Telescope Comparison
Askar 103APO vs Vixen SD81S
The specs are close. The experience isn't.
First light
Askar · 103mm · £1,199
The custom-rig optical tube
- 103mm refractor — optical tube only, no mount included
- 700mm focal length at f/6.8
- Requires a compatible mount before you can observe anything
- Best for: observers who already own a suitable mount or are building a specific imaging rig
- Not a complete purchase — budget at least £100–300 extra for a mount before observing
Vixen · 81mm · £1,199
The custom-rig optical tube
- 81mm refractor — optical tube only, no mount included
- 625mm focal length at f/7.72
- Requires a compatible mount before you can observe anything
- Best for: observers who already own a suitable mount or are building a specific imaging rig
- Not a complete purchase — budget at least £100–300 extra for a mount before observing
The full picture
The numbers that separate these two scopes — and what they mean at the eyepiece.
Aperture
Askar 103APO gathers 1.6× more light. On bright targets — Moon, Saturn, Jupiter — you won't notice. On fainter targets — dim galaxies, faint globular clusters — the gap is real.
Focal length
Askar 103APO's longer focal length reaches higher magnification with the same eyepiece — better reach for planetary detail. Vixen SD81S's shorter focal length gives a wider true field — better for large open clusters and extended nebulae.
Focal ratio
Askar 103APO's faster f/6.8 delivers wider fields with any eyepiece — better for open clusters and large nebulae. Vixen SD81S's f/7.72 provides more magnification per eyepiece — better for fine planetary detail.
Mount type
Neither scope includes a mount — both require a separate purchase before you can observe.
Weight (OTA)
Vixen SD81S's optical tube is 1.8kg lighter. Relevant if you plan to use it on multiple mounts or carry the tube to dark-sky sites separately.
Optical design
Both are refractors — no mirrors to collimate, good contrast, colour-free stars with ED or APO glass. The differences between them are in aperture, focal ratio, and glass quality.
At the eyepiece
| Target | Askar 103APO | Vixen SD81S |
|---|---|---|
| Planets | ||
| Moon | Excellent 103mm aperture delivers sharp crater detail and clean terminator views; the ED triplet produces essentially no chromatic fringing on the bright limb | Excellent 81mm aperture with superb colour correction delivers crisp, fringe-free lunar detail; f/7.7 supports rewarding high-magnification views |
| Saturn | Good Rings clearly defined, Cassini Division visible in steady seeing; 700mm focal length supports useful magnification but aperture limits fine banding detail | Good Rings clearly defined, Cassini Division visible in good seeing; 625mm focal length limits image scale but clean optics compensate |
| Jupiter | Good Two main equatorial belts and GRS visible; 103mm resolves some secondary belts in good seeing but can't match larger apertures for fine atmospheric detail | Good Main equatorial belts and GRS visible with high contrast and no false colour; aperture limits finer belt detail |
| Mars | Moderate Small disc visible with polar cap detectable near opposition; 103mm and 700mm focal length limit the detail available on this demanding target | Challenging Small disc visible at opposition with possible polar cap hint, but 81mm aperture cannot resolve surface albedo features |
Deep sky | ||
| Orion Nebula (M42) | Excellent 103mm gathers plenty of light and 700mm focal length frames the full nebula complex well; Trapezium resolved and nebulosity extends visually | Excellent 625mm focal length frames the nebula well; 81mm gathers enough light to show core structure and nebulosity wings |
| Andromeda Galaxy (M31) | Excellent 700mm focal length keeps the full extent of M31 in the field; 103mm aperture shows the bright core and hints of dust lanes | Excellent 625mm focal length captures the galaxy's full extent; core and dust lanes visible, though outer halo is faint at 81mm |
| Open clusters | Excellent 700mm focal length and wide true field frame showpiece clusters like the Double Cluster and Pleiades beautifully | Excellent Wide true field at 625mm beautifully frames clusters like the Pleiades and Double Cluster with pinpoint stars |
| Globular clusters | Moderate 103mm shows a granular, textured ball but cannot resolve individual stars in the core; M13 and M3 appear mottled at best | Challenging 81mm cannot resolve individual stars — globulars appear as fuzzy, concentrated glows |
