Deep Sky Object (DSO) Imaging
My personal taste is for long exposure DSO work. The time you can put into this can collectively come to an enormous amount, and that’s just on the data gathering stage. Ive seen images on social media (Facebook, Instagram, Twitter etc) where this stage alone has surpassed 20 hours. These are usually where the astronomer / astrophotographer has collected multiple types of narrowband data, such as Ha (hydrogen alpha), Oiii (oxygen) and Sii (sulphur) on top of their usual RGB. I’ll revisit narrowband imaging once I actually start doing some of this myself. But suffice to say, that where we could gather say 5 hours of normal RGB data on an object, for each of the bands above, you would do the same (this is just a rule of thumb.) So, using the example above, that’s already a massive 20 hours of data gathering. At the moment. my biggest data magnet has been 17 hours in total on the Heart Nebula in Cassiopeia.
Now you’re not going to gather that amount of data by just pointing a camera at the night sky and clicking it a couple of times. In order to obtain those crazy amounts of data, you’re going to need some equipment and software. Now, when I first started this 5 years ago, I was using a standard tripod and a crop-sensor DSLR that I had previously been using for semi professional photography before I moved across full time to astrophotography.
So, what do you need? Well, that camera and tripod did me well for the better part of a year and I learned all sorts of things, such as the “500 rule” for example, how to focus at night etc. But I was restricted in what I could shoot. So I gradually upgraded to the kit that I use nowadays. Bearing in mind that this is still “entry level” stuff almost, but for the time being, it seems to work well for me.
Astrophotographers the world over will say two words when asked what the most important part is – “the mount.” Now there are all sorts of mounts and they all have their pros and cons, but if you want to start shooting long exposure astrophotography, then you’re going to need something that tracks the stars and counteracts the earth’s rotational effects. So you’re looking at what’s called a GEM, or German Equatorial Mount.
Because of the way the earth rotates and the manner in which the stars move across the sky, it all follows an arc, and a GEM will follow that arc once you have it all set up and guiding properly. I won’t cover guiding here as I’ve already done that in a previous walkthrough on the subject. Another type of mount, the Alt-AZ, will still track, but without something called a wedge, it tracks the stars differently and limits the amount of exposure time you can obtain. There are also small portable trackers, that you can take on field trips with you, such as the SkyWatcher Star Adventurer. I have all three of these types, but my primary DSO rig is the GEM, a SkyWatcher EQ5 Pro. My children, my ikkle astrohearts, use the other two for the time being, and I’ll cover those in separate sections.
For the DSO work, which is what this section is all about, I use what’s called a widefield refractor, in this case the SkyWatcher 72ED. With it’s focal length of 420mm and an apeture of 72mm it’s ideal for widefield nebula and large galaxy work.
Up until recently, I was using a CCD CLS filter, but after recently relocating to Class 4 skies, I’ve passed the filter back to my children to use in their Class 5. I’ve yet to buy one for my modified Canon 450D.
The target that I’m shooting will determine the camera that I use. For something large like Messier 31, The Andromeda Galaxy, I’d use the 450D. For something like Messier 101, or Barnard 33, I’d use the Altair Hypercam 183C Pro. But then if I were shooting those wider, then I’d stay with the 450D.
Miranda is my primary rig and it’s her that I’m usually out with. I do have another setup that I’m in the process of building, which will be used for lunar and planetary imaging, as well as (I hope) faint fuzzies. I’ll cover her in another section. And you guessed it, she also has her own name.
In order to achieve long exposures when you have stars that constantly move, you need to be able to keep those stars absolutely rock steady otherwise you’ll merely have a mass of blurred lines. To achieve this, you need to be able to do something called guiding, which I cover in another walkthrough. In summary though, you have another, smaller mini telescope and camera sat on your main one, and this, using software running on the laptop, which you have all of this plugged into, locks onto a star. The software looks for movement in that star and sends correction commands to the mount in order to keep it as solid as possible. Depending how good your mount and setup is, you can achieve exposures of 20 minutes and over with a single frame. Mine can hit 10 minutes, although not consistently, and so I keep it to 5 minutes for the time being.
This smaller scope and camera is something I often refer to as my “guiding solution.” There are a number of solutions that others use, but ideally you’re looking to use something with a 50mm apeture and 200mm focal length, although there are smaller and quite succesful ones available. Mine consists of the SkyWatcher 9×50 finderscope and the ZWO ASI120mm (mono) camera, which is the perfect combination for me.