8 Inch vs 10 Inch Dobsonian Telescope: What the Extra 2 Inches Actually Cost

The 8 inch vs 10 inch Dobsonian telescope question usually gets framed as portability against power, with the 10-inch cast as the scope you have to measure your car for. For the two solid-tube Dobsonians Explore Scientific makes in these sizes, that picture is mostly wrong. The 10-inch tube is 1.25 inches longer than the 8-inch, the complete scope weighs 7 pounds more, and because both have a focal length of about 1,200mm, every eyepiece you own gives the same magnification and the same field of view in either one.

What the extra two inches really cost is girth, half again the price, and a faster f/4.7 mirror that asks more of your collimation and your eyepieces. What they buy is about 47 percent more light on every faint object. Every weight and dimension below comes from Explore Scientific's instruction manuals and product pages for the three Dobsonians we carry in these sizes, and the optical figures are calculated from them.

TL;DR: Quick Summary▾

The 10-inch costs you girth, weight and fussier optics; tube length and eyepieces stay the same.

Choose the 10-inch f/4.7 if the scope can live on the ground floor or in a garage. It puts about 47 percent more light through the same eyepieces, for 7 pounds more.

Choose the 8-inch f/5.9 if you carry it up and down stairs or this is your first reflector. Its pieces are lighter, its tube is slimmer, and its optics tolerate about twice as much collimation error.

Choose the 10-inch hybrid truss if storage space is the limit. It breaks down small, but it weighs more than the solid-tube 10-inch.

Compare all three in our Dobsonian telescopes collection.

Table of Contents▾

• 8 Inch and 10 Inch Dobsonians at a Glance: What the Numbers Mean
• 8 Inch vs 10 Inch Dobsonian: The Head-to-Head Comparison
• What Can You See With an 8 Inch or 10 Inch Dobsonian?
• Real-World Scenarios: Which Size Fits?
• Best 8 Inch and 10 Inch Dobsonians We Carry
• Verdict: Should You Buy an 8 Inch or 10 Inch Dobsonian?
• Frequently Asked Questions

8 Inch and 10 Inch Dobsonians at a Glance: What the Numbers Mean

A Dobsonian is a Newtonian reflector on a rocker base that you move by hand. Four numbers explain how the 8-inch and 10-inch differ, and our guide to telescope specifications covers each term in more depth.

Aperture. Explore Scientific's "8-inch" mirror is actually 208mm (8.2 inches) across, and its 10-inch is 254mm. The bigger mirror has 49 percent more area. Subtract the shadow each scope's secondary mirror casts (46mm and 64mm across) and the 10-inch collects about 47 percent more light. In star terms that is about 0.4 magnitude: from the same site, the 10-inch shows stars roughly 0.4 magnitude fainter.

Focal length and focal ratio. The 8-inch has a 1,219mm focal length and the 10-inch 1,200mm. Focal ratio is focal length divided by aperture, so putting a wider mirror behind the same length of tube makes the 10-inch "faster": f/4.7 against f/5.9. Magnification is the telescope's focal length divided by the eyepiece's, so near-identical focal lengths give near-identical magnifications. Explore Scientific's own figures for the supplied 25mm Plössl are 49x in the 8-inch and 48x in the 10-inch.

Exit pupil. Divide an eyepiece's focal length by the focal ratio and you get the width of the beam of light leaving the eyepiece. With that 25mm Plössl it is 4.3mm in the 8-inch and 5.3mm in the 10-inch. The magnification is the same, so the wider beam is where the extra light goes.

Resolution. By the Dawes limit, the 8-inch can split stars 0.56 arcseconds apart and the 10-inch 0.46. In practice the atmosphere usually decides: Al Nagler wrote in Sky & Telescope that telescopes larger than about 9 inches seldom reach their theoretical resolution because of atmospheric seeing. On most nights the 10-inch's real advantage is the light.

8 Inch vs 10 Inch Dobsonian: The Head-to-Head Comparison

The table sets the three Dobsonians we carry in these sizes side by side. The 8-inch Ruby Red edition shares the black 8-inch's optics and tube, so one column covers both.

Weights, dimensions and focal lengths come from Explore Scientific's instruction manuals and product pages; light, Dawes limit, magnification and exit pupil are calculated from them. The hybrid's product page lists a heavier mirror box (about 35 lb) than its manual does, so treat its weights as approximate. Prices are MSRP.

