Choosing a Telescope for EAA

Almost any telescope can be used for EAA.  There are a few telescopes which will not work at all although some will require modification and a few will not provide the best user experience.  Any telescope that will work for astrophotography will also work for EAA, but a high end astrograph is certainly not a requirement for EAA.   When we talk about telescopes, here we are talking about the optical tube only, not the combination of an optical tube and mount which is often how telescopes are sold.  Three important general considerations for choosing a telescope for EAA are focal ratio, aperture and focal length.  Another factor that must be considered is whether or not the telescope can be focused with a camera attached as some Newtonian telescopes will not. 

​Focal Ratio (f ratio)
EAA is all about viewing as much detail as possible in real time.  This is not astrophotography where lots of sub frames are captured for post processing the next day to obtain a high quality image suitable for publication.  Even when using live stacking the objective is to enjoy the view in real time.  To achieve this a fast telescope is preferred or at least one which can be made faster with the addition of a focal reducer.  In fact, having the ability to work at two different focal ratios with the addition of a focal reducer provides greater flexibility in choosing targets of different size.  A fast system allows for shorter exposures and more immediate views.  Shorter exposures also put less demands on the mount's tracking accuracy which means a less expensive mount can be used.  It also means that a good, but not necessarily a precise polar alignment is required when using an EQ mount.  And, as discussed in the blog, "Choosing Mounts for EAA", shorter exposures make it practical to use Alt-Az mounts for EAA.

The speed of the optics is determined by its focal ratio which is the focal length divided by the aperture.  The focal length is the length of the path the light travels from the primary optical element (objective or mirror) of the telescope to the focal plane.  An 8" SCT has a focal length of 2000mm and a 20 mm aperture.  It has a focal ratio given by the formula:
                 
                      Focal ratio  =   Focal Length / Aperture   =   2000 / 200 = 10

which is written as f/10.  For comparison, a 4" (100mm) refractor with a focal length of 500mm has a focal ratio of f/5.  One might be inclined to think that because the 8" SCT has a larger aperture it would require a shorter exposure time to achieve the same image quality since it should capture more light with the bigger aperture.  But the fact is that the f/5 refractor has the faster optical system and requires the shorter exposure to achieve a pleasing image.  How much shorter?  The exposure is reduced by the square of the ratio of the focal ratios:

                                  Reduced Exposure   =    t  x  (5/10)^2  =  0.25 x t

where t is the exposure at f/10.  So with the f/5 refractor the exposure required is only 25% as long as at f/10.  A 20 second exposure is reduced to 5 sec.  The faster the focal ratio the shorter the exposure no matter the size of the telescope aperture.  You can immediately see the benefit of shorter focal ratios for EAA.  So how is it possible that the larger aperture telescope requires a longer exposure compared to a smaller telescope with a faster focal ratio?

To understand this we must compare the image scales for these two telescopes.  Image scale is defined as the amount of sky in arcsec which is focused on an individual pixel.  It is a measure of the concentration of photons falling on a pixel for a given camera.   Image scale is determined by the focal length of the telescope combined with the size of the pixels in the camera used.  Longer focal lengths produce smaller image scales for a given pixel size.  

     Image Scale (arcsec/pixel)  =   205  x Pixel Size (microns) /  Focal Length (mm)

Let's compare image scales for our 8" SCT at f/10 to our 4" f/5 refractor.  Using the ASI1600MC sensor which has a 4656 x 3520 array of 3.8 micron square pixels the refractor has an image scale of 1.56 arcsec/pixel while the SCT has an image scale of 0.39 arcsec/pixel.  The area of sky covered by the sensor is 127.3 arcmin x 96.2 arcmin for the refractor and 31.8 arcmin x 24 arcmin for the SCT.  The area of the sky covered by the SCT is 1/16th the area covered by the refractor.  Now since the 8" SCT has an aperture twice the size of the 4" refractor it has 4X the light gather area.  The net is that the SCT has 4X the amount of light per area of sky but collects light from only 1/16th the area of the sky.  Hence, the SCT requires 4X the exposure of the refractor.  If we add a focal reducer to the SCT to change its focal ratio to f/5, the image scale increases to 0.78 arcsec/pixel.  The area of the sky covered by the SCT increases to 63.6 x 48.1 arcmin which is now 1/4th that of the refractor but since 4X as much light is gathered over that area, the exposure is now the same as for the refractor.  When it comes to exposure time, focal ratio is the only thing that matters.  The larger aperture of the SCT provides a higher magnification at the same focal ratio so the image appears 4X larger in the SCT given the image scale.

