The most common advice given to anyone thinking of getting into astrophotography is to invest most heavily in a good equatorial (EQ) mount with excellent tracking stability for the best chance of success. This is because astrophotography requires the telescope to remain fixed on an object within a fraction of a pixel during exposures many minutes long. And this requires the mount to track the rotation of the earth relative to the stars with a motor driving the Right Ascension (RA) axis of the mount at the same rate as the earth's rotation. Otherwise, stars will move relative to the pixels and they will appear elongated in each frame and not round as they should be. Furthermore, astrophotographers often employ a separate guide camera to continuously make small corrections to the mount's tracking accuracy to keep the stars fixed to the sensor in hopes of obtaining pin point stars in their images. Without a doubt, an equatorial (EQ) mount is an absolute necessity for astrophotography and typically the most expensive component in a serious imager's setup.
Fortunately, for EAA the requirements of a mount are not quite as demanding, although a stable tracking mount is still essential for success and an EQ mount will provide the greatest flexibility. Neither is a guide scope necessary. This is because EAA typically involves much shorter exposures than those used for astrophotography. Exposures are much shorter than 1 min, and often less than 10 sec when using live stacking software to get a pleasing view of a Deep Sky Object (DSO) in real time. Because of the short exposures used, EAA can even be done with an Alt-Az mount despite the fact that it's tracking axes do not fully counteract the effect of the earth's rotation on the apparent movement of the stars.
Visual observing is very forgiving of the motion of the sky, but astrophotography and EAA are not. As the earth slowly rotates on its axis at 15 arcseconds per second, the view through a telescope will begin to appear to rotate. This field rotation makes the constellation Orion appear to rise above the eastern horizon on his back while rotating to set in the west on his face. Our eyes do not mind if the object in the eyepiece moves slowly across the Field of View (FOV) while we observe. The human eye integrates the light over extremely short time periods so we are able to adjust to the motion and do not see a blurred image or trailed star patterns. And, we can manually adjust the mount to keep the object relatively centered in the FOV while observing visually.
However, this is not the case when using a camera to capture an image a few seconds to tens of seconds long. With a non-tracking mount the image will suffer from objectionable star trailing with even a very short exposure. The number of pixels traversed by an object during the exposure defines the amount of star trailing. It depends upon the declination of the object (dec), the focal length (f) of the optical system, the size of the pixels (Pxl) in the camera's sensor, and the length of the exposure (t):
Star Trail Length in pixels = 2 Pi x f x t x cos dec /(86.2 x Pxl)
where f is in mm, t is in seconds, dec is in degrees and Pxl is in microns. As an example, assume we are using a camera with 4 micron pixels and are trying to image an object at 60 degrees declination. Using an 8" SCT at f/5 (1000mm focal length), a star will cross a path 9.1 pixels long during a 1 sec exposure. This is more than enough to create noticeable star trails and cause blurring of the deep sky object. In fact, 5 or more pixels is enough to result in objectionable star trailing. Even an 80mm refractor at f/5 (400mm focal length) produces a star trail 3.6 pixels long with a 1 sec exposure. The situation gets worse closer to the celestial equator and better toward the pole. Since many exposures will be 5 sec or longer, a mount that will track the motion of the stars is an absolute necessity for EAA.
An Equatorial mount (EQ) solves this problem as the mount rotates in RA at the same rate as the earth' rotation canceling out the apparent motion. To do this, the mount must have its RA axis aligned with the celestial pole and the better the alignment the better the tracking accuracy. Since EAA employs much shorter exposures than astrophotography one of the lower cost mounts will provide sufficient tracking capability to get the job done. Also, while a good polar alignment is still important, it is not necessary to obtain as accurate of an alignment as for astrophotography to have success with EAA. This is particularly true when using very short exposures and live stacking. For certain, a well polar aligned and solidly built EQ mount will provide the longest single frame exposure and the longest stacked frame total exposure without star trailing. And an EQ mount is the only choice for EAA if you think you would like to also try astrophotography with the same setup at some point in the future.
