Well, it’s official—APO refractors are like Crack—you can’t get enough! I liked the William Optics ZenithStar 66SD refractor so much I started saving my pennies for a larger APO scope for astrophotography. There are lots of choices out there and I looked around for a while (do you have any idea just how MANY pennies are needed to buy an APO refractor?).
I quickly realized that I needed to sit down and determine what characteristics I was looking for in a new refractor. A serious look at my proposed use was in order to make sure I spend my hard saved pennies wisely. Those that know me will realize that my major use of the scope will be for astrophotography with a very secondary use for visual observing. Really, the little 66 will probably see more visual observing. It is so small it is easy to throw on a photo tripod and take on a short trip. On the other hand—good visual color correction and sharp high contrast views are what APO scopes are all about. The scope should have some serious aperture—larger than 80mm at least. It should be small enough to cluster on my mount with other telescopes (Hmmm.... A hint at future projects....). Focal ratio should be between f5 and f7 to keep exposures on extended objects reasonably short and still maintain good color correction. Focal length.... hmmmm.... The focal length of the 66 is about 320mm with the Field Flattener in use. A reasonable focal length would be about double that to properly frame objects that are just too small for the 66’s field of view.
So far I determined that I wanted a scope that was at least 90mm in aperture, and no more than about f7 in focal ratio. This still left a bunch of choices out there. Several of the StellarView telescopes are in this range and they have an extremely good reputation. There are also the AstroTech scopes—some new ones coming up are very interesting. Tele Vue has a number of contenders—the TV85 and NP101 both fit my criteria. TMB Signature Series has a 92mm that looks interesting—but it is not available yet. William Optics had a 90mm Megrez model and a new 110 Megrez model that were both intriguing.
Getting down to choices—the Tele Vue models are very well made and very well corrected. They are also very expensive in comparison to the others that I was looking at. Stellarview has a great reputation but I have never seen one in the flesh so to speak. AstroTech also enjoys a good reputation but the 90mm scope is just out (I don’t know if there have been any delivered yet) and the 100mm is “coming.” AstroTech and Stellarview do not have local dealers so I could not actually see one of these anywhere around here. William Optics.... I already own one WO scope, the 66mm SD, and I am impressed with the build and optical quality as well as the “value” (bang for buck). I saw two of the 90mm Megrez scopes at the FAAC swap meet in the winter so they have been around for a while. Riders Hobbies in Livonia is a William Optics dealer so I can have a local connection for the scope—an important consideration IMHO.
Between the two WO scopes—which one should I buy? The introductory price for the Megrez 110 puts the cost only about 200.00 higher than the Megrez 90. That seems to be a bargain for an additional 20mm of aperture. I suspect that I would be happy with either one and “The Enabler” at Riders was tempting me with both models. The Megrez 90 is an f6.9 APO doublet using an FLP53 glass element. The Megrez 110 is an f5.9 APO doublet using an FLP51 glass element. The slightly longer focal length and the FLP53 glass would seem to give the 90 a slight edge in color correction for visual use. A call to William Optics (by the enabler) confirmed that the 110 was really intended for astrophotography use as opposed to visual use.
So now you are thinking—Clay does astrophotography —of course he bought the 110! Well..., I bought the 90. My thinking was that either scope will work for astrophotography but for those times that I will be looking through it I want the best color correction that I can get. The 90 has been around for a couple of years without any systemic problems that I could find on the internet—and I did find some excellent reviews of the scope.