Now, as they say, there is no free lunch.  With focal reduction comes a concomitant reduction in the field of view (FOV).  For focal lengths greater than 200mm the following equation is a very good approximation for the FOV in degrees:

                                                     FOV  ~ 57.3  L  /  F

where L is the length of one side of the sensor chip in mm and F is the telescope focal length in mm.  Let us continue with the example of the ASI1600 which has the Panasonic MN34230 CMOS sensor with dimensions of 11.7mm x 13.4mm.  Also, let us look at 3 different focal ratios for the SCT, the native f/10, f/6.3 which can be achieved with a Celestron or Meader 0.63X focal reducer, and f/2 which is obtained using the Hyperstar adapter.  Taking the long side of the chip we can calculate the FOV for these 3 focal ratio configurations of the SCT as well as for the 4" refractor.
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​Looking at the table we see that for a given aperture, the FOV increases as the focal length decreases.  In other words, more of the sky is focused onto the sensor for shorter focal lengths.  ​Now with the shorter focal length and wider FOV, the image captured is smaller than for a longer focal length.  Looking at the above table we see that the FOV at 2000 mm is 5X smaller than the FOV at 400 mm.  So the object will appear larger in the longer focal length image.  The trade-off with focal reduction is an increased speed but a reduced image size.  As a general guide for EAA, most DSOs are best viewed at f/4 to f/6, saving the much smaller (distant) objects for larger focal ratios.  So when choosing a telescope it is important to look for a naturally fast scope or one which can easily be reduced to f/4 to f/6 with a focal reducer.  This is why SCTs and ACFs are popular for EAA as they can be used across a wide range of focal ratios.

On the other hand, telescopes with focal ratios greater than f/10 are not preferred for EAA.   Examples are the Matsutov-Cassegrain design like the Celestron 4SE, Meade ETX 90 observer and the Explore Scientific First Light 127mm with focal ratios of f/13 to f/15.  ​Yes, they can use focal reducers too, but too much focal reduction can result in severe vignetting causing the light intensity to fall off toward the edge of the FOV which distracts from the quality of the image viewed.  So a good rule of thumb is to select a telescope with a native focal ratio of ~f/6 to f/7 or less, or one which can use a focal reducer to obtain a focal ratio of f/6 or less.

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​Aperture
Aperture is the the size of the primary optical element in the telescope.  For a refractor it is the lens or lenses at the front of the telescope and for a reflecting telescope (SCT, ACF, Newtonian, etc.) it is the large mirror at the back of the telescope.  As discussed above, contrary to first thought, while a larger aperture does collect more light, it does not ensure shorter exposures.  Focal ratio alone determines the length of the exposure.  Aperture determines the minimum resolvable detail or magnification of an object.  For a given focal ratio, aperture determines the focal length which in turn determines the image scale or magnification of the object to be viewed.  The focal length of a 16" f/10 SCT is 4000mm while the focal length of an 8" f/10 SCT is 2000mm.  That means that the resolution of the 16" SCT is 1/2 that of the 8" SCT and the area of the sky covered by the 16" SCT is 1/4 that covered by the 8" SCT which is a magnification of 4x.  So the larger aperture telescope provides more resolution or detail than the 8" SCT at the same focal ratio so long as the seeing conditions are sufficient to support the smaller image scale of the 16" SCT.                          
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Aperture is the primary driver of the cost of a telescope.  Prices for refractors increase almost exponentially as the size of the aperture increases.  Typically, as aperture increases so does the complexity of the objective going from a singlet to doublet and even a triplet objective designed to improve or eliminate chromatic aberrations.  The same is also true for the prices of SCTs and Newtonians but not as dramatically as for refractors since mirrors are less costly to produce in general than lenses which need 2 sides figured and polished versus a single side for a mirror.  The price increases fast as the objective or primary mirror size increases due to the increasing difficulty of manufacturing larger optics.

Large apertures also mean more weight.  My 9.25" SCT weighs a mere 20lbs, while my 11" SCT weighs 27.5 lbs.  After nearly 10 years I decided to sell my 14" SCT because I could not safely mount and demount it at 45 lbs.  Not only does the 14" weigh 45 lbs, but because most of the weight is in the primary mirror which is at the back end of the telescope, it is awkward to handle with the uneven weight distribution.

