Focal reducers and flatteners are useful tools for astrophotography with refractor and Cassegrain-type telescopes. While many focal reducers are available and many seem very promising on the surface, it is important to understand what separates a good one from a borderline scam and what you can expect these devices to actually be capable of. No telescope or accessory can defy the laws of physics, try as the manufacturers might to convince you otherwise.
What a Focal Reducer Is & Isn’t
It is important to understand what focal reducers can and cannot do.
Focal reducers aren’t magic. Fundamentally, taking the light cone coming out of a telescope and compressing the rays is not an easy task to do, let alone with a cheap, simple, or compact series of lenses. A common mistake made by beginners is to buy a cheap 0.5x focal reducer and attempt to screw it onto their eyepiece or camera. At best, you will get a vignetted and low-quality image; at worst, it probably won’t reach focus at all. Not only is 0.5x too extreme in most cases to ever work, but the optics required to do so would be very expensive and bulky. The focal length and f/ratio being reduced also affects aberrations like coma, astigmatism etc.
Scaling the focal length back will reduce the illuminated area of the focal plane by some amount, which is one of the big reasons aggressive focal reducers and/or those that attach directly to an eyepiece are usually out. Even in the best cases with quality focal reducers, you won’t be able to use an f/6.3 SCT reducer with a 2” eyepiece without vignetting, and many refractors will vignette with larger camera sensors if used in conjunction with a focal reducer. This is simply due to the physical path light has to travel through the telescope and focal reducer. Widening the corrector, baffles, and focuser inside the telescope can alleviate this issue, but it comes with other compromises, the biggest being cost.
Focal reducers designed for more than one type of telescope should pretty much always be avoided, and visual focal reducers for non-catadioptric telescopes are essentially not viable. The only focal reducers that are actually worth buying are those for refractors, Ritchey-Chretien telescopes, and Schmidt-Cassegrains, and most are primarily designed for photography or for niche visual purposes such as binoviewers.
So in any event, if you get a good one, what do these devices accomplish? Well, a few things.
- Faster f/stop – Good focal reducers are usually 0.6x to 0.8x and will thus speed up the normally “slow” f/8 to f/11 Schmidt-Cassegrain telescopes making them more suitable for astrophotography. Likewise, you can turn an f/8 refractor into something a little faster, provided you don’t run into vignetting issues with your camera.
- For night vision or EAA use, lower-quality or more aggressive focal reducers are not as much of a problem due to the limited resolution of many of these devices, and serve to provide brighter images with less gain or exposure time.
- Shorter focal length – For visual use, an f/6.3 or f/7 SCT reducer can be used to achieve a similar field of view with wide-angle 1.25” eyepieces as you would get with a very low power 2” eyepiece. This used to be popular when acceptable 2” eyepieces were more expensive as it also allowed for a secondary use for the reducer. However, for higher magnification viewing you need to unscrew the reducer and then greatly adjust the telescope’s focus afterward which can be annoying, and 2” eyepieces are a lot cheaper nowadays.
- 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 are of an aplanatic design such as a Petzval or EdgeHD variant respectively, have an issue known as field curvature, where different parts of the field of view focus at different points. The effect is 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 field curvature by essentially inducing the opposite amount of field curvature, which cancels out the field curvature of the telescope.
Our Best Focal Reducer-Flatteners Recommendations
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.
#1. Starizona SCT Corrector 4 – 0.63x Reducer and Coma Corrector
The Starizona SCT Corrector is the least aggressive focal reducer for SCTS, as it actually 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 will work 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.
#2. Best for EdgeHD/ACF – Optec Lepus 0.62x Reducer
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.
#3. Best for Visual – Baader Alan Gee Mark II Telecompressor
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.
#4. Best Value: Celestron f/6.3 Focal Reducer Corrector for SCT Telescopes
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.
#5. Cheapest: Astromania f/6.3 Reducer Corrector for C Series Telescopes
The Astromania f/6.3 reducer is quite simply a generic clone of the Celestron reducer with worse 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.
Refractor & Ritchey-Chretien Reducer-Flatteners
Ritchey-Chretien Cassegrain telescopes produce a similarly curved field to refractors. As such, a flattener or reducer-flattener may be warranted, and what works for refractors almost always works with these scopes as well.
Many refractors, especially those with more complicated optical designs such as Petzvals, are best with their own dedicated reducer or reducer-flattener. Manufacturers usually provide information on where to obtain these on the product page for their telescopes.
#1. Starizona Apex ED 0.65x Focal Reducer & Field Flattener
Starizona’s 0.65x reducer-flattener is expensive, but the 0.65x speed reduction is extremely useful and works well even with faster refractors. APS-C and smaller sensors up to a 30mm image circle work just fine, though you almost always need at least 55mm of back focus with these scopes. The S version is best with short focal length refractors of 90mm and smaller apertures while the L version works with larger and longer focal length scopes. The Apex also works well with the Meade ACF aplanatic Schmidt-Cassegrain telescopes, though not Celestron’s EdgeHD.
#2. Best Value – Orion 8894 0.8x Focal Reducer for Refractor Telescopes
Orion’s 0.8x reducer-flattener is a bargain option which works well with most slower refractors above f/6.5 and most smaller sensors such as APS-C. The exact fully illuminated field will depend on your telescope’s specs such as focuser design, back focus and focal length/f-ratio.
Orion also sells a dedicated flattener for use with faster refractors from f/5 to f/7.5 which features similar performance for APS-C and similar sized sensors.
#3. William Optics Adjustable 0.8x Focal Reducer & Field Flattener
While sold specifically for William Optics telescopes, the William Optics 0.8x reducer-flattener works well with most refractors. It works with a wider range of f/ratios and larger sensors than the Orion model, though it will vignette a little with a full-frame sensor depending on your spacing, focal ratio of the telescope, and other tolerances.
#4. Long Perng 2” 0.6x Field Flattener / Reducer for f/5 – f/7 Refractor Telescopes
Long Perng’s 0.6x reducer-flattener isn’t quite as sharp as the more common budget 0.8x units but works well if you don’t mind having to crop part of your images.
#5. SVBONY SV193 Focal Reducer 2 Inch 0.8X Field Flattener
Similar to the Orion 0.8x reducer-flattener but with often debatable quality control, the SVBONY SV193 reducer-flattener works, but like Astromania’s SCT reducer the amount of defective units shipped seems to be unusually high and you get what you pay for.