Light Gathering: 47 Percent More on Every Faint Object

Light is the one advantage the 10-inch delivers every clear night. Its extra 0.4 magnitude reaches stars the 8-inch can't, resolves more of the stars in a globular cluster, and makes a galaxy's faint outer glow easier to hold in view. Because the two scopes share a focal length, the same eyepiece gives the same magnification and the same patch of sky in each, and the 10-inch fills that view with about half again as much light.

Magnification and Field of View: Your Eyepieces Work the Same in Both

Fast focal ratios are often sold as wide-field optics, and against a long-focus telescope they are. Against this 8-inch they aren't: true field is set by focal length, and the 10-inch's 1,200mm is within 2 percent of the 8-inch's 1,219mm. An eyepiece collection built for the 8-inch moves to the 10-inch unchanged, and both take 2-inch and 1.25-inch eyepieces.

The hybrid truss is the outlier. Its 1,270mm focal length gives 4 to 6 percent more magnification, and a correspondingly narrower field, with the same eyepiece: 51x with the 25mm.

Weight and Size: 7 Pounds and 2 Inches of Girth

Each solid-tube scope comes apart into a tube and a rocker base, so you carry it in two trips. The 8-inch tube weighs 20.0 lb and its base 18.7 lb; the 10-inch's weigh 24.2 lb and 21.5 lb. That is 4.2 lb more in your arms on the heavier trip. Length barely changes (45.5 inches against 46.75), but the 10-inch tube is 11.9 inches across against 10, and that extra girth is what you notice in a doorway or on a staircase.

The 10-inch also raises the eyepiece: 48 inches off the ground with the tube pointed straight up, against 44.5 inches for the 8-inch.

The hybrid truss saves space and adds weight. It breaks down into a mirror box, a rocker base, a short upper tube and eight 25.2-inch truss poles, so there is no 4-foot tube to store. But the mirror box alone weighs 31.5 lb by Explore Scientific's manual, more than either solid tube, and the whole scope comes to about 54 lb. Choose it to fit a small car or a closet.

Collimation: The 8-Inch Is About Twice as Forgiving

Collimation means lining up a reflector's mirrors so their axes meet, and how precisely you have to do it depends steeply on focal ratio. Collimation specialist Vic Menard puts the tolerance for the primary mirror's axis at 0.005mm times the focal ratio cubed. That works out to about 1.0mm of allowable error at f/5.9, 0.6mm at f/5 and 0.5mm at f/4.7, so the 8-inch gives you roughly twice the margin of the solid 10-inch.

In practice the 8-inch stays sharp through more bumps between checks, and the 10-inch rewards a quick check at the start of each session. Neither is hard to collimate. Both primary mirrors come center-marked, and Explore Scientific's manual advises, "Close enough is good enough, especially for an inexperienced person." The solid tubes don't include a collimation tool, though a simple collimation cap is enough to start. The hybrid includes a tool for turning its primary mirror's collimation screws, since it is reassembled every session. Our guide to Newtonian collimation walks through the procedure.

Coma and Eyepieces: What f/4.7 Asks of You

Every Newtonian shows coma: stars away from the center of the field stretch into small comet shapes. Coma grows with the inverse square of the focal ratio, so at the same distance from center the f/4.7 shows about 1.5 times the coma of the f/5.9. The diffraction-limited center of the field, where stars stay pinpoint, is about 12.6 arcminutes across in the 8-inch and 6.7 in the 10-inch. Optical designer Al Nagler, quoted in Astronomy magazine, explains that coma "becomes bothersome to most people when the mirror has a focal ratio of f/5 or faster."

Simple eyepieces add to it. Astronomy Now rates budget Plössls as working well "down to around f/6", which puts the 8-inch at the edge of their comfort zone and the 10-inch past it. Explore Scientific's own manual warns that with cheap eyepieces, edge-of-field stars "will be even more distorted because the off-axis aberrations of the telescope and the eyepiece will add up."

The supplied 25mm Plössl handles finding and framing in either scope. For sharp field edges at f/4.7, plan on better wide-field eyepieces. If you want pinpoint stars all the way to the edge, the HR Variable Coma Corrector ($249.99) is a photo-visual design that works at the eyepiece as well as with a camera. Our guide to choosing eyepieces covers apparent field and eye relief.