Since reflecting telescopes with apertures of 10" or more are common, one also needs to consider cool down times.  The primary mirror is a large piece of glass and since glass is not a good thermal conductor, very large mirrors can take a few hours to cool down especially if the day time and night time temperatures are very different.  This is especially true for any sealed tube design like and SCT where there is very little natural air flow.  Cool down cycles can be increased with the use of fans attached to the backsides of mirrors.  Truss tube telescopes allow for more air flow and will cool down faster.  

Another consideration is dew formation as this becomes more likely the larger the aperture.  Dew shields will help but in many cases dew heater straps have to be used around the optical element in the front.  Solid tube Newtonians have less of a problem since the tube acts as a very long dew shield.  SCTs are particularly susceptible to dew on the front corrector plate, including on the inside of the corrector if a dew shield and/or dew strap is not used.

Focal Length
While focal length is determined once focal ratio and aperture are defined, it is still worth looking at the impact of focal length separately.  As we discussed above, a short focal length telescope provides a large FOV which is much less demanding on the mount's tracking capability and the quality of the polar alignment (assuming an EQ mount).  Larger FOVs make it a lot easier to find and identify DSOs.  A shorter focal length, less than 600 - 800mm is less impacted by the seeing conditions since it means a larger image scale.  Very long focal lengths, greater than 2000mm, can make the image you view a bit soft appearing to be slightly out of focus and displaying less detail due to the effect of turbulence in the air with poor seeing conditions.

Very short focal length telescopes are better suited for very large objects, like the North American Nebula, or for sweeping views of a rich field of DSOs such as a  cluster of galaxies like Markarian's Chain.  On the other hand, there are many DSOs which are better viewed at higher magnification such as the Ring Nebula or the Whirlpool Galaxy.  The long focal length images, while more challenging to obtain, will show richer detail including star clusters and nebulae in nearby galaxies which are not possible to see in wide field views.

Short focal length telescopes typically have short tubes, are lighter and less expensive.  These can be mounted on a lighter and less expensive mount while achieving good tracking capability.  A short focal length telescope, like an 80mm refractor with a focal length of 600mm or less is a good choice for a beginner providing an easier and less costly introduction to EAA.

​Focus
If a telescope will not come to focus with an attached camera it obviously will not work for EAA.   In order to focus, the camera must be placed at the back focus position which is the distance from the back of the optical tube to the point where the image is focused.  The backfocus position changes if an additional optical component such as an eyepiece, focal reducer or tele-extender is placed in the optical path.  If the telescope's focuser  does not have sufficient travel inwards or outward it will not bring the image to focus on the camera's sensor.   A Schmidt-Cassegrain (SCT) or Meade ACF telescope has a large focus range since focus is achieved by moving the primary mirror so it does not have a problem achieving focus with cameras used for EAA.  The proper threaded spacers will be required, but those are commonly available.

Unless a refractor is designed for astrophotography, called an astrograph, it likely will not have sufficient outward travel to achieve focus without the addition of extension tubes or a diagonal.  Once again, extension tubes are readily available for this and refractors are another common choice for EAA.   Extension tubes are preferable to diagonals as they eliminate the need for another optical surface, the diagonal's mirror, in the optical path which will slightly reduce the light intensity reaching the camera.  Also, extension tubes are available which either slip into or thread onto the refractor's focuser.  Threaded connections are preferable as they create a more rigid connection and better ensure the camera is centered in the optical path.  Even slip fittings with a centering ring and a thumb screw to lock the extension in place can sag under the weight of the camera, reducer and filters.

In contrast to refractors, Newtonian telescopes often do not have sufficient inward travel to achieve focus.  In this case a diagonal or spacers will not help since it is necessary to position the camera closer to the primary mirror not further away.  A few very small analog cameras like the Revolution Imager I and the cylindrical shaped Lodestar X2 digital camera can slide into the focuser sufficiently to achieve focus on some Newtonians, but none of the more recent vintage of CMOS and CCD cameras are small enough to achieve this feat.  In that case, the only solution is to modify the telescope by moving the primary mirror closer to the secondary mirror and, in so doing, push the image plane further outside the focuser enough to achieve focus with the camera.  There are a few truss tube Newtonians which are designed with the ability to move the primary mirror toward the secondary mirror to switch from visual mode to imaging mode and these will work for EAA.  Otherwise, for EAA it is necessary to purchase an imaging Newtonian or Newtonian astrograph.  These have the primary mirror correctly spaced for imaging and EAA.