An Alt-Azimuth (Alt-Az) mount will also track the stars but because it does not move along the same axes as the earth's rotation the stars will slowly drift tracing out an arc. However, one can get away with exposures of 30sec or less with an Alt-Az mount without appreciable star trailing. The length of exposure possible depends upon where in the sky the telescope is pointed as well as the focal length of the telescope. Star trailing is worse when pointing due south or due north and least pointing due east or due west. Star trailing increases with altitude, i.e. directly overhead, and least at the horizon with an Alt-Az mount. Longer focal length telescopes will also make star trailing more obvious since a smaller portion of the sky is focused on the sensor. In other words, the optical system is working at higher magnification. Just like an EQ mount, Alt-Az mounts can be used with live stacking software to obtain total exposure times of many minutes without suffering from significant star trailing due to the fact that the software accounts for the field rotation when stacking individual image frames on top of one another.
Alt-Az mounts for EAA have several advantages compared to EQ mounts. First, they tend to be much less expensive than EQ mounts. For instance, a Celestron 6SE is a combination 6" SCT with an Alt-AZ mount which can be had for $679 while the same 6" SCT on Celestron's least expensive EQ mount sell for $1329, or nearly 2X the price. Second, Alt-Az mounts tend to be much lighter than EQ mounts making them easier to transport from house to back yard. Also, since Alt-Az mounts cannot be polar aligned, they are very simple and quick to setup requiring only a couple of starts for a good GoTo alignment. This is why Alt-AZ mounts are increasingly popular for EAA, especially for those on a limited budget. However, keep in mind, that an Alt-AZ mount will not work for astrophotography if you think you may wish to move in that direction later. Another disadvantage of many At-AZ mounts is that they cannot be pointed very close to the zenith since the camera can crash into the base of the mount unless a diagonal is used. This is because the way that the optical tube is attached to many Alt-Az mounts like the Celestron and Meade. Mounts like the iOptron Cube models are slightly better in this respect since the mounting arm is to the side, however, a long tube OTA can still crash into the tripod legs of the mount unless an extension tube is used to attach the mount to the tripod.
Whether an EQ or an Alt-Az mount, it is important to match the mount's rated load capacity with the OTA and any additional equipment including camera and adapters which the mount will carry. In general, it is assumed that all but the very high end mounts overstate their load capacities so it is probably a good idea to assume no more than 50 - 70% of the stated capacity, not including counter weights, as the true capacity for EAA. To get maximum benefit it is also important to do a good job of balancing an EQ mount in both axes and an Alt-Az mount in its Alt axis.
Most mounts with tracking, whether an EQ or an Alt-AzA motorized mount , will also have GoTo capability. GoTo allows one to tell the mount where to point the telescope in the sky through a hand control or software on a computer connected to the mount. This enables the user to swiftly and painlessly find and view any number of objects in the night sky over the course of an evening. While the GoTo feature is not absolutely essential, it will certainly make it a lot easier to locate and center deep sky objects quickly and allow you to spend more time observing instead of searching. One reason for this is that many cameras used for EAA provide small FOVs. Typical analog cameras use a sensor with a diameter of 6 mm (Revolution Imager I and II) or 8 mm (Mallincam Xtreme or Xterminator). The corresponding FOV is similar to that of an EP of the same focal length as the sensor diagonal. For instance, a camera with a 1/2" CCD on an 8" SCT at f/5 produces a FOV of 16.6 x 22.1 arcmin (28 arcmin diagonal) which is very close to the 29 arcmin FOV of an 8 mm EP. This is about the size of the moon which is much harder to manually point to with a telescope than one thinks. Cameras with the Sony 1/3" CCDs on the same scope produce an even smaller FOV of 12.6 x 16.8 arcmin (21 arcmin diagonal) which is similar to the FOV of a 6 mm EP. While the popular new breed of CMOS cameras such as the ASI1600, ASI294 and ASI071 with sensor diagonals of 22.2 to 28.4mm have FOVs 2X to 4X those of analog cameras, GoTo will still make life much easier to quickly hunt down DSOs and spend one's time observing rather than looking around for dim objects.
