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Focal Reducer for Meade 7" MAK

Posted by Steve Hollenbach   06/18/2008 07:00AM

The Meade 7” Maksutov, an Experiment...ooooh
Steve Hollenbach

I’ve been an amateur astronomer for about thirty years, and now live in a light polluted Phoenix suburb called Apache Junction. Even so, the night sky still has a lot to offer. My interests cover the planets and large deep sky formations. I’ve written twice before on this scope or I should say OTA as I use a German Equatorial mount. Rather than reiterate what others have said let’s just say the Meade 7” MAK is a great instrument, and you don’t need a second mortgage to buy one.

The OTA:
The 7” aperture of my “Big MAK” is small enough for beating turbulent air and big enough to let some light get through in the light polluted suburbs. It has a fair amount of focal length; 2670mm. As you would expect from a Maksutov, it has the ability to magnify, but a narrower field of view than most popular 8” SCTs. That’s easy to solve with a focal reducer, and that’s today’s subject.

The Experiment:
How to get the maximum possible True Field Of View (TFOV) from the Meade 7 “ Maksutov

Why do this at all? I chose a larger MAK based on the clarity they offer. The focal length lends itself to magnification. Rather than adding a Barlow to a short focal length OTA, I wanted less glass in the light stream when viewing the moon and planets. The clarity there was important to me. What about the rest of the sky? A focal reducer would work, but most reducers are built as a thread-on unit. I wanted a barrel. A surprising number of people told me I was barking up the wrong tree. However it made sense to me to use the extra glass for large deep sky objects on those few occasions, rather than using a Barlow all the time for the planets. I have a 60mm finder, but the aperture falls short for a really good view.

Exception to the Rule
The Pleiades, the Vail Nebula and the Andromeda galaxy are examples of the few things we have difficulty with in long focal length scopes. For most of us the desire to view those objects in their entirety is compromised by the equipment we need to see the rest of the heavens. Even wide angle eyepieces fall short of encompassing very large celestial objects. Because seeing these was only an occasional activity, it was considered an exception when I chose the scope.

Rule for the Exception
Only a few formations are so large as to pose some difficulty. Long focal length scopes like Meade’s 7” MAK at 2670mm offer a challenge in that regard. The Meade 56mm 4000 series eyepiece worked very well on the double cluster with room to spare. Just to be fair the TeleVue 41mm Panoptic was slightly wider, but having the Meade in hand I was satisfied. A long EP can do a lot, but another way was needed to beat what was readily available. I had to intercept the light cone well ahead of the eyepiece. I needed a focal reducer.

The Experiment:
The Celestron 6.3 focal reducer intercepts the light cone just as it exits the OTA. When combined with a TeleVue 27mm Panoptic EP it gave me about 1.2 degrees TFOV without any serious vignetting or coma. By using a spacer between the EP and the diagonal, Vega took 6 minutes 12 seconds to transit. Granted it’s well north of the equator, but it worked out to about 1.5 degrees. However, the edges were starting to break down. I knew I was near the practical limit for that OTA.

If you use the transit time of an object to judge your True Field Of View (TFOV) remember the further north or south of the equator you go the longer the object will remain in view. If you wanted to time the transit of Polaris, you’d need more than a stopwatch! Objects at the equator work best.

The Celestron and the nearly identical Meade focal reducers have to be screwed onto the visual back. I wanted a convenient “slip-in” unit like an eyepiece. Once the limit of the field was established, then I could refine it to find the best TFOV. In other words how much I could get and still have a good flat crisp view.

The Celestron 6.3 focal reducer with the 27mm Panoptic on a spacer was amazing. The seven primary stars of the Pleiades fit nicely into the FOV and as it reached the zenith some nebulosity showed even here in the 'burbs. It just wasn’t well framed, and the mechanical process of installing and removing the reducer in the dark was far from enjoyable. I still sought an easy to use reducer that fit like a Barlow; an “Anti-Barlow”.

I experimented with two Antares focal reducers; their .50 and .70 in the 2 inch format. These are meant for imaging, but can be used visually. They also have 48mm filter threads. The .50 turned out to be not enough when placed at the EP and too much when screwed onto the diagonal. The .70 did the trick. The following parts are available from Scope Stuff and possibly other vendors:

Anteres 2" x.70 focal reducer #FR27
Filter Threaded Eyepiece barrel extender #EPE2
2 inch Eyepiece Barrel Extension #EEX2

With these three components an Anti-Barlow or X .70 Reducer can be constructed. All the components are high quality and total cost in 2007 was about $130.

With the “Reducer” between the eyepiece and the diagonal you get about a x.67 reduction in focal length. With it between the OTA and the diagonal you get about a x.36 reduction. Removing the spacer or screwing the reducer onto the filter threads of either the diagonal or the eyepiece reduces the effect. So, many combinations are possible with the same optical element.

With my trusty 27mm Panoptic EP and the Reducer assembly at the OTA end, I was able to time the transit of Mars in January at 7 minutes 21 seconds. Keep in mind Mars was near the ecliptic and not the equator. Some difference, but it worked when viewing the Pleiades. All the associated stars were well within the TFOV.

Using Mars as my unit of measure, there was significant vignetting at about 6 arc minutes from the edge and coma starting at about 1 arc minute from the edge of field. At 24 arc seconds from the edge there was noticeable diffraction. Within 10 arc seconds of the edge the view really broke down. This really only makes a difference when you try viewing an object right at the edge. With an estimated 1.7 degrees TFOV the defects were so far out at the edge they failed to compromise the intended wide field target. The Pleiades was well framed and clear. I have not compared it to a corrected digital image, but my interests are mainly visual observation. For all I know I’ve distorted space-time. But… the view was fantastic.

In conclusion:
For me the experiment was worth doing and worth sharing. I now have an optical piece that adds even more utility to the ones I own. If any of you happen to own a long focal length telescope with a 2” visual back and are looking for a simple method to reach its maximum wide field, this is the way I found. The parts are available at a reasonable price from a reputable dealer.