Orion EON f/6.25 Doublet Refractor

Putting it All Together
While focal ratio, aperture and focal length are the key attributes which determine a telescopes performance capabilities, cost is typically the biggest factor affecting the final choice.  As the aperture increases the price increases quickly.  Fortunately, there are good telescope options for EAA in all price ranges and telescope types as we will discuss below.   The best option may be to use a telescope already on hand to get started before investing in additional equipment.  In that case, you will want to consider your next possible telescope choice before investing in a mount and camera to make sure that they are compatible with your future telescope purchase.
 
The choice of telescope type, reflector or refractor, will play a major role in the price of the telescope for a given aperture with refractors costing the most per inch of aperture compared to reflectors.   Reflectors can be broken into multiple sub-types with the two most of interest to EAA being the Newtonians and Schmidt-Cassegrains.  Newtonians tend to be priced lower at a given aperture than SCTs.  And Newtonians typically have natively fast focal ratios compared to SCTs.  On the other hand, SCTs are more compact for a given aperture and can be more versatile given the options for focal reduction.  Refractors are much more expensive per inch of aperture than reflectors but provide some of the sharpest images given that they have no obstructions (secondary mirrors) in the light path.  Given the variety of sizes and brightness of DSOs there is no single perfect telescope.  That is why more advanced EAA'ers often have two or more scopes to choose from, or use focal reducers for greater versatility with a single telescope.  

As of this time, most telescopes are in high demand because of the Corona Virus social distancing protocols which means most are on back order.  Apparently some retailers have raised prices substantially so shop around before you buy.


Refractors
  Refractors with fast focal ratios (f/3.9 to f/6) or even moderate focal ratios (f/7 to f/7.5) are well suited to EAA.  The fast focal ratios provide a wide field of view which enables large objects like the North American Nebula or the Andromeda Galaxy to fit into the image frame.  Fast focal ratios as mentioned above will enable the use of shorter exposures to achieve pleasing images quickly.  And short exposures mean less demand on the mount's tracking capability and less precision required in the polar alignment with an EQ mount.   Like other telescope designs, native focal ratios can be further reduced with the addition of a focal reducer which typically come in the 0.7X or 0.8X range for refractors.  These can take an f/7 or f/7.5 refractor down to ~f/5 or f/6 speeding up the system by a factor of ~2X. 

Refractors inherently suffer from three optical aberrations: chromatic aberration, spherical aberration and field curvature.  Chromatic aberration is the most significant and results from the fact that a single lens does not bring all wavelengths of light to focus at the same plane.  Red, green and blue light focus at different focal plane distances so that bright objects will appear to have a purple halo caused by the out of focus blue and red light in a low end refractor.  In addition to the halo, extended objects may appear to have a "soft" or incomplete focus.  A partial solution to this employed in Achromats is to use two glass lenses in the objective, one made of low dispersion crown glass and the other made of high dispersion flint glass.  This helps to focus the red and blue light closer to the same focal plane while the green light is still focused in front.  Chromatic aberration can be further improved with the use of a yellow or green filter or a UV-IR filter which helps by eliminating the extreme red and blue wavelengths of light which appear most out of focus.  Apochromatic (APO) refractors use two (doublet), three (triplet) or four (quadruplet or Petzval) lenses of different shapes with at least one lens made from higher quality and more expensive Extra-Low dispersion (ED) glass.  This enables all three wavelengths of light to focus close enough to the same focal plane so as to produce little or no discernible chromatic aberration.  The best APOs use the highest quality ED glasses in one or more of the lenses.  While a triplet should show less chromatic aberration than a doublet it really depends upon the quality of the ED glasses used and the overall design of the lenses themselves.  As the complexity of the design increases the cost of the refractor goes up quickly.  High quality refractors are noted to produce tack sharp in focus stars.

Spherical aberration results from the fact that light entering the objective near its edge focuses closer to the lens than light entering closer to the center of the lens.  This problem is solved by using multiple lenses with different curvatures and is likely part of an objective design to eliminate chromatic aberration.

