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Megrez 90 FD - Fluorite doublet review
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Megrez 90 FD – Fluorite doublet
by Gianluca Rossi
First of all I want to clarify that I am not an expert in star testing telescopes nor I am linked to William Optics in any way. I just want to share my experience and impression of this telescope hoping to give some useful information to other amateurs who may be interested in this product. I got a second hand Megrez 90 from an amateur of a local astronomy club. I just wanted a grab&go telescope to do some good visual observing of the planets, double stars and star clusters and to do some wide field imaging with both a CCD and a DSLR and felt that an Apochromatic refractor would be the right choice. After doing some research I became more and more intrigued by this little refractor, an FPL-53 doublet by William Optics. The specimen I purchased was only 6 months old and the warranty was still in place at the time of the purchasing. The owner decided to sell the scope in favor of bigger aperture and then I was able to get the scope at a fairly good price.
The lens of the Megrez 90 appears to have smooth coating and the telescope looks very well made. Every year or two you
may consider cleaning the objective to eliminate some minor white spots caused inevitably by dew.
At first sight the telescope looks very well made, the lens has a smooth coating, the focuser seems rock solid and you get a nice overall impression. While the clear aperture is actually 90 mm I found the focal length of my scope to be around 570 mm instead of the claimed 621 mm. The effective focal ratio is then f/6.3 instead of f/6.9. I have found that many people who own this telescope have measured a shorter focal length of their specimens and some people say that the whole old version of the Megrez 90 was launched on the market with an error in communicating the actual focal length.
The very first light of my Megrez 90 took place on the same night of the purchasing. I was able to watch the moon with a Pentax XW7 mm and it looked beautiful, crisp and bright with no trace of false color near the limb. Nothing comparable to my old 80 mm achromatic refractor. After looking at a bright star however I got the impression that there was something wrong with the alignment. I did a star test later at home with an home-made artificial star and my suspicion was confirmed: the telescope was misaligned. Luckily the lens cell of the Megrez 90 is equipped with 6 collimating screws but their adjustment requires some experience as they are radially mounted to the lens. To solve the issue I ended up sending the scope to a local dealer for servicing.
To assess optical quality I did various star tests on both real stars and indoors with my home-made artificial stars. The telescope snaps into focus quite easily and on real bright stars (Capella and Rigel) I could detect at about 250x only a tiny amount of false color that does not affect significantly the observing. Out of focus images of these bright stars display some pale green and some violet blur. Even if the lens has one element made of synthetic fluorite the secondary spectrum is still detectable. Violet fringes however cannot be corrected in doublets as the laws of physics teach us that to bring violet to the same focus of red and blue we need a triplet design. However as the eye has a small sensitivity to violet this is not a significant problem in visual observing. As said above only the brightest stars are affected. Artificial star test confirms the presence of pale green and violet around bright sources but indicates also that the majority of this effect comes from the combo barlow + eyepiece used (Televue 3x barlow + Pentax XW 7 mm). In fact in focus and out focus images of artificial stars taken through a mirror telescope show a lot of false color so the Megrez contribution may be limited to just a pale pinkish fringe around the brightest sources.
Home-made artificial star test indoors reveals some false color around bright sources. Even if the effect here is boosted by
the high magnification and by the afocal method (DSLR behind eyepiece and barlow), the Megrez 90 does display some
false color around real bright stars. As discussed the majority of this effect comes from the eyepiece and barlow and it does
not significantly affect the performance of the scope
Home-made artificial star test performed under the same conditions but through a Newtonian telescope. The contribution of
the eyepiece and barlow is significant
Out of focus home-made artificial star test through the Newton boosts false colors caused by the lens in the optical train
(barlow + eyepiece)
As said above none of these false colors affects significantly the visual performance of the Megrez 90 at the eyepiece when observing perfectly focused stars, just a tiny color fringe around the brightest stars is detectable at the highest practical powers. One point to note is some small lateral color visible around off centered bright stars using wide field eyepieces such as the Meade SWA 24.5mm, but again it could be mostly due to the eyepiece. As regards spherical aberration I compared the diffraction images inside and outside focus. As the telescope is affected by some spherochromatism I used a green filter to do a better assessment. Through a green filter on real stars I estimated spherical aberration to lay at 1/4 wave or perhaps slightly better by comparing the similarities of the inside and outside of focus images. Although I didn't do an interferometric test I compared my Megrez 90 star test to the Megrez II 80 FD tested by expert Wolfgang Rhor.
