Astrophotography
Lunar close-ups and mosaics.
The Moon presents a bright and easily accessible target for astrophotography. Indeed, there are numerous ways to image it, ranging from wide-field scenes captured using a smartphone to impressive close-ups taken with a planetary imaging setup.
Close-up images require long focal lengths and thus ideally a high frame rate camera. The reason for this is that using a long focal length creates a highly magnified image in which you are likely to see the blur caused by atmospheric seeing. A high frame rate camera takes lots of images in rapid succession, which can then be analysed and processed by a freeware registration-stacking program such as AutoStakkert! or RegiStax. These programs are capable of determining the highest quality frames and then averaging them together to produce an image with a higher signal-to-noise ratio. The net result is a cleaner image suitable for further manipulation such as sharpening.
High frame rate results can be emulated to a degree with a stills cameras such as a DSLR by taking a number of images one after the other and running them through a similar registration-stacking process. However, the best method is to use a high frame rate planetary camera, designed to take hundreds or even thousands of shots in a single, short imaging session.
Both colour and mono cameras are suitable, although if the Moon is low you may find that atmospheric dispersion reduces quality. This effect, which gets worse with lowering altitude, spreads the light of an object into a rainbow, resulting in colour fringing. In this instance it’s worth obtaining an atmospheric dispersion corrector; an accessory designed to specifically reduce this effect. For lunar mosaics, opt for a mono high frame rate camera fitted with a red or infrared pass filter. The reason for this is that the longer wavelengths at the red end of the spectrum are more resilient to the effects of atmospheric seeing.
Once you’ve chosen your camera you’ll need to decide on your focal length. This can be calculated by the simple formula fl=3.6xDxP, where D is your scope’s aperture in millimetres and P is the size of your camera sensor’s pixels in microns. An optical amplifier such as a Powermate or Barlow lens will help you achieve the right ball-park value; don’t worry about matching the calculated value precisely.
Aim to capture around 800-1,600 frames; the higher the imaging scale, the more frames you’ll need. Monitor exposure and ensure that you don’t over-saturate to white. Aiming for a value of 70-80 per cent saturation is a good strategy.
Moving methodically across the Moon can be challenging if you’re not familiar with its features. Using an equatorially driven mount, the best technique is to orientate the camera so the Moon moves parallel to the bottom of the image frame when you slew in RA. Once achieved, you can work across the area required in strips, overlapping by 15-20 per cent at the edges as you go. Once a strip has been completed, slew in declination to the next strip, ensuring a 15-20 per cent vertical overlap. Work your way along the next strip, and repeat until the entire area has been recorded.
Before you can piece together a lunar mosaic – the subject of this month’s Step by Step – you need to process each capture file into a single still image. Programs such as AutoStakkert! can do this automatically, achieved by simply dragging all of the capture files onto the main program window. You’ll have to monitor the processing of the first capture and suggest settings where required but after that the other captures will be processed without further ado.