William Optics Megrez 88 FD OTA Impressions

[Canopus Lim]

http://img126.imageshack.us/img126/2538/img0984qt6.jpg

http://img357.imageshack.us/img357/1925/img0987jm7.jpg

http://img295.imageshack.us/img295/9793/img0989eq0.jpg

http://img58.imageshack.us/img58/5176/img0992az6.jpg



Ok.. I received my Megrez 88FD a week back. I bought this scope for as I wanted a good grab and go refractor for astronomy and terrestrial. My requirements are very good optics, largest aperture in a small package, a refractor, short focal length (for wide field of view) and must be able to fit into my existing dry cabinet. The weather has not been too good and the seeing is from average to poor for many days but recently the seeing has improved and I was able to do proper optical evaluations. These are my impressions I have for these few days.

Specifications of Megrez 88FD
• Air space doublet with Ohara FPL53 extra low dispersion glass
• f/5.6, 498mm focal length
• 88mm Clear Aperture
• 3.3kg and 40cm length
• 2 speed focuser
• 2 knife edge baffles

Having only 498mm focal length, with my 35mm Panoptic, it can achieve 4.5 degrees true field!

1. Mechanical and finishing
- Fit and finish is excellent. I really like the beautifully powder coated finish as it looks professional. I also have a preference for it being matt in colour.
- The focuser is very smooth and solid without any bumps. I won't say it is smoother than feathertouch but it is close.
- The rotating function of the focuser is good without any bumps and a nice feature.
- I really like its compact size of length only 40cm and it fits into my dry cabinet.

2. Optics
I did both star tests and testing of distant lights (about 20km away from Batam island) using 50x per inch (200x magnification).
I also did some observations of the moon, some star clusters and Jupiter.

The Strehl ratio is an expression of the amount of light contained within the Airy disk as
a percentage of the theoretical maximum that would be contained within the disk with a perfect optical system. Hence, the Strehl ratio is the measure of how close the optics is to the ideal (perfect/ theoretical) optics by design that is limited by physics. A strehl ratio of 0.95 means that the optic is 95% to that of the theoretical performance; 5% of the perfect/theoretical Airy Disc energy is scattered to the diffraction rings.

According to RF Royce he states and describes about optics quality:

http://www.rfroyce.com/standards.htm

Strehl Ratio
0.85-0.9 – good
0.9 to 0.95 – very good
0.95 to 0.99 - excellent
1.0 – perfect…..if there is such thing

To me if the optics is >0.9 it is a very good performer..

2a. Strehl Ratio (on-axis) estimation:
Based on in-focus and defocus analysis of the diffraction rings, for Sodium light D-line (street lights) coming from Batam island the spherical correction is very good. The defocus diffraction rings are very similar, and uniform in brightness. There is only a hint of spherical aberration (being one side of focus slightly less bright). I would estimate it is about 0.95 strehl or better at that wavelength. The scope is probably optimized at that wavelength which is a traditional primary wavelength to be optimized. Looking at other lights in green and blue wavelength from Batam island, the spherical correction is very good (prob about 0.9 to 0.95) but it does suffer a bit of astigmatism. I would think the strehl is probably slightly above 0.9 for blue and green wavelengths if factor in the slight astigmatism.

During a night when there was good seeing, I was able to analyze the in-focus star. The seeing was good enough that the focused star does not waver about and the diffraction ring is steady. At office, I did some simulations using Zemax optical design software of the PSF (Point Spread Function – which is the diffraction pattern of a focused star) for different Strehl values. By analyzing the thickness and spacing of the diffraction rings, I can estimate the Strehl value.

Testing a few bright stars at Scorpius and Centaurus, and testing on white stars (so that it carries the whole visual wavelength information), I found out that the overall strehl ratio to be estimated around 0.94; which is very good. I was worried that the slight astigmatism in the green and blue wavelength will affect the Strehl but was happy to see that the in-focus star exhibited a very good diffraction pattern. The Airy disc was bright, circular and the 1st diffraction ring was faint and thin. The spacing between the 1st diffraction ring and the Airy disc was also wide. I could make up the 2nd diffraction ring and it is very faint and very thin. All these information is a good sign of very good optics and by comparing the diagram that I came up with, I estimated the optics to have an overall 0.94 strehl ratio which is very good especially for such a fast doublet.

2b. Colour correction
The scope looks like it is optimized to bring to focus blue, green, yellow and shorter red wavelength as CA is deep red in colour; so it means shorter wavelength red is still focused but probably suffers from lower strehl. This is my first ED refractor I own, so I am not too sure what is called 'good colour correction'. I have owned Newtonians and SCT which are colour free. From what I observed, CA (deep red colour) is noticeable (though slight) at 25x per inch onwards on the limb of the moon. However, even at 50x per inch, I do not see the CA on the moon's terminator or on the features itself. On Jupiter at 50x per inch, I will see slight CA of deep red too.

