Review of the William Optics Megrez 90 APO Refractor

In February 2009 I made the decision to purchase a William Optics Megrez 90 APO refractor telescope. The aim was to upgrade in optical quality from my existing William Optics Megrez 80 SD refractor. This page provides some information on my thoughts of how the megrez 90 compares to the megrez 80 and general reviewing comments about the Megrez 90 APO refractor.

Images

Click here to see images of my William Optics Megrez 90 refractor

The Package

I purchased the package which contained:

• William Optics Megrez 90 APO OTA (the B version / second version)
• Hard travel/carry case
• William Optics 90mm mounting rings

Upon unpacking the box and Megerz it was immediately obvious this was a William Optics telescope. The case, OTA and rings all have that sturdy construction, well machined and manufacturered look about them.

The case is of extrodinary strong construction, almost too big and strong for the telescope in my opinnion. There is absolutely no doubt the case is sufficient to send the telescope in airline luggage etc, it really is the most sturdy telescope case I have seen. It's bright orange colour stands out a little, but that won't bother me considering it will live in my observatory or car for short trips. The case is considerably large (actual dimensions available on WO website) and I think too large for the telescope. For someone like me who needs to fit this in a small car the extra space taken by padding in the case really seems unnecessary. For example the entire lid is foam padding (5cm or so) and considering the strength of the case I think that is excessive. There is also nowhere inside the case to put eyepieces. This is quite different from the soft carry case my Megrez 80 SD came with. The soft case may not have protected the telescope as well but it was definitely sufficient while also having space for 5 eyepieces and taking about half the space in my car.

The OTA feels strongly constructed and wel machined. The finish of the outside is a matt white rather than the gloss white of my Megrez 80 SD. It almost has a chalky feel to the matt white. The lens cap is a little hard to get on and off, being an extremely tight fit. I expet the fit of the cap will losen over time.

The rings have multiple attach points to secure them to the mount's plate, and to stack another set of rings atop. They seem of sturdy construction and secure the telescope satisfactorally.

Optical Quality

I am not an expert at determining optical quality so this is somewhat of a casual statement of what I think of the optical quality of the Megrez 90 APO.

The field is sharp from edge to edge. As with my Megrez 80 there is no doubt the wide field views through my TeleVue Panoptic 22mm are excellent. Stars ar pinpoint and easily focused to that sharp point.

To the eye through an eyepiece I cannot say with any confidence there is false colour on bright objects. There might be, slightly, but I don't notice it. With my Megrez 80 SD when pointed at the Moon or a bright star it would show yellow fringing around the objects. The Megrez 90 APO does not show any false colour on those objects that I can see.

Using my Canon 350D DSLR camera on the Megrez 90 telescope (which I actually did before using an eyepiece on it), I notice the following from a photographic point of view:

• There is some edge distortion in the corner 1/4 of the FOV which for good photo's will need the Field Flattener to be removed. The flatness is perhaps slightly better than my Megrez 80 SD but is much the same.
• Bright stars appear more blue than I was expecting. I have used a William Optics 110 FLT previously to take photo's of the Eta Carina nebula and I am quite sure that in comparison the Megrez 90 APO shows blue around the brighter stars. This is quite disapointing, I was expecting better from the other reviews I had read.
• Blue fringing around bright stars apparent in the Megrez 90 APO is enormously less significant than the blue fringing around bright stars in my Megrez 80 SD, to the point that it is really not noticeable. There is absolutely no doubt that the Megrez 90 APO is a huge improvement on my Megrez 80 SD. Resulting photographs do not obviously have any CA problem.
• The amount of blue fringing around bright stars in the Megrez 90 APO is probably OK to leave as-is without processing to remove it from images, as it is not large. If chosen to be removed, I think some light processing would be able to remove any existing fringing as compared to the Megez 80 SD where no amount of processing could satisfactorally remove the blue fringing around bright stars.

I plan to include here some comparison images taken with the Megrez 80 SD, Megrez 90 APO and FLT 110 in the future. Hopefully soon (March or April 2009). Contact Me if you want them and they aren't here.

Update April 2010: I now use a William Optics Field Flatenner v3 to try and flatten the field of the M90 which I became unhappy with for photographic results. Unfortunately the FFv3 does not entirely remove the problem - it changes the problem. I now have distortion in a rotational direction around the image, caused by the FFv3 over-correcting the field. This problem remains unsolved. I'm told it's due to camera spacing from the FFv3, but how's a guy like me without engineering experience meant to solve the problem of changing camera-FFv3 spacing by factions of a millimetre?

Update May 2010: I have now reached the limit of what the FFv3 can do and am looking for an alternative, probably soon to purchase either a TeleVue 0.8x reducer/flattener or a Borg DGL flattener. I have in the last month tried adjusting spacing between camera and WO FFv3 without any improvement in the images. It seems the default spacing is best for the Canon 350D combination.

The distortion visible when using the FFv3 (and example pictured below) is evident in anyexposure length (given bright enough stars to illustrate it). The below is from a 3 minute exposure, but exactly the same is apparent in a 5 second exposure.

Below is an exmaple of the distortion still visible in the top left corner of my images when using the William Optics Field Flattener III (v3) with the Megrez 90:

Focuser

The focuser seems to be of considerable debate when reading the reviews on the web. I can understand why now, because it is different to what you might expect on a refractor like this.

