Is a Schmidt-Cassegrain telescope good for everything?
Are Schmidt-Cassegrains expensive?
What makes them different to refractors and reflectors?
These types of scopes are different because they use a mish-mash of glass and mirrors.
Did you know that they’re exceptionally lightweight?
Have you noticed that the specs versus the physical dimensions are deceiving?
There’s a place for SCTs in the market and they’re very desirable telescopes to own. Here is your SCT 101 buying guide and a shortlist of some affordable models that won’t break the bank – too much.
Best Schmidt Cassegrain Telescope In 2021
What is a Schmidt-Cassegrain telescope?
First and foremost, an SCT (Schmidt Cassegrain Telescope) is a catadioptric telescope – a type of optical system that uses a combination of mirrors and lenses. It uses a Cassegrain optical path with a Schmidt corrector plate.
What the regular consumer would be used to seeing in this category are compact SCTs. This is where the focal length is much longer than the actual physical length of the tube.
This is why SCT scopes are very portable and lightweight options for travel.
Who are SCTs good for?
Beginners would find it easier to start off with regular refractor telescopes or reflectors, and as they come into telescope skills/knowledge, they’ll find that maybe their interests lead into planetary observation or imaging which SCTs can be a good consideration for.
What are some good Schmidt Cassegrain telescopes?
SCTs come at multiple price points and with various types of mounts. How you want to narrow it down for various areas of interest will be determined by the specs and mount type.
Here are a few at different price points that are great for observation only or for imaging with computerized EQ mounts.
Best Schmidt Cassegrain Telescope Reviews
1. Celestron NexStar 8SE
The NexStar series is an incredibly popular line of telescopes. You may not know that they’re SCTs, so that just might add to the allure.
Pros & Cons
✔️ 8” aperture
✔️ StarBright XLT coatings
✔️ Limited astrophotography
❌ No manual slewing
The NexStar 8SE obviously has that nice, big 8” aperture, and there is a difference when it comes to visibility. You can see more, see fainter objects, and get more resolution from what you’re seeing. With StarBright XLT coatings, you can also feel confident that the optics are making the best use of available light to bring the clearest, brightest picture to your eyes.
Like many SCTs, this is on a computerized GoTo mount. The mount is a fork-arm which is typical of a GoTo SCT, but you will be limited to alt-az movement unless you use a wedge. By the way, it can be polar aligned, so no issue there – just use the included hand control to help get it done.
While computerized movement is desired for imaging, the mount has a lightweight payload capacity. The only reassurances you may have is the fact that Celestron tends to be pretty liberal about their payloads, so tread carefully and watch your worm gears.
Everything is extremely lightweight, so transporting the NexStar will be a breeze. Getting under dark skies with an 8” should be on your bucket list. Not going out under dark skies with this visual superstar would be a crime.
2. Meade 8 LX200 ACF
The price for the LX200 may instantly cause your jaw to drop. Yep – it’s an SCT on steroids that provides some of the best visibility and imaging benefits capable of being provided by an SCT variant.
Pros & Cons
✔️ 8” aperture
✔️ ACF optical system
The LX200 isn’t cheap, and it’s why it’s a dream telescope for many. But, is it really an SCT? It’s often compared to a Ritchey-Cassegrain, but it’s not an RC. So, what it is? It’s an SCT with a hyperbolic secondary mirror. Since the RC uses two hyperbolic mirrors, it’s safe to assume that the primary mirror in the Meade telescope is still a spherical one.
This is okay because you can expect excellent spherical and coma control that even some SCTs struggle to correct. ACF? Advanced Coma Free optics. That’s a tall order to deliver.
With 8”, you’re in real wow-factor territory, and with a wide-field of aberration-free seeing, the quality of your visual or imaging sessions will be unparalleled. To get those images, you’ll need a GoTo mount. The Meade scope comes on a cast-aluminum double tine fork mount. Instantly, you should recognize it has alt-az movement, so you’ll need a wedge to make the most of your imaging goals.
