Takahashi 180 Mewlon

Takahashi 180 Mewlon Review
Ed Kessler

This review has been a work in progress over the period of several months of having owned the Mewlon. I wanted to thoroughly use the scope in various settings so that what I wrote would be more objective and less prone to prejudice caused by "new scope excitement."

I was not able to find much information on the internet regarding the Mewlon when I was looking at upgrading my telescope so I hope this review will be helpful to those considering this Dall-Kirkham from Takahashi or one of its larger siblings.

A Bit of Background:

I am primarily a planetary and lunar observer although I occasionally turn my scope to the brighter deep sky objects. In fact, since purchasing the Mewlon I have spent more time viewing DSOs, since the scope performs so well. I especially like to view globular clusters and interesting open star clusters. Over the years I have owned a number of SCTs (a 10" Meade 2010, a 12" LX200, a Celestron 9.25" and a C8) and a number of refractors (Genesis SDF, Celestron CR150-HD that I used with a Chromacor). My current, most used scope is a TAK FS-102 which I love. I have a pair of 25x100 binoculars for widefield viewing, and also enjoy the challenge of taking the best photographs I can using a simple Coolpix 990 camera. Someday I may pick up a 10- 12" Dob. I own a Denk II binoviewer / power switch and with the exception of widefield views I use it almost exclusively.

I observe mostly from my backyard where on nights of good transparency I can see the Milky Way quite well, but my views to the south are hampered by sky glow from the city of Harrisburg, Pennsylvania. I'm mostly a grab and go observer and both of my scopes are setup so that I can carry them out on their mounts and be observing in minutes.

Basic Specifications:

The Mewlon is a Dall-Kirkham design. Without getting into details, a Dall-Kirkham looks like a Schmidt Cassegrain without the corrector plate. Technically, the Mewlon is not a catadioptric telescope, but a reflector of the classical cassegrain venue, but because it looks like a Schmidt Cassegrain at first glance, it is sometimes listed with them. The Mewlon consists of a primary mirror figured to f/3 and a secondary mirror at f/4.

Here are the basic specs for the 180 Mewlon:

• Effective Aperture - 180mm.
• Focal Length - 2160mm (or 1763mm with Takahashi's focal reducer).
• Focal Ratio - F/12 (f/9.8 with the reducer).
• Diameter of the main tube - 210mm (about 8.25 inches).
• Secondary mirror diameter - 54mm (or 30%).
• Weight of main tube assembly - 6kg (or 13.2 lbs.).
• Length of the main tube assembly is about 22 inches .
• The 180 Mewlon sells for about $2600.00 retail.

Takahashi makes the claim that with its relatively smaller secondary, without the need for a corrector plate, and with a careful choice of primary and secondary mirrors with 96% reflective coatings the Mewlon provides "refractor-like performance in a larger folded optic reflector design." That sounded just like the scope that I was looking for and I decided to test that assertion.

A Box at My Door

The Mewlon came supplied with an excellent 7x50 finder scope which doubles as a handle, a 2" visual back with a 1 1ž4 inch adapter, and a dovetail adapter plate, affectionately known as "the hockey puck." Takahashi's 7x50 finder is the clearest and best finder that I have ever looked through. It is easy to adjust and has a very smooth focus. However, after using the scope for several nights, I decided that I definitely needed the optional illuminator for the finder. I simply couldn't see the crosshairs at night when pointed at the dark sky, and since the Mewlon has a relatively narrow field of view, pointing was a bit difficult. The illuminator made a world of difference, albeit at the price of an additional $80.00. (This is my only gripe with Takahashi - everything is expensive. However, having nearly pulled my hair out working with cheaper, less expensive instruments and mounts, I've decided that the extra cost is worth it. Everything about Takahashi, including this little illuminator exudes quality and generally works very well from the start and for a long time.) The shipping box came padded with foam, tailored for the OTA, so I had a local carpenter build a wooden box to slip the shipping carton into, making a nice case for storage and transportation.

