Real Time Viewing with Video Cameras
The technique of using a camera to enhance real time views of deep sky objects
(DSOs) far beyond what is visible with an eyepiece (EP) can be traced to the late 90s with the introduction of the SBIG STV in 1999. The STV was the first video camera capable of integrating the light collected by its B & W CCD sensor over many 1/60sec video frames, effectively creating a single exposure from several seconds to as long as 10min. It was this integration ability which made it possible to observe detail in DSOs like M1, M51, and even the Horsehead Nebula with exposures of 10 to 40sec using modest size telescopes from a light polluted suburban backyard. The STV consisted of a CCD camera attached by a cable to a large control box used to adjust camera settings and which could display the image on the optional internal LCD screen or an external TV connected via a video cable. The STV had a Thermo Electric Cooler and the capability for live dark frame subtraction to minimize thermal noise from the CCD. It also had a feature called "Track and Accumulate", which internally aligned and stacked up to 10 frames in real time further reducing background noise and increasing image detail. Packed with such advanced features, the STV was certainly a camera ahead of its time, but at just under $2000 the STV was out of reach for most amateurs looking for better views than they could get through their EPs.
In 2001 and 2002 the Stellacam and Mallincam lines of integrating video cameras were introduced at prices well under $1000 enabling more amateurs to explore camera assisted viewing. These and the other astronomy video cameras introduced over the next 15 years were modified versions of analog security cameras designed with high sensitivity for low light situations making them ideally suited for astronomy. The most significant modification made for astronomy was the ability to integrate successive video frames of 1/60sec (or 1/50sec PAL) to collect sufficient photons to get an image of DSOs better than the "faint fuzzies" typical with an EP. The earliest Stellacams and Mallincams were only capable of exposures up to 2.1sec limiting them to the brighter DSOs. Subsequent models introduced over the decade used more sensitive CCDs and extended exposures to 8sec, 54sec and eventually provided unlimited exposure capability enabling all of the Messier and most of the NGC objects to be viewed in real time with a video camera. Other advances included color, Thermo Electric Cooling and in-camera frame averaging for noise reduction but without the alignment capability of the STV. With these video cameras real time color views of galaxies and nebula which would only be possible with telescopes 3-4X the size when viewed with an EP were suddenly available with very modest equipment. Stars down to 18.9Mag are visible with an entry level video camera mounted on a 9.25" SCT with a 17sec exposure. These technical advances opened up a whole new branch of amateur astronomy often called Electronically Assisted Astronomy (EAA), Deep Sky Video Astronomy, Near Real Time Viewing or Camera Assisted Viewing (CAV). Regardless of the name, the common thread is a camera with the sensitivity to capture and view amazing deep sky images in a few seconds to a few minutes.
Over the first decade of this century, video astronomy slowly grew in popularity as it occupied a unique spot somewhere between live observing with an EP and
astrophotography with more sophisticated and expensive cameras. While the images viewed live on a monitor could not compete with a traditional astrophotograph, they did provide immediate satisfaction without the hours of post processing. And while the views were not instantaneous as with an EP, they actually took no longer than a seasoned observer would take to tease out all the detail possible through an EP. While the images viewed through a camera can be impressive, they do have their limitations. First, with CCDs with less than 0.4MPixels, these cameras produce images with very low resolution compared to today's HD resolution televisions and computers. This results in stars with a "blocky" appearance when viewed at full screen on a typical computer monitor. Also, analog video is subject to video artifacts, the most nefarious of which, dark halos around bright stars, is commonly referred to as "raccoon eyes". Most video cameras also suffer from "amp glow", a bright background at an edge of the sensor caused by IR radiation from the readout amplifier.
Video camera viewing requires additional considerations beyond what is required when observing with an EP. Both the camera and the monitor need power, usually 12V d.c., as well as, separate cables for each. A third cable is needed if the camera is controlled with a wired remote or with a computer, requiring careful cable management to avoid snags and drag while tracking. Also, the control menus are designed for use as security cameras making setup for astronomical viewing cumbersome and confusing for the first time user. Fortunately, there are only a few controls which need to be changed for a successful night's viewing and there are several home grown manuals available which help to demystify these controls. And there is free software available for some camera models which translate the menus into a layout friendlier to astronomy.