Alhough the Megrez II 80 FD displays worse color correction it shows neat Airy disks and has a Strehl ratio of 0.92 at 550 nm. As the Megrez 90 also shows neat Airy disks on real stars I guess it might have a comparable Strehl ratio together with slightly better color correction.
How does the Megrez 90 perform visually? I watched the moon several times with different eyepieces and power. The moon appears crisp and bright even at very high powers and chromatic aberration is practically negligible. Perhaps just a slight yellowish tint around the edge of the moon in the 250x-570x range with a Televue 3x barlow and Pentax XW 7 mm and Plossl 3 mm eyepieces respectively, that most probably may have been caused by the combo eyepiece and barlow. Even at this incredible power the view is really stunning. The planets? Mars at 14” shows the polar cap and some dark areas on the disk at 162x and 244x (Pentax XW 7 mm and Televue 2x and 3x barlows). Jupiter is a beauty. On nights with good seeing I was able to count 5-6 bands that appear colored in the Megrez 90. I could also see details in the 2 bigger bands. Saturn reveals the Cassini division easily and some details of the disk are visible. Luckily, despite star test results, color fringes are not visible around the disks of the planets up to about 250x.
Boosting higher powers is possible and desirable to split close double stars. I was able to resolve 12 Lyn mag.5.4 and 6.0 sep. 1.7” and 57 Cnc mag. 6.1 and 6.4 sep. 1.5” using 380x and 570x with a Plossl 3 mm and a Televue 2x and 3x Barlow respectively. At 190x double star 12 Lyn is resolved but to see clearly the Airy disks I needed to boost 380x. Surprisingly the images were still good even at 570x, an outstanding outcome for a 90 mm scope. Perfect focus at these high powers is easy to achieve with the dual speed focuser that comes with the scope. However I found the dual speed fine focusing knob to have a ratio 1:9 instead of the advertised 1:10 but for visual use it works well anyway.
Of course deep sky observing with this scope is limited by its aperture but under dark skies in the mountains (1,600 mt above sea level) I was able to see the two bigger arms of the Veil nebula, NGC 6960 and NGC 6992 without the use of filters. Star clusters such as the double cluster in Perseus show pinpoint stars across the field and the view is really pleasing. Overall, for visual observing a very good telescope for the price, probably inferior to costly triplets, but really a big step forward compared to Achromatic refractors. Just one point to note is the use of either a star diagonal or an extension tube to reach focus.
Astrophotography with this telescope opens another realm. First of all the focuser. What I have found to be a good focuser for visual observing turned out to be inadequate for serious astrophotography. The focuser is advertised to be Crayford design and looks very sturdy but it isn't either. The design is actually rack&pinion that is prone to flexure under the weight of a DSLR or CCD with filter wheel as there are no ball bearings in this design.
The original power linear focuser that comes with the Megrez 90 is a rack&pinion design with dual speed knobs
To reduce the effect William Optics sent me a couple of screws with nylon tips to be put on top of the focuser body but tighten these will result in blockage of the fine focusing knob and they cannot be used if you rack the focuser more than 60 mm out.
Two steel screws with nylon tips have been mounted on the focuser body of the Megrez 90 in an attempt to reduce flexure. As
discussed in the article for serious astrophotography the original focuser must be replaced
The focuser has also a rotatable feature that is actually useless. There are 3 nylon screws as well as one locking screw on top of the body of the focuser but rotating the focuser will easily result in loss of orthogonality between camera and optical axis. The nylon screws wear out quickly and the locking screw cannot hold the camera firmly. Moreover focusing a CCD or a DSLR is a matter of mere microns and it is a pain to achieve such a high level of accuracy by hand even with the fine focusing knob. I ended up replacing the original focuser with a motorized feather touch focuser and building an appropriate spacer that I blackened with black velvet internally. Of course the focuser is now completely blocked to the OTA via 3 steel grains so I have no rotatable feature but flexure is no longer a problem. To compose a picture I simply rotate the whole corrector and camera body train and once I am done I just focus the scope. I dare say that a good quality motorized focuser is a must if you plan to use this scope mainly for astrophotography.
