Object Basics

Taking astro-photos is a long and involved process. There's a lot to learn and there's multiple techniques to getting to the end result. There's no right or wrong way to process the final image and there's also a lot of personal preference with how it will look. Experiment, learn and keep refining your own process and see where it takes you! There are already a lot of very good websites, YouTube channels and other resources out there to help you learn. Below, I've listed a few basics that I learned along the way that should help get you going.

There's different techniques for photographing planets and for deep-sky objects like galaxies and nebula: 

Planets

In simple terms, planets are typically photographed by taking very high frame rate short videos and using software to find the best few frames then stacking them together. This is known as 'lucky imaging'. The purpose is to find several 'lucky' images that have the least amount of atmospheric aberration. Stack these together and you can get a nice result. A fast frame rate also helps stop blurring of the image from the planet's rotation.

The best results come from when the planet is high in the sky, so you're looking through the least amount of atmosphere. The atmosphere is unstable and the more you look through, the more blurry the images.

Galaxies

Galaxies are a very long way away and can be quite dim (with a few notable exceptions!). As such, it can take a while to collect enough light to really capture them. The technique to capture images of galaxies is to take lots of pictures, known as 'frames', and stack them together using software. The longer you can keep the camera shutter open for each frame, the more light you can collect in each frame. However, keeping the camera shutter open for long periods presents other issues (star trails, object rotation, tracking, auto-guiding, camera sensor heating up, etc) which are mentioned later.

Nebula

The basic technique for imaging nebula is the same as for galaxies. However, there are some subtle differences. Some nebula are 'emission' nebula, meaning they emit light from the hot gas they are made up of. There are also 'reflection' nebula that reflect the light from surrounding stars. Getting good images of these different nebula requires the use of light filters in the imaging train. Light filters will be mentioned later.

Optical Tube Assemblies and Mounts
A 'telescope' typically has 2 main elements, the 'optical tube assembly' and the 'mount'. Often these will be bought and supplied together, but they don't have to be. It is possible to mix and match. However, when getting started, I found it easier and quicker to buy the two together as a package.

'The OTA'

The tube that contains the mirrors, lenses, or both, often thought of as 'the telescope', is known as an Optical Tube Assembly, or 'OTA'. These tubes have many different designs and come in many different shapes and sizes. Some are better at planetary imaging, some are better at deep sky object imaging, some are better for wide field (big objects, such as the Andromeda galaxy, that are very 'wide') images, some are better for very high magnification. Some have optical trains (lenses, mirrors, etc) that provide very 'fast' imaging, sometimes with an 'f' number as low as 2, which collect a lot of light very quickly. Some collect light more slowly and have a corresponding higher 'f' number. 

I chose to start with a 'Schmit Cassergrain Telescope' (SCT) OTA design as they are considered to provide the best the compromise between many designs. They are small and compact enough to be easy to handle, can provide very long focal lengths (longer focal length = higher 'resolving' power, or magnification power) and are quite good at visual astronomy and capturing reasonably good astrophotography results.

My first OTA was a Celestron 6" SCT. In 2021, I upgraded to a Celestron 9.25" Edge HD SCT.

'The Mount'

There are 2 basic types of mount for the OTA. These are 'AltAz' mounts and 'Equatorial' mounts.

AltAz, or Altitude/Azimuth mounts, move the OTA vertically and horizontally (up/down and left and right). Pros - easy to set up and 'align' (point in the right direction), great for beginners to get going quickly, great for optical astronomy. Cons - won't rotate the OTA/camera as they track objects across the night sky so the object slowly rotates in the frame. Issues with 'star trails' (elongated stars) can occur with anything other than very short exposures.

GEM, or German Equatorial Mounts' (often just referred to as Equatorial mounts, or just EQ mounts), rotate the OTA around the RA (Right Ascension) axis and DEC (Declination) axis. This enables the OTA to match the path objects take across the sky as the heavens rotate around the Celestial North Pole (CNP). The CNP is very close to the North Star 'Polaris'. Pros - Track objects and their rotation as they progress across the sky throughout the night. Cons - more involved set-up requiring 'polar alignment' before use.     