I guessed that probably because it is very fast at f/5.6 that is why the CA is seen from 25x per inch onwards on bright targets. Overall, I would say the images are sharp and has very good contrast. However, being unsatisfied to just guess the reason for the deep red CA, I did some rough designs of lenses similar to what the Megrez 88FD; Same focal ratio and same aperture. From these rough designs, I come to the conclusion that depending on the mating glass element to the FPL53 glass, the red colour will tend to be defocus or having lower Strehl value (spherochromatism) at best focus (with respect to green). For blue, green, yellow and shorter red wavelengths the strehl can be high. Hence, it is possible that the optics designer designed this scope to enable it to have very good strehl values for blue, green, yellow and sacrifice the longer wavelength red so that the overall strehl of the scope can be better; I did such optimization with the software and it will help in the overall strehl by allowing red to be less good. Actually, because it is just only a doublet and at such a very fast F ratio, it is not possible to have colours from blue to deep red being focused at the same plane or having a focus spot small enough to have a high strehl ratio (having low spherochromatism); put it this way, a doublet has limited bandwidth. One would need a triplet or quadruplet lens to have better colour correction at this F ratio. This is also what I got from the design I did. Being only a visual observer, I actually prefer that the CA is being in the deep red colour as our eyes are most sensitive to blue, green and yellow at night. Red is hard to see and carries little visual information; that is why we use red flashlights to help preserve our night vision. In focus, stars that are not bright will not show any CA at all. Therefore, design wise this scope is very good and slight CA is expected and I would trade the slight CA for the portability and very wide FOV. I also do not find the slight CA obtrusive in visual observation. From what the reviews I read, the Takahashi Sky 90 which has about the same specs (I guess that the 88 FD was made to compete with it), do also have slight CA.

2c. Off axis performance

Off-axis with my 7 and 11 Nagler Type 6, star is sharp from edge to edge. With my 17 Nagler T4, the extreme outer edges have a bit of field curvature and that is mainly attributed to the fast focal ratio of the 88FD; majority of the field (2.8 degrees) is sharp. With my TV plossls, the stars are sharp but do have slight astigmatism (due to the fast focal ratio) at the very edge of field but nonetheless it gives a nice image. Using my Baader orthoscopic eyepieces for higher magnification, stars are also sharp but less sharp off-axis than my TV plossl; because TV plossls are the best corrected Plossls in the market and have an edge in off-axis than traditional Abbe type orthos. Therefore, for such a fast scope, for viewing DSOs and taking in large swath of FOV, the best kind of eyepieces are modern widefield types that are well corrected for off-axis. For higher power viewing (above 25x per inch), I would use my 2x TV Barlow for my 8mm TV Plossl, 6mm Baader Ortho and 5mm Baader Ortho. With the Barlow in place, the star images are sharp from edge to edge as the Barlow increases the effective focal length and thus the eyepiece is able to perform much better off-axis.

In summary, from my few days of viewing, I am very happy with the scope as it has met my requirements of being compact in size, great mechanics, very good optics and great price. I bought this scope to replace my small SCT that I sold more than a year ago as grab and go and it wins it hands down..  I would recommend this scope to anyone looking for a very good grab and go scope.

[acc]

Nice review and a great looking scope. Seems its a really compact scope given the comfy way it nestles inside its case. Is that a Pelican?

[VinSnr]

Bro, don't worry so much about all the Strehl and whatever CA there is. As long it looks good , make you feel good and give you nice images, just enjoy!!!

And welcome to the world of WO. They are simply lovely scopes.

[ariefm71]

I don't care about the look of my scopes though, I usually look through them, not at them

As long it looks good

[Canopus Lim]

Nice review and a great looking scope. Seems its a really compact scope given the comfy way it nestles inside its case. Is that a Pelican?

Haa.. it is not a Pelican. The brand is Vanguard and the case is waterproof, airtight and very tough. Nice case and has customizable foam.

- I bought it at Alan Photo at Funan..and John3:16 shop was not open on Sunday..guess the boss in church hor.

[Canopus Lim]

Bro, don't worry so much about all the Strehl and whatever CA there is. As long it looks good , make you feel good and give you nice images, just enjoy!!!

And welcome to the world of WO. They are simply lovely scopes.

Haa yup join the WO club
And I don't have to worry about its optics after the test. Just enjoy the views now..

[VinSnr]

Nice review and a great looking scope. Seems its a really compact scope given the comfy way it nestles inside its case. Is that a Pelican?

Haa.. it is not a Pelican. The brand is Vanguard and the case is waterproof, airtight and very tough. Nice case and has customizable foam.

- I bought it at Alan Photo at Funan..and John3:16 shop was not open on Sunday..guess the boss in church hor.

Bro, doesn't the scope comes with a case?

[VinSnr]

I don't care about the look of my scopes though, I usually look through them, not at them

As long it looks good

oh man,you just broke the hearts of the scope designers.

If you tell this to Normand Fullam, he will

[ariefm71]

If I were Gandalf, I would love to have one of Fullum's scopes

[Tachyon]

I personally think Fullum's scopes are over the top... too 'loud' for my taste.