Telescope Accessories Buyer's Guide: Focusing on Focal Reducers & Flatteners
Who needs magic when you've got science? Focal reducers and flatteners may not be the most glamorous additions to your astro gear, but these useful tools can take your refractor or Cassegrain-type telescope to the next level. Let's dive into the mysterious world of focal reducers and examine what they do, how they work, and how to choose the right one for your setup.
What's a focal reducer and what isn't?
First things first: a focal reducer isn't a magic wand that will instantaneously pluck those elusive deep-sky objects from the heavens. Fundamentally, it's a device that contracts the light cone coming out of your telescope, making the stars and galaxies appear closer together. However, this task isn't as simple as screwing on a cheap, compact set of lenses. Be warned: beginners might be lured into snatching a cheap, 0.5x focal reducer and slapping it onto their eyepiece or camera, only to end up with a vignetted, low-quality image. Or worse yet, they might not even be able to get it into focus! The truth is, 0.5x is too extreme for most situations, and the optics required to achieve this would be extremely expensive and bulky. The focal length and f/ratio being reduced also impacts aberrations such as coma and astigmatism.
So, what can we realistically expect from a focal reducer?
A faster f-stop
Good focal reducers usually have a reduction factor of 0.6x to 0.8x, making those "slow" f/8 to f/11 Schmidt-Cassegrain telescopes faster and more suitable for astrophotography. Likewise, you can turn an f/8 refractor into something a little speedier, provided you don't run into vignetting issues with your camera. For night vision or EAA (Electronic Astronomical Imaging) use, lower-quality or more aggressive focal reducers serve to provide brighter images with less gain or exposure time.
A shorter focal length
For visual use, an f/6.3 or f/7 SCT reducer can be employed to achieve a similar field of view with wide-angle 1.25" eyepieces as you would get with a very low power 2" eyepiece. In the past, this was popular when acceptable 2" eyepieces were more expensive; it also allowed for a secondary use for the reducer. However, for higher magnification viewing, you will need to unscrew the reducer and greatly adjust the telescope's focus afterward, which can be somewhat annoying. These days, 2" eyepieces are much more affordable. For astrophotography, a shorter focal length from using a focal reducer of course gives you a wider field along with a faster f/stop.
Field flattening
Refractors and Schmidt-Cassegrain telescopes, unless they're of an aplanatic design (such as a Petzval or EdgeHD variant respectively), typically suffer from field curvature—a problem where different parts of the field of view focus at different points. This often manifests as a "fishbowl" or "barrel" effect, with blurry stars towards the edges of the field of view. Most eyepieces compensate for this inadvertently, but a camera cannot. A side effect of most good reducers (which is why they are referred to as reducer-flatteners) is that they usually fix this issue by essentially inducing the opposite amount of field curvature, which cancels out the field curvature of the telescope.
Our top focal reducer-flattener recommendations
Now that we've covered the basics, let's take a look at some of the best focal reducer-flatteners out there for your Schmidt-Cassegrain telescope.
Schmidt-Cassegrain focal reducer-flatteners
With the exception of the Optec Lepus 0.62x, none of the reducers listed here are compatible with EdgeHD or Meade ACF aplanatic Schmidt-Cassegrain telescopes. Celestron sells EdgeHD reducers for each size telescope, while Meade ACF owners will need either the Optec Lepus or the Starizona Apex reducer.
The Starizona SCT Corrector is the least aggressive focal reducer for SCTs as it only converts most SCTs to around f/7 (except the Celestron C9.25 and C11) due to the position of the primary mirror and corrector required to reach focus. However, it is the only corrector with a 27mm illuminated image circle, which works okay with APS-C sized camera chips and the widest-angle 1.25" eyepieces, along with even some 2" eyepieces, without appreciable vignetting. It also does an excellent job removing field curvature and coma with minimal effects on resolution with a high-power eyepiece for visual use.
The Optec Lepus 0.62x Reducer is the only one in our list that works with both aplanatic EdgeHD or ACF scopes. It is designed to perform best with aplanatic Schmidt-Cassegrains but works acceptably with regular SCTs too. It delivers a sharp, 22mm image circle and has plenty of back focus. However, this reducer is not designed for visual use and is purely a photographic accessory.
The Baader Alan Gee Mark II Telecompressor puts the corrector lens assembly inside your Schmidt-Cassegrain telescope's baffle tube, reducing vignetting issues and allowing it to be a little more aggressive with a 0.59x reduction factor. It will have absolutely no vignetting issues with 1.25" eyepieces and is a little more forgiving with camera sensor size while also being, of course, a little bit faster than the standard 0.63 to 0.7x. The 18mm fully illuminated field for imaging purposes does, however, limit you to smaller sensors.
Celestron's f/6.3 reducer-corrector is designed for their regular Schmidt-Cassegrains (dedicated f/7 reducers for each EdgeHD model are also available), and it works with non-ACF Meade Schmidt-Cassegrains as well. It's not perfect and doesn't do a great job flattening the field, but for casual imaging purposes and for visual observers looking to increase the field of view with 1.25" eyepieces, it does the job. It will vignette with APS-C sensors quite a bit, however, and only work well with smaller camera chips.
The Astromania f/6.3 reducer is quite simply a generic clone of the Celestron reducer with notoriously poorer quality control between units. If you are willing to take this risk to save money, it is a bargain, but you can easily get a defective product.
Remember to choose a focal reducer that's compatible with your specific SCT model and provides the correct back focus distance for your camera setup. A focal reduction of 0.63 to 0.8 is common, but you can find reducers with different reduction factors depending on your needs. Consider the image quality, as well—opt for a flattener that provides uniform illumination and minimizes distortion across the field. With these factors in mind, your Schmidt-Cassegrain telescope can take you on even more enchanting journeys across the cosmos.
Happy stargazing! 🌟🔭🚀
- Understanding focal reducers and flatteners is essential for enhancing the performance of refractor and Cassegrain-type telescopes in astrophotography.
- A focal reducer is a device that contracts the light cone coming out of your telescope, making the stars and galaxies appear closer together without being a magical wand.
- Good focal reducers usually have a reduction factor of 0.6x to 0.8x, making telescopes faster and more suitable for astrophotography, and providing sharper images with reduced field curvature.
- For visual use, an f/6.3 or f/7 SCT reducer can be employed to achieve a similar field of view with wide-angle eyepieces as a very low power 2" eyepiece, while for astrophotography, a shorter focal length from using a focal reducer gives a wider field of view along with a faster f/stop.
- Field flattening is a feature of good reducers that usually fixes field curvature, a problem where different parts of the field of view focus at different points, often caused by refractors and Schmidt-Cassegrain telescopes.
