A Brief Overview of Deep-Sky Imaging Equipment

Abstract

The process of imaging a deep-sky object is in theory very simple. It involves a telescope or a basic camera and lens that tracks the movement of the object across the sky for as long as it takes to gather enough light to produce an image.

The process of imaging a deep-sky object is in theory very simple. It involves a telescope or a basic camera and lens that tracks the movement of the object across the sky for as long as it takes to gather enough light to produce an image.

A deep-sky imaging system therefore has to be capable of accurately following the path of an object across the sky. This is the job of the telescope drive system. The drive has to be aligned with the polar axis of the earth, as it is the earth’s rotation that we need to counteract. This all has to be done very accurately for extended periods of time. Otherwise, tracking errors will show up as star trails and blurry images that lack the fine detail we are trying to record. Fortunately, even at the budget end of equipment, it is possible to produce fantastic results.

Deep-Sky Imaging Setups

The following is a description of imaging setups capable of producing truly remarkable results. Which of them is the most suitable for you depends on the objects that you are interested in imaging. Like any hobby, it is best to start with basic equipment so you can decide if it really is for you. I would therefore suggest that you start with either the basic Wide-Field Imaging System or The Minimal Deep-Sky Imaging System. These are both excellent starters that will teach you the basic skills needed to setup and align an imaging system and are capable of producing great results.

Wide-Field Imaging Setup

• Camera and lens

• External interval timer if your camera is not equipped with one

• Simple equatorial drive system

• Tripod

• Power Supply

The picture in Fig. 2.1 shows an autoguider, which is desirable but not completely necessary. This type of setup does not have to be aligned quite as accurately as a full imaging setup, as it generally uses a shorter focal length, meaning the images will be of a lower magnification and tracking errors will not be quite as obvious. A wide-field imaging setup can also be quicker to set up and is not as heavy as a full imaging setup. It can therefore be very handy for star parties or even on holiday, where space and weight may be limited. In its smallest form a camera with a wide-angle lens and one of the many new lightweight drives is an economical, easy, and effective introduction to astrophotography. For these reasons, many astrophotographers have a wide-field system in addition to their usual deep-sky imaging system.

Fig. 2.1

Wide-field imaging setup. (Credit: Dave Fielding)

For deep-sky imaging, a wide-field setup is more suited to larger objects, such as M31, the Andromeda Galaxy, and M33, the Triangulum Galaxy. The Rosette Nebula, North America Nebula and Pelican Nebula are also great targets, to name just a few.

This setup will indeed image many of the smaller deep-sky objects, however, to see them in any great detail requires a longer focal length system hence—a telescope—due to the small size they will present on the imaging chip of the camera. You can appreciate this limitation using a Field of View app or webpage calculator, which will show an accurate representation of how a given object will fill the frame of your camera. This is very simply done by entering the focal length of the lens or telescope and camera and then choosing an object you are interested in imaging.

Here is the address of an online FOV Calculator/Telescope simulator:

• https://www.skyatnightmagazine.com/astronomy-field-view-calculator/

Minimal Deep-Sky Imaging Setup

• Camera

• Telescope

• Equatorial drive system

• Solid tripod

• Power supply

A minimal deep-sky imaging system can also be a good starting point for the aspiring astrophotographer. The setup needs to be done with care and accuracy to get the best results. It is capable of taking excellent images and carrying out serious work if desired. Many astronomical objects will be within reach of this type of setup depending on your local sky conditions. It is a great start for the experienced visual astronomer looking to transition to imaging, and you may even have most of the requirements already.

This is also a good choice of imaging setup for taking to dark sky locations and star parties. It is however slightly more advanced. The main difference between this and the wide-field imaging setup is that this one enables you to image smaller and fainter objects due to the higher magnification and greater light-gathering power of the telescope. Many more objects are within its capabilities. This type of system forms the backbone of an imaging setup and can be expanded on and upgraded as required.

This setup requires a very good polar alignment and an accurate telescope drive, which limits the duration of exposures that can be taken before star trailing starts to become a problem. It also relies on the object to be located and positioned in the frame manually. This will require practice and possibly a technique called star hopping, which is explained later.

Basic Deep-Sky Imaging Setup

• Telescope

• Camera

• Drive system preferably GoTo

• Autoguider (this can be standalone or requires a computer)

A basic deep-sky imaging setup uses a GoTo drive to help locate objects for you and has an autoguider that is locked onto a star to send corrections to the drive and keep the object from drifting in the frame. The GoTo telescope drives are generally controlled by the use of a handset. This however has started to change, and it is becoming more common to have drives that connect to a mobile phone or tablet computer and are controlled by the use of an app. This method of control is usually carried out by a wireless connection using a Wi-Fi hotspot generated by the telescope drive.

These additions make it much easier to find an object to be imaged, and much longer exposures can be taken. This in turn means that much fainter objects can be imaged without star trailing. This is the level at which so many images are successfully taken by astrophotographers the world over.

Note that the system still requires an accurate polar alignment and also some form of star alignment that synchronizes the sky with the telescope controller. The object may need to be positioned in the center of the camera frame manually depending on the GoTo accuracy. One method to improve the accuracy is to slew the telescope to a nearby bright star, center that in the field of view and use the synchronize function, and then make the short slew to the object being imaged. This will result in accurate slews to the required object.

Advanced Deep-Sky Imaging Setup

• Telescope

• Imaging Camera

• USB filter wheel

• USB focuser

• GoTo Drive system

• Camera for Autoguiding

• Computer

The advanced deep-sky imaging system differs from the others in that it uses a computer to control the telescope instead of a hand controller, which becomes redundant. This is achieved using a special computer cable that typically plugs into the handset controller socket on the telescope drive system and then a USB port on the computer. This allows the telescope to have many more functions available than are built into the handset. The system may then be capable of locating and centering objects in the field of view, and choosing a guide star locking onto it for autoguiding, then taking the desired images. It can also focus and change filters under computer control. In its most advanced form, it can also be capable of following an imaging sequence or script containing a list of objects, the exposure times and the filters, when to check the focus working autonomously until all the required images have been captured (Fig. 2.2).

Fig. 2.2

Advanced imaging setup

As you can see, moving away from the hand controller to computer control adds massively to the telescope system capabilities by allowing you to use a multitude of programs to control many or all aspects of your imaging system. There are several commercial software packages available, but many of the popular programs can be used free of charge under a GNU license.

This is definitely a system suited for a more permanent setup of the dedicated imager, as it requires a lot of initial setting up. Additionally, the software elements can seem a little daunting at first, but they are actually quite straightforward if you follow the instructions carefully. There is also a vast amount of help available online if you get stuck. Once that is all out of the way, the entire system is relatively easy to run. However, it does require a logical approach to use and some problem solving when things don’t go according to plan. I have written a guide to problem solving in Chap. 10: Troubleshooting.

This type of setup is capable of very high quality imaging once it has been fully mastered and can image most objects, dependent on the local sky conditions. It can carry out fully automatic imaging runs without any intervention by the imager.

A setup like this can take a few years to fully master and tune to reach its full potential. It is the ultimate challenge and joy of amateur astrophotography.