Long exposure Astrophotography brings out details and colours in faint objects and can create stunning images.
This project could keep you enthralled for your entire observing career!
Deep Sky objects are galaxies and nebulae many light years from Earth that are not visible to the naked eye and are often too faint to be viewed through binoculars or a telescope. The long exposure times possible with cameras attached to telescopes allow the details of these objects to be seen.
Connecting your DSLR camera to your telescope
To achieve good results in deep sky imaging we need to rigidly attach your camera to your telescope and keep it stable throughout the exposure, which an be many minutes in duration.
We can do this in a couple of ways. The simplest is to use an adapter that replaces your camera lens with an eyepiece barrel. These adapters typically come in two components: A T Ring suitable for your camera; and a T thread barrel.
The T ring I’d a round disc with a fitting that matches your camera on one side and a hole in the middle that has a standard T thread.
The barrel is simply a 1 1/4 inch or 2 inch eyepiece barrel with a T thread on the end.
Screw the barrel into the T ring, remove the camera lens and attach the T ring/Barrel to the camera. Keep a cover on the eyepiece barrel to protect the innards of your camera.
Now, remove the eyepiece from your telescope and replace it with the camera, tighten the thumbscrew on the eyepiece holder and you’re good to go.
You may need to rebalance your telescope and you may find that you need to adjust the spacing of the camera by removing the telescope diagonal, or using a barrel extension tube.
Focus the image in the camera by pointing the scope at a bright star and use the live view feature of your camera to zoom the image, then adjust the telescope focus until the star shows as a bright, sharp point. Lock the focus if you can.
Start with a nice,easy target like the Orion Nebula by pointing your scope toward the middle star in Orions sword, set your camera to Manual mode and Bulb exposure and use a remote Shutter Release to take a 10 Second exposure at ISO 1600.
if everything is working nicely and you scope is well aligned you should see a view of the nebula that shows a bit of colour. It may be offset from the centre of the frame, if so, adjust the telescope to centre it, taking more shots to confirm you composition.
Now try a longer exposure. Jump up to 20 seconds and see what a difference it makes. You should see the central core of the nebula starting to overexpose, showing as a white blob and the outer reaches of the nebula starting to become visible.
If your image is starting to show trails in the stas, then your mount is not precisely aligned. Most astrophotographers use an Autoguider to ensure that their scope correctly tracks the desired object throughout an exposure.
The darker the sky is at your site, the longer an exposure you can take before the glow of the sky overwhelms the image. Try some longer exposures and see what you get.
if you are seeing a lot of noise (speckles) in your image, reduce the ISO setting and compensate but increasing the exposure length to see what combination gives you the best results.
Darks, Flats, Bias, Subs
These are the magic words of astrophotography.
Many things can introduce noise into your images: faulty pixels in your camera; hot pixels created during long exposures; hot pixels created by data transfer in your camera’s electronics; and dirt in your optics.
A dark frame is simply an image taken with the same settings as one of your ‘real’ images and with the camera at the same temperature but with the telescope covered so no light enters. This creates an image with the distinctive pattern of faulty and hot pixels for this exposure setting. Typically at least 1 dark frame will be taken with each set of exposures.
A flat frame is an exposure taken of a neutral white surface, often created by stretching a white cloth over the end of the telescope. This shows the distinctive pattern of dirt in your optics ( very scope has some!) The duration of this exposure is not important, but the orientation of the camera in the scope must be identical to that in your ‘real’ exposures for the dirt to appear in the same place in the image. Typically at least 1 flat frame will be taken at the beginning of your imaging session.
A bias frame is just like a dark frame, but is a short exposure taken with the telescope covered so no light enters. This frame shows any noisy pixels created by the camera’s electronics. Typically at least 1 bias frame is taken at the beginning of your imaging session.
Subs are simply your ‘real’ exposures taken multiple times with the same settings.
After your imaging session, you can process your images to improve the final result. For each exposure you can subtract the dark, flat and bias frames to remove noise and dirt from your image. Then you can combine your subs (through stacking) to sharpen the image. Finally you can combine images of different exposure lengths, and different filtering to capture detail only available in specific exposure lengths.
Processing your images can be a painstaking and time consuming affair, but the result are worth the effort.