I started my astrophotography journey using my wife's DSLR, a Canon EOS 5D Mk III. I've never been a photographer and never knew much about it. Once I'd figured out how to connect the DSLR to the telescope (you need something called a t-ring - it's an adapter that connects the camera to the back of the telescope and is specific to each make of camera), and found the right software to start capturing images (Backyard EOS, in my case), it worked very well. However, as I progressed with my understanding, I switched to a dedicated astro-cam.

Back focus

One of the main issues to understand at the beginning is back focus. This is the point at which the light coming through the telescope focuses. It is typically some distance behind the telescope and requires an adapter to make sure the light is precisely focused onto the imaging sensor of the camera. If the back focus is not correct, it will be impossible to properly focus. The industry standard is 55mm of back focus from the back plate of the telescope. Achieving this requires a 'T' adapter, which are specific to the make of the camera and telescope type. 

When I moved from the 6" SCT to the 9.25" SCT, the back focus changed from 55mm to 142mm. This required a different 'T' adapter. When introducing a filter into the imaging train, this also slightly changes the back focus. Adding a filter will mean having to potentially change the back focus distance and refocussing.

DSLR or dedicated AstroCam?

It is possible to achieve very good results with a DSLR as the main imaging camera. I used one for over a year. The reason I switched to a dedicated astro-cam was to get the built-in sensor cooling system as well as having something optimised for astro-imaging. CCD or CMOS imaging sensor chips, as found in modern digital cameras, can get hot when they're used for extended periods, such as when imaging for several minutes at a time for long exposure astrophotography. Hot imaging sensors introduce a lot of electronic 'noise' into each frame. Dedicated astro-cams have built-in cooling systems to help minimise this noise. In addition, astro-cams are optimised for the typical light found when astro-imaging. DSLR's tend to be made to have a very broad sensitivity, so they can be used for a very broad range of photography purposes.

One-shot-colour or Mono?

Dedicated astro-cams come in two basic flavours, monochromatic (black and white) and 'one-shot-colour' (OSC). Both types use the same type of imaging sensor, all of which are monochromatic. The only difference is a OSC version has a 'bayer matrix' in front of the sensor. A bayer matrix is a pattern of red, green and blue filters that sits over the top of the imaging sensor and enables colour images to be captured. This matrix has a recurring RGB pattern that sits over the top of every 4 pixels in a square. This means a OSC can capture 'true colour' images with one frame, it also means the effective 'resolution' of the sensor is reduced by a factor of 4.

A monochromatic camera needs to have at least 3 frames of the same object to achieve the same thing, as it needs one frame with a separate red, green and blue filter applied. These types of cameras require a filter wheel in the imaging train and at least 3 times as many frames to create the final image, if you're trying to capture 'true colour'. The advantage of a monochromatic camera is that they don't have a bayer matrix so they produce the highest possible resolution.

Why didn't I go with a monochromatic camera and filter wheel set-up? Firstly it's about time. The number of occasions when you can get multiple hours of clear skies when you're available and free to image are few and far between. Secondly, having studied the difference in image quality of both types of camera, I struggled to see the difference. Thirdly, it's about the law of demishing returns. An additional set of equipment (the filter wheel and all the filters) is not only expensive and time consuming to set up, operate and subsequently process, the results were not that different to my eyes, nor is my skill and experience high enough to warrant the additional investment. It's a lot quicker to capture good frames with an OSC, it's quicker to process, it's easier to set up and manage and it's cheaper to purchase. Seemed like the logical choice!

Maybe one day I'll be good enough to warrant a monochromatic camera and filter wheel and maybe I'll have the time to justify it. Or, will the OSC's continue to deliver amazing results with minimal fuss, very little real difference in the end result and a lot better value for money? I'm expecting it'll be a OSC for me for the foreseeable future!