Low-Power Eyepieces: The 10-Inch Tops Out Sooner

The fast focal ratio has a second effect. Your dark-adapted pupil sets the widest beam your eye can use, and it shrinks with age: in a 2011 study of 263 people, Bradley and colleagues measured average dark-adapted pupils of 6.6mm in people in their 30s, 5.8mm in their 50s and 5.6mm in their 60s, with wide variation between individuals. An eyepiece whose exit pupil is wider than yours wastes aperture. For a typical observer in their 50s, the longest eyepiece that still uses the full mirror is about 34mm in the 8-inch, 29mm in the hybrid and 27mm in the 10-inch f/4.7.

A 32mm eyepiece in the 10-inch f/4.7 produces a 6.8mm beam, so that same observer uses only about 216mm of the mirror at that power, roughly what the 8-inch offers. Build the 10-inch's low-power end around a 24mm to 27mm wide-field instead.

Cool-Down: Give Either Scope Half an Hour

A mirror warmer than the night air makes the view shimmer until it cools. Explore Scientific recommends letting either solid-tube scope sit outside for 20 to 30 minutes before observing. Neither has a cooling fan, and Explore Scientific doesn't publish mirror thickness, which Bryan Greer's thermal research for Sky & Telescope found matters far more than diameter, so there is no measured cool-down time for either.

What Can You See With an 8 Inch or 10 Inch Dobsonian?

Under the same sky, the 10-inch shows everything the 8-inch does, and more of it. How much more depends on the target, and on the sky more than the mirror.

The Moon and planets. Both scopes show Saturn's rings, Jupiter's cloud belts and fine lunar terrain, and on steady nights both reach the Cassini Division. On the planets the atmosphere usually sets the limit before either mirror does.

Star clusters. In Telescopic Watch's review of the Sky-Watcher 8-inch Classic 200P Dobsonian, Zane Landers reports that globular clusters such as M2, M3, M5, M13, M15 and M92 "show a fair amount of stars" at that aperture. At 10 inches (his review of Celestron's StarSense Explorer 10-inch) his assessment moves to resolving "most of the globular star clusters in the Messier catalog and many of the brighter NGC globular clusters." This is where the extra 0.4 magnitude shows most plainly, because a globular's stars are points of light and the extra reach applies to every one of them.

Galaxies. Spiral structure needs both aperture and a dark sky. Roger Clark compared M51, the Whirlpool Galaxy, through several apertures under dark Colorado skies: at 8 inches "considerably more detail on the spiral arms is visible" than at 6, and at 12.5 inches "smaller detail in the spiral arms becomes apparent." The 10-inch sits between those steps. Landers puts it in practical terms: an 8-inch "can just show a hint of spiral arms" in M51 from a dark site, while a 10-inch can show the spiral arms of "M51, M81, M101, and M33" to "a careful eye" under dark skies. Clark's drawings show the progression better than any description.

Under a bright sky. Every one of those galaxy observations depends on a dark site. From a light-polluted backyard both scopes still deliver the Moon, planets, double stars and bright clusters, but faint galaxies fade in both, and Landers notes that a few galaxies, such as M74 and M83, "may be difficult to see if you suffer from light pollution" even at 8 inches. If your sky is bright and you rarely travel, the 10-inch's advantage narrows to star clusters and the brighter deep-sky objects. Our guide to the Bortle scale helps you rate your own sky.

Real-World Scenarios: Which Size Fits?

"It will live in the garage or on the ground floor." Take the 10-inch f/4.7. If you carry it 20 feet rather than up a flight of stairs, its extra 7 pounds stop mattering, and the 47 percent more light is there every night.

"I carry it up and down stairs." Take the 8-inch. Its heaviest piece is 20.0 lb against 24.2, and its tube is 10 inches across instead of 11.9, which is easier to hold close on a staircase.

"This is my first reflector." Take the 8-inch. Its collimation tolerance is about twice the 10-inch's, and its f/5.9 mirror is kinder to the supplied Plössl and to whatever budget eyepieces you add first. Learning collimation on the 10-inch is entirely doable; the 8-inch simply forgives more while you learn.

