I thought I would share my impressions with my latest telescope purchase. I saw an earlier thread regarding if this scope was any good. Jim Riffle
Spectrum Optical Instruments --- ExploraPro 70mm Maksutov
Specs: 70mm x 750mm f/10.7 or just (2 3/4” x 30” fl.)
Measured diameters: Entrance dia. of Mak corrector 70mm. Exit diameter of corrector is 72mm. Diameter of primary mirror is 78mm. Painted aluminum tube 3 1/2” dia. X 7” long. Supplied diagonal is std 1 1/4” roof prism type for correct image orientation. 3 nice metal barrel eyepieces of 20mm, 10mm and 4mm focal length + 2x Barlow lens. Secure snap-on main lens cover. Finder is red dot type with variable brightness adjustment.
One arm fork mount is nice diecast aluminum. Elv and Az slow motions are by metal worm gear sets with clutches that must be loosened to move mount and re-tightened for worm gears to grip. OTA is centered over tripod and the outward fork allows for beyond zenith viewing. Nice size center leg lock tray with 3 eyepiece holes. Tripod/mount weighs 5 pounds. OTA is 2 pounds. Both together easily a one handed carry in and out.
Tripod legs are extruded aluminum and very solid, with only 1 single leg extension for a short tripod, but fine for a low chair/stool use. Rubber tips at ends.
Overall impression: The box was so big, I thought for sure that they sent a refractor by mistake. Turned out to be just good packing with a fully assembled mount/tripod. The better instrument quality became obvious as I unpack.
Bench tested optical tube in double pass auto-collimation against an optical flat with 200 line Ronchi light. Very good correction is noted. Made sure to focus at infinity prior to test and diagonal removed for test. Noted slight collimation error which was adjusted out with 2.5mm hex wrench (not supplied).
Daytime visual @ 2 mile target. Focus was smooth and slow motions had no noticeable cross talk when reversing directions (good). Focus was very touchy/critical with a slightest motion making a difference in sharpness. Some annoying azimuth torquing at leg mounting hub was minimized with ¼ turn to tighten 3 leg attachment bolts with 5mm hex wrench (not supplied). Noted some stray light from inside of baffle tube. It was threaded, but with too fine of threads to be effective. Close focus distance tried and came in at just 68”!!!! FOV at that distance is just barely a silver dollar. Macroscope!.
The cell phone adapter has a really solid bayonet system of attachment to the eye end of 20 and 10 mm eyepieces. You need to remove their rubber eyecups for access. Unfortunately the eyepiece projection distance is not adjustable and I needed to make a 0.20” thick spacer washer and then find a longer coupling 1/4-20 screw. Now I can get the full-circular evenly illuminated field view on my camera screen. The Sarblue 60 “out of the box” remains my favorite for daytime cell photos. The Bluetooth shutter fob worked great.
Evening Astronomical use: I had a cheap 25mm eyepiece on hand I used for deep sky. The Pleades fit in nicely in a non cramped field at 30x. I could just see the elliptical companion galaxy next to M31 as well at the elongation plane of M31. The finder had a fat red dot and also some parallax which annoyingly diminished it's accuracy. The slow motion controls were nice to use. Next Jupiter. The 10 mm (75x) showed the 2 main cloud bands and pin point moons. With the 2x Barlow @ 150X it was just as sharp. I tried the 4 mm @200x with less detail (ran out of light/contrast). Might have been my 1948 eyesight. I noticed no image flaring or degradation from this roof prism diagonal. The slow motions were invaluable at centering and tracking. The elevation flex shaft was very close to the side of my head and precluded using my left eye unless I rotated the diagonal to the left side some. The moon views were very crisp. There is no discernible image shift focusing at high power. Image damping in about 2 sec.
Mods: Aside from tightening the 3 tripod leg bolts, next day I loosened the worm mesh on both axis (not for the average hobbyist to try). When the morning temp got down to 35F the focusing became very/too stiff. I changed out the greases on sliding baffle tube with light oil and it was still stiff after cooling. The plastic rear cell hole had shrunk on the brass focusing bushing as the temp dropped. Next morning I had to carefully machine about.0015” from the O.D of the brass to allow good low temp focusing. Also had to remove grease from focusing screw and use light oil. I also removed a primary mirror baffle extension tube which just screwed on (it wasn't needed and so lets in more light now). This small 70 mm Mak is now a nice little keeper to me.
Tip: Mirror alignment adjustment is by 6 screws on plastic rear plate. The 3 higher screws are locking screws. The lower 3 are for adjustment. To readjust, lightly loosen the top 3 firstly before adjusting a lower screw and then re-tighten top locking screws lightly at completion. The mirror assembly weighs just a few ounces so go easy on tightness. 2.5 mm wrench needed.
Overall: I am nicely impressed with the engineering in this. They had needle bearings on hardened races for the slow motion clutches. Nice solid metal worm gear sets with necessary adjustments. Great optical quality, nice eyepiece set, secure cellphone coupler method, though not at the correct spacing for my I-phone. I like the tripod leg stability and useful eyepiece tray. I like the fact the OTA is centered over the tripod for minimal vibration. I especially like the low price and high overall quality. Mine cost $138 + tax on EBAY with 3 day delivery.
But this is just a small/inexpensive 2 3/4” dia. Maksutov, however with optical quality par with a 3 1/2” Questar I owned 35 years ago. I said optical quality, not performance.\
About the reviewer: I have been more of a deepsky observer/astro-imager over the years and not so much a visual solar system viewer. I do appreciate sharp-coma free optics and tiny tight stars. I have professionally built and sold many 12” Maksutov telescopes in the past. I currently own a Celestron 127mm Mak that has very fine optics and a older C-8 used for Fastar CCD imaging from the past. I also have both 60 & 70mm Sarblue Maksutovs and a custom 50mm Mikro-Mak on a Mikro-mount.
As a side note, I personally do not favor the Maksutov Cassegrain optical design for daytime spotting telescopes due to their required internal light baffling and secondary mirror shadow. Both of these issues lower contrast. Refractor optical designs are superior in comparison as visual spotter scopes.
One image shows Sarblue 70 on right with Spectrum 70.
Edited by Jim Riffle, 03 December 2022 - 12:55 PM.