| Faint galaxies | Moderate 103mm aperture detects brighter Messier galaxies as smudges but struggles with fainter NGC targets visually | Moderate Brighter Messier galaxies visible as diffuse patches; 81mm lacks the light grasp for structure or fainter NGC targets |
| Milky Way / wide field | Good 700mm is slightly long for sweeping starfield views but still delivers rich fields; a reducer brings it closer to wide-field territory | Good 625mm focal length is moderately wide; rich starfields are enjoyable but the scope is too narrow for grand sweeping views |
Other | ||
| Double stars | Good 103mm resolves doubles to about 1.1 arcsecond; f/6.8 is not ideal for high-magnification splitting but the clean optics help | Good Clean optics and near-zero chromatic aberration make this a satisfying double star scope; Dawes limit around 1.4 arcseconds |
| Astrophotography (deep sky) | Not recommended No mount or tracking included; with a suitable equatorial mount this scope would rate Excellent — f/6.8, 103mm aperture, and ED triplet design are ideal for deep-sky imaging | Not recommended No mount or tracking included; when paired with a suitable equatorial mount this would rate Excellent (81mm, f/7.7, superb correction) |
| Astrophotography (planetary) | Moderate 103mm aperture captures reasonable planetary detail with a high-speed camera, but aperture and focal length limit resolution compared to larger scopes | Moderate Clean optics suit planetary capture, but 81mm aperture and 625mm focal length limit resolution and image scale |
| Emission nebulae (imaging) | Excellent 700mm at f/6.8 frames large emission nebulae like the Heart, Soul, and North America Nebula well on APS-C sensors; tight star correction across the field with a matched flattener | Not applicable |
| Galaxy groups (imaging) | Good 700mm focal length provides enough scale for galaxy groups like the Leo Triplet or M81/M82 on common sensor sizes while keeping good signal-to-noise at f/6.8 | Not applicable |
The real tradeoff
Both scopes are capable. The question is which one fits the way you actually observe.
Askar 103APO
- You'll capture noticeably more signal per frame — 103mm collects 62% more light than 81mm, and at f/6.8 you're gathering photons faster than the SD81S's f/7.7, which means shorter sub-exposures or deeper stacks on faint nebulae and galaxy fields.
- You'll also feel every gram of that extra aperture when you're loading gear into the car — the OTA is roughly double the weight of the Vixen, which means your mount needs to be a solid mid-range EQ platform like an HEQ5 or equivalent, not a lightweight travel mount.
- You're buying a triplet ED design, so colour correction is handled at the optical level with less compromise — when you're stretching histogram data on bright star fields, you'll appreciate the absence of residual chromatic aberration that even good doublets can show.
Vixen SD81S
- You'll build your imaging sessions around portability — at 2.4kg, the SD81S rides comfortably on smaller equatorial mounts like the Star Adventurer GTi or AZ-GTi in EQ mode, which means you're more likely to actually get out on a work night.
- You'll find yourself reaching for an eyepiece more often than you expected — Vixen's SD glass delivers genuinely crisp, high-contrast lunar and planetary views that punch above what 81mm typically offers, making this a surprisingly satisfying visual scope between imaging runs.
- You'll pay for the Vixen ecosystem — the proprietary dovetail may need adapters for your existing mount, and the dedicated SD flattener is another £250–£350 on top of the OTA price, but you're buying into a system where every component is designed to work together without fuss.
The dark side
Every scope has a personality. Here’s where each one gets difficult.
Askar
Askar 103APO
At f/6.8, the 103APO is moderately fast but not a speed demon — if you're coming from an f/5 Newtonian astrograph, you'll notice longer exposures are needed to reach the same signal-to-noise on faint emission nebulae.
The OTA weighs 5–6kg fully loaded with camera and accessories, which rules out lightweight travel mounts and pushes your total imaging rig cost well past £3,000 once you factor in a capable equatorial mount, guide scope, and flattener.
You'll need Askar's matched field flattener or reducer for sharp stars at the edges of APS-C or larger sensors — the native field curvature without one will show as elongated stars in your corners.
Vixen
Vixen SD81S
This is an SD doublet, not a triplet — while colour correction is excellent for a two-element design, it is not truly apochromatic across the full visual spectrum, and aggressive stretching of bright star data may reveal slight residual colour the Askar's triplet avoids.