"My backyard is in town." Either works. The 10-inch still shows more stars in every cluster, but the galaxy detail it is bought for needs a dark site, so if you never travel, the 8-inch gives up less than the numbers suggest.

"I drive to dark sites." Measure your car first: the 10-inch tube is 46.75 by 11.9 inches, the 8-inch's 45.5 by 10. If the 10-inch fits, take it, because a dark site is where its extra light pays most. If neither fits, the hybrid truss packs down.

"I live in an apartment with little storage." The hybrid truss packs into a mirror box, a rocker and a set of poles, with no 4-foot tube to store. Accept that it weighs more in total and needs assembly and a collimation check each time you set it up.

Best 8 Inch and 10 Inch Dobsonians We Carry

All four carry Explore Scientific's limited lifetime warranty for the original owner, with two years on electronic parts such as the finder.

Best 10-Inch for a Ground-Floor Home: Explore Scientific 10" f/4.7 Dobsonian

At $899.99 MSRP, the Explore Scientific 10" f/4.7 Dobsonian is our pick for anyone who doesn't have to carry far. Its 254mm mirror sits in a rolled-steel tube barely longer than the 8-inch's, with a 2-inch Crayford focuser and a spring-tension system that keeps the tube balanced when you add heavier eyepieces. It ships with a 25mm Plössl (48x), a finder, a smartphone adapter and a dust cover, and Explore Scientific's video below shows the two-piece assembly.

The trade-off: f/4.7 is the most demanding focal ratio here. Plan on a collimation check each session, and on better eyepieces or a coma corrector if sharp field edges matter to you.

Best 8-Inch: Explore Scientific 8" f/5.9 Dobsonian

At $599.99 MSRP, the Explore Scientific 8" f/5.9 Dobsonian is the lighter, more forgiving choice and the one we'd point a first-time reflector owner toward. Its 208mm mirror sits in a rolled-steel tube on the same style of rocker base and spring-tension system as the 10-inch, with a 2-inch Crayford focuser, a 25mm Plössl (49x), a finder, a smartphone adapter and a dust cover.

The trade-off: the 10-inch collects about 47 percent more light. The 8-inch costs a third less and weighs 7 pounds less.

Best 8-Inch Starter Kit: Explore Scientific 8" f/5.9 Dobsonian (Ruby Red)

At $699.99 MSRP, the Explore Scientific 8" f/5.9 Dobsonian (Ruby Red) has the same optics as the black 8-inch in a red finish, and the extra $100 buys an upgraded eyepiece kit: a 25mm 70-degree wide-angle in place of the Plössl, a 10mm Plössl and a 2x Barlow. That gives three magnifications out of the box: 49x, 122x and 244x, with the 1.25-inch Barlow on the 10mm. At 49x the 70-degree eyepiece takes in about 1.4 degrees of sky, against about 1 degree for a standard 50-degree Plössl.

The trade-off: if you already own eyepieces, the black 8-inch is the same telescope for less.

Best 10-Inch for Tight Storage: Explore Scientific 10" f/5 Hybrid Truss Dobsonian

At $899.99 MSRP, the Explore Scientific 10" f/5 Hybrid Truss Tube Dobsonian puts the same 254mm aperture on a short solid upper tube and eight truss poles, over a mirror box with a 9-point mirror cell. It ships with more than the solid tubes: 25mm and 10mm Plössls (51x and 127x), a finder, a collimation tool, a moon filter, a moon map and a planisphere. Zane Landers of Telescopic Watch scored its optics and mount 5 out of 5 and wrote that its smooth motions make panning around while looking through the eyepiece "an absolutely pleasurable experience."

The trade-off: it weighs more than the solid 10-inch (about 54 lb in total, with a 31.5 lb mirror box), it needs assembly and a collimation check each session, and its open truss lets stray light and dew reach the optics. Explore Scientific's Dobsonian shroud for 10" and 12" truss scopes ($99.99) closes the truss.

Truss designs are one way Dobsonians grow past 10 inches; our 16-inch Dobsonian deep dive covers what that size asks of you.

Verdict: Should You Buy an 8 Inch or 10 Inch Dobsonian?

Choose the 10-inch f/4.7 if the scope can live on the ground floor or in a garage. For 7 extra pounds and the same eyepieces, it collects about 47 percent more light, which under a dark sky brings out more detail in a galaxy's spiral arms. The Explore Scientific 10" f/4.7 is the pick.