Fortunately, the mount can make good use of one, and various types of imaging techniques can be performed and supported on this setup. Getting it setup and taken down is much easier than its larger alternatives, but it still weighs a hefty amount of 66 lbs at the end of the day. Good thing you’re not hauling it one piece, right?
The LX200 is a scope worth saving up for. There are some naysayers out there, but when they’re not imaging with it, what would they know? This is a serious scope for a serious amateur to get serious with.
3. Celestron Advanced VX6
The Advanced VX6 may seem like an expensive purchase considering there are 6” apertures for much cheaper, but when you have an SCT and a quality mount on your side, it’s worth the price.
Pros & Cons
✔️ 6” aperture
✔️ EQ mount
❌ Need more accessories/power supply
The Advanced VX6 has the classic specs of an SCT with its f/10 focal ratio and a good 6” aperture. It’s on the smaller side for an SCT, but so is the price. When you’re considering the fact that it comes with a heavy-duty EQ GoTo mount, you’re set from the get-go to get imaging.
The mount is Celestron’s Advanced VX mount that has EQ movement, a 30 lb payload capacity, and the NexStar+ hand control. Precision and accuracy is paramount for imaging and the VX delivers.
While the total weight comes in pretty heavy, no one part weighs over 20 lbs, so there’s plenty of reason to travel and get under dark skies. Portability is a go.
However, only one eyepiece is included in the package, so you may want to get another eyepiece to give yourself some versatility for observations. Since imaging is a go, you may also want to experiment with a Barlow lens or autoguiding. Regardless of what accessories you buy, you will need a dependable power supply as a car adapter may not be enough.
Take a cue from the masses – the VX6 is worth it!
4. Celestron NexStar 6SE
The NexStar 6SE is far more attractive in price than its 2” larger sibling, so what’s the difference? What are you missing out on?
Pros & Cons
✔️ 6” aperture
✔️ StarBright XLT coatings
✔️ Limited astrophotography
❌ No manual slewing
To jump right in, the main difference between the 8” and the 6” is the aperture. The extra 2” does get you a lot more in optical seeing and that extra bit of resolution. However, that would be the only noticeable difference between the two models. Are you willing to spend the extra for the benefits of a larger aperture? Only you can answer that.
Another difference is that the 6” is only slightly lighter and more portable than the 8”, but even that small difference makes the 6” better suited to the mount. Yes – both models use the same mount. There will be some more reliable performance seen with this model.
When it comes to imaging, they both have the same limitations – alt-az movement unless you incorporate a wedge. When you do, keep an eye on tracking inaccuracies caused by any excess strain due to the heavier loads you’re using. Experienced users can add GPS and take images of planets with a planetary camera.
The 6SE provides a good database of objects for beginners to find, and the NexStar+ hand control will help them get it done. Thinking about it this way for newbies negates the fact that it can’t be slewed manually.
The 6SE provides excellent value for its features and is more in line with what many beginners are looking for in an upgrade. Considering that it costs significantly more to upgrade to the next size up with the same mount makes the 6” model even more appealing. Is it lacking anything? Certainly not.
5. Orion StarMax 90 mm Tabletop
This will throw you for a loop. Not only is it significantly cheaper than what you’re probably used to seeing, this is a Mak-Cass scope and not an SCT. What’s the difference and why is it here? Here’s my thought process.
Pros & Cons
✔️ Mak-Cass design
✔️ Good for planets
✔️ Extremely portable
❌ Not for DSO observation
The StarMax is included in this lineup to open your mind to variety. Mak-Cass (Maksutov-Cassegrain) telescopes are different to SCTs but they’re still within the catadioptric family. The optical path is the same, but the optical components are slightly different. The good news is, you’re spending a whole lot less, gaining a lot more in portability, and you’re not compromising on planetary observational quality.
This little scope was made for seeking out the planets. Its slow focal ratio speaks to that and even though its aperture is 90 mm, the planets are bright – so you’ll be able to locate them if you know how to find them.