Field curvature is caused by light entering the objective at different angles which results in a curved focal plane.  This is not a problem for visual work but will produce soft or out of focus images with a camera.  A higher quality refractor will be designed through the use of multiple lenses to minimize field curvature.  A field flattener can be used to correct for field curvature if the native design is not sufficiently flat.  The Petzval design incorporates a field flattener in its objective.

Refractors are widely available ranging in objective size from 2" to 6".   There are far too many manufacturers and models to give a comprehensive list here so we will discuss a few different options.  Keep in mind that an refractor with higher quality ED glass will do a better job of suppressing chromatic aberration, eliminate spherical aberrations and provide a flatter field than one with a cheaper type of ED glass.  Also note that some doublets are advertised as APOs and others as Achromats.  If the doublet doesn't have ED glass it should be considered an Achromat as it will be expected to have noticeable chromatic aberration.

For lower cost wide field views Williams Optics has a 61mm f/5.9 and a 71mm doublet at $538 and $648, respectively.   At 80mm models include the Williams Optics (f/6.9) and SkyWatcher (f/7.5)  doublets for $800 and $825 while Explore Scientific has an f/6 triplet model for $850.  Moving up in price, Explore Scientific, Meade and Orion all offer 80mm f/6 triplets at $850 and $1000.

There are quite a few options for 4" or ~100mm refractors.  At the lower cost end is the Explore Scientific 102mm f/6.5 doublet Achromat for $550.  Explore Scientific, iOptron and Orion offer a 102mm f/7, a 108mm and a 110mm f/6 refractor in the $1200 to $1300 price range.  The Explore Scientific is a triplet while the other two are doublets all with different ED glasses to qualify as APOs.  Meade and Explore Scientific have triplets at $1900.  At the high end in both price and performance of this size apeture are the Takahashi f/8 100mm and the Televue f/5.4 NP101is.  The Takahashi is a doublet with the most expensive ED glass made of fluorite for $2900.  The Televue uses 4 glass elements in a Petzvalwhich for $4000.

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Explore Scientifc 127mm f/7.5 CF Triplet Refractor

Moving to 4" models (100mm to 110mm) Explore Scientific has a doublet at f/6.5 using non ED crown and flint glass at $550 and a triplet with one ED element at f/7 for $1200.  iOptron has an f/6 doublet with ED glass for $1250 while Orion has a f/6 doublet with ED glass for $1300.  Higher end models range in prices from $1900 like the Explore Scientific f/7 triplet or their carbon fiber tube version at $2200, the Takahashi f/7.4 doublet with expensive fluorite ED glass at $2400 and the Televue quadruplet f/5.4 model at $4000.   You can find still more expensive models than these but even the higher priced ones described here are better suited to astro-imaging and may be overkill for EAA.

A 5" refractor starts to get fairly large but can certainly be used for EAA.  One of the less expensive models is the Explore Scientific f/7.5 ED127 Essential triplet with ED glass at $1900 which I purchase for my son has and which I have used on occasion for EAA.  In addition to Explore Scientific, Meade, Orion, SkyWatcher and Williams Optics all have 5" models at f/7 to f/7.5 for under $3000.  Notice that as the aperture increases the focal ratio tends to increase as well.  While f/7 is not fast it is perfectly useable for EAA and these telescopes can be used with focal reducer/field flatteners, typically 0.7x to 0.8X, to get the focal ratio down to f/5 to f/6.

6" refractors can certainly be used for EAA just as they are for astrophotography but their cost and size goes well beyon what is needed for EAA.
 
There are many reasons why a quality refractor is an excellent choice for EAA.  In addition to fast optics, refractors are lightweight, especially in comparison to reflectors.  For example the Explore Scientific 80 mm weighs 7.5 lbs and the 127 mm weighs 18 lbs, making them very easy to carry and mount.   However, the large aperture refractors have long tubes such as the Explore Scientific 127 with a 34" tube length.  The longer the tube the larger the moment of inertia.  This must be taken into account when selecting an accompanying mount since the  larger moment requires a stronger mount compared to a shorter tube telescope with similar weight like the Celestron 9.25" SCT  which weighs 20lbs but is only 22" long.