Another issue is that even with a wide FOV, DSOs can be difficult to see with very short exposures. It may take 30 sec or longer to verify that the object is within the field of view. Without a GoTo telescope it is likely that the object will not be in the FOV and you will have to make adjustments in the telescope position, take another exposure and repeating the process until it is. This can be very frustrating and waste a lot of precious time under the night sky. With GoTo and a good pointing alignment, one can be more confident that the DSO of interest will be somewhere in the FOV.
GoTo or pointing alignment and PA are not to be confused as they are two completely different things. You can have a very good GoTo alignment with a poor PA and vice versa. A good GoTo alignment is possible on both an EQ mount and an Alt-Azimuth mount while PA is only possible with an Equatorial mount. A GoTo alignment is obtained by pointing the telescope at a number of bright stars or planets in the night sky, centering them in the FOV and letting the mount know that it is centered. This is done either through the hand control or through software which is connected to the mount and taking the place of the hand control. Typically 1 -4 stars are used but a high end telescope using TPoint modeling can use hundreds of points for more accurate alignment. For instance, my Celestron 6SE asks for 1-3 bright objects, my Celestron AVX and GCE use between 2 and 6 stars and my Software Bisque MyT uses TPoint and anywhere from a couple of dozen to a few hundred point for a TPoint model. A GoTo mount uses its internal readings for the RA and Declination (Dec) of the centered start to build a model of the sky which enables it to GoTo and put any object in the sky somewhere in the FOV or very close to it.
In the next sections we will review the different mounts available. Keep in mind that manufacturer's constantly update, replace and introduce models to keep their offerings current and up to the start of the art. The mounts discussed below are a snapshot of what is currently available. Also, pairings of telescopes to mounts given below are estimates based upon the mounts rated capacity and the weights and lengths of the telescopes. Some may prefer to stay well below the rated mount capacity for best overall performance and others may choose to push the limits. If care is taken to carefully balance the load, shield the telescope against winds and vibrations, add additional weights to the bottom of the tripod and even use an auto focuser, one can achieve the best results for each combination.
Just as an equatorial mount (EQ) is an excellent choice for astrophotography, it is also an excellent choice for EAA. And if you expect EAA to eventually lead you into astrophotography an EQ mount is the right choice. Since the EQ mount's RA axis can be accurately aligned to the earth's rotation by performing a polar alignment, it will keep an object fixed in the FOV enabling long exposures with sharp images. In addition to longer exposures than an Alt-Az mount, an EQ mount does not have any trouble with targets approaching and at the zenith, whereas cameras and/or optical tubes will run into the base of many Alt-Az mounts when approaching the zenith. The main downsides of EQ mounts are their higher cost compared to an Alt-Az mount with similar capacity and higher weight of the mount and tripod. The need for polar alignment of an EQ does make setup a bit more tedious compared to an Alt-Az mount, but with experience and the simple polar alignment routines in EAA software like Sharpcap the process is greatly simplified and shortened.
While there is a continuous range of EQ mounts in terms of cost and quality we will look at mounts priced up to $4000 and break them down into 4 classes for simplicity:
1) Budget Under $750 and capacities of 11 to 22lbs
2) Moderate $800 to $1150 and capacities ~30lbs
3) Intermediate $1500 to $2900 and capacities of 40 to 50 lbs
4) High End $3400 to $3600 and capacities of 60 to 75 lbs
Keep in mind that new mount designs appear over time so the list below may not be accurate several years from now. And this list is does not include every EQ mount as that would be overwhelming. Finally, capacities are often overstated, especially in the lower to mid-end mounts.
Budget mounts are the least expensive but also the most limited in overall capability including weight capacity and tracking accuracy as these are made with the cheapest components to keep costs down. However they will work for EAA and may be the only option for someone on a very limited budget. A few models in the $400 to $725 price range are available such as the Explore Scientific iEXOS100, the iOptron Smart EQ Pro, and the SkyWatcher EQM 35. These have capacities limited to 11 to 22lbs. The low capacity ratings limit the choices of telescopes which can be paired with these mounts. On the lower capacity end an small 80mm refractor or camera lens attached to a camera for wide field viewing would be appropriate while on the higher capacity end a 6" or smaller SCT, Newtonian, or a 90mm refractor would be possible. These are extremely light with weight mounts ranging from 13 to 23lbs including tripods so they are highly portable. Tripods are made with 1.5" or smaller diameter legs and may not provide as solid of a footing as needed without hanging extra weight from the bottom to provide more stability against vibrations. These would not be good options if astrophotography is in your future. This class is best for anyone with a very tight budget and a need for an extremely light weight setup.
