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Jaspal Chadha reveals how you can capture quality deep-sky images under light-polluted skies
For the last few years I've been taking images of the night skies. But there's a problem: I live in London, one of the most light-polluted areas in the UK. This makes it hard to achieve one of the main requirements for a good astro image ? a high signal to noise ratio. While the best way to increase that ratio is to reduce the noise by imaging from a site with darker skies, there are various things you can do to capture decent images from under the urban lights in the middle of a city.
After months of trial and error I finally settled upon a setup that works for me. When I started I used a DSLR and color single shot CCD. The results weren't what I expected. The images lacked detail and were often filled with the orange glow of light pollution, despite my best attempts to reduce it. Even with exposures of four hours I wasn't happy with the amount of detail being captured and trying to remove the glow with photo-editing software was a long process that still didn't get me the results I wanted.
Sacrifice color for clarity
Things improved when I switched to using a monochrome CCD camera, an option that retains the main advantage of a CCD: its sensitivity. The more sensitive the camera, the shorter the exposure required to detect faint detail. CCD cameras have a greater dynamic range than DSLR cameras, meaning there's a larger range of luminosity that they can detect. The CCD can more easily capture both faint and bright detail in a single exposure, rather than needing several images to bring out different elements.
However, a mono CCD only detects the brightness of the light, not its color. If you want to bring out the color of images you have to use filters. In normal color imaging, three filters are used to separate the primary colors of the visual spectrum. Red, green and blue (RGB) filters are designed to approximate the color sensitivity of the human eye, so that the resulting image is true color.
When using RGB filters to create a broadband image, all types of wavelengths are captured across the entire visible spectrum so this picks up a lot of light pollution from the surrounding city lights. This is usually most visible as green and magenta gradients in the images. To reduce this I use a simple CLS CCD light-pollution filter.
There are a few things to remember when using light-pollution filters, however. First is that they're not 100 per cent foolproof when it comes to light suppression. These filters are helpful, but they have their drawbacks. All of them are designed to block out only particular wavelengths of light and there's one overriding factor to consider when deciding how effective they'll be for you. They're designed to block the wavelengths emitted by low-pressure sodium-vapour lamps ? the orange type. If the location where you make your observations from is lit by newer LED street lamps, then light-pollution filters will be useless. They also cut down the total light getting to the sensor, so the exposures required will be longer. But they should increase the ratio of useful image information compared to background glow, so overall should result in an improvement.
Go deeper by narrowing down
For those times when I want to bring out the finer detail of a deep-sky object, I attach narrowband filters: the light-pollution filter's green tint is easy to remove in photo-editing software during the image-processing stage. These filters enhance the contrast of emission objects by accepting only a narrow range of wavelengths around the emission lines of certain gasses within the objects, such as hydrogen-alpha (Ha), doubly ionised oxygen (OIII), ionised sulphur (SII) and others. During the image-processing stage, data from each emission line is assigned a certain color band ? red, green or blue ? and these are combined later in a graphics editor to create the most stunning images.
As narrowband filters only pick up a tiny portion of the available light, they can be used to take astrophotos even when the Moon is up as well or from locations that are usually plagued by light pollution. Narrowband filters commonly come with a bandwidth of 5nm or 3nm at the emission line they let through; you can expect to pay a higher price for the lower bandwidth filters.
Much time and effort will be required to capture data from all three filters. So little light gets past the filter that imaging requires very long exposure times. Typically I'll spend at least three to four hours' imaging time on each filter, with some single exposures of up to an hour. The most rewarding results come from the Ha filter. The image is readily visible and has much detail. The OIII and SII filters produce dim, noisy images that are frustrating to work with, but are needed for a complete image.
Staying on target
Keeping the telescope on track when narrowband imaging takes some practice too. I use an autoguider, and finding a stable guide star can be a challenge thanks to the fact that so little light reaches the guide camera's sensor. Some narrowband imagers choose to use a separate camera on a guide scope. This has several advantages, the biggest being that since the guide CCD is not looking through the narrowband filter, the stars appear considerably brighter and finding a suitable guide star is easy.
The drawback to a guide scope though is flexure, where different parts of the telescope setup move by different amounts over the course of the night. This means that the main telescope may not be perfectly aligned with the guide scope during the course of the exposure. I use an off-axis guider that allows you to guide your telescope through the same optics that are taking the picture. This eliminates any possibility of guiding error.
However, the main difference between narrowband and broadband images comes when you combine the colors for the first time using image-editing software. Generally I use the standard Hubble palette combination where SII is red, Ha is green and OIII is blue. Since the Ha is usually a much stronger emission line, the result comes out very green. So it's a good idea to assign the SII and OIII a stronger combine factor, or do an equivalent in Photoshop to 'push' the SII and OIII data before combining them. Even when that is done, it's likely you'll want to do more color adjustments, such as adding a Selective Color adjustment layer, before the results become pleasing.
Though I try to cut down on light pollution, there's always a little that gets through. To cope with this, I use a powerful program called Gradient Xterminator (www.rc-astro.com/resources/GradientXTerminator) when processing data. This Photoshop plug-in destroys light wash and amplifier glow from surrounding lights that may have been introduced while imaging.
- Plan your imaging in advance using a planetarium program to work out where your target is going to be in the sky and how much time you have to capture it. This will also help you work out where the best place in your garden or observing site will be to both avoid light pollution and get the best view.
- Image when your desired object is just past the meridian line in the sky. This will ensure you have the best sky conditions and will help shy away from light pollution.
- Invest in a decent mount that will allow you to track for a longer period if you're aiming to do long-exposure astrophotography.
Reducing light pollution
Hiding away from light pollution is much more effective than editing it out afterwards.
Shielding your equipment from stray light can be as simple as adding a cardboard cuff to the end of your scope, or by extending the dew shield.
A lot of stray or unwanted light comes from security lights. If you have them, turn them off while you're observing. If your neighbour's security lights are troublesome, politely ask if they can be turned off during your observing sessions. An offer to show the neighbours what you're looking at can work wonders and you never know, you might convert them to your hobby.
Stray light gradually decreases as the night goes on. More people head to bed, turning off the lights in their homes as they go, and some local authorities turn street lighting down or off after midnight. If you're able to stay out late, you'll probably find that after midnight the amount of stray light around seems to be less than earlier in the evening.
Even if you have streetlights shining into your garden, it's usually possible to find a spot that's not illuminated by them, giving a good view of the sky. Getting into the shadow of a brick wall or a tree can help here, though this can block your view of a large part of the sky, so you may need to hunt around for the best spot in your garden.
One of the most difficult forms of light pollution comes from artificial light being shone directly into the sky and reflecting back off dust and water vapour, filling the night with a haze of light. High humidity or prolonged dry spells when dust can be thrown up into the atmosphere will seem to make the situation worse. Check weather reports and wait for stable conditions with low wind speeds to get the darkest skies.
About The Writer
Jaspal Chadha has been an astro-imaging enthusiast for two and a half years. Observing from London, he has to overcome heavily polluted skies.
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