​Since refractors do not use mirrors, they do not have coma, an optical aberration which makes off axis stars appear to have tails like comets.  Since there is no obstruction in the optical path like that of the secondary mirror of a reflector there is no light lost which produces the highest contrast telescope design.  Refractors rarely, if ever, need collimation and have short or no cool down times compared to large mirror reflectors.  Because they typically have smaller apertures they also have smaller cross-sections which minimizes vibrations which can elongate stars on a windy night.  Also, their sealed tubes tend to keep out dust, dirt and bugs.    And most refractors come with a convenient slide out dew shield.

When selecting a refractor there are other important considerations including whether or not the telescope tube is made of aluminum or the more expensive carbon fiber and the size and type of focuser.  Carbon fiber tubes have a lower coefficient of thermal expansion than aluminum and therefore exhibit less focus shift during the night as the air cools.  Carbon fiber tubes are also much lighter than aluminum which reduces the weight of the optical tube which can allow for the use of a less expensive mount.   Focuser types and sizes can vary greatly ranging in size from 1.25" up to 4" with the larger focusers available on the higher priced telescopes..  The larger the camera chip size the larger the focuser required to avoid vignetting of the image.  The focuser should be mechanically sturdy to handle the weight of the camera, focal reducer, etc. without any sag in the optical train.  A two speed focuser allows for finer focus adjustment.  
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Inch for inch, refractors tend to be significantly more expensive than reflectors.  The table below shows the cost per inch of Achromat doublets and APO triplets from Explore Scientific, Imaging Newtonians from Skywatcher and SCTs from Celestron.  Achromat doublets are 50% more expensive on average than Imaging Newtonians and nearly as expensive as SCTs.  APO triplets are more than 3X as expensive as the Imaging Newtonians and almost twice as expensive as SCTs.  This reflects the higher cost to manufacture lenses compared to mirrors which climbs the more elements and the higher quality the glass used.​ ​

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Orion StarBlast 6" f/5 Newtonian

 Newtonians
As the table shows, measured in cost per inch of otpics, Newtonians are the least expensive telescopes and are therefore a very good option for EAA.  As noted above, care must be taken to be certain that a camera will come to focus either by design (imaging Newtonians or astrographs) or by moving the primary mirror closer to the secondary.  Some truss tube Newtonians are designed for adjustment of the primary mirror distance so do not need user modificaiton.  

Newtonians are typically designed with fast optics with f ratios typically between f/3.9 and f/5.3.  Because of the fast optics coma is an inherent problem with Newtonians.  This is caused by light from off axis angles coming to focus at different distances and results in stars looking like comets toward the outside edge of the FOV.  A coma corrector is typically desired to eliminate or minimize this.

The long tubes of a Newtonian help to minimize dew formation but acts as a sail on windy nights.  And the long moment arm of most Newtonians requires a sturdier mount per inch of aperture compared to the more compact Schmidt-Cassegrain designs.  The open tube helps to minimize thermal stabilization time but also allows dust and bugs to get into the telescope so covers for the primary and secondary mirrors are recommended. 

Newtonians require frequent collimation (maybe at every use) which can be challenging at first but with practice this can be done in short order.  The central obstruction of the secondary mirror results in reduced contrast compared to refractors.  Straight vanes used to hold the secondary will produce diffraction spikes around bright stars which you may or may not find objectionable, but curved vanes will not.

At the lowest price range, 6" f/4 imaging Newtonians from Apertura and Orion are available for $299 and $400, respectively.  These are light weight at 9.6 and 12.7lbs each with tube lengths of 22.5" so they are relatively easy to handle.   Expect to sacrifice build and optical quality at this price range, but for the very price limited case these may be worth considering.

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SkyWatcher Quatro 8" Imaging Newtonian

​An 8" telescope tends to be a sweet spot in terms of light gathering capability versus size and weight.  One can find quite a few options at this aperture including three at or just under $500 from Apertura (f/4), Orion (f/3.9) and Explore Scientific Bresser 208mm (f/3.9).    SkyWatcher has an 8" Quattro at f/4 for $640 while Explore Scientific has an f/3.9 carbon fiber tube model for $1000.  Weights are ~ 20lbs and tube lengths ~30" so with the longer tube lengths and heavier weights, these telescopes will require a higher capacity mount.

A 10" aperture begins to get heavy, 25 to 36lbs, and tube lengths approach 39" in length.  These require a higher end mount to avoid vibrations and tracking issues.  Because Newtonians are relatively inexpensive to build, one can find multiple 10" models  for under $1000 including  an f/4 from Apertura, and f/3.9 from Orion and and f/4 from SkyWatcher.  

