Pros: Compact, easy to use, relatively inexpensive, does what it claims
Cons: No real cons
The Bottom Line: The Reducer Corrector lets a Schmidt Cassegrain Telescope become a wide-angle telescope with a nice flat image and no residual color at 10% of the cost of a second scope.
Pirich's Full Review: Celestron 94175 (050234941754)
This device is an accessory for use with Schmidt Cassegrain Telescopes. It is intended to convert the telescope into a shorter focal length wide-angle field of view telescope.
Celestron advertises the f/6.3 focal reducer as being the one accessory needed to turn one of their f/10 Schmidt Cassegrain Telescopes ( SCTs ) into another telescope. In fact, they even claim its potency is so great that a shorter focal length version of their telescopes is redundant, so they don't make them since one of the f/10 telescopes can be turned into the other telescope at will at 10% or less of its cost.
I finally broke down and got one of these and I've spent over a year experimenting with this device on a 5" C5 and an 8" diameter NexStar 8GPS.
Background
One of the problems in observing is the built-in specialization in telescopes. A single optical system can be optimized for wide-angle views, but then will have difficulty reaching high magnifications. On the other end, a telescope designed to perform well at high magnifications can't go down to low power and wide fields of view.
One partial answer is using a Barlow lens, which can boost the effective focal length by a factor of two to four to make it behave as if it were a longer focal length telescope. However, the extreme curvature in a very short focal length telescope with large aperture produces errors in the optical figure, which just get magnified with the image. So, there is a limit to the practical magnification unless the telescope has extremely perfect (read incredibly expensive) optics. Or, to put it another way, magnification boosting with a Barlow lens works better on something with a moderate focal length.
The SCT has an intermediate focal length at f/10. Putting a regular 2X Barlow lens in one immediately turns it into an f/20 telescope with a narrow field of view, and lets an observer use eyepieces with lower focal lengths (like a 15mm eyepiece in stead of a 7mm eyepiece). This works fairly well, but the remaining problem is getting the telescope to widen its field of view.
Generally, devices to do this are called focal reducers, since they are designed to pull in the focal length of the telescope. A second class of similar devices is what is called a field flattener. Unlike a focal reducer, the primary objective of a field flattener is to take fish-eye type curvature out of the image. These are more often seen for refractors with very short focal ratios, which have built-in curvature in their image. The intent is to prepare the light path to focus on camera film or on a CCD.
The reduce/ corrector is a set of lenses in a metal collar which threads on to the back of the telescope before attaching a visual back, for doing visual observing with the eye, or attaching a camera. This device is to shorten the focal length of the telescope and reduce magnification. The difficulty in doing this is squeezing a larger image into a smaller area tries to curve it, or give it a fisheye effect. The reducer corrector corrects to take the distortion out of the image, so it reduces focal length and corrects image distortion in one step. So, there is no need for a separate field flattener.
Description
The Celestron Reducer/Corrector is a collar a bit larger in diameter than the visual back, or eyepiece holder, which usually is threaded onto the back. The corrector has a threaded socket on the bottom and a threaded collar on the top identical to the set of threads on the back of a C5 or C8 tube. Larger Celestron tubes have a collar built in to the back to accept even larger devices, and this threads onto that.
The collar has a grip around the outside to make threading easier. If you are going to attach an eyepiece, the visual back threads straight on to the focal reducer after it is threaded on. It's important to take care to make sure everything is well attached. I have threaded on the focal reducer, put on the visual back, and then had strange vigetting on one side of the image until I realized I hadn't threaded the visual back all the way on.
The unit is very compact and is obviously of very sturdy construction. It is clearly intended to be able to survive years of use. It comes with lens caps for both sides to collect the large aperture lenses inside. The lenses have green reflections indicating they are multi-coated to reduce internal glare.
Observations
I had thought at first this would be something I would be trying to put on and take off repeatedly in a night of observing, but this is a little difficult to do in the dark since I want to be careful not to cross-thread the reducer in place. The other worry is accidentally dropping the device while doing this. I have found I will use both hands to thread and remove the focal reducer.
One thing to keep in mind is the focal reducer and either a visual back or photographic attachment collar (T thread, usually) are threaded the same way, so you need to be careful what you are actually turning when stacking on the focal reducer. Here, it is fairly clear the rubber grip on the reducer is intended to make it easier to hold on to while adding or removing other accessories downstream from it. This said, a little care and this is no issue. I am pretty sure one drop on to concrete would be the end of the reducer, so I am always careful when doing this.
