Advanced deep-sky photography

 

Capturing an image of The Wall in LRGB produces striking colours and sharp contrasts

In part six of our guide to astrophotography, we’ll be taking deep-sky imaging to the next level.

In part five, we looked at producing deep-sky images with DSLR cameras and one-shot colour CCD cameras.

This time we’ll be concentrating on deep-sky imaging with mono cameras and external filters.

These don’t rely on built-in colour filters to encode the colour data, as well as having increased sensitivity, enabling you to produce sharp, exquisitely detailed deep-sky images.

MASTERCLASS

Imaging The Wall, the brightest part of NGC 7000, using LRGB filters

Here we’re going to show you how to produce a full colour image using LRGB filters and a mono CCD camera.

A mono CCD camera is a very versatile piece of equipment; it lets you capture data using a wide range of filters to achieve different results.

Add to this the ability to take long exposures with reduced electronic noise, thanks to these cameras’ in-built Peltier cooling, and it becomes a very powerful tool.

Your choice of filter size will be dictated by the size of your sensor as well as your pocket. Although it is possible to remove the camera and replace each filter in turn, this is really not recommended!

It becomes a tortuous task to make sure that everything goes back in place correctly to ensure that the individual groups of images line up with one another, so a filter wheel is a real must-have.

Although a software-controlled electronic wheel is very nice, a manual wheel removes a level of complexity from the system and is pretty much guaranteed to have repeatability of filter placement each time.

Despite ranges of good-quality filters being designed to be parfocal, wise astrophotographers will always check focus when swapping to the next filter so unless you have completely automated focus as well, the appeal of an electronic filter wheel diminishes further.

However, electronic wheels do have one great advantage over their manual siblings: once the filters are loaded in they are light- and dust-tight because there is no wheel edge projecting out of the casing for you to manually revolve.

It is useful to plan an imaging run carefully in advance to make sure that you make the best of your available time.

In LRGB imaging, your luminance data is the most important as this decides the deepness and detail features of your image, but how you capture your RGB data can have a great effect on the final image too.

What you’re trying to achieve in RGB imaging is a finished picture that replicates the colours seen by the human eye through matching the spectral sensitivity of the CCD’s sensor to your eye.

The human eye is most sensitive to green light whereas a CCD sensor normally has its highest sensitivity in red light, which is great for imaging emission nebulae.

However, the use of filters and a naturally occurring effect known as ‘atmospheric extinction’ that reduces the brightness of night-sky objects, will further skew the sensitivity of the sensor to red, green and blue.

If you were to take equal length exposures for each of your three colour filters your images could end up with a colour cast to them (which is why DSLR cameras have an automatic ‘white balance’ feature built in).

To compensate for this skewed sensitivity, it is necessary to either take different length exposures for each colour or to adjust an image’s colour balance later in postprocessing.

The atmosphere naturally scatters blue light more than the other colours so if you can reduce the amount of atmosphere that the blue light has to travel through, your blue data will be crisper and less noisy.

Aim to capture your blue data when your chosen object is at its highest in the sky.

Just as important as your image data are your calibration frames. Bias frames are not dependent on the filter in use but your flat frames most certainly are, especially if you are hoping that these frames will remove any ‘dust bunnies’ in your images.

Although dark frames are not directly dependent on the filter in use, bear in mind that if you have used different exposure lengths for each filter, you will need dark frames of a matching exposure length for each filter too.

STEP BY STEP

Create a spectcular image of The Wall using LRGB captures

1 Luminance

With the luminance (IR) filter selected, good focus achieved and the autoguider running, start your imaging run. It always pays to capture luminance first so that if the clouds roll in during the session, at least you’ll have something worthwhile for your trouble. Take at least 10, 480s exposures.

2 Red channel

With the luminance data under your belt you’ll have a pretty good idea of what your image is going to look like even though it will only be in mono, so change the filter to red and carefully re-check the focus, adjusting it if necessary. Take six 240s exposures, binned 2x2.

3 Blue channel

Assuming that about now, NGC 7000 is high in the sky, change filter to blue and re-check the focus. If you are using an ED doublet refractor there is a good chance the focus will match that of the red filter and with a triplet refractor, it should be bang on. Take six 360s exposures, binned 2x2.

4 Green channel

The final part of the capture is through the green filter. If the cloud rolls in before you capture this set, there is a way of producing a full colour image from what you’ve already captured. Check focus again because if any colour will be out, it’ll be this one. Take six, 300s exposures, binned 2x2.

5 Calibration

If you have an electro-luminescent panel or light box, capture your flat frames immediately after you’ve taken each set of filtered images, or take them the following day without disturbing the focus. The bias and dark frames can be taken inside at any time. Calibrate each set of filtered images.

6 Stack, align and combine

Stack the images into four masters, double the size of the red, green and blue masters and align each with the luminance in Photoshop. Produce an RGB file the same size as the luminance and populate each channel with the matching colour. Paste the luminance channel on top and set the blend mode to Luminance.

EXPERT TIP

Focal reducers and field flatteners

Many deep-sky objects, particularly nebulae, are quite large, requiring a large sensor and a short focal-length telescope to fit them into the field of view.

Large sensors tend to accentuate a problem inherent in most telescopes - that of field curvature.

Most telescopes produce a curved focal plane yet a CCD’s sensor is flat, so if accurate focus is achieved with stars at the centre of the field of view, those out towards the edges will be out of focus and will take on a stretched appearance.

This effect is made worse by using ‘fast’ optics, yet fast telescopes with their small focal ratios of around f/5 and f/4 are very desirable for capturing the dim light from deep-sky objects.

Luckily there is an elegant solution to this, which is to use a combined focal reducer and field flattener.

These special double-element optics are installed between the CCD camera and the focuser, where they not only reduce the apparent focal length (thus reducing the focal ratio) but also correct the field curvature thanks to the way their optics are figured.

The end result: far sharper stars at the field edges.