The William Optics 0.8 Focal Reducer II is a photographic-only focal reducer for small refractors such as the William Optics ZenithStar 66 SD APO or the Astronomy Technologies AT-66ED. This reducer also works with larger scopes like the Celestron 80ED with focal lengths up to 600mm. In terms of performance, on the smaller AT66ED, this reducer works quite well; it produces a flatter field than the telescope alone, with pinpoint stars out to the edges of the image in an APS (28mm) sized Digital SLR. I have more general information on telescopes in my review on Picking a Telescope.

Background

For telescopes, the focal ratio governs the fundamental performance of the instrument for how much it attempts to magnify an image of incoming light or how much it amplifies the brightness of the image. The focal ratio for a telescope is used a bit differently from a camera's because they don't have an iris, but instead have a fixed frontal aperture.

The focal ratio for a telescope is therefore defined as the front diameter divided by the focal length. So, a telescope like the time tested Celestron NexStar 8SE has a 203.2mm diameter and a 2032 focal length, so its focal ratio is 2032mm/203.2= f/10, and if you attach an SLR camera to it, it behaves like a 2032mm telephoto camera lens set at f/10. Unlike a camera lens, which changes aperture to change the focal ratio, telescopes change the f number by focal length change. So, for example, installing a 2X Barlow Lens increases this to f/20, at which point the telescope will behave like an f/20 lens. To go in the other direction requires a focal reducer, which needs to shorten the focal length to change the focal ratio, in this case to f/6.3.

The driver for focal reducers in telescopes is the desire to produce a brighter image able to capture dimmer objects. A change to lower the focal ratio can mean an astrophoto only goes to 14th magnitude stars instead goes to 16th, A series of astronomical photos can mean hours of work to get a usable series of exposures after adjusting the mount for perfect tracking, careful focusing, and producing test images. The ability to get a brighter image in less time is an enormous benefit since it means more objects can show up in the same series.

The actual change in brightness the camera sees as a function of focal ratio is proportional to the inverse square of the f number. So, for example, at f/6.3, the image is 2.52 times as bright as an f/10 image.

For the William Optics ZenithStar 66 SD APO or the Astronomy Technologies AT-66ED, the initial focal ratio is a comparatively fast f/6, so the reducer which only changes the focal ratio by 0.8 of its original value takes the system to f/4.8, but the change in brightness is an additional 56%.

Note the following observations about this process:

(1) Changing the focal length of the optic means the magnification and field of view are changed with the image brightness.

Example A: use a barlow to double the focal length:

Magnification: 2X over original magnification. If the original magnification was 14.7X, the telescope will now be at 29.3X, so objects are twice as large in the image.

Field of View: Field of view is half of the original field, so if it started at 3.4 degrees, it is now at 1.7 degrees.

Brightness: The change in brightness moves with the inverse square, so if you had started at f/6, the system is now at f/12, with 1/4 the original brightness. Or, to put it another way, four times the exposure would be needed to get the equivalent brightness seen at f/6.

Example B: use a Focal Reducer to shorten the focal length by 20%:

Magnification: 0.8X over original magnification. If the original magnification was 14.7X, the telescope will now be at 11.7X, so objects are 80% as large in the image.

Field of View: Field of view is 25% larger the original field, so if it started at 3.4 degrees, it is now at 4.3 degrees (in astronomical terms, both of these are large fields of view).

Brightness: The change in brightness moves with the inverse square, so if you had started at f/6, the system is now at f/4.8, with 156% of the original brightness. Or, to put it another way, 64% of the exposure would be needed to get the equivalent brightness seen at f/6, but the same exposure used at f/6 now yields the equivalent of a 56% longer exposure.

(2) The native focal ratios for telescopes tend to be a little slow by camera standards, with f/5 being the floor for conventional telescopes. Only specialist photographic designs have focal ratios as low as f/2.8.