Visual Use
The reducer is often thought of as a photographic accessory, but it also works very well with eyepieces. The effect is very much the same as if the telescope really was built with a shorter focal length . The main difference is it starts vignetting the field of view with eyepieces over 32mm focal lengths.
When looking at objects like the Pleiades in a C5 without the reducer, they barely fit in the field of view with a 40mm eyepiece. The reducer takes the focal length from 1250mm to 780mm, so any eyepiece will operate at lower magnification with a wider field of view. With the focal reducer in place, the Pleiades fit in the field of a 25mm eyepiece. At first glance, it seems the 25mm and the 40mm are the same, since 25mm/.63= 39.7, but a 25mm plossl eyepiece has a wider field than the 40mm plossl (52° vs 46°), so the resulting true field is larger (1.7° vs 1.47°). With a 32mm eyepiece, the field is equivalent to using a 50mm eyepiece, a 2.1° true field.
The resulting images are brighter as well as being wider. There isn't obvious distortion in the image- in a lot of ways, it really is like having a second telescope appear at 1/5 the cost of the optical tube. In the past, Celestron did make an f/6.3 version of the C5, which had the problem of not being able to go up in magnification since its large secondary obstruction degraded the image.
The results are much more compelling with larger telescopes such as the 8" SCT . I used this device with a NexStar 8GPS telescope. With the 8" SCT , the focal length of 2000mm means even low power eyepieces don't give a magnification low enough to put the Pleiades or Double Cluster into a field of view. Unlike the C5, where the extra field is nice to have but not really needed, it changes everything for the C8 as many objects normally unable to be seen at once will now fit into a single field of view. Star clusters such as the Pleiades and the Hyades are now visible with the 156% greater light gathering capability of an 8" telescope.
Using the focal reducer with a C8 really is like having a second telescope. The most serious limitation of a 2000mm focal length is the narrow field of view it enforces. Being able to step back to 1280mm puts the telescope very close to the C5's focal length and field of view, but with much greater light gathering ability. Especially out in the countryside with dark skies, the focal reducer makes it possible to pull in large star clusters and nebulas otherwise too large in scale to see at the same time.
Photography
When trying to photograph an object such as a planet, the moon, or the sun, there really is no reason to use the focal reducer. The largest of these are only a half-degree across, and so they will readily fit into a single field of view for SCTs up to 12' in diameter.
The picture changes for deep space objects, such as nebulas , galaxies and star clusters. All of these have very low surface brightness compared to planets and other solar system objects. Many of them span large sections of the sky. Since greater magnification reduces image brightness, stepping down the focal length brightens up images for cameras. This allows brighter images to be recorded with lower exposure times, and also lets dimmer parts of an object shine through as their light is concentrated.
The results I have had taking photographs with the focal reducer have been very gratifying. On the Pleiades , the reducer made it possible to get the entire cluster into a smaller area on the C5. On the C8, the biggest treat was being able to get Double Cluster into a single frame. The best part is even stars out at the edge of the image are little dots instead of being elongated by image effects. In other words, the best thing about using this device is it is passive- you just put it in and operate everything as you otherwise would.
The only criticism I could arrive at would be wanting to go to a lower focal ratio for even larger objects, but this isn't really a problem with the NexStar 8GPS since it is designed to convert to an f/1.95 telescope by removing the secondary mirror and installing a Fastar lens for a CCD camera to see the primary mirror. So, if you really get into wide-angle photography, there is a way. It's a lot more expensive than the focal reducer, so it'll be a while before I seriously think about doing that.
Summary
The Celestron Reducer simply screws on the back of any Celestron SCT and instantly drops its focal length to 63% of the original. As claimed by the manufacturer, it doesn't put in false color or give the image fisheye curvature. It is easy to use- for all practical purposes you have nothing unusual to do while using the reducer. The only difference is in the performance of the telescope.
In performance, it does just what it promises. The effect of having the focal length reduced is more noticeable in larger telescopes where specific objects go from being too wide to see in one image to fitting comfortably in a field of view.
Photographically, it makes getting larger objects in a single frame much easier since even if an object would have fit in a single frame previously, the focal reducer makes it possible to have a brighter image of dim objects like galaxies, nebulas , and star clusters so it is possible to get a photograph with a shorter exposure, or get a brighter image on the film or CCD with the same exposure time. The fact there is no residual color or distortion makes using this device worthwhile.
Though it isn't quite the same as having two telescopes in one, it is very close.
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