Nearly two years ago I obtained my first USB camera, an ASI224MC sold by ZWO.  ZWO had introduced this camera in July 2015 for $350 and it was an immediate hit with the EAA crowd for good reason. The ASI224MC uses a 1/3" format Sony IMX224 color CMOS sensor with 1.27 Million 3.75 micron square pixels.  This provides round, pin-point stars with high resolution unlike typical analog video cameras which have rectangular and much larger pixels.  With a high sensitivity rated at 2350mV this Exmor sensor combined with an extremely low read noise of 0.55e to 3e means (depending upon the gain) the camera is well suited to live stacking of many short exposure images.  This is very helpful for Alt-Az mounts where single frame exposure lengths are limited by field rotation to around 30sec.  When combined with the powerful and free Sharpcap software this camera shows great detail in a wide range of deep sky objects.

Since this is a USB camera and not an analog camera, a computer is required to operate the ASI224.  A single USB cable from the camera to the computer is all that is required for both camera power and control.  One less cable to deal with and to potentially snag on the mount compared to analog cameras is a pleasant advantage.  Sharpcap is probably the most commonly used software for the ASI224 and it provides control over all of the camera menu settings.  It provides for image display, capture, histogram stretching, dark frame subtraction and on-the fly alignment and stacking among other nice features.  Sharpcap can be downloaded from the ZWO web site along with the native driver necessary to connect the camera to the computer.  To use other software you will have to load the ASCOM driver also available on the ZWO website.

Camera settings include an exposure range of 32micro-sec to 1,000sec which makes the camera highly capable for both planetary and deep sky viewing.  The camera has a gain setting range of 0 - 450.  Lower gain provides higher dynamic range while higher gain provides the lowest read noise.  Typically, people report working with gains in the range of 300 - 350.  This camera also has the capability for 2x2 binning which makes the pixels effectively 7.5 x 7.5 microns, increasing the sensitivity and speed of the camera at the cost of resolution.

With a sensor diagonal of 6.09mm, the ASI224 has a field of view and magnification factor similar to a 6mm eyepiece.  This can make it challenging to place a faint object in the field of view unless the mount's GoTo alignment is very good.

The ASI224 comes with a 2m USB cable to connect the camera to a computer along with a 1 1/4" nose piece adapter to connect the camera to the telescope like any eyepiece.  It also comes with an f/2 all sky lens with a 151 degree field of view for detecting meteor showers.  This must be removed before attaching the 1 1/4" nose piece.  The ASI224 is also designed to work as an autoguider when used with a program like PhD.  As such, it has an ST4 guide port and comes with a 2m cable to connect to the mount.  While the camera comes equipped with a 1 1/4" nose piece to fit inside a standard 1 1/4" focuser like any eyepiece, the camera can also be inserted into a 2" focuser since the camera body has a 2" diameter collar in the front.  This is helpful to achieve additional in-focus when using Newtonians and Dobsonians which are not designed for astrophotography. In addition, the front of the ASI224 has an M42 x 0.75 thread making it ready to be used with T-adapters. 

The ASI224C was upgraded in April 2016 with anti-amp glow circuitry.  Amp glow is caused by the heat produced in the read out circuit of the sensor during long exposures and is common in analog video cameras.  It produces a bright glow at the edge of the image making that part of the object appear over-exposed.  Short exposures and/or dark frame subtraction are commonly used to eliminate or minimize the impact of amp glow.  With the ASI upgrade this is no longer necessary.   The USB connection was also upgraded from USB2.0 to USB3.0 to support better download rates.  In addition, the price of the ASI224 has been reduced to $249.99 as other cameras with the IMX224 sensor have become available.  A cooled version of this camera, the ASI224MC-C was available for a while and sold for $599, but has since been discontinued. 
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​As can be seen from the images here, the ASI224MC camera performs very well for all sorts of deep sky objects.  It produces nice round, pin point stars and has very good color saturation.  All images were captured using Sharpcap and have had no post capture image processing.  What you see here is what you would have seen on my computer screen live. 

While ZWO and QHY make their own cameras with the Sony IMX224 sensor, there are many other retailers who appear to re-brand cameras from a Chinese company called Touptek and sell them under their own label.  These include three cameras from Rising Tech in China, and one each from Mallincam in Canada and the U.S., Orange County Telescopes in the U.S. and Altair Astro in the U.K.