(3) While Barlows naturally flatten the image, focal reducers tend to produce a curved image, or fisheye effect. In an astronomical image, this will cause stars towards the edge of the frame to be distorted into arcs. A well designed focal reducer also acts as a field flattener, so the entire field of an image will be distortion free whether you look at the center or the farthest corners.


William Optics appeared on the astronomy scene in 2000 and quickly garnered attention since everything they built had clearly superior fit and finish, previously only seen in the most expensive makes of telescopes such as Astro Physics, Tele Vue, Takahashi, and Stellar Vue. The company was having their hardware made in Taiwan, and early models had a lot of commonality with StellarVue for focusers and smaller diameter optics. The ball really started rolling with a product called the Megrez 80, which combined a well made ED achromatic telescope with a well made optical tube. This combination of good with good entered a world of extremes, where if we were talking cars, there were Neons as the low end refractors, MacLarens as the apochromatic refractors, and the Sport Utility of the world was the Schmidt Cassegrain telescope (SCT). But there was nothing out there combining very good performance and quality for someone who wasn't quite prepared to spend the astronomical money for the very highest end until William Yang started William Optics.

Since then, their product line has undergone rapid development with a large number of telescopes and eyepieces, all of which have superb build quality and solid, straightforward designs. They have also been archetypes of pragmatism over snobbery by incorporating some neat tricks to make their hardware usable outside their own product line. For example, the back end of the SD66mm telescope's focuser termiates in a Schmidt Cassegrain thread rather than a simple 1.25" collar. Because of this, a small telescope which is easy to piggyback on a telescope like the Celestron CPC 8" SCT can use the same accessories as the larger telescope, making it extremely easy to swap roles between being a guide scope or being a wide angle photography scope.

This has been such a successful design, it has been picked up by siblings of the William optics line, such as the Astronomy Technologies AT-66ED, which I have been using the Willaim Optics Field Flattener II with, including operation piggybacked on a Celestron NexStar 8 GPS SCT.

Description and Usage

I ordered the William Optics Field Flattener II directly from William Optics since I had a problem: I had looked at the data sheets, and though the focal ratio, diameter, and focal lengths matched between my Astronomy Technologies AT-66ED telescope and the William Optics ZenithStar 66 SD APO, I didn't know if the Focal Reducer II would be compatible (if the focuser had a different range, for example, it would be impossible to come to focus). The fellow at William Optics did not know, but said if I tried it and there was a problem, they would give me a full refund with the return, and they did want to know if it would come to focus. So, I ordered the focal reducer, and had it within three days (when they mean same day shipping, they aren't kidding).

The reducer came well packaged in a black box with padding inside designed for long-term use. The reducer comes attached to a 2" focuser adapter, which has a conic section so if you put it into a standard set screw focuser like on the back of a Celestron 80ED, the collar will pull itself in tight against the back of the focuser for perfect alignment. The front end of the collar accepts standard 48mm thread filters used with 2" sized astronomical eyepieces.

The focal reducer itself is in two sections with SCT thread attaching a locking rotating collar on the front end. The back end is the reducer itself, which terminates in standard T-thread used with SLR cameras. The focal reducer can therefore attach by using the 2" collar adapter, or you can remove the 2" collar and thread it directly to SCT treads, like those found on the back end of the At66ED, or on William Optics' ZenithStar 66 SD APO telescope.

Unlike the Celestron 0.63 reducer for Schmidt Cassegrains, which uses a long T adapter tube between the reducer and a camera, the Field Flattener II attaches directly to the camera's T-adapter ring. As a result, the reducer is a short connection between the camera and the back of the telescope. I can't be sure if it is coincidence, but when mated to the Minolta Maxxum 7D I have been using, the locking screw for the reducer's camera rotation feature is clocked straight up at the front of the camera, the most convenient location possible.

