Review on 🔭 Celestron C90 Mak Spotting Scope (Black) for Advanced Optical Exploration by Justin Santiago

My device was defective. It had an off-center primary septum. This resulted in poor illumination of one side of the eyepiece field. However, I can make the following observations: The optics were sharp with the supplied eyepiece. The eyepiece looks like a GSO/Celestron Omni Plössl, but with a standard coating instead of a multi-coating. In that respect it's not bad. The only difficulty is that the opening in the deflector (16mm) is not wide enough to support the full field of the eyepiece. The outer 5-10% of the field only darkens. Skywatcher sells the same optics with the 25mm Plössl, which is a better choice. Additionally, discussions on astronomy websites suggest that the C90 may have a longer focal length than advertised. Closer to 1450mm not 1260. What else is there looks normal. Contrast isn't great, but Astrozap's dew guard helps, as does the flocking on the inside of the main bezel. Viewfinder isn't the best. I don't see a way to focus the crosshairs. They look a bit sharp when I use the close focus lenses on my trifocals. The direct method is suitable for ground-based observations, but is much less efficient when pointing the telescope at stars. The Orion 6x30 rectangular correct viewfinder is a good substitute. This is Maksutov's design. You'll never get by on low power, and a 40x or 50x low zoom (depending on which focal length you count) is pretty big for terrestrial viewing. There is a way to improve this, but it will cost you money. You will need an adapter that allows you to use standard Schmidt-Cassegrain accessories, a Celestron f/6.3 corrector and a 1.25" Celestron focus back. This reduces the low-power magnification from a claimed 39x to 25x, while reducing the unilluminated portion of the eyepiece field by more than half. You get something close to a 2 degree field of view, and it's sharp. So here it is, a cheap, imperfect scope that could use more money and be upgraded. Update from December 10th, 2016: One of the three screws for attaching the main mirror and focus unit was broken off at the factory, put back into the hole and the scope sent on its journey. With 100% defects in two scopes, I shoot a star. Update December 14, 2016: The third scope has arrived. He's wrong too. The half inch of the front panel is very shiny because the manufacturer Synta never bothered to paint this piece black. The rating has been reduced by another star as this is unforgivable. Correcting a septum defect with a septum lock worked well, but the scope has the added problem of spherical aberration. The C90 does not focus at 100x. The images below looked blurry. Star tests show extreme non-uniformity on either side of focus. It was one of the worst star tests I've seen in 50 years of watching. This third scope is back. Edit 10-2017: In December 2016 I bought an Orion (USA) 90mm Maksutov. Like the C90, it is made by Synta. He also suffers from a deviated septum. The disadvantage is by far not as serious as with the first copy of the C90. However, if you want to check the collimation using the daylight reflection method, you will get wrong results. I ended up collimating with a star. The result is a scope with a magnification of 160x. The contrast is good, even without dew protection. Has proven to be a good travel option. It provided a beautiful view of the Sun during the August 2017 eclipse, slightly showing the Sun's grainy surface.