Even larger and more costly Newtonians are available but will not be discussed here since they are not widely used for EAA.

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Celestron 9.25" SCT

​Schmidt-Cassegrains
Schmidt-Cassegrains (SCTs) are a specific type of reflector consisting a primary and secondary mirror like the Newtonian but with the addition of a corrector plate at the front of the telescope.  The corrector plate is designed to eliminate spherical aberration caused by the use of a spherical primary mirror which is easier to form than a parabolic mirror keeping the cost of SCTs down.  SCTs may be the most widely used telescopes for EAA due to their relatively low cost per inch of aperture and their versatility to function at a range of focal ratios from f/10 to f/2.  

The design of an SCT is compact for its focal length as the optical axis is folded upon itself.  This results in a shorter tube compared to a similar focal length Newtonian which helps to make the large aperture SCTs easier to handle and less susceptible to winds.  Because an SCT has a corrector plate the optical tube is sealed against dust, bugs and dew, although the corrector itself will still collect dew.  Because there is no air flow inside the tube a large primary mirror will have a long cool down time which can be mitigated by after market fans like the Tempest fans from Deep Space Products.   

Because SCTs are focused by moving the primary mirror they have a lot of focus range to easily accommodate cameras, filters and focal reducers in the optical train.  The downside is focus shift where the object being viewed shifts in the FOV during focusing as the mirror shifts a bit on its rails while focusing.   This is more of an annoyance for EAA than a serious problem.  Also, the weight of the secondary mirror can cause it to move or flop as the telescope rotates across the sky which results in another shift in the image within the field of view.  This is less of a problem for 9.25" and smaller SCTs but can be at least partially mitigated if the telescope has mirror locks.  Also, since EAA does not require hour or longer imaging on a single object, mirror flop is less of a problem than it is for astrophotography.  Despite these nuisances, SCT are still very common for EAA.

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​SCTs also exhibit coma and field curvature unless they are corrected with a focal reducer/field flattener like the Celestron and Meade f/6.3 reducer/correctors.  Modifications of the SCT designs such as the Celestron Edge and Meade ACF add optical elements in the light path to correct for coma and flatten the field so that stars are sharp to the edge of the FOV.  Since SCTs have a large central obstruction they provide the least contrast compared to Newtonians and refractors. 

Because SCTs use a primary mirror at f/5 and a secondary mirror at f/2 (the Celestron 14" has a secondary mirror at f/1.9), the native focal ratio is f/10.  While this works well for very small DSOs, it requires long exposures and is too much magnification for many objects.  Fortunately, the SCT focal ratio can be easily reduced with the aforementioned f/6.3 reducers from Celestron or Meade.  With a Celestron Edge telescope a more expensive f/7.5 reducer must be used instead due to the complexity of the Edge optics.









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​Celestron has two lines of f/10 SCTs, one with and one without Edge optics.  The non-Edge SCTs have apertures of 6', 8", 9.25", 11" and 14" with prices, weights and tube lengths shown in the table above.  The Edge line of SCTs has an additional optical element after the secondary mirror designed to provide a flat focal plane and reduced coma out to the edge of the FOV, hence the product name "Edge".  In addition, the Edge line includes filtered tube vents at the rear of the optical tube to help reduce cool down time and tension clutches which help to reduce mirror flop.
Both the Celestron Edge and non-Edge designs from 6" to 14" can replace the secondary mirror with the optional Hyperstar adapter from Starizona which reduces the focal ratio to f/2 (f/1.9 for the 14" aperture) making for a FOV 5 times larger and imaging speed 25X faster than at its native f/10.  Many, but not all, of the older non-Edge SCTs can be retrofitted to accept the Hyperstar.  This is one reason for the popularity of these scopes. 

As the apertures of SCTs approach 12" weights become challenging to carry and attach to the mount as most of the weight is in the primary mirror at one end of the optical tube.  Having used a 14" SCT for 10 years both at home and in the field I can attest that such a large scope is a wonder to use but a challenge to handle for all but the very sturdy individual.  Compared to Newtonians, SCTs have tube lengths which can be as much as ~50% shorter making it possible to use a larger aperture SCT on a given mount.

