the qualities of the caffenol granulation.

The result was thus incredibly pleasing image of the moon, which I hereafter title “Moon through a dirty window”

Chemigram caffenol contact printLooking at the results obtained shows that the idea of using caffenol in a chemigram process has great possibilities for image making. On its own it can be semi-controlled to create attractive abstract images, while when combined with a regular B&W printing process it turn an otherwise plain image into an intriguingly textured pleasing artwork. I’m very much looking forward to getting back in the darkroom to further work through the possibilities this offers