Choosing and Using 2" Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

By: Manish Panjwani and Brian Ventrudo
March 20, 2019

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Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 1 – 2" SCT diagonals from (left to right) Celestron, William Optics, and GSO. The telescope side of each diagonal has 2"-24 threads that attach directly to the rear of the optical tube of most Schmidt-Cassegrain telescopes.

1. The Advantages of Using a 2" Diagonal

Most Schmidt-Cassegrain telescopes (SCTs) come with an OEM 1.25" star diagonal and a 1.25" visual back. The diagonal, which is essential to enable a comfortable viewing position, connects to the telescope with the help of the 1.25" visual back that threads onto the back of the telescope (see Figures 2 and 3). Most standard-issue 1.25" diagonals are generally of good quality, and they allow visual observation with any 1.25" eyepiece.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 2 – A schematic showing how a typical 1.25" OEM diagonal connects onto the back of the optical tube using the included 1.25" visual back.
Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 3 – An 8" SCT with the OEM 1.25" visual back and diagonal as sold with many popular SCT and Maksutov-Cassegrain telescopes.

Once they gain some experience with their telescope, however, many observers may wish to upgrade to a 2" star diagonal for their SCT. These diagonals allow the use of larger 2" eyepieces that provide a larger field of view. They are more mechanically robust and securely hold larger eyepieces and accessories. And many 2" diagonals are also of superior quality to many standard 1.25" diagonals, while still accommodating 1.25" eyepieces with a 2"-1.25" adapter.

Upgrading to a 2" diagonal for an SCT (or a Maksutov-Cassegrain telescope) is not difficult. But many beginners, and even some more experienced astronomers, have some confusion over the choices involved. The key lies in understanding that when you remove the standard 1.25" visual back, you expose a larger 2"-24 male SCT thread (with scopes up to 9.25" aperture), or a 3.25" or 3.28" male SCT thread (on larger scopes), that offers three options to add a 2" diagonal. These options are:

  • Using a dedicated 2" 'SCT' diagonal specifically designed to thread onto Schmidt-Cassegrain telescopes
  • Using 2" 'refractor diagonal' that slides into a 2" visual back threaded onto the scope that replaces the 1.25" visual back
  • Directly threading a 2" diagonal body onto the back of the SCT
  • Installing an external 2" focuser on the back of an SCT or MCT telescope that will accept 2" star diagonals.

In this article, we look at the first three solutions to determine which is the best choice for your situation. The fourth solution, installing an external focuser, was covered in a previous article at this link.

2. Installing Dedicated 2" SCT Diagonals

The two main configurations of 2" star diagonals, refractor diagonals and dedicated SCT diagonals, are shown in Figure 4.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 4 – A dedicated 2" SCT diagonal (left) and a 2" refractor diagonal (right). The SCT diagonal has a threaded collar that connects to the back of an SCT optical tube. The refractor diagonal has 2" metal tube that inserts into a 2" visual back on an SCT or into an external focuser.

As described in the next section, 2" refractor diagonals, which are more commonly available, interface to the telescope with a 2" tube that slides into a 2" visual back.

A dedicated 2" SCT diagonal, by contrast, is connected to the telescope by way of a collar or adapter with 2"-24 female threads that connect directly onto the male 2"-24 threads on the back of the SCT tube itself (Figures 5 and 6). The diagonal can be threaded and adjusted in any orientation to suit the observer. The standard 1.25" visual back must be removed before you can install the SCT diagonal.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 5 – A schematic showing how a dedicated SCT diagonal threads onto the back of the optical tube.
Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 6 – A dedicated GSO SCT diagonal screwed directly onto an 8" Meade SCT.

Dedicated SCT diagonals will not work with SCTs that have external Crayford or rack-and-pinion focusers. If you have an external focuser on your SCT, you will need to remove it, or you will need to use a 2" refractor diagonal that slides into the external focuser itself.

Although SCT diagonals have female 2"-24 threads, they can also be installed on larger SCT scopes with 3.25" (used by Meade) or 3.28" (used by Celestron) threads using a 3.25"/3.28"-to-2" adapter that is included with the telescopes by the manufacturer. These adapters are not currently available from third parties.

Dedicated 2" SCT diagonals generally cost about the same as refractor diagonals. They offer a simple solution for observers who wish to upgrade because there's only one new component to install. However, they can only be used on Schmidt-Cassegrain telescopes; they will not work with refractors.

