Deep Sky DSLR imaging
This guide has been split into three sections: how to assemble your imaging set up, what setting you'll need for your camera, and how to take images. All of these are equally important and, while it all may initially seem complicated, the settings remain the same (and stored in your camera's memory), the set up will seem very intuitive (easy to remember) and once these are sorted, taking the images is simplicity itself.
How to Assemble Your Imaging Set Up
Primary Scope Set Up
Firstly, you need to remove your scope's 1.25" adapter if it has one, so that your field flattener can be slotted into your scope's 2" fitting. The field flattener will be held in place either by a screw fitting or brass compression rings that are tightened with a screw.
Secondly, the T-ring adapter will screw into the other end of the field flattener. At each step here, make sure everything is tightly secured as you don't want any expensive equipment falling from your scope!
Finally, remove the lens from your DSLR and attach it to the T-ring as if it was another lens.
Once again, ensure everything is securely fitted and won't fall out if your scope points straight upwards.
At this point your scope is the camera lens.
Guide-scope Set Up
The guide-scope set up is far more simple to assemble but is the crucial element that lets you take very long exposures.
Firstly, you want to secure your guide-scope to you primary scope in one of the following ways:
Piggy backing - stacking your guide-scope on top of your primary scope by securely screwing your guide-scope's tube rings to your primary scope's tube rings.
Side-by-side - using a mounting plate that allows you to screw both your scopes side-by-side to create an 'H' shape.
Your guider of choice simply slots into your guide-scope's 1.25" fitting and connects to your computer via a network or USB cable. From here you can control the exposure settings, lock onto a star and set it guiding. The guider locks onto a star and tells your mount (via another cable) to adjust its position if that star deviates from the dead-centre of the guide-scope.
This allows your mount to fix both scopes fields-of-view precisely, so that you can take long exposures without incurring any star trails or blurring of the images (assuming you've focused properly!)
Guiding in this way allows you to take exposures of many hours if you wish, but you'll most likely take 5 or 10 minutes exposures (or subframes/'subs') and stack them together with software.
What Settings You'll Need For Your Camera
White Balance - Set to 'Cloudy' or as close to 6500K as you can get. The 'Shade' setting, if you have it is also acceptable.What Settings You'll Need For Your Camera
Picture Style - This needs to be set to 'Faithful'. Getting the Picture Style setting wrong can lead to all sorts of colour aberrations.
Drive Mode - If you have a remote timer or shutter release use the Self-timer/Remote control' setting. If not select the 'Timer' setting so that you can push the shutter release and give the camera and scope time to stop wobbling before it opens the shutter. *
Mirror Lock Up - If your camera has a 'Mirror Lock Up' feature use it! This feature will move the 45 mirror in front of your camera's sensor a fraction of a second earlier than it opens the shutter to prevent camera shake from creating a blurred image.
How to Take Images
The exposure setting will already defaulted to f/0 because your camera will not recognise the lens it now has attached (your scope). So your exposure is now governed by your scope and you can't/won't need to adjust this.
The ISO setting you want will be either 400 or 800. The lower the ISO the greater the contrast; the higher the ISO, the greater the detail. But higher ISO settings introduce more noise. Once you get the hang of it you may decide to try additional 200 or 1600 ISO images for advanced Photoshop merging. But for now 400 or 800 will probably suffice.
The shutter speed will always be set to 'bulb'. If you have a remote timer, this will allow you to set it for as long an exposure as you wish otherwise you'll can use a remote timer to finish the exposure manually*.
The length of your exposures will depend upon the level of light pollution and how much detail you need to capture. If you're imaging a faint object (such as The Pinwheel Galaxy) you'll want very long exposures of 10 or even 15 minutes. Combining 10 or 20 very long exposures in stacking software will give you tons of detail in your images - but, if you have any light pollution, this will start to saturate the frames in such long exposures. Slight light pollution can be mitigated with a filter (see image right) or by taking shorter exposures of 5-10 minutes.
* You want to avoid ever touching the camera or scope during the exposure, as you can introduce shake or blurring into the images. So I can't recommend getting a remote timer or shutter release enough. A simple infrared remote shutter release can be bought from Amazon for under £5 and a digital timer (which will take multiple exposures at your desired length) can be bought for £15.