Here Comes Digital
For ten years, analog video cameras were the only cameras sensitive enough to
capture pleasing images of DSOs with very short exposures. But in 2009 camera
assisted viewers discovered that a digital camera from Starlight Xpress designed as an auto-guide camera could provide similar real time views without the video artifacts inherent to analog images. The Lodestar from Starlight compared well with analog cameras at short exposures because it used the same highly sensitive Sony ICX829 CCD as the analog video camera from Mallincam called the Xterminator. The Lodestar still suffered from blocky looking stars like analog cameras because of the rectangular pixels used in its CCD, and it had a high noise floor, but it did represent a step forward in camera assisted viewing. Because it is a digital camera the Lodestar requires a computer to operate, which many real time viewers prefer to avoid. But, because it is digital it uses a single USB connection for power, control and viewing thus minimizing the cable management task inherent to analog cameras.
The Lodestar was to be the first of a wave of digital cameras which would come to be used for real time viewing over the next decade. However, it wasn't until 2015 when other suitable digital cameras became available. The ASI224MC was not only the first camera with a CMOS sensor to be widely used for camera assisted viewing, it marked the beginning of a serious transition to digital. While it used a small sensor it did provide a great improvement in resolution over the Lodestar with 1.24MP. The Atik Infinity and Starlight Xpress Ultrastar cameras came shortly after and were the first digital cameras with CCD sensors to be successfully marketed for "near real time viewing" and "live viewing", respectively. Together, all three cameras had sensors with more than 1.2MP providing 3X the resolution of the best analog video cameras available. And all three came with free software for camera control, live image processing and image capture, as will be discussed below, simplifying camera controls and greatly enhancing the live viewing experience. With these three cameras, the digital revolution in camera assisted viewing was finally in full swing and the number of amateurs engaged in real time viewing has steadily increased.
Software Enhances Viewing
As the transition from analog to digital was underway, another key innovation in camera assisted viewing was taking place through the use of software developed to control the cameras and capture and display the images. In 2105 Robin Glover in the U.K. added live stacking to his free Sharpcap software application. Live stacking takes successive frames from a camera and automatically rotates and translates each to align the stars to the first frame. Once aligned, the frames are added together to reduce background noise and enhance image detail. This is exactly what T&A did in the STV, but without a limitation on the number of frames stacked. Live stacking makes it possible to watch as stunning real time views unfold over tens of seconds to several minutes. With its capabilities uniquely suited to real time viewing, Sharpcap was quickly adopted by the camera assisted community. Robin added automatic dark frame subtraction and flat frame scaling along with live histogram stretching to Sharpcap in subsequent updates. These are all techniques commonly used to post process astro images, but in this case they are performed live and on the fly enabling one to get near astrophotography image views of DSOs in real time.
In parallel with the development of Sharpcap, Paul Shears, also in the U.K., developed an application he called Lodestar Live with similar live stacking and dark frame subtraction features specifically for the Lodestar camera. This was later adopted by Starlight Xpress, renamed Starlight Live, and expanded to work with the SX Ultrastar cameras used for live viewing. Likewise, ATIK created their own application for their Infinity camera with similar easy to use capabilities for live stacking and histogram stretching on the fly helping to make the Infinity a popular live viewing camera. Both Starlight Live and the Infinity applications work only with their respective manufacturer's cameras, while Sharpcap works with a wide variety of cameras including those from ZWO, Starlight Xpress, QHY and any camera with an ASCOM driver. Not only do these SW applications greatly enhance what could be seen but they also simplify camera controls eliminating a significant hurdle to further adoption of the technique.