Rising Tech sells three versions of the IMX224 based camera.  The USB 2.0 version, Model GPCMOS, comes without the amp glow reduction and sells for $162 making it the cheapest IMX224 based camera currently on the market.  This camera also has a lanyard which can be used as a safety strap to attach the camera to the scope.  Model G3M224C has the USB 3.0 connection and the amp glow reduction circuit for $245 and has a slightly larger body than the GPCMOS camera. However, both models are small bullet shaped cameras which can fit well inside a focuser making it easier to achieve focus with Newtonian telescopes which are not designed for astrophotography.

Model G3-1200KPA also has both the USB 3.0 and the amp glow reduction, but comes with a cooling fan for $295.  The G3 also has a much larger camera body and has M42 X 0.75 threads making it straightforward to attach a t-adapter to the camera.  All cameras come with camera control software called Rising Sky, which appears to be a re-branded version of Toupsky camera control software.  Toupsky has many of the same features found in Sharpcap such as image display and capture, on the fly stacking, dark frame subtraction, histogram stretching, etc.  

​Altair Astro's GPCAM3 224C has a USB 3.0 computer interface and comes with a UV-IR filter and AltairCapture software.  The camera body has cooling fins and the same bullet shaped front as some of its competitor.  The GPCAM3 sells in the U.K. for £249.  It also comes with a 1 year license for the Pro Version of Sharpcap.

​Orange County Telescopes sells the Revolution Imager 224 (R224) but does not hide the fact that this USB 2.0 camera is manufactured by Touptek.  The R224 makes no mention of the amp glow circuitry, comes with the ToupSky software and sells for $250.  The R224 is another of the bullet style cameras.  It comes with a UV-IR  filter, a 0.5X focal reducer and a soft carrying case for the camera, cables and accessories.

Newtonian and Dobsonian telescopes are commonly used for astronomy because of their relatively low cost and fast focal ratios, typically f/5 or faster.  Dobsonians are just Newtonian optical tubes on an Alt-Az mount after the style made popular by John Dobson.  There is a critical consideration when choosing a Newtonian for deep sky video astronomy, Electronically Assisted Astronomy, or Near Real Time Viewing, whichever you call it.  Newtonians and Dobs, unlike Schmidt Cassegrains and refractors, do not have a lot of in-focus travel which is required to focus analog cameras like the Revolution Imager, Mallincam Xtreme, Strellacam, Samsung, etc, or digital USB cameras like those from ZWO, Atik, Rising Tech,etc.  This is especially the case if using a focal reducer to speed up the optical train and/or achieve a large field of view.  When there is not enough focus travel, modification of the optical tube may be required to achieve focus with a camera.  Fortunately, there are many Newtonians and Dobs which are able to achieve focus with a camera without the requirements for scope modification.  These are often, but not always, sold as imaging Newts.  The table below shows a list of Newts and Dobs that have been reported on Cloudy Nights as compatible for use with various cameras without modification to the scope.  There are certainly many more than listed here, but these are the ones I am currently aware of.  They span the range from 100mm aperture  to 305mm and all are fairly fast even without a focal reducer.  Several of these scopes have also been verified to work with a focal reducer  without the need for any modifications.  This is shown in the "Focal Reduction" column.

When a Newtonian scope does not have enough in-focus travel to work with a video camera, there are two options.  First, modify the scope by moving the primary mirror closer to the secondary.  Some truss tube collapsible scopes make this easy to do with a specific configuration for imaging which brings the primary mirror closer.  But the solid tube Newtonians do not have this option and require mechanical modification to bring the mirror closer. ​ Many have done this modification to make their existing Newts compatible for use with cameras.  It will require re-doing the collimation, but Newts seem to require frequent collimation anyway. 
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If shifting the primary mirror position is not something that you are comfortable with, there is another option.  There are many bullet style cameras with 1 1/4" diameters either their full length or enough of it that they can fit further down the focuser, bringing the camera's imaging sensor close enough to focus, even with the addition of a focal reducer.  Examples include the Starlight Xpress X2C, Revolution Imager  IMX224, and the Rising Tech IMX224.  It has been reported that this works with the 8" f/6 and 12" f/5 GSO Dobsonians along with a 16" f/4.5 Meade Lightbridge Truss Dob.  These can even achieve focus with a 0.5X focal reducer by using a 2" negative profile adapter like this one from ScopeStuff which allows the camera body to slide even further inside the focuser.

 Even though cameras like the Mallincam Micro Ex, LnTech300, Revolution Imager 1 are not bullet shaped, they are small enough to fit down inside a 2" focuser allowing these cameras to achieve focus with many Newts.  However, with both the bullet style cameras and these small rectangular cameras, be careful that they do not slip down and contact the secondary.  The 1/4" x 20 mounting bracket found on the Micro, etc. is useful as positive stop to prevent this from happening.