Moderate class mounts can be found for ~$800 to $1150 and include examples like the Celestron AVX, Meade LX 85, Orion Sirius EQ-G, iOptron GEM28 and the Explore Scientific EXOS-2GT among others. Rated capacities are ~30lbs which opens up the options on telescopes with which these can be paired. The mechanics on these mounts is better than the Budget class mounts but these mounts are still best suited to short exposures and light weight optics such as a 6" Newtonian, an 8" SCT , a 4" refractor or smaller telescopes of each type. These mounts are generally light ranging from 23 to 43 lbs making them easy to transport assembled from inside the house to the backyard. These can be used for astrophotography, but would be considered lower end performers.
Intermediate mounts can be classified as those with stated capacities of 40-50lbs. There are multiple models from most manufacturers in this range such as the Celestron CGEM II, the Losmandy GEM811G, iOptron CEM40, Atlas II EQ-G, Sky-Watcher EQ6-R Pro. Prices vary from $1500 to $2900. These mounts have better mechanical tolerances than the Moderate class mounts so they will provide more precise tracking and GoTos. Mounts in this class will weigh significantly more than in the moderate class with a range of 31lbs to nearly 60lbs. While they are still well suited for transport to dark sites and star parties, they need to be disassembled and reassembled even if they are being taken from the garage to the backyard. Many people have some sort of cart which can be used to move the assembled scope and mount from inside to outside in one piece. With their higher capacities these mounts can handle up to 9.25" SCTs, up to 8" Newtonians and up to 5" refractors. Tracking and GoTos are excellent provided one does a good job of PA and GoTo alignment. These are also quite capable for astrophotography.
The last classconsists of mounts with capacities of 60 to 75lbs. These include the Losmandy G11 at $3395 and the Celestron CGX-L at $3600. These larger mounts can handle up to 14" SCTs, 10" Newtonians and 6" refractors. Mounts in this class, have superior mechanics providing the best tracking capability and stability among the four classes. The capabilities of these mounts represent significant overkill for EAA unless one has the desire to use one of the larger OTAs. Some will find these a challenge to transport to dark sites given their total weights. That was certainly the case for me with the CGX-L, which I owned briefly, as it consists of a mount weighing 53lbs and a tripod weighing 46lbs. Add to that the counterweights and telescope and we are talking about over 100lbs total that has to be transported. Obviously these mounts are well suited to astrophotography if that is a future option.
Then there are mounts with still higher capacities at sky rocketing prices which are well beyond the needs of someone starting out in EAA. We will not consider these here. As one can see, there is an EQ mount for every budget, weight limitation, and carrying capacity. The class of mount that is best for EAA depends on the desired telescope that will be used with it. It is essential to match the mount to the telescope which rides on top of it. Because an SCT is very compact for the size of its optics, one can use a smaller mount than an equivalent sized optics in a Newtonian or refractor due to the fact that they tend to have longer optical tubes. The key is to have a scope/mount combination which will track accurately and be stable against vibrations and wind. In choosing a mount it is also important to look to the future. It is better to wait and save for a higher class mount if you expect to use larger aperture telescopes in the future rather than buy something cheaper right away and out grow it quickly.