The focuser used on the William Optics Megrez 90 APO (the version I have) is a dual speed rack and pinnion. You can feel the slight "bumps" of the rack and pinnion style focuser as you rack the focuser in and out using the cause focusser knob.

The dual speed aspect of the focuser is almost a requirement considering the type of focuser used. On my Megerz 80 SD which was a standard crayford style focuser I didn't believe and still don't believe that a dual speed focuser is requried because such fine adjustments can be easily made. However with the focuser on the Megrez 90 I find I must use the fine focusing knob to achieve accurate focus, it is extremely hard without that fine adjustment due to the behaviour of the focuser at cause speed.

Motorising the focuser of the Megrez 90 APO is going to be a bit more challenging than that of the Megrez 80 SD which I easily attached an Orion AccuFocus unit to. This is because you can't just take one of the knobs off and have access to the simple focusing shaft, because there is the dual speed aspect to the focuser. I am yet to consider in detail how I will motorise the focuser but I suspect it might be by way of a strap/band/belt connecting the motor to the fine adjustment focuser knob. It is a shame that William Optics does not offer an electronic focuser for the telescope. I believe a RoboFocus could be relatively easily adapted to fit the Megerz 90 by way of attaching to the fine adjustment focus knob with a rubber belt.

Using the fine focus adjustment it is easy to achieve accurate focus on the Megrez 90 APO. The ratio seems sufficiently fine that this is not a problem.

One problem I have not solved with the focuser on my Megrez 90 APO is that there appears to be some slight backlash and slight image shift when changing focus direction. This is very disappointing. The Megrez 80 SD focuser has no backlash and no image shift. Why would a crayford style focuser have image shift problems? this seems odd. What do I mean by image shift? Well, when changing focus direction and moving the fine adjustment knob there is a slight shift in the position of the stars in the FOV as you see it through the eyepiece. This might prove to be problematic for astrophotography, time will tell, I have done relatively little with the telescope so far. This image shift might be related to tension adjustments of the focuser, I'm not sure.

It is disappointing that the telescope does not come with any instructions regarding maintenance and adjustment of the focuser. There are obviously adjustment points on the focuser, and I have already needed to adjust them to increase tension on the focuser, but there are no instructions on the adjustment of this, what is expected, how to maximise performance, etc.

Something I think is sub-standard of the focuser for a William Optics product is the amount of slop exhibited when the focuser is loosened such that it can be rotated. What happens is that the focuser assembly (the black potion) drops down slightly (at an angle, not away from the white OTA) when the adjustment screw is loosened. The change in angle is about a degree. This raises the question: When is the image plain of the camera in the focuser draw tube actually perpendicular to the focal plane?? I can only guess when the focuser is held "up", pushes tight against the white OTA. The below images show what I'm talking about (note, the position of the OTA/mount was not changed between photo's):

Taken before loosening the focuser for rotation (32.1 degrees)	Taken after loosening the focuser for rotation with it still loosened (32.3 degrees)	Taken after re-tensioning the focuser to fix it at the current rotation (31.4 degrees).

You can see in the above examples how the angle of the focuser with respect to the OTA (which trust me, didn't move) changes.

As far as carrying capacity goes the focuser of the Megrez 90 does appear to be able to hold a good amount of weight without slipping, when correctly tensioned using the allen/hex screw on the underside. With my Canon 350D attached movement of the focuser down is easier than movement up, but once stopped moving down/out the focuser does not slide/slip, which is good.

It is obvious that the focuser on the Megrez 90 is sufficient for precise focusing for astrophotography but it is also obvious that it is far from perfect.

Weight & Size

I was interested to find that the tube ring size of the Megrez 90 is in fact the same 90mm as the tube ring size of my Megrez 80. The objective at the end is larger, 90mm vs 80mm, but it seems by the time the light path reaches the main tube where the rings are positioned it doesn't need to be that diameter. I imagine this helps the weight considerably also.

The Megrez 90 is heavier than my Megrez 80 but not considerably so. It's still easily handled by my Losmandy GM8 with all the other gear I have mounted at the same time.

Sample Images

For sample images I suggest you google my site for "megrez 90": try this link There are plenty on my site, some are cropped others are not, but in most you will get a good feel for the CA and distoration characteristics of the Megrez 90 telescope.

Conclusion

Officially, I am yet to come to a conclusion as to how much I like the Megrez 90. It certainly isn't the "wow, fantastic!" I had hoped for, but if it performs well for astrophotography is yet to be determined. The focuser has disappointed me, but the case, weight, and otherwise general build quality have impressed me.

Update April 2010: I am reasonably happy with the Megrez 90 now. It is largely a "set and forget" situation where I have it set up in my observatory and simply use it night after night to image objects. The problem of field flatness is the main one that remains - the FFv3 has not removed the problem, and so this field rotation and field un-flatness is a problem in almost every photograph taken. That said, if suitable to crop to 3/4 frame, images are great, but this isn't always appropriate for the photographed object. Overall I'm quite happy with the Megrez 90 but think there might be better alternatives that I would instead go for if given a second choice. If it weren't for the field flatness, I would say definitely go with the M90.