(info and picture from manufacturers web site)
|Objective Type||Doublet FPL-53 Air Spaced|
|Focuser||360° Rotating Camera-angle|
|Adjuster||1:10 Dual Speed Fine Focus|
|1.25” Adapter||Brass Compression Rings|
|Tube Length||430mm (Fully Retracted)|
|Tube Weight||7.1lbs. (3.2kg)|
|Case||High Quality Aluminum Case|
|Case Dimensions (WxHxD)||23.2” x 8.3” x 7.1”|
|(59cm x 21cm x 18cm)|
It is a very nice looking telescope with a white pebble finish powder coat paint and gold colored accent rings. The end cap is also gold and sports the WO swan logo prominently on the front. The supplied rings are a nice black finish and are made very nicely. The crayford focuser is a typical William Optics masterpiece being butter smooth and the rotate is very nice for photo composition. The scope features a “foot” mount like some of the smaller WO models including the 66mm. In my opinion this scope is too heavy for practical use of the foot mount. I personally would eliminate it. It would have been more convenient for mounting if the bottom of the foot was at the same level as the bottom of the rings. This is a minor quibble and I will take care of this in the adapters that I use when I build a permanent mounting to my G11. Until then, the 90 mounts well on my G11 with the little 66 riding on top.
Optically this scope performs! To say that it “snaps” into focus would be an understatement. The dual speed focuser is overkill on this scope—there is no question where the focus point is located. The first time I used it I was looking at Saturn. I stepped up through my eyepieces until I was looking through my 4.8mm Nagler at a totally sharp and high contrast Saturn surrounded by a seeming cloud of faint moons. I actually had to go and dig out my seldom used 3mm Radian just to see how the scope handled it. No problem! 206X was a breeze and Saturn looked great!
I started out using the WO66 as a guide scope and immediately noticed some flexure (the astrophotographer’s evil foe!) in the mounting. I worked on this for a couple of nights and then I said to myself “Hey—I bet my Taurus Tracker will fit on the 90!” Defeating flexure is the reason that I bought the Taurus Tracker. Problem solved! Springtime is the “time of the galaxies” not the best of targets for a 620mm focal length scope and a DSLR. I did, however, manage to get a shot that I have wanted to take for a while. Markarian’s Chain in Virgo.
Not to worry—the Milky Way is rising earlier every day. I really like the Milky Way, there are so many targets suited to the focal length of this telescope. I have managed to take some shots over the last few weeks.
One of my favorite targets—the Lagoon Nebula. The Megrez shows excellent color correction around the bright stars. Globular cluster NGC6544 looks good and NGC6559 shows both red and blue nebulosity. You can even see a very small planetary nebula PLN 6-2.1. I am sure the processing could be better but this is my best shot of this object to date.
The next two shots are segments of the veil nebula. I have attempted to shoot these at various times with my achromatic refractor but with no where near this success.
Both are very faint objects with a large H-alpha and O III component. They are also surrounded by a large number of stars. The 90mm APO gave me great color correction so I avoided the dreaded blue halos and it also allowed me to reproduce the various star colors quite nicely. My latest shots include M13:
This is a highly cropped portion of the total frame. The size of M13 in a 600mm focal length instrument is very small. The Megrez 90/10D combination is sharp enough to allow for quite a bit of enlargement.
I really like the star colors that I am getting with this telescope. Much better than blue halos! Like most doublets it really should have a field flattener to give it’s best results. William Optics says that one is in the works and will be released later this year. Until then I will crop my frames to remove the majority of the field curvature. Even with the field curvature this is the nicest astrograph that I have ever had.
Beyond the foot mount height I found that the focuser did not seem to lock down very tightly. In looking at the arrangement the lock screw is bearing on a plastic ball that acts as a brake on the focus drawtube. A call to William Optics got me instructions on adjusting the focus tension. The tension is set pretty loose as the scope comes from the factory. It is good for visual use but much too light for imaging. Once I adjusted the focus tension it was much easier to lock the focus down securely. This also “tightens up” the focus action—not a bad thing for me but a purely visual astronomer might want to leave the focus tension looser. The nice thing is that the adjustment is there! I only wish the manual covered the adjustment better (or at all!).
My $1200.00 was very well spent! I am quite happy with this refractor and I am looking forward to using it to shoot many of the summer Milky Way objects in the coming months. It was great to be able to get this locally—having local dealer support is a good feeling when you are plunking down cash for a purchase like this. John at Riders was a peach to deal with as always. I should mention that John currently has one of these in stock if you should want to take a look at one. All I can say is “Come On New Moon Weekends” !!!!!