Alt-Az mounts have become increasingly popular for EAA in the last 5 years. This is because they are typically less expensive compared to an EQ mount and they are generally much lighter. And because they cannot be polar aligned they are simpler and faster to setup for a night of EAA. However, those same advantages also limit the ultimate capability of the mount for EAA and make them totally impractical for astrophotography. An Alt-Az mount has mechanical axes of rotation horizontal (Azimuth) and perpendicular (Altitude) to the plane of the earth. So it cannot completely track the rotation of the sky which is tilted relative to that plane according to the latitude of the observer. So, 30 sec is about the maximum practical exposure to avoid the effects of field rotation. This is long enough to view many deep sky objects of interest. The actual maximum exposure depends on the observers latitude, the altitude of the object being observed and the azimuth angle of the object (see my discussion of field rotation elsewhere on my website). While an Alt-Az mount cannot be polar aligned it can be GoTo aligned similar to an EQ mount so that the observer can accurately slew to objects in the night sky and expect to find them in or very near the FOV avoiding wasted time hunting for objects instead of viewing them.
The ~30sec limit on exposures can be increased to more than 5 min, well beyond what is required for EAA, with the use of live stacking software. One popular version is Sharpcap which, like the others, will electronically rotate and translate successive image frames to align the stars with the first frame and stack the frames into a single combined image. This can continue for as long as the observer likes, but again, there is a practical limit which may be anywhere from 5 min to 20 min, much longer than needed for EAA. We will discuss live stacking in detail in a later Blog.
Motorized Alt-Az mounts are often sold as a package with an included telescope such as an SCT or a refractor which are good options for low cost EAA. If starting from scratch and knowing that astrophotography will not be in your future, these combinations are the most cost effective choices and simplest way to get started in EAA. These also can also be broken down into 4 classes:
1) Budget $349 to $700 with 80 & 100mm refractor & 5" SCT
2) Moderate $800 to $1300 with 80mm refractor & 6 to 8" SCT
3) Intermediate $1700 to $2900 with 108mm refractor & 8" to 12" SCT
4) High End $3000 to $4600 with 9.25" to 12" SCT
There are 7 combinations listed in the table below in the budget category for less than $700. These include the Meade Star Navigator 102 Refractor, and the Meade ETX 80 Observer. Also available are the Sky-Watcher Star Travel 102 AZ-GTe and the iOptron 80mm SmartStar Cube Refractor, both with modest 80mm refractors at f/5. These are very light weighing 9 to 14lbs including mount, tripod and scope which makes them ideal for travel. Since these include both the mount and OTA for under $500 you can expect to sacrifice some elements of performance although all will work for EAA when budget is so constrained. I have used the Meader ETX 80 and while I found I had to add a hanging weight on tripod to improve stability and had to use a careful touch when focusing I was able to do EAA of many of the brighter DSO objects. Also in this category are the 102mm refractors from Sky-Watcher and Celestron and the Celeston Nexstar 5SE SCT. If you can afford to, plan to purchase one of the next category of models as they will add a lot more capability for the price.
Mount/scope combinations in the price range of $800 to $1300 make up our moderate class. These include the Celestron NexStar 6SE and 8SE, the Meade 6" and 8" LX65 ACF, and the Celestron 6" Nexstar Evolution all with f/10 SCTs. Refractor combinations include the SkyWatcher EvoView Pro ED 80mm. SCTs tend to be excellent choices for EAA as they are native f/10 for small DSO and can be reduced to f/6.3 with a focal reducer to fit larger DSOs into the field of view and also reduce exposure times by speeding up the optics. The Evolution mount has improved mechanics compared to the SE mount from Celestron. This class provides a big step up in capability while still keeping the overall cost down. Weights of the mount, tripod and scope range from to 28 to 38 lbs so these are also very portable combinations.
The Intermediate class includes mount/OTA combinations in the range of $1700 to $2900. Among these are the Celestron Nexstar Evolution 8" and 9.25" and 8" Edge along with the more stable but heavier dual arm fork mount CPC 8" and 9.25" SCTs. From Meade one can choose between the 8", 10" and 12" ACFs on the LX90 mount along with an 8" ACF on the heavier duty LX200 mount. iOptron offers a 108mm ED APO refractor on their AZ Pro mount. An 8" SCT is considered a "sweet" spot by many who practice EAA. The dual arm fork mounts provide a more stable platform than the single arm mounts but at the cost of more weight since the optical tube and the mount cannot be disassembled like the single arm mounts. The Celestron Evolution models are single arm so the OTA is easily disassembled for transport but stability can be compromised with an optical tube larger than 8" compared to the dual fork designs. Weights for the optical tube and mount range from 23lbs for the refractor, up to 60lbs for the larger scopes on dual fork mounts.