Live stacking software also greatly simplifies the equipment and setup requirements for camera assisted viewing. With the software doing the work of aligning each successive frame, an equatorial mount is no longer required. Inexpensive Alt-Az mounts like the Celestron Nexstar SE or the larger capacity Nexstar Evolution are now very popular for real time viewing. In addition to the cost savings, Alt-Az mounts eliminate the need for a precise polar alignment and can be GoTo aligned in just a few minutes using 2 or 3 bright sky objects. So long as individual frame exposures are kept under 30sec, the live stacking software will successfully stack images for many minutes without noticeable star trailing. Eventually, you will see a picture frame effect along the edges of the stacked image which will be more noticeable the longer the total stacking time. This effect is worse the closer to the meridian and zero declination the object being viewed and the more misaligned the optical axis is from the celestial pole.
CMOS, The Next Wave
Orange County Telescopes introduced a new analog video camera in 2016 called the Revolution Imager II for just $100 and a complete viewing kit with all accessories required for a nights viewing for $300. The low price point attracted even more converts to real time viewing with a camera. But with the announcement by Sony in 2015 that they would cease production of CCDs and switch to the more widely used CMOS sensors by 2017, the Revolution Imager appears to be the last of the analog astronomy cameras. Early in 2020, the other major CCD manufacturer, On Semiconductor, announced that they would also switch all manufacturing from CCD to CMOS. These decisions to switch to CMOS sensors were possible because their sensitivity was fast approaching that of CCD sensors and their manufacturing cost was much less.
A major advantage of CMOS over CCD is its extremely low read noise approaching 1e. This makes it practical to reduce exposure times to sub 10sec while using live stacking software to gather additional photons over several minutes without having read noise impact the overall image noise level. This makes it possible to use less expensive EQ mounts and reduces the requirements on the precision of the polar alignment. It also eliminates the need for auto-guiding as the stacking software makes sure the stars are aligned from frame to frame. As noted before, shorter exposures also make Alt-Az practical for camera assisted viewing.
The CCD used in the SBIG STV was small with a diagonal of ~ 6mm while those used in the Stellacams and Mallincams have a diagonal of ~8mm. These provide FOVs equivalent to 6mm and 8mm EP, respectively. With such small FOVs it can be very frustrating to locate faint DSOs without a good GoTO alignment. With the advent of digital cameras like the Infinity and Ultrastar, sensors with 11mm diagonals became commonplace providing larger FOVs making it much easier to find faint objects in the night sky. The larger sensors also accommodate larger DSOs for a given telescope focal length. Another significant leap in sensor size occurred in 2016 when ZWO introduced a 16MP CMOS camera, the ASI1600, with a 21.9mm diagonal to be followed soon after by ATIK’s Horizon camera with the same CMOS sensor. In the years since, ZWO, ATIK, QHY, SX, Mallincam and others have introduced additional models of digital cameras with the latest versions of CMOS sensors available.
Digital cameras used for camera assisted viewing are now packed with more and more features greatly improving both the images viewed and the process for capturing them. Faster USB3 interfaces and internal memory to avoid lost frames during transfer from camera to computer are essential with the larger sensors now used. USB hubs on the camera allow easy connection of accessories minimizing additional cables hanging from the scope. In camera binning is commonplace allowing resolution to be traded for sensitivity and tailoring of the image scale to different optical setups. Most cameras also now come with 2-stage cooling for better thermal noise control and heated windows over the sensor to keep dew in check. And the CMOS sensors have deeper wells, lower read noise, greater sensitivity and clear apertures from 6mm to full frame.
Digital cameras eliminate most of the objectionable image defects common with analog cameras. Smaller, square pixels ensure that stars no longer appear pixelated. Video artifacts like “raccoon eyes” are also gone. Since the signals are now digitized, noise picked up on the cable between the camera and the computer can no longer produce stray black lines in the image viewed. Finally, since these new cameras are designed for astronomy, the camera menus are intuitive and simple to operate.
While analog cameras are still available from Mallincam and OCT they are more often the cameras of choice for public outreach events as digital cameras, especially CMOS based ones, are now the new norm for camera assisted viewing. When combined with on the fly image processing software, real time viewing of the deep sky has evolved tremendously from its analog origins two decades ago. With the current technology it is even possible to straddle two once vastly different objectives. One can use on the fly processing to get the immediate enjoyment of observing deep sky objects in real time, while saving individual frames for later post processing off line to obtain astrophotography quality images.