Several manufacturers including William Optics, GSO, and Celestron make dedicated 2" SCT diagonals. Learn more about the specific brands and choices for 2" SCT diagonals at the link below:

https://agenaastro.com/optical-accessories/diagonals-prisms/sct-diagonal.html

Baader Planetarium diagonals are popular with many observers. While Baader offers two 2" refractor-type diagonals, they do not offer any SCT-type diagonals. However, there are adapters available to replace the removable nosepiece of a Baader diagonal with a female threaded SCT adapter for direct coupling with 2"-24 threads of the telescope (Figure 7).

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 7 – A dedicated Baader 2" refractor diagonal (left) and the same diagonal converted into an SCT diagonal (right). This conversion was achieved by removing the 2" Baader nosepiece and replacing it with GSO's SCT adapter # F228 (available separately from Agena AstroProducts).

3. Installing Refractor-Type Diagonals and a 2" Visual Back

A 2" refractor-type diagonal is another good option for use with an SCT. If you own a refractor, you may already have one. Or you may wish to upgrade both your refractor and your SCT with a premium 2" diagonal that works with both telescopes.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 8 – A schematic showing a 2" visual back which threads onto the telescope and a 2" refractor diagonal which slides into the visual back.

To use a 2" refractor diagonal on an SCT, however, you must first remove the 1.25" visual back on your telescope and thread on a new 2" visual back that can accept the larger nose piece of the2" diagonal (Figures8 and 10). There are many types of 2" visual backs available for SCTs, from simple thumbscrew models to more sophisticated "Clicklock" backs from Baader Planetarium, William Optics, Kasai Trading and others (Figure 9). Meade makes a refractor-type diagonal that also includes a 2" SCT visual back.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 9 – A variety of 2" SCT visual backs are available from Agena AstroProducts that allow use of 2" refractor diagonals on 2" SCT threads.

You can see a range of available visual backs for Schmidt-Cassegrain telescopes at this link:

https://agenaastro.com/parts-accessories/adapters/visual-backs/shopby/sct_thread_female.html

Installing a 2" visual back and sliding in a 2" refractor diagonal is a simple process, but there are a few disadvantages. The larger 2" visual back results in a slightly longer path length, which means the effective focal ratio (and focal length) of your SCT telescope will increase slightly. It may also cause a problem with clearance with SCTs mounted on fork mounts: the visual back and diagonal may protrude far enough from the back on the telescope to make it impossible for the telescope to 'swing through' the fork mount (see section 5.2 below).

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 10– A 2" refractor diagonal mounted in a 2" Kasai Trading visual back threaded to an SCT optical tube (left); a Baader 2" Clicklock visual back on an 8" SCT (right). Image credit: Kasai Trading and Baader Planetarium

Once you have a 2" visual back mounted, you can insert a 2" refractor diagonal from the many types available including those from GSO, William Optics, Baader Planetarium, and Tele Vue Optics.

4. Direct Coupling the Diagonal Body with the Telescope

The solutions in the last two sections will work with SCT scopes on a GEM (German equatorial) or alt-azimuth mount, and even on many fork-mounted SCTs. However, on some fork-mounted SCTs, like the Celestron Nexstar scopes, you will likely not have sufficient clearance from the base of the fork mount for the optical tube when it is aimed parallel to the arms of the fork. That means you cannot use even a 2" SCT diagonal and expect to be able to point the scope at or very close to the zenith (or towards the poles if the fork mount in on an equatorial wedge) because the larger diagonal body will hit the base as your scope slews in that direction.

The solution? Thread the 'telescope side' of a 2" diagonal body directly onto the back of the telescope. This isn't possible with most diagonals. In fact, there is no thread standardization with diagonals. Some like Celestron and William Optics use an M48x0.75 female thread that will not fit on the back of SCT telescopes. Others might use a different thread that will also not thread directly onto the scope.

Baader Planetarium, however, equips their 2" diagonals with an SCT female 2"-24 thread on the telescope side of the diagonal body to allow you to thread the diagonal directly onto the SCT (Figure 11).

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 11 – A 2" Baader diagonal with the 2" nosepiece removed to show the female 2"-24 SCT thread at lower right.

At one point, GSO diagonals had an SCT female thread on the body on the telescope side. But a few years ago, GSO changed the pitch of this thread with the result that their newer diagonal bodies now no longer feature a standard SCT thread pitch and cannot be directly coupled with SCT scopes.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 12 – A schematic showing how to attach a 2" diagonal body directly to the back of a Schmidt-Cassegrain telescope using an SCT lock ring to control its orientation.