The High End class consists of combinations ranging in price from $3000 to $4600. Among these are the Celestron CPC dual arm fork mounts with the 9.25" and 11" Edge SCTs and the 11" non-Edge SCT. Meade offers 10" and 12" ACF SCTs on their heavier duty LX200 mount in this class. At weights of 58 to 75lbs for the mount/scope combination these are much more challenging to transport and set up. Some sort of wheeled system is generally helpful to transport these from inside to outside the house.
If you already own a telescope, or do not want one of the Alt-Az packages you can still purchase a standalone Alt-Az mount. There are a lot less Alt-Az mounts to choose from compared to EQ mounts. SkyWatcher has two models below $400, the AZ-GTe and the AZ-GTi both capable of 11lbs payload and weighing only 8.6lbs. The GTi is the same as the GTe but with built in WiFi. iOptron has the Cube Pro GoTo for less than $428 with a payload capacity of only 8lbs. These are capable of a very light weight scope like an 80mm short tube refractor or a camera attached to a wide field lens instead of an optical tube. iOptron has a heavier duty mount, the AZ Mount Pro which can handle 33lbs for $1300 so it can handle an 8" SCT, 6" Newtonian or 4" refractor. The tripods that come with these mounts are very thin so that stability is compromised when the legs are fully extended. iOptron does offer a larger tripod option on the AZ Mount Pro Model which increases the price to $1600. Again, hanging a weight to the bottom of the tripod always helps with stability. These are all good travel mounts with the mount and accessories fitting nicely into a small carry on case but with the tripod requiring a separate larger bag. I used the Cube Pro GoTo as my light weight scope when traveling by airplane to view the last total solar eclipse. These mounts allow the optical tube to be mounted off to one side so it will not crash into the mount when pointing to the zenith, but can crash into the tripod legs if the scope is too long or an extension tube is not used to attach the mount to the tripod.
The other style of Alt-Az mount uses a single fork arm to attach the telescope over the center of the mount. Celestron has two models, the Nexstar SE and the higher end Nexstar Evolution for $430 and $1200, respectively, and with capacities of 12lbs and 25lbs. These are designed for 6" and 8" SCTs respectively. Meade has a similar offering in their LX65 model for $500 with a capacity of 14lbs.
A computerized mount with both tracking and GoTo capability are essential if you want to do EAA. You do not need to use a computer to use these capabilities as the hand control will accomplish everything you need to do. The "computerized" nomenclature refers to the mount itself, not a separate computer. However, you can connect your computer to the mount and control it either with software from the mount manufacturer like CPWI from Celestron or third party software such as The Sky X or Cartes du Ciel. Just as for astrophotography, invest the most you can in the mount as its ability to keep the target centered on the camera is key to avoiding unpleasant star trailing and disappointing images. The lowest cost option for EAA is an Alt-Az mount. These have become very popular for EAA in recent years due to the ability of live stacking software to offset the fact that an Alt-Az mount does not perfectly track the earths rotation. However, if you plan to try hour hand at astrophotography at some future date or do not want to be limited to exposures of 30sec or less, an EQ mount is the right choice. Whether you choose an EQ or an Alt-Az mount pay careful attention to the rated capacity to make sure it will be stable against vibrations and be able to handle the weight of the telescope you choose to use.
Mounts & Mount Telescope Combinations I have used successfully for EAA:
Celestron 6SE - good low cost and light weight telescope/mount for EAA
Meade ETX80 Observer - very light and very low cost but limited long term
iOptron Cube Pro - easily fits in a carry on case minus the tripod
Celestron CGE - now discontinued but a good choice used
Celestron CG5 - upgraded and now named AVX
Celestron CGX-L - large capacity, extremely stable, but very heavy mount/tripod
Software Bisque MyT - portable but overkill for EAA unless using an 11" scope
Software Bisque MX - replaced by the MX+ but overkill for EAA. Not very portable
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