Choose the 8-inch f/5.9 if you carry it up stairs or this is your first reflector. Lighter pieces, a slimmer tube and about twice the collimation tolerance make it the easier scope to live with. Get the black 8-inch if you own eyepieces, or the Ruby Red edition if you don't.

Choose the 10-inch hybrid truss if storage space is your hard limit. It breaks down to fit a small car or a closet but weighs more than the solid 10-inch. The Explore Scientific 10" f/5 Hybrid Truss is the one for that situation.

Still deciding? Ask where the scope will live before you ask how far you can carry it. On the ground floor, the 10-inch's extra weight is a non-issue; up and down stairs every night, the 8-inch is the scope you will actually use. Still comparing telescope types? Our telescope buyer's guide covers refractors and catadioptrics alongside reflectors.

Frequently Asked Questions

Is a 10-inch Dobsonian worth it over an 8-inch?▾

For buyers who don't carry the scope far, yes. Comparing Explore Scientific's two solid tubes, the 10-inch collects about 47 percent more light through the same eyepieces at the same magnifications, for 7 pounds more and half again the price. The gain shows most on globular clusters and on galaxies under dark skies. If you carry the scope up stairs every session, or observe only from a bright city sky, the 8-inch gives up less than those numbers suggest.

How much does an 8-inch or 10-inch Dobsonian weigh?▾

Explore Scientific's 8-inch f/5.9 weighs 38.7 lb in total: a 20.0 lb tube and an 18.7 lb base. Its 10-inch f/4.7 weighs 45.7 lb: a 24.2 lb tube and a 21.5 lb base. The 10-inch hybrid truss comes to about 54 lb, and its heaviest piece is a 31.5 lb mirror box. Each solid-tube scope separates into tube and base, so you can carry it in two trips.

Can one person carry a 10-inch Dobsonian?▾

It is built to be carried in two trips: the Explore Scientific 10-inch separates into a 24.2 lb tube, 11.9 inches across, and a 21.5 lb base. The tube's girth is the awkward part on stairs, which is why we point stair-carriers to the 8-inch and its 20.0 lb tube.

Will my eyepieces work in both scopes?▾

Yes. Both Explore Scientific solid tubes take 2-inch and 1.25-inch eyepieces, and their focal lengths (1,219mm and 1,200mm) are within 2 percent of each other, so any eyepiece gives the same magnification and field in either. At low power the 10-inch tops out sooner: for a typical observer in their 50s, the longest eyepiece that uses its full aperture is about 27mm, against 34mm in the 8-inch.

Do I need a coma corrector for a 10-inch f/4.7 Dobsonian?▾

It isn't required. Coma, the comet-shaped stretching of stars toward the edge of the field, is about 1.5 times stronger at f/4.7 than at f/5.9, and Al Nagler puts the point where it becomes bothersome to most people at f/5 or faster. If you want pinpoint stars across a wide-field eyepiece, a photo-visual corrector such as the HR Variable Coma Corrector removes it. The 8-inch, at f/5.9, sits above Nagler's threshold.

What magnification can I use with an 8-inch or 10-inch Dobsonian?▾

The textbook maximum is twice the aperture in millimeters, about 416x for the 8-inch and 508x for the 10-inch, but the atmosphere rarely allows it. Al Nagler's guide for the most planetary detail is 20x to 30x per inch of aperture: roughly 160x to 250x for the 8-inch and 200x to 300x for the 10-inch. On an average night, expect to work at the lower end of those ranges.

Can you do astrophotography with a Dobsonian?▾

For the Moon and planets, yes. Both Explore Scientific solid tubes include a smartphone adapter that holds a phone at the eyepiece for snapshots. A manual Dobsonian doesn't track the sky's motion, so long-exposure deep-sky imaging needs a tracking mount.

What can you see with an 8-inch Dobsonian?▾

The Moon, Saturn's rings, Jupiter's cloud belts, double stars, star clusters and the brighter galaxies and nebulae. Telescopic Watch's review of the Sky-Watcher 8-inch Classic 200P Dobsonian says the aperture "is enough to show you the entire Messier catalog", with globular clusters such as M13 beginning to resolve into stars. A 10-inch can show the spiral arms of a few galaxies to a careful eye under dark skies.