Deep-sky astrophotography produces some of the most spectacular images in astronomy. Here we’ll show you the best way to set up your gear, capture the data and process it to create your own deep-sky images.
Capturing and processing your images presents a whole new set of challenges for both you and your equipment.
Capture the globular cluster M13
Here we’ll show you how to capture an image of the popular globular cluster, M13, in the constellation of Hercules.
You can use a one-shot colour CCD or a DSLR camera for this object.
Focus your camera on mag. +2.8 Zeta (ζ) Herculis, the star at the bottom right of the Keystone asterism in the centre of Hercules.
Remember to remove your Bahtinov mask if you used one and slew up towards mag. +3.5 star Eta (η) Herculis at the top right-hand corner of the Keystone asterism.
M13 is to be found two-thirds of the way between the two stars.
The cluster’s apparent diameter is 20 arcseconds, so a focal length of 650-1,200mm would be ideal – well within reach of many popular reflectors and refractors.
A smaller focal length may not work as well, as globular clusters lose their visual impact if the field of view is too wide.
Framing isn’t too critical for this circular object but you might wish to include the two contrasting colour stars in your image – blue to the south and reddish to the east – or even the magnitude +11.6 galaxy, NGC 6207 to the northeast.
M13 is fairly bright at mag. +5.9 but it has a bright core so you’ll need to be careful not to overexpose it and burn out the centre.
Exposures in the region of 60 seconds at an ISO of between 800 and 1600 for a DSLR camera, or 120-150 seconds with a one-shot colour CCD camera, should capture some good detail.
Take at least 10 images but preferably more – up to about 30 – at these settings, using RAW mode on your DSLR camera or unbinned if you are using a CCD camera.
You can automate the process with the software that controls your CCD camera but if you are using your DSLR camera without a laptop, a programmable remote shutter release (readily available from camera stores) will do this for you.
If you are manually operating your DSLR camera be sure to use a remote shutter release, as a minimum, to avoid camera shake.
Complete the session by taking 16-20 dark frames, bias frames and if possible, flat frames.
Using suitable software such as MaximDL or Deep Sky Stacker, you should now calibrate and stack your images.
Deep Sky Stacker will import your light, dark, bias and flat frames and automatically carry out the various processing tasks for you to give a calibrated, de-Bayered (to generate the colour channels captured by the Bayer filter) and stacked image ready for importing into a photo-editing program like Photoshop or GIMP.
MaximDL, on the other hand, will break the process down into modules.
The first operation stacks the calibration frames into master frames.
These masters will then be applied to each of your light frames. Save the result in a new folder if you wish.
Stack the calibrated light frames to produce a final FITS-format file, then save a copy in TIFF format for final processing in Photoshop or GIMP.
STEP BY STEP
Imaging the globular cluster M13
To ensure that your mount tracks the sky as accurately as possible and to help your Go-To system to locate objects first time, make sure that you carry out an accurate polar alignment.
Using a polarscope is quick and easy, so don’t compromise as it will be a little time well spent.
Your mount will work most efficiently if you limit the amount of load it has to move.
Getting an accurate balance will go a long way to achieving this.
Balance the dec axis first with all of your equipment installed and the camera at approximate focus, then balance the RA axis.
Slew to Zeta Herculis and focus
Align on a bright star near Hercules, attach your Bahtinov mask and adjust focus until the cross is bisected by the line.
Remove the mask and slew to Zeta Herculis, centering it on your camera’s sensor.
If your mount has the facility, synchronise on this star and slew to M13.
Capture your light and calibration frames
Start your imaging run, aiming for as many images as possible.
Unless you already have a library of bias and dark frames (at the correct exposure length), leave time for these.
Flat frames must be taken on the night or the next day without touching the camera or focus.
Calibrate, de-Bayer and stack
Using your stacking software, apply your calibration frames to your image data to remove unwanted artefacts.
Unless your software does this automatically, de-Bayer your calibrated frames and stack them using the ‘SD Mask’ or ‘Kappa Sigma’ option.
You should already be seeing a reasonable image now but this is just the starting point.
Export your image as a TIFF file (preferably 16-bit) and load it into either GIMP (8-bit images only) or Photoshop (16-bit).
Start by applying a gentle ‘Levels’ adjustment to release detail in your image.
NEED TO KNOW
Camera control software
If your camera software allows you to take exposures of a minute or more, use it to set the ISO value to 1600, white balance to auto, file format to RAW, noise reduction (if available) to off and shutter speed to 60 seconds.
If not, set these values manually, then set the shutter speed to ‘bulb’ and use a manual or programmable remote shutter-release cable to trip the shutter for a 60-second exposure.
There’s no aperture setting because telescopes don’t have a variable iris.
If your software doesn’t allow a ‘live view’ image on your laptop, take a series of 10-second exposures using a Bahtinov mask and adjust the focus in between shots.
CCD cameras need software to control them.
Programs such as MaximDL and Nebulosity will do this.
Some CCD cameras have a gain control but use with caution to avoid increasing noise levels from the camera’s sensor.
Set exposure length to between 100 and 120 seconds for the imaging run but for focusing on a bright star, 6-second exposures with 2×2 binning will suffice.
Unlike a DSLR camera, there’s no image processing built in, so images are automatically taken in a RAW state.
If you have set-point cooling, set it to -25º and set the binning mode to 1×1.