The Optical (Flex)Tube Assembly
The SkyWatcher FlexTube 200P or Sky-Watcher’s 8″ Collapsible Dobsonian is an 8” f/6 Newtonian (203mm Aperture, 1200mm Focal Length), with the same optics as those found in Orion, Apertura, Zhumell, SkyWatcher classic, and GSO dobsonians. The main difference between this telescope and others of its size is the collapsible “FlexTube” functionality. This feature allows it to be stored more compactly than a traditional dobsonian. The OTA is extended by untightening three thumbscrews, pulling up the secondary mirror assembly until there’s a subtle click into place, and then retightening the thumbscrews.
The OTA measures 9.375” diameter and 44” long. It’s 33” when collapsed. The optical tube is 24 lbs, which is actually not lighter than the other 8” tubes (the missing mass of the optical tube is made up for by the large metal struts).
The secondary mirror assembly contains a finderscope mount, a 2” Crayford focuser, and a 1.88” secondary mirror. The primary mirror assembly carries the locking thumbscrews, collimation thumbscrews on the backside, and naturally the 8” primary mirror. The two assemblies are joined by three black metal rods.
The focuser is easy to use and very smooth. It’s a single-speed 2” Crayford focuser, and occasionally you may need to adjust the tension thumbscrews. But it’s easy enough to use and we occasionally let guests with poor vision focus the telescope themselves. Sometimes guests will nudge the eyepiece and push the focuser in, though, which is a small unfortunate side effect of the Crayford design. It accepts 2” barrel eyepieces, though I’ve never used them in this telescope. 2” eyepieces allow for wider apparent fields of view.
To summarize the optical tube review of SkyWatcher 8″ collapsible dobsonian, the optical quality is very good, and I’ve never had a problem getting some spectacular views of the planets and Moon. Some observatory guests claim some objects look better in the 8” dob than the 24” CDK in the dome! The telescope excels for both deep sky objects (even at a moderately light-polluted suburban college) and planets.
A Dobsonian mount is the best bang for your buck. A large tripod mount capable of carrying an f/6 8” Newtonian would cost another few hundred dollars and would not be portable. John Dobson pioneered making large Newtonian telescopes with cheap but very sturdy and easy to use wooden rockerbox/lazy-susan bases. The rockerbox of the SkyWatcher 200P (Classic and FlexTube models) is a sleek curved shape made of painted particle board. The mount must be assembled by the user, but this is not a particularly difficult task, and the tools to assemble it are included. It takes about an hour, maybe two.
As opposed to the simple Teflon strips of a traditional Dobsonian’s altitude bearings, the SkyWatcher 200P uses handle/knobs sticking out the side which are screwed through the rockerbox onto the optical tube assembly. The altitude bearings can be tightened or loosened by twisting the handles. When fully tightened it’s nearly impossible to change the altitude of the telescope; when fully loosened, it is very sensitive to changes in balance from heavy eyepieces. You should be untightening to move and then retightening when you’ve found the object, but in practice, I end up just keeping it at some middle-ground tightening. Forcing the telescope to move when the bearings are too tight can wear the bearings out over time, so keep that in mind.
To aim the telescope, sight down the back of the tube towards the part of the sky you’re looking for. You can look behind the finderscope with one eye, looking for the object with the other eye. Then you look into the finderscope to finish aiming.
It is not very difficult to nudge the telescope in the direction of the sky’s motion, even when tracking at very high magnifications.
The mount is very easy to use, and within minutes of being introduced to one, most people are able to at least find the Moon or bright stars.
Finding objects must be done manually, requiring you to learn starhops to find any objects you can’t see by the unaided eye. This is actually a good thing. It’s a lot more fun for the beginner as a way to learn the night sky! GoTo functionality would double the price of the telescope anyway.
The only difficulty with dobsonians is a fundamental flaw of alt/az telescopes: Dobson’s Hole. Pointing very near the top of the sky (the Zenith) vastly reduces the accuracy and ease of pointing the telescope, since you have to rotate very far around the azimuth axis to move a small amount to the other side of the Zenith. It’s also difficult to get your head behind the telescope to sight along the tube.
The front of the mount has a sturdy plastic handle and a metal eyepiece rack, both of which are useful.
The FlexTube 200P comes with the standard fare for a large dobsonian: a 25mm Plossl and a 10mm Plossl. These are appropriate for deep sky viewing and some introductory planetary viewing and provide 48x and 120x respectively.
The FlexTube 200P also includes a very useful 8×50 Right Angle Correct Image Finderscope (RACI Finder). It provides 8x magnification and a view bright enough to see many deep sky objects you might be looking for as dim blobs. The right angle of the eyepiece is incredibly useful. It really saves you from neck and back strain!