When connecting to a telescope, the most convenient connection is with the 2" collar, since this just slides into the back of a 2" focuser and locks in place. This is also the easiest way to use a filter with the reducer, since the front of the 2" collar is threaded for standard 2" format filters. Attaching to SCT threads is more difficult since the adapter has to screw on to the back of the telescope. You can choose between threading the reducer on and then locking your camera to the back, or attempting to thread the reducer while the camera is attached. The advantage of attaching the camera to the reducer is the front of the camera is protected from dust while making the connection, but the camera rotation collar is stiff enough to make using it to thread on impractical, so you end up spinning the whole camera.

The biggest problem with using the reducer with its SCT connection is the difficulty of inserting filters, such as Hydrogen alpha filters to block light pollution and photograph the ionized star forming gas in nebulas. The problem is simply there isn't anything to attach it to. The threads in the back of the filter can attach to 1.25" eyepieces, and the filter is just big enough to cover the entire field of an APS (28mm) sized camera, but there isn't an interface to attach to. The best I have been able to come up with for this system is using a piece of dense hobby foam to make a squeeze-in filter support. Using that method, I have been able to take a photo with Hydrogen Alpha and Oxygen III filters, which is a good illustration of what this reducer does:

http://www.buytelescopes.com/gallery/view_photo.asp?pid=15288&c=35913

The first thing I would like to draw attention to is the field of view this focal reducer produces is flatter than the field the telescope has without it. A good example of the field without the reducer is seen here:

http://www.buytelescopes.com/gallery/view_photo.asp?pid=10915&c=35913

In the second case, there is some image stretching thanks to field curvature visible in the upper left corner. In comparison, the wider field produced with the 0.8 reducer/field flattener has sharp pinpoint stars all the way to the edge of the frame.

In the photo I have from the reducer, I have been restricting the light coming into the camera with the filters for hydrogen alpha and Oxygen III, and the large hit to total brightness is fairly easy to see. For completeness, I am planning a series using the reducers and the less aggressive Baader Neodymium filter, which should produce a brighter image, though I expect light pollution to take a heavy toll on the image quality. As I contemplate how the only only person in my neighborhood to have had a break-in was the joker with a brilliant blue-white mercury vapor streetlight outside his house, I can only despair at the irony of how we have an energy crisis largely fueled by this provably incorrect approach to security.

However, given the sharp results of this image, I am planning to make the reducer a standard part of the imaging setup using the 66mm telescope. The other configuration I am planning to try out is attempting to use the reducer with the Celestron 80ED. This setup will have this mechanically simple set up with a focuser rotator for the first time, which will make framing photos much easier. for example, Orion is at an angle in the sky, so turning the camera is necessary to line up the image in a direction to make the best use of the frame. The biggest change for this telescope will be going from its native f/7.5, 600mm focal length to a much brighter f/6, 480mm focal length configuration. One benefit here is the telescope has a 2" focuser, so 2" sized filters will just attach to the front of the focal reducer's 2" mounting collar instead of having us use my ad-hoc internal filter arrangement. As soon as I have had a chance to try out this setup, I will post an update to this review with the results.

The flexibility of the William Optics 0.8 reducer largely comes from giving a camera enough light for you to choose the exposure and ISO settings to optimize a photograph rather being forced to go to the maximum sensitivity and hoping for the best. Unfortunately, astrophotography is a time-intensive activity, so with the large number of possible setups, I will only be able to try out a sampling of them on a smattering of objects unless I get an observatory built so I can leave a mount and telescope set up between nights. In the meantime, I will continue to take photos of the night sky as I am able. And I expect the AT66ED with the 0.8 focal reducer to be a star in this effort.

Conclusion

If you are getting serious about astrophotography, focal reducers are an enabling piece of equipment for dim objects in the deep sky because they make it much easier to get dimmer details to appear. The William Optics 0.8 Focal Reducer and Field Flattener is beautifully made, easy to set up, and works very well. The only complaint I can imagine for it is the difficulty in inserting filters when using the small AT66ED. But in reality, what I really need to do is get this combination out into the countryside where it can perform to its true potential. I'll post an update when I get that done.

Recommended: Yes