Figures 12 and 13 illustrate how this method works. The nosepiece of the 2" diagonal must be removed to expose the female 2"-24 threads, and you must remove the visual back from the telescope. You will likely also have to remove the eyepiece holder on the diagonal to prevent it from hitting the focuser knob as you thread the diagonal onto the body. Then you can thread the diagonal body to the back of the telescope until it's tight. Once the diagonal is installed, you can re-attach the eyepiece holder.

While you can thread the diagonal directly to back of the telescope until it is tight and secure, it will likely end up in an inconvenient orientation for observing. This problem can be fixed by threading an SCT lock ring onto the back of the telescope before threading on the diagonal body. The diagonal will thread onto the telescope until it's tight against the lock ring. If you need to adjust the orientation of the diagonal, you can back the diagonal off, orient it correctly, then tighten the lock ring. SCT lock rings are available from Baader Planetarium and Blue Fireball.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 13 - A Baader diagonal threaded directly to the back of a SCT with a lock ring to fix the diagonal in a convenient position for observing.

This link has more information about how to attach a Baader Planetarium diagonal to an SCT.

5. Diagonal Clearance Considerations

Adding a 2" diagonal to an SCT is a worth while upgrade. However, there are two clearance requirements you need to consider before making the switch to a larger diagonal:

  • Clearance of the diagonal from the focus knob of your telescope on small SCTs
  • Clearance of the diagonal (plus 2" visual back, if used) from your telescope mount if your SCT is fork-mounted

These factors are discussed below.

5.1 Telescope Focus Knob Considerations

Before purchasing a 2" diagonal, you must ensure that it's not so big that it interferes with the scope's focus knob. Generally, this is not an issue with SCT telescopes with an aperture of 5" or larger, but this can become a serious impediment in smaller 3" or 4" scopes.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 14 – A schematic diagram of the rear view of an SCT or Maksutov-Cassegrain telescope showing the width of a diagonal and how it might impinge on the focus knob.

Figure 14 will help you understand the measurements to ensure a diagonal will fit. In this figure:

W = Width of the diagonal body as measured from the rear
C = Distance of the main focus knob from the center of the telescope backplate

For a given diagonal to fit on your telescope, you simply need to ensure that W < 2C so that the diagonal body is not too wide.

If a diagonal is too wide for your small telescope, you may wonder if you can simply insert an additional SCT-threaded spacer between the telescope and the diagonal to move the diagonal behind the focus knob. Unfortunately, this is seldom feasible in practice since the heavy weight of the diagonal and accessories may create enough torque to unravel the connection. This spacer would also further increase the effective focal length of the telescope (discussed in section 3 above).

5.2 Telescope Mount Considerations

If you use a fork-mounted SCT, it's important to carefully measure the available fork clearance before selecting a diagonal. That is the distance from the SCT threads on the rear of the telescope tube to the base of the fork mount with the telescope tube pointed parallel to the arms of the fork (which means straight up in the case of an alt-azimuth fork mount). This is especially critical if you use a 2" refractor diagonal and a 2" visual back with your telescope. Because of the limited fork clearance, especially on the smaller-aperture SCTs, you will be restricted in your diagonal choices and range of use.

It's easy to determine whether a given configuration of visual back-plus-diagonal will clear your telescope's fork.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 15 – A schematic showing the available fork clearance 'F', an external visual back and refractor diagonal, or an SCT diagonal showing the dimensions that are relevant to fork clearance.

For a refractor diagonal with a visual back, if F is your available fork clearance, then:

F > (X + Y + M)

Where:
X = Mechanical path length of the 2" visual back
Y = Horizontal length of the refractor diagonal body (note that this excludes the nosepiece)
M = A safety margin. This may be required, for example, if your diagonal's thumbscrew extends beyond its triangular body, or if additional cables or accessories are plugged into your fork's base (as is often the case with Celestron SCTs).

One other minor point to keep in mind is to ensure that the length of your refractor diagonal's nosepiece is shorter than the insertion depth available in the visual back it is being inserted into. Otherwise, your diagonal will not fully seat into the visual back, and the amount by which the nosepiece sticks out will need to be factored into the equation above. However, this is not an issue with most but the shortest visual backs.

For an SCT diagonal, this equation reduces to

F > (Z + M)

Where:

Z = Horizontal length of the SCT diagonal body plus its SCT collar fully extended.

Some SCTs will have trouble clearing the fork with an external diagonal, even if you use the shorter SCT diagonals, so you have to be prepared to give up on observing objects that are without 10 degrees of the direction in which the tube and fork arms are parallel. With an altazimuth fork mount, for example, that means you won't be able to observe objects with an elevation greater than about 80 degrees.