81mm of aperture is a hard physical limit — no amount of optical quality compensates for the light-gathering deficit against a 103mm scope, and faint galaxies or planetary nebulae will simply require significantly longer total integration times.
Vixen's proprietary dovetail and accessory mounting system means you'll likely need adapters to fit this OTA to common third-party equatorial mounts, adding cost and potential points of flex to your imaging train.
Which is right for you?
Two different buyers. Two different right answers.
The custom-rig optical tube
Askar · Askar 103APO
You're an intermediate astrophotographer who's outgrown an 80mm refractor and wants a meaningful step up in light grasp and resolution without jumping to a full-frame Newtonian rig. You already own — or are ready to invest in — a solid mid-range equatorial mount, and you're building a system around deep-sky imaging rather than casual visual observing. You don't mind the extra weight and setup time because you're chasing fainter targets at longer focal lengths, and you want a triplet's colour correction to hold up when you're stacking hours of data on emission nebulae or galaxy groups.
The custom-rig optical tube
Vixen · Vixen SD81S
You're an astrophotographer who values actually getting out under the sky over maximising aperture on paper. You want a lightweight OTA that pairs with a portable mount, fits in a carry-on bag, and still delivers genuinely high-quality data on widefield nebulae and starfields. You also enjoy visual observing between imaging sessions and appreciate clean, contrasty views of the Moon and planets — the SD81S rewards that dual-use approach. This isn't for you if chasing faint targets is your priority, because no amount of optical refinement makes 81mm collect as many photons as 103mm.
Our verdict
At similar price points, these scopes offer different amounts of aperture per pound. The Askar 103APO gives you more light-gathering for your money — and for visual observing, aperture per pound is the most useful single metric.
For pure optical value, the Askar 103APO is the stronger pick. The Vixen SD81S compensates with other features — decide whether those trade-offs justify the premium. If I had to choose: the Askar 103APO — more aperture per pound means more sky.
Askar 103APO
View Askar 103APO →Vixen SD81S
View Vixen SD81S →Deep field: Full specifications
Every data point, for those who want to go further.
Full specifications
Fields highlighted in blue or amber indicate the better value for that spec. Data is manufacturer-stated and may vary.
How much can it see?
| Spec | Askar 103APO | Vixen SD81S |
|---|---|---|
Apertureⓘ The most important spec — bigger = more light = better views | 103mm | 81mm |
Focal Length Longer = more magnification potential | 700mm | 625mm |
Focal Ratio Lower f-number = wider field of view; higher = more magnification per eyepiece | f/6.8 | f/7.72 |
Optical Design The type of optics — each design has different strengths | Refractor | Refractor |
Coatings Better coatings = more light transmission through the optics | Fully multi-coated ED triplet on all air-to-glass surfaces | Fully multi-coated SD (Super Duplex) glass doublet |
How do you point it?
| Spec | Askar 103APO | Vixen SD81S |
|---|---|---|
Mount Type The mechanical system that holds and moves the telescope | None (OTA only) | None (OTA only) |
GoTo Computer-controlled pointing — finds any of thousands of objects automatically | ||
Tracking Motor keeps objects centred as the Earth rotates — essential for astrophotography |
The focuser
| Spec | Askar 103APO | Vixen SD81S |
|---|---|---|
Focuser Size 2" accepts wider eyepieces and gives better low-power views | 2" / 1.25" | 2" |
Focuser Type Rack-and-pinion is standard; Crayford and dual-speed are smoother | Dual-speed Crayford 2" (10:1 reduction) | Dual-speed Crayford (with 1.25" adapter) |
Size & weight
| Spec | Askar 103APO | Vixen SD81S |
|---|---|---|
OTA Weightⓘ Optical tube only — useful for comparing mount load capacity | 3.8kg | 2kg |
Tube Length | 550mm | 540mm |
Tube Material | Aluminium | Aluminium |
What's in the box?
| Spec | Askar 103APO | Vixen SD81S |
|---|---|---|
Diagonal Tilts the eyepiece 90° for comfortable viewing — useful on refractors |
Blue highlight: Askar 103APO advantage · Amber highlight: Vixen SD81S advantage · Greyed cells: equal or subjective.