My Basic Setup:

My 180 Mewlon rides on a Vixen GPDX mount which handles the weight of the scope and accessories very well. As shipped, the scope came with a dovetail attached to the tube which is an almost direct fit to the Vixen mount. The dovetail doesn't quite "bottom-out" however. There is about a 1/16 inch gap between the bottom of the dovetail and the mount itself. Still when tightened the scope is quite secure. Equipped with my 2" diagonal, the OTA is easy to balance on the GPDX, and the more I use the scope, I appreciate how well it balances in contrast to the C8 that I also used on an equatorial mount. My eyepieces of choice are Tele Vue plossls and I use a 2" Tele Vue Everbright Diagonal. I now do most of my planetary viewing with a Denkmeier Denk II binoviewer and power switch.

First Light:

The 180 Mewlon lives up to Takahashi's reputation for quality. The first impression one gets when unpacking the Mewlon for the first time is that the scope is solid, beautiful, and well made. One look down the tube at the primary gives one the impression that this is a serious instrument. Takahashi figures both mirrors to 1/20^th wave!

During the first night out with the scope I had brief opportunities to look at Saturn and Jupiter, although seeing was dismal. Cool-down took about twenty minutes, and even though dew can be a problem here in south-central Pennsylvania, I have yet to have a problem with it on the Mewlon's mirrors, even though on some nights the scope itself is covered with dew. I consider this a definite plus for the Mewlon. With my SCT I always needed to be conscious of dew, but now I generally ignore it.

When I first pointed the finder scope to Jupiter and looked through the telescope I saw nothing but stars in the telescope. The finder needed alignment which is accomplished by using three set screws (an Allen wrench is supplied with the scope). This alignment arrangement allows for precise and secure adjustments. Aligning the finder took about fifteen minutes, and I then took a look at Jupiter.

I discovered that I could not find a critical focus and that while I was seeing some planetary detail, the view was not as crisp as I expected. Jupiter's moons were also a bit fuzzy; not pinpoints of light. I checked the collimation and sure enough it was off - considerably. Takahashi states that the scope is collimated at the factory before shipment, but in my case it arrived out of collimation. I suspect that this sort of experience is normal.

The scope is supplied with an Allen wrench that fits the alignment bolts of the secondary mirror. Access to the collimating bolts is through a cover that threads off the back of the secondary. (Before I go any farther, let me give two cautions. Experience is talking here. First, before doing anything at the front of the telescope, make sure that it is pointed parallel with the ground or downward. That way if anything is dropped it will not fall into the telescope, hitting the primary mirror. That will ruin anyone's day. Second, I attached a piece of string to the Allen wrench that adjusts the collimation bolts and then used it as a bracelet to prevent me from dropping the wrench onto the ground or into the telescope. You've been warned!) In the reviews and comments that I did find on the web about the Mewlon, most praised its performance, but cursed its need for precise collimation - that it was more sensitive to mis-collimation than other designs. As a refractor user those comments had me concerned, but after thinking about it, I came to the conclusion that any telescope with a secondary mirror is going to perform poorly if not collimated well and that while some telescopes are easier to collimate than others, the Mewlon could not be much more difficult than collimating an SCT at F/10.

So with just a bit of anxiety, I removed the secondary cover, pointed to a 2^nd magnitude star and began the collimation procedure. I specifically followed the directions in the small manual that came with the scope. The Takahashi manual is quite terse! The bottom line is that in about ten minutes I had collimated the scope to the point where I was seeing excellent detail on Jupiter and its moons were now pinpoints of light instead of fuzzy dots. Collimating the scope proved to be sort of a boogeyman. It was actually easier to collimate the scope than it was to align the finder. My word of advice is to make very small adjustments at a time, checking things as you proceed. I also noticed that after collimation the finder needed to be tweaked again.

Additional Note on Collimation: On a cloudy night I decided to check the collimation of the scope using a well-made single-point laser collimator. The directions were to align the reflected beam from the secondary with the primary beam coming from the laser. In other words, centering the return point of the laser on the initial point was to provide collimation. Using this procedure I centered the return spot, seen on the face of the laser, as best I could. On the next clear night, I checked the collimation on a star and discovered, to my surprise, that it was significantly off. I suspect that because it is difficult to see the return beam when it gets very close to the initial beam, this method is not very precise. Indeed, the Mewlon was much easier to collimate on a star and it has held collimation very well.

Once the mirrors were collimated I concentrated on a brief star test and was very pleased to see a textbook diffraction pattern for a scope with a central obstruction. That was one of those moments when I said, "Cool!", to myself and I knew that this scope was going to perform well.

