Head to head
A four-element Petzval astrograph with the flattener built in, against a four-inch apochromatic triplet that can do almost anything competently. They cost similar money and they answer completely different questions.
Last updated · first published
Specifications on this record are compiled from public manufacturer documentation and have not been independently re-verified. Tell us if something is wrong.
A tiny, expensive, almost foolproof wide-field astrograph. You are paying for the fact that nothing about the optical train can go wrong.
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The classic do-everything four-inch apo: excellent visually, capable on nebulae with a flattener, and never the cheapest way to any single goal.
Links open at the retailer in a new tab. We do not publish prices, because a price we recorded last month is worse than no price at all.
The RedCat is a wide-field imaging instrument with no eyepiece and no ambitions beyond that; the ED102 is twice the aperture, works beautifully at the eyepiece, and needs a flattener and a heavier mount before it images. Choose the RedCat if imaging wide nebulae is the goal, the ED102 if you want one telescope that does several things well.
These two telescopes do not have a single winner. Choose a purpose and the verdict, the winner, the scores and the order of the specification table all change.
Compare for a purpose
The verdict, the winner, the score and the specification order all change with this choice.
Explore Scientific ED102 FCD-100 (OTA) leadsWhat matters for visual →
The RedCat is camera-only by design — there is no visual back and no eyepiece position. The ED102, by contrast, is one of the more enjoyable small visual telescopes available: 102 mm of unobstructed apochromatic aperture at 714 mm focal length, cooling in minutes, never needing collimation, splitting close doubles cleanly and holding excellent contrast on the Moon and planets. If any part of your plan involves looking through the telescope, this is decided before you start.
Derived comparison: Explore Scientific ED102 FCD-100 (OTA) leads by 5.8 points. The editorial verdict above takes precedence over the arithmetic.
Visual observing
Not applicable
William Optics RedCat 51 is a camera-only instrument. There is no eyepiece position, so it cannot be used for visual observing at all — this is a property of the optical design, not a shortcoming to be scored.
Visual observing
5.8 / 10
Confidence: high
Unobstructed, colour-free and long enough for real high-power work; only aperture and price keep it from a higher mark.
| Factor | Score | Weight |
|---|---|---|
| Aperture | 3.7 | 30% |
| Optical qualityeditorial | 9.0 | 20% |
| Ergonomicseditorial | 7.0 | 15% |
| Portability | 6.9 | 15% |
| Mount usabilitySold without a mount — the mount is a separate purchase and a separate decision. | 2.0 | 10% |
| Valueeditorial | 6.0 | 10% |
Weighted total: 5.8. Read the full methodology.
| Specification | William Optics RedCat 51 | Explore Scientific ED102 FCD-100 (OTA) |
|---|---|---|
| Optics | ||
| Aperture | 51 mm (2") | 102 mm (4")Leads here |
| Aperture sets how much you can see. Nothing else substitutes for it. | ||
| Optical design | Four-element Petzval with integrated flattener | Air-spaced apochromatic triplet |
| Focal length | 250 mm (9.8")Depends | 714 mm (28.1")Depends |
| Neither is better — it decides magnification range, not quality. | ||
| Focal ratio | f/4.9Depends | f/7Depends |
| Focal ratio does not make a telescope brighter visually; aperture and magnification do. | ||
| Telescope type | Petzval refractor | Apochromatic refractor |
| Resolving power (Dawes, calculated) | 2.27" | 1.14"Leads here |
| Imaging compatibility | ||
| Usable visually | No — camera only | YesLeads here |
| Mount and tracking | ||
| Mount included | No — OTA onlyDepends | No — OTA onlyDepends |
| A complete package gets you observing tonight. An OTA lets you choose a better mount, at extra cost. | ||
| GoTo | Depends on your mountDepends | Depends on your mountDepends |
| GoTo finds objects; it does not make them brighter. It is a convenience feature, not a purpose. | ||
| Tracking | Depends on your mount | Depends on your mount |
| Size and weight | ||
| OTA weight | 1.6 kg (3.5 lb)Leads here | 4.5 kg (9.9 lb) |
| Tube length | 280 mm (11")Leads here | 730 mm (28.7") |
| Product | ||
| Price band | Premium | Premium |
| We publish bands rather than prices, because a price we scraped last month is worse than no price at all. | ||
| Status | Current | Current |
William Optics RedCat 51 leadsWhat matters for nebulae →
At 250 mm and f/4.9 with a built-in flattener covering a full-frame sensor, the RedCat is a complete wide-field imaging system that runs on a battery-powered star tracker. The ED102 at 714 mm and f/7 needs a separate flattener, a mount rated well above 4.5 kg, and roughly twice the integration time per unit of signal — and it still frames a much smaller piece of sky. The ED102's 0.7× reducer closes the speed gap but not the framing gap.