The telescope has a dust cap with a removable secondary cap to act as an aperture stop for viewing bright objects like the Moon, Venus, Mars, and possibly Jupiter. Using the aperture stop does however limit the useful magnification to about 100x. Due to the open tube, it can be attached either at the top of the telescope or at the front of the primary mirror assembly. When at the front of the primary mirror tube (middle of the telescope), it will reduce the image brightness by twice as much as if it’s at the front of the secondary mirror assembly (top of the telescope) as the light gets blocked both coming in the telescope and coming out the primary mirror in the former configuration.
What to get next? You’ll want to acquire a large collection of other eyepieces over time. The telescope is capable of a maximum magnification of 400x, which is about the limit of atmospheric seeing effects even on very good nights. 400x can be attained with a 3mm eyepiece, or perhaps a 6mm with a 2x barlow. Minimum magnification of 29x is achieved with a 40mm eyepiece. The largest useful 1.25” eyepiece is 32mm, so you’ll want a 2” barrel eyepiece for the 40mm eyepiece. Avoid Plossls under 10mm focal length, instead, you’ll want a more specialized planetary eyepiece. The Goldline-type eyepiece provides a relatively cheap, wide-angle alternative.
The Collapsible Tube: Pros & Cons.
The most notable aspect of this telescope is its collapsible “FlexTube.” The advantage of portability is significant.
With a traditional dobsonian, the optical tube must be stored separately from the dobsonian base when height is an issue, as in a car–the base must be stored upright and the tube stored sideways.
However, the FlexTube is short enough that it can be stored fully assembled upright in an SUV; or in a smaller car, sideways along the back seat.
At 50 lbs, the telescope is heavy, when fully assembled. It could be taken off of the mount and the two taken out separately, but you’d have to be careful not to lose any of the handles–it’s not really intended. If you must carry the telescope more than a few feet, you’ll need a cart, a wheelbarrow, or something.
Big Newtonian telescopes tend to need a lot of time for their mirrors to cool off before they can operate at peak performance. This can take 30-60 minutes for a big 8” telescope. However, the FlexTube’s open optical tube does help the telescope cool off a fair bit quicker than normal, so you’ll be getting crisp views sooner.
There are, however, some problems with an open tube design. Light from other parts of the sky can leak in through the sides of the tube and make the sky background appear brighter than usual, lowering contrast. It has the effect of removing about an inch of aperture. It also makes it impossible to safely use a solar filter–even a safe full-aperture solar filter will only cover the top of the telescope, leaving the sides open to dangerously concentrated sunlight.
The good news is both problems can be solved with a “sock,” or a large black fabric tube to wrap the sides of the telescope up with, without sacrificing the collapsibility of the telescope.
What can you see?
Goodness gracious, what can’t you see? That’s a bit of an exaggeration perhaps, but this is a very capable instrument. The telescope does struggle to show very wide-field views–the Pleiades, the entirety of the Beehive, the Coat Hanger, they are all incomplete through this telescope (at least, when seen with a 1.25” barrel eyepiece, you might fit the whole thing in the FOV if you use an aftermarket 2” eyepiece)
This telescope will show all of the Messier objects aside from a small handful even in suburban skies. It can split dozens of double stars, a good handful of which are very pretty.
You’ll see the rings of Saturn including the Cassini division, the moons and cloud banding of Jupiter, Jupiter’s Great Red Spot, Jovian transit shadows. Easily see the Venusian phases, barely see the Mercurian phases. Subtle albedo features on Mars will be visible.
The Moon is shown in phenomenal clarity and is always an impressive sight.
Some of my favorite objects to view through this telescope are M42 (The Great Nebula in Orion), M13 (the Great Globular Cluster in Hercules), the double star Alberio, Saturn, and the Double Cluster of Perseus (h & χ Persei).
The weight concern, which applies pretty much equally to all full-size dobs, means that it’s not necessarily a good fit for all observers. If you must keep the telescope up in the house and must take it downstairs to get it out to a place you can observe with, or you’re physically disabled, or out of shape, or you’re buying for a child; you may find yourself rarely using the telescope–in that case, a tabletop dobsonian such as the AWB OneSky/SkyWatcher Heritage 130P, Orion StarBlast 4.5 or 6, or Zhumell Z130 might be a better option due to their Grab-N-Go capability.
Pretty much any 8” dobsonian you can buy is going to have pretty much identical optical performance. What you’re differentiating is the value of different accessories and the price. GSO in Taiwan makes the telescopes that are sold in the US under the names Zhumell and Apertura. Orion also makes some good dobsonians.
Other quality telescopes in this price range will have smaller apertures. Telescopes like the Omni XLT 150 newtonian, the Omni XLT 102 refractor, provide quality serious instruments on high quality German Equatorial Mounts. However, such telescopes are not good beginner instruments as their mounts are difficult to understand and set up. GoTo scopes in this price range will be even smaller, to the point where there’s not really anything you can see with GoTo scopes of this price that can’t easily be found manually. (Besides, a GoTo manual isn’t very fun to read compared to a finder chart or an observing guide.)