After all this I turned the scope back to Jupiter and I was amazed at the amount of planetary detail I was seeing. The GRS was prominent, I was seeing detail in both the North and South Equatorial Bands. There were hints of banding in the temperate and polar regions and there were several festoons visible as well. And this was a poor night! Subsequent observations on Saturn, Jupiter, M13, M57 and Albireo have all been superb. M13 was nicely resolved. I could see some structural detail in M57. The contrast of the yellow and blue of Albireo was excellent, and the Mewlon has provided better planetary views of any scope I have ever owned. On a ensuing night I pointed the scope on a 5-day old moon, but seeing was poor (3-4/10). Nonetheless, I was able to nicely resolve Rima Petavis, Rima Janssen and was seeing nice detail in Vallis Rheita. I'm going to have a lot of fun viewing the moon with the 180 Mewlon! Alas, its been cloudy and raining here for about two weeks and I haven't been able to do any more extended viewing on some other DSO's. (I also can't help wondering what a 210 Mewlon would be like . . . but for reasons of domestic tranquility, I best not go there, at least not yet.)

More Details

Another potential problem that had me concerned was mirror flop or movement during focusing. Some users reported that it was minimal; other stated that it was annoying. In my scope, while mirror movement is noticeable, it is also minimal. When focusing an object at high power in my Coolpix camera, there is some noticeable movement but it is not bothersome. I suppose if one was working with a CCD camera with a small chip it may prove frustrating, and if one buys a Mewlon to do astrophotography (which they do very well, I'm told) Takahashi's optional rack and pinion focuser may be necessary or desirable.

After about two months I did, however, install a Moonlite Dual Rate SCT focuser on my scope. I wanted more control over the fine focus for taking photos with the Coolpix. The excellent craftsmen at Moonlite tailored one of their SCT adapters to fit the non-standard thread on the back of the Mewlon, and also set up the focuser to minimize the distance between the telescope and the eyepiece. I earlier had discovered that the limited back focus was an issue with both binoviewing and attaching an aftermarket focuser. My first attempt was to use an SCT adapter plate (the WU adapter, see my comments below) with a Starlight Instruments Feathertouch Focuser and their standard SCT adapter. With that arrangement I could not reach focus with my eyepieces or even with my Denkmeier binoviewer with the lower power setting on the Power Switch. After one tiny bump in the low power setting of the power switch (1.3x to 1.4x), the new setup with the Moonlite Focuser enabled me to reach focus with everything I have tried so far.

An additional note regarding installation of the Moonlite focuser: To achieve an optimum installation of the focuser, it is necessary to remove the Mewlon focuser knob. At first this puzzled me. There are three set screws beneath the rubber grip on the focuser knob. After loosening them, I still couldn't pull the knob off. Finally, I discovered by accident that the knob threads onto the focus shaft. Thus, the procedure for removing the knob is fairly simple as are most things when one knows what he is doing. To remove the knob, first turn the focus counterclockwise as far as it will go. Then peel up the rubber cover on the knob from the bottom toward the top about half way up the focuser knob. Loosen the three set screws considerably but not so far that they fall out. At this point the knob simply unthreads, again in a counterclockwise direction. Removing the knob allows the Moonlite focuser to be tightened as closely to the telescope tube as possible.

The standard focuser on the Mewlon is smooth and fine adjustments are relatively easy; however, the focuser is tight, meaning that it requires a little more force to turn when compared to the last SCT that I owned. Perhaps this will ease up in time, but this is a minor complaint that I have about the scope.

The claim was that the views would be refractor-like, and I can honestly say that they are. I have never looked through a 6" APO so I have no scientific basis to make this next statement, except that I know what I can see and what the views are like through my TAK FS-102, but my intuition tells me that side by side with a larger APO the 180 Mewlon will hold its own when well-collimated. Contrast is excellent, the background sky is dark, stars are pinpoints even at high powers, and I am observing more detail than I have ever seen, even in comparison to the bigger SCTs that I've owned.