The Megrez 90 equipped with a good focuser for photography, a digital motorized feather touch focuser. The use of an
appropriate spacer is necessary to reach focus.
You need a field flattener for photographing with this scope. William optics was unable to deliver a specific flattener for this model as it is the old version of the Megrez 90 that is no longer produced. After trying various correctors of other brands I solved the problem with one cheap brandless universal field flattener/focal reducer suitable for refractors between 400 and 600 mm of focal length that is perfect for my needs. It practically corrects nearly all of the field covered by an APS size and optimal back focus is 62 mm instead of the nominal 55 mm. Then I can use a custom off axis guider that enables me not to overload my mount, to eliminate flexure and to save a guider scope. Under these conditions the Megrez 90 works very well. I get no significant color fringes around the stars, another point that seems to confirm that most of the color aberration of the star test does not come directly from the Megrez 90, and results are practically comparable to costly triplets. With my Canon 40D I generally use total exposures between 3 and 5 hrs with 20 mins subs with satisfactory results. After 5 hrs (15 x 20 mins subs) I measured a limiting magnitude of about 19 with a DSLR operating at ambient temperature of 1°C, something that a couple of decades ago was simply unthinkable for an aperture of 90mm.
The Pleaides taken through the Megrez 90 and the universal brandless field flattener, 15X1200 secs for a total exposure of
5 hrs with an unmodded Canon 40D. The image is a crop of a larger frame as I experienced a lot of flexure with the original
focuser. Some distortion of stars is still visible at the corners of the image
NGC7000, the North America nebula, taken with the same scope and camera but with the feather touch focuser , 9x1200
secs for a total exposure of 3 hrs. No crop here the actual field of view is about 3° at the diagonal. The nebula appears
pinkish as the camera was unmodded
I also use the Megrez 90 for doing some scientific work in the field of photometry. Under urban conditions and near full moon I can shoot variable star fields with exposures up to 300 secs through V and B photometric filters and an SXVF H9. Under these conditions I can measure and study variables from 10th to 15th magnitude, a remarkable outcome.
The star field of variable star S Aurigae taken with a single exposure of 60 secs through a photometric V filter and the
Megrez 90. The red arrow indicates the variable at 11.5 mag. The brightest stars of the frame are 5th mag stars while the
faintest are 15th mag stars. The shot was taken under urban sky and with full moon above the horizon.
The same star field taken, with the same setup and under the same conditions, through a photometric B filter with a single
exposure of 300 secs. Being S Aur a red Mira variable it is now hardly visible (below 16th mag in blue light).
One thing I have noticed is the long time needed by this scope for reaching thermal equilibrium. It takes more than an hour to produce steady images so for visual observing at high powers (250x or more) you are advised to let the scope acclimate outdoors accordingly. In astrophotography I measured a focus shift of about 180 microns after 4 hours of observing and a drop of ambient temperature of about 6°C. Checking focus every half an hour is therefore a must for serious astrophotography.
The Megrez 90 FD is a very good grab&go telescope for doing any kind of astronomy, from visual observations of the planets to wide-field DSO photographing to photometric measures. Although this fluorite doublet displays a bit of false color around the brightest stars, mainly caused by the eyepiece however, this feature does not affect significantly its visual and photographic performance. If you want a higher level of color correction you should consider a triplet design bearing in mind however that complete elimination of color aberrations is not possible in lens objectives and that not all the triplet design always show a more reduced secondary spectrum.
A preliminary star test is necessary to check collimation and to assess the amount of false color around bright stars. If you are interested in astrophotography replacement of the focuser with a good quality real Crayford motorized focuser is advisable. The use of a field flattener is also a must for photographing and you may encounter some difficulties in getting the right model, at least with the older version of the Megrez 90.