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​Meade has their own versions of modified Cassegrain telescopes with their Advanced Coma Free optics, hence the moniker ACF for this line of telescopes.  These are offered in four different groups.  The LX65 and LX85 have the same optical tube but the LX85 comes with 2 eyepieces instead of one and an 8 x 50 finder instead of a unity finder for a small price differential.  The LX200 series steps up to a Losmandy dovetail instead of the Vixen used on the LX65 and LX85.  It also has an oversized mirror which allows for better performance to the edge of the FOV.  And the LX200 has mirror locks to prevent shifting of the primary mirror.  The fourth version has three key differences.  First, it has a slightly faster focal ratio of f/8 which provides a 25% larger FOV and a 56% faster imaging system.  It also has a more rigid mirror mechanism which uses a 2 speed focuser instead of the single speed version on all of the other versions of ACFs providing finer focus adjustments.

GSO 10" Truss Tube RC

 Other Options
There are many other variations of reflecting telescopes which are not common for EAA but cannot be ruled out.  The least expensive among these are  Maksutov-Cassegrains (MCTs) but these tend to have even higher focal ratios, f/12 to f/15, than the SCT so they are not common for EAA. 

Guan Sheng Optical (GSO) produces a relatively inexpensive line of f/8 Ritchey-Cretien (RC) telescopes with  6", 8", 10", 12", 14" and 16" apertures priced from $399 to $6995.  You will find re-branded versions from iOptron, TPO, Orion, etc.  RCs are designed to eliminate coma and are natively faster at f/8 compared to SCTs.   The 6" version comes with a steel tube while the 8" version comes in a both a steel tube and a carbon fiber model for $500 more.  The 10" and larger models use an open truss tube design to minimize weight.  Focusing is done with a separate focuser as the primary mirror is fixed eliminating the possibility of mirror shift or flop.

Perhaps the most interesting option for EAA is the Rowe-Ackerman Schmidt Astrograph (RASA) from Celestron.  The design has no secondary and, like a Hyperstar, places the camera where a secondary would be which provides very fast f/2 optics.  The RASA has filtered mag-lev fans for cooling and a modified focusing system to minimize focus shift and mirror flop.  Celestron has an 8" version for $1700 which weighs only 17lbs while the 11" version for $3500 weighs significantly more at 43lbs.  The downside is that it can only be used as a wide field telescope since the focal length cannot be modified.

Another option is to start with very wide field EAA using a 100mm or 200mm telephoto lens.   These need one of the readily available adapters to connect the lens to the astro camera and you will need a clamp or other method to mount the lens to a mount.  Because of the light weight and short focal length, this type of setup can use one of the less expensive Alt-Az or EQ mounts to obtain satisfying results.  And this approach has the advantage of being light weight and highly portable.

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Summary
The good news is that there are many options available in telescopes for EAA.  The bad news is that so many options can make for difficulty deciding, or paralysis of analysis.  If you already have a telescope and mount the best advice is to start with what you have.  If you will have to purchase a camera to go along with your current telescope, take into consideration any future telescope purchase you might already have in mind.  Using what you have now will help you get your feet wet and make it less likely you purchase something that will not work well for your interests.

If you do not already have a telescope it is difficult to go wrong with an 8" SCT.  An 8" SCT has the versatility of multiple focal ratios, has a light weight and compact design making it easy to transport and setup and is relatively inexpensive.   If you can afford the cost, a Celestron Edge version will give you the ability to work at f/2 if you purchase the Hyperstar adapter now or at a later date.

Another good option if you are starting fresh is an f/6 or faster triplet 80mm refractor which will likely stay with you for a lifetime.  With its fast focal ratio and wide field it will be easy to get started avoiding many of the frustrations of long focal length imaging.  Once your skills improve and you begin to get aperture fever you can continue to use the 80mm as your scope of choice on those nights when you want to view large DSOs.

Telescopes That I have used successfully for EAA:

Celestron 9.25" SCT - excellent aperture to weight combination, reasonable cost
    Available from High Point Scientific
Celestron 14" Edge SCT  - large aperture, excellent optics, but heavy
    Available from High Point Scientific
Celestron 11" Edge SCT -  good compromise between aperture and weight
     Available from High Point Scientific
Orion ED80T - this is the carbon fiber version whereas mine has a metal tube
     Available from High Point Scientific 
Explore Scientific 127mm -  very nice sharp stars 
     Available from High Point Scientific

If you are interested in other telescopes you can find lots of options in each category of telescope on the High Point Scientific web sites.

​OPT and High Point links are Affiliate links.