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Mewlon 250S and EM-400 for Imagers

Posted by John O'Neill   12/29/2007 00:00:AM

Mewlon 250S and EM-400 for Imagers
Mewlon 250S and EM-400 mount for Imagers

I just finished reading James Edwards’s excellent review and wanted to chime in since he is a visual observer and I am an imager. I thought that imagers might be interested in learning more about this scope as an imaging platform. Any of you who have read my first review (on this scope) knows that this is my second Mewlon 250, (actually it’s my third, I’ll explain later). James is absolutely correct, the Mewlon 250 is without a doubt the finest 10” scope money can buy. I noticed that the 10” Mak Cass Astro Physics that is being auctioned off is going for over $20,000. Somebody will pay that but in my estimation they won’t be getting a better scope.

Without being redundant, since I have covered quite a bit in my first review, I do want to list all the reasons I think makes the Mewlon an excellent imaging platform.

Dall Kirkham Optical Design – Many may want to dispute what I am about to say but the proof is in the pudding. Besides the Takahashi quality of the two mirror surfaces used in this design the fact that there is no meniscus I feel is a big plus, less glass to distort the image. Anyone who has spent any time imaging knows that you will get star bloat with some models of Schmidt Cassegrain’s, and Maksutov Cassegrain’s. With the Mewlon you see none of that. I use an ST-8XE with the Tak .8 reducer (2,400mm) to image and see none of the chromatic aberration and coma as has been reported in other reviews. (see image of M1). Is the entire field completely flat, NO but certainly expectable for such a long focal length scope.

Electric Focuser – Smooth image shift free focusing with ability to change, in degrees, the speed of the focus. Has a very slow mode for fine focusing which can be done fast by using a bright star and intersecting the three diffraction spikes. The best part is you can do this from your table, watching your computer screen, to see the changes. The electric focuser solves a BIG problem with some scopes and that is mirror flop. The Mewlon 250 has a fixed mirror.

Dew Control Through Design – The front of the tube is rounded eliminating dew buildup. The secondary mirror and focuser are recessed so dewing is minimal. The open tube can cause dew to form on the primary (dew in Florida is a big problem) but a few minutes with the hair dryer and your back in business.

Removable Back for Quick Cool Down
The Mewlon has a removable plate, at the back of the scope that exposes the mirror to outside air, for a quick cool down, important here in Florida. (again)

The scope weights only 27 lbs so it can be easily mounted to the scope, for those who have to image at remote sites. The EM-400 mount is also exceptional in this area. It comes in four pieces with the largest piece weighing a hefty 45 lbs but surprising easy to lift and mount on the mount cradle.

As I mentioned earlier, this is my third Mewlon 250. Those of you who have been imaging a long time realize that there is NO substitute for good equipment. The most important is the mount and tripod. The mount must be large enough to carry all the equipment with room to spare (less stress on the motors and bearings) and track with the least amount of periodic error possible. The tripod must be capable of supporting all the weight with absolutely no flexure and be capable of holding a star in the guider during a breeze. The stronger and beefier the tripod the better the results. The scope’s optics obviously are important but even a badly balanced setup will give poor results.

All the preceding is the reason I purchased the third Mewlon. This Mewlon is the 250S, which means that the scope doesn’t have the hockey puck on the bottom so it has to be supported with rings, the same rings that come with the BRC 250. You may wonder what was wrong with the second Mewlon. The answer is nothing but I decided to add a guiding scope so I could image in Ha and narrowband and not have to worry about finding a guide star. It wasn’t long before I realized that by adding the rings, dove plate, mounting plate for the guide scope, the guide scope, the guider and extra counterweights, I was way over the capacity of the EM-200. The EM-200 quickly was posted on Astromart and sold. I purchased the EM400 and tripod and soon realized that an extra weight was needed (will this spending never end?) I went through two guide scopes before I realized that my flexure problems where caused by inadequate focusers. The purchase of a FS-78 (with a solid rack and pinion focuser) solved the final piece of the imaging puzzle.

The EM-400 is extremely well made (as are all Tak products) and designed for the astroimager who needs a strong mount that is still portable. It comes with 4 main parts, the main head, the mount cradle, rotate-able base and screw-in weight bar. The weights have two locking screws for added safety. The medium wooden tripod is just the right height for field use and very strong. Takahashi has a new metal tripod that is even better.
The polar scope is more accurate than the one in my older EM-200 and literally takes a minute to align. Just set the offset for your area (see note) adjust the Ra axis until the bubble is centered, rotate the dial for the time and day of the month (make sure you use standard time), place Polaris in the 2007 area of the rectangle, sync on a star and you’re up and running. I ran the Polar Align program and found that the polar alignment needed no adjusting. Go To is determined by the polar alignment and I found it to be dead on with this mount.

Note: For those of you with an EM- mount. The manual is wrong on the +5 deg for the offset scale for Houston. It should read -5. That means you need to reverse your setting for your location. I live in west Florida, which is -7 on the offset scale. This obviously will improve your polar alignment and go-to capabilities.

Well how does the new setup perform? Better that I ever imagined. The mount is capable of guiding with a PEC of +/- 4 arc sec. At times of perfect steadiness the guiding scale in Maxim was showing +/- 2 arc sec, which many will find hard to believe but I have many witnesses who will attest to this. The Starfish guider works flawlessly and in combination with the excellent motors in the mount keeps the guide star in its place. Now all I need is one of those BIG chip cameras.