As you can imagine, as a small 90 mm scope with a tabletop mount, it’s wins the portability and lightweight aspect of telescopes of this type. Its mount is Dobsonian-style, so it swivels for azimuth movement and you can move the tube up/down for altitude motion.
What’s great about this scope is that it’s inexpensive enough to be used by the entire family including kids but it still has good quality about it that you’re not compromising on anything.
Since it does have a narrow field of view and small aperture, it’s not the best for large and faint DSOs. This is a great example of how different scope designs have strengths and weaknesses. The StarMax is a planetary rock star!
What to Look for in a Schmidt Cassegrain Telescope
Everything you need to know about a Schmidt-Cassegrain is laid out here. From how it works to what you can do with it, you can read the obvious and between the lines to get clued in.
How an SCT Works
As part of the catadioptric type of telescopes, an SCT uses both lenses and mirrors. In this case, you have a Cassegrain optical path and a Schmidt corrector plate.
You have a spherical primary mirror and a thin corrector lens (Schmidt corrector plate) that serves to correct for this expected spherical aberration. A convex secondary mirror, which also acts like a field flattener, is mounted to this very thin corrector lens.
It lays flat to the primary to reflect this cone of light back through a hole in the primary mirror (Cassegrain folded light path) where light rays fall onto a focal plane behind the primary. This formed image is seen through the eyepiece that is attached to the visual back on the end of the tube.
Now, where are they in relation to their positions in the tube?
The primary spherical mirror with the hole is mounted on the end of the tube where the visual back and eyepieces go – much like looking through a refractor. The secondary mirror and corrector lens are located at the top of the tube.
There is a long list of benefits to owning a Schmidt-Cassegrain telescope. Since they’re good for both visual and imaging with various accessories and equipment while being lightweight and portable, it’s no wonder why they’re very popular telescopes.
- Short tube length
- Good for viewing planets
- Easy to use/handle
- Visual back SC threading allows a variety of accessories to be used
- Can be fast and easy to setup
- Good for visual
- Good for imaging
- There are some drawbacks to consider as no one telescope does it all.
- Narrow fields of view
- Tend to be “slow” telescopes
- Large secondary mirrors
- Requires collimation
- More moving parts in the OTA
- *More expensive
- *SCTs tend to be more expensive than regular Newtonians with the same aperture. However, the unique benefits the SCT provides may be enough to justify the price jump on an SCT.
What’s nice about the SCT is that it can come in aperture sizes larger than a refractor, but it will be more expensive than a typical reflector like a Dobsonian. Why the difference? An SCT is extremely versatile.
You may find that an 8” SCT is a very popular size, they come in sizes from 5” to 16”. Obviously, the larger you go, the more expensive they become. 6” to 8” offers a good balance between versatility, size, and cost usually starting around $1000.
SCT Focal Length
Together, we’ve already established that the folded light path actually ends up providing a focal length that is much longer than the physical length of the tube. What exactly happens here?
Remember that convex secondary mirror? Not only is it reflecting light back through to the perforation in the primary mirror, it’s also adding power to it. See, when light bounces off the primary mirror, it comes to a focal point sometime down that focal path.
By placing the secondary mirror between the primary mirror and this focal point, the light cone created is at a much narrower and smaller apex angle than it would have seemed to have been without the secondary mirror where it is.
Because of this narrower light cone and magnification factor all created by the secondary mirror, the focal length is much longer but with the benefit of a compact tube.
As a result, focal lengths can be very long in an SCT. It is normal to see focal lengths anywhere between 1200 mm to 2800 mm.
SCT Focal Ratio
Closely related to focal length, the focal ratio can give you an idea of optical speed necessary to determine the field of view and image scale for imaging.
In a compact SCT, the primary mirror usually has a focal ratio of f/2 whereas the secondary mirror will likely have a focal ratio of f/5. This brings up the focal ratio to a final f/10.