Choosing and Using 2-inch Diagonals for Schmidt-Cassegrain and Maksutov-Cassegrain Telescopes

Figure 16 – A GSO refractor diagonal attached to an 8" Meade LX200 (Classic) SCT with a Baader clicklock visual back. Note that this combination easily clears the fork base (even though the diagonal does not fully seat into the visual back) with the telescope pointed at the zenith. However, this will be a challenge in many other fork-mounted SCTs.

If your telescope sits atop a German Equatorial mount, then the length of the diagonal/visual back is not an issue.

6. Attaching a 2" Diagonal to a Maksutov-Cassegrain Telescope

Many Maksutov-Cassegrain telescopes can also work with a 2" star diagonal, and the solutions in the three previous sections apply. The challenge is to make sure there are 2"-24 male threads on the back of the optical tube. Most Maks do not have such threads, and there is little standardization of thread sizes across manufacturers. There are at least four different thread sizes on Maksutov-Cassegrain telescopes. But each has an adapter you can buy separately that will accept 2"-24 threads from a visual back or diagonal.

  • The smallest Mak scopes, such as the Celestron C-90 spotters, the Celestron Nexstar 4SE, or the Meade ETX scopes all use a small 1.375" diameter thread. The conversion adapters from Baader (#2958500A) and Meade (#07036) will convert the rear 1.375" NX4/ETX threads to 2"-24 SCT threads.
  • Some intermediate sized Maks like the Celestron Nexstar 127 use a 44.5mm diameter thread that many refer to as a 'Maksutov thread'. The Blue Fireball C-07 adapter will convert this to a 2"-24 SCT thread.
  • A few less popular Maks have a T/T2 42mm photographic thread. Adapters for this thread are made by Baader (#2958242) and Blue Fireball (T-13).
  • A handful of large Maksutov telescopes, such as the Sky-Watcher 180mm Mak and others from Orion, feature the same 2"-24 male SCT threads discussed above. If you have one of these scopes, 2" diagonals can be used on them as described in the previous sections without the need for an extra adapter.

7. Final Considerations

Here are three more important points to keep in mind as you plan your upgrade to a 2" star diagonal for your SCT or Mak.

If you plan to attach an external SCT focuser on the rear of your scope, or you already have one, you can no longer thread an SCT diagonal to the telescope (unless you remove the focuser). Your only option is to use a refractor diagonal.

If you thread a 0.63x focal reducer onto the back of your SCT, the suggestions in this article remain the same. The male SCT thread on the scope effectively gets replaced by the male SCT thread on the focal reducer after, so you can use dedicated 2" SCT diagonals. Or you can install a visual back to use refractor diagonals.The only difference is the additional clearance for fork-mounted telescopes because the focal reducer adds to the length of accessories extending from the back of the tube.

Finally, for smaller telescopes, it is also important to make sure your telescope and mount can support the extra weight of a diagonal plus larger eyepieces/accessories on the back of the scope.

8. Conclusion

Adding a 2" star diagonal to a Schmidt-Cassegrain (or Maksutov Cassegrain) telescope is usually very easy and an affordable way to step up to 2" eyepieces and accessories. Using a refractor diagonal lets you use the same diagonal on other scopes, but requires a 2" visual back as an accessory. A dedicated 2" SCT diagonal, which attaches directly to the optical tube, offers a shorter external path for the light but can't be used with an external focuser or on a refractor telescope. The choice may primarily be driven by fork clearance, your budget, or your current inventory of 2" star diagonals.

Manish Panjwani
About the Author

Manish Panjwani has been an active amateur astronomer since before Halley's Comet last swung by our neighborhood. A former wireless communications consulting engineer and management consultant to various Fortune 500 companies, Manish started Agena AstroProducts in 2003. Since then, Agena has become one of the leading online retailers of telescopes and astronomical accessories worldwide. Besides observing from his heavily light polluted backyard in Los Angeles, Manish enjoys conducting astronomy outreach programs in local schools. Manish holds a Master's degree in Electrical Engineering from Virginia Tech and an MBA from the Kellogg School of Management at Northwestern University.

Brian Ventrudo
About the Author

Brian Ventrudo is a writer, scientist, and astronomy educator. He received his first telescope at the age of 5 and completed his first university course in astronomy at the age of 12, eventually receiving a master's degree in the subject. He also holds a Ph.D. in engineering physics from McMaster University. During a twenty-year scientific career, he developed laser systems to detect molecules found in interstellar space and planetary atmospheres, and leveraged his expertise to create laser technology for optical communications networks. Since 2008, Brian has taught astronomy to tens of thousands of stargazers through his websites OneMinuteAstronomer.com and CosmicPursuits.com.