Binoviewer Saga

The scope will not come to focus with my binoviewer without the use of an OCS when using the supplied 2-inch visual back and a 2-inch diagonal. It is that simple. I tried everything. I tested the 1 1ž4 inch visual back with a 1 1ž4 inch diagonal, but had similar results. There simply was not enough back focus; it was close, but I could not bring things into focus. Another attempt was to purchase a WU adapter for Mewlons (TAK part #TCD0003). At $102.00 this little adapter is expensive. It works with the Mewlon's visual back allowing one to attach SCT accessories, providing an additional 1.5 inches of back focus. I then purchased a 2" diagonal designed for SCTs to reduce the focal length of the binoviewer system as much as possible, but I still couldn't reach focus. After that, I gave up! I contacted Russ at Denkmeier and he put together a power switch for my binoviewer that switches in a minimum,1.3x, OCS. (I later had this changed to 1.4x to compensate for the Moonlite focuser added approximately 0.5" of back focus.) That is the best lowest power arrangement that I could put together. I have since learned that the 180 Mewlon has the least amount of back focus of the Mewlon family of scopes, and wish I had done a bit more homework before I spent a few hundred dollars experimenting. Nonetheless, views through the 180 with my Denk II binoviewers and 32mm Tele Vue plossls are superb. (For this observer there is no going back to viewing with a single eyepiece except for objects that require a relatively large FOV.)

Widefield Viewing

That brings me to the subject of widefield viewing. The Mewlon has a relatively narrow FOV. I needed to get something in the 30mm-40mm range with a 70 degree or more AFOV for a maximum field. The maximum field that one can hope for in the Mewlon is somewhere around 1.3-1.5 degrees. I did not buy the Mewlon to do widefield viewing anyway; that's why I own a pair of large binoculars. However, I after doing some research I settled on a 55mm Tele Vue plossl which provides a 1.3 degree field at about 39x, and in my opinion is an excellent value. It provides a pleasingly sharp view across the entire field, and is less expensive than a Panoptic, Nagler or UWAN. (Incidentally, the 55mm TV plossl provides a true field of 3.4 degrees at 14x in my FS-102). Other eyepieces like the Konigs should perform well with the Mewlon at f/12. I also picked up a Siebert Observatory 34mm eyepiece that provides a nice crisp 1.10 degree FOV at 64x - a very nice lightweight eyepiece. One additional note regarding the Mewlon's FOV - I had one of Harry Siebert's focal reducers designed to use with a binoviewer on an SCT. The reduction is 0.6x. On a whim, I decided to thread this into the end of my 26mm plossl to see if I could come to focus on the Mewlon. It worked! Although I was viewing Jupiter at the time, I could not see any image degradation. I'll need to check its performance on a star field, but the results looked promising. This may be a much cheaper way to reduce the focal length of the Mewlon without spending nearly $375.00 on Takahashi's focal reducer. Since, I can now adapt SCT components to the Mewlon, it may be a good idea to test a focal reducer designed for SCTs as well, although I've been told that back focus will again be a problem.

One thing that did annoy me at first, being a refractor fan, was that I could see diffraction spikes from the secondary spider on the brighter stars and Jupiter. In the Mewlon there are six! Takahashi uses a very thick-vaned spider. I could not see the spikes on Saturn. I suppose viewing Venus may prove troublesome, but that is something I haven't tried. However, I was surprised at how quickly I was able to overlook the spikes when viewing Jupiter - the planetary detail was so interesting and the planet was so bright that I simply forgot about the spikes. They are also less noticeable at higher power, although their visual effect is certainly there even if not obvious. This is perhaps the one issue that may turn you away from the Mewlon, but I have not found the diffraction spikes to be ultimately bothersome. While viewing the brighter objects in focus the presence of the diffraction spikes will be the only tell-tale sign that you are not looking through a refractor.

Bottom Lines:

What I liked about the 180 Mewlon:

• Excellent fit and finish, true to the Takahashi pedigree.
• Crisp, high contrast views and dark sky backgrounds.
• An excellent finder.
• The minor mirror movement when focusing.
• The light weight and ease of balance.
• Excellent portability.
• The relative ease of collimation and the stability of the secondary mirror.
• No worries about dew.
• The cost - in comparison to a larger APO.

• The diffraction spikes.
• The higher cost of TAK accessories.
• A focuser that is a bit too tight.
• Limited back focus.