The narrower field of view is excellent for viewing and imaging planets. Although contrary to popular belief, you can also get some DSO benefits out of it too. With narrow fields of view, you can seek out DSOs like planetary nebulae.
While faster refractors may be able to get a better look at DSOs, can they compete with the large aperture of the SCT? Fast Newtonians may be able to, but will it be good for imaging as well? While SCTs are often touted as do-it-all telescopes, it’s not without some compromise.
Depending on the type of imaging you’re doing, you can remove the secondary mirror from the tube and use your camera lens in its place. This will essentially turn your f/10 SCT into an f/2 SCT.
Some may add a Barlow lens and find it’s enough to reach focus on some slightly faster SCTs. Others may need to add a reducer to the mix. Some experimentation is called for, but users with this skill level are no strangers to patience and experimentation.
SCTs can be mounted to AZ and EQ mounts, but you’ll likely see them on fork-arm mounts with GoTo. This implies that they have computerized alt-azimuth motion. While the GoTo is good for imaging, you’ll be limited with the AZ movement. You may want to look for an equatorial wedge to add additional astrophotography benefits to your setup.
There are computerized EQ mounts with SCTs, but they can be very expensive because of the extreme precision and heavy-duty requirements.
If you’re not in a rush, you can always buy the SCT as a standalone buy and then choose the mount of your choice later down the road as the mount will determine your imaging capabilities.
How does a Schmidt Cassegrain Telescope Work?
Schmidt Cassegrain telescopes use mirrors and lenses to create a folded light path that provides a long focal length in a compact telescope tube. It involves a spherical, perforated primary mirror and convex secondary mirror with a very thin corrector lens.
Light enters the tube and bounces off the primary mirror and towards the secondary mirror which sits in between the primary mirror and the focal point that meets behind the secondary. The secondary mirror reflects towards the primary a narrow and less steep cone of light that enters the perforated part of the primary mirror.
This light cone falls onto a focal plane behind the primary mirror where an image is formed and seen through an eyepiece.
What is the Difference Between Maksutov Cassegrain and Schmidt Cassegrain?
Both Schmidt Cassegrains and Maksutov Cassegrains are part of the catadioptric or compound type of telescopes with very similar benefits and drawbacks. While they use the same light path, and so the optical system essentially yields the same results, they go about it with different materials.
The Schmidt-Cassegrain uses a spherical mirror, convex secondary mirror, and a thin corrector lens. They can be made larger in aperture.
Maksutov-Cassegrains have the perforated primary mirror and a corrector lens, but the lens is a thick, highly curved spherical lens called the meniscus corrector. Instead of a secondary mirror, the corrector has an aluminized spot that serves as the secondary mirror to reflect the cone of light through the hole in the primary mirror.
Maksutov-Cassegrains may come in smaller apertures than Schmidt-Cassegrains, but they can provide a brighter and sharper image depending on the quality and precision manufacturing of the corrector lens.
Do you Have to Collimate a Schmidt Cassegrain Telescope?
Yes, a good Schmidt-Cassegrain telescope should allow for collimation and the user must collimate it. Unlike refractors and reflectors that focus via the drawtube being moved in and out, the SCT requires the primary mirror to be moved forward and backward within the tube. The way the secondary mirror is held in place also raises collimation concerns. Vibration and temperature changes can be causes of misalignment.
Learning how to collimate an SCT is a must-have skill. You can also look for mirror lock features to help provide some alignment security.
It’s easy to see why SCTs are a popular telescope type among amateurs and experienced astronomers. They’re available at good price points with extra features that make them attractive. They’re exceptionally lightweight and portable. They also provide better aberration correction benefits over a wider field of view than refractors or reflectors alone.
SCTs are sometimes called the do-it-all telescopes, but with an in-depth view, we see that’s not true. However, if you know what you’re getting into and what you can get out of it, you can determine if the SCT is appropriate to fulfill your astronomy goals.