Derived comparison: William Optics RedCat 51 leads by 2.7 points. The editorial verdict above takes precedence over the arithmetic.
Nebula imaging
8.4 / 10
Confidence: high
Fast, flat, full-frame and light enough for a tracker — the whole design points at exactly this job, with only aperture holding it back.
| Factor | Score | Weight |
|---|---|---|
| Focal ratio | 5.0 | 25% |
| Corrected field | 9.5 | 20% |
| Image circle | 10.0 | 15% |
| Focal length fit | 10.0 | 15% |
| Mount demand | 10.0 | 10% |
| Imaging workfloweditorial | 9.5 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 8.4. Read the full methodology.
Nebula imaging
5.7 / 10
Confidence: medium
Excellent star quality with a flattener, but f/7 is slow and the field is narrower than dedicated wide-field astrographs.
| Factor | Score | Weight |
|---|---|---|
| Focal ratio | 3.0 | 25% |
| Corrected field | 5.0 | 20% |
| Image circleImage circle not published; rated from stated APS-C support. | 7.0 | 15% |
| Focal length fit | 8.7 | 15% |
| Mount demand | 5.4 | 10% |
| Imaging workfloweditorial | 7.0 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 5.7. Read the full methodology.
| Specification | William Optics RedCat 51 | Explore Scientific ED102 FCD-100 (OTA) |
|---|---|---|
| Optics | ||
| Focal ratio | f/4.9Leads here | f/7 |
| A faster system reaches a given signal level on extended targets sooner. | ||
| Focal length | 250 mm (9.8")Leads here | 714 mm (28.1") |
| Scored on fit to large nebulae (roughly 200-600 mm), not on raw length. | ||
| Aperture | 51 mm (2")Depends | 102 mm (4")Depends |
| For extended nebulosity, focal ratio and field matter more than raw aperture. | ||
| Optical design | Four-element Petzval with integrated flattener | Air-spaced apochromatic triplet |
| Telescope type | Petzval refractor | Apochromatic refractor |
| Resolving power (Dawes, calculated) | 2.27" | 1.14"Leads here |
| Imaging compatibility | ||
| Corrected image circle | 44 mm | — |
| Compare against your sensor diagonal: 28.3 mm for APS-C, 43.3 mm for full frame. | ||
| Field correction built in | YesLeads here | No |
| Built-in correction removes a corrector, a spacer stack and a backfocus problem from the optical train. | ||
| Largest sensor supported | Full frameLeads here | APS-C |
| Corrector required for imaging | NoLeads here | Yes |
| Backfocus | 55 mm | 55 mm |
| Backfocus is a hard constraint, not a preference. Your camera, filter drawer and adapters must add up to it. | ||
| Reducer available | No | Yes (×0.7) |
| Usable visually | No — camera only | Yes |
| Mount and tracking | ||
| Mount included | No — OTA onlyDepends | No — OTA onlyDepends |
| Tracking | Depends on your mount | Depends on your mount |
| GoTo | Depends on your mountDepends | Depends on your mountDepends |
| Size and weight | ||
| OTA weight | 1.6 kg (3.5 lb)Leads here | 4.5 kg (9.9 lb) |
| Tube length | 280 mm (11")Leads here | 730 mm (28.7") |
| Product | ||
| Price band | Premium | Premium |
| Status | Current | Current |
Explore Scientific ED102 FCD-100 (OTA) leadsWhat matters for galaxies →
Both are too short for serious galaxy work. The ED102 at 714 mm reaches about 1.09 arcseconds per pixel with a typical sensor, which handles the larger Messier galaxies acceptably; the RedCat at 250 mm gives about 3.1 arcseconds per pixel, at which nearly every galaxy is a few dozen pixels across. Neither has the aperture to reach faint outer structure quickly. If galaxies are the priority, both of these are the wrong purchase.
Derived comparison: Explore Scientific ED102 FCD-100 (OTA) leads by 1.3 points. The editorial verdict above takes precedence over the arithmetic.
Galaxy imaging
5.3 / 10
Confidence: high
At 250 mm focal length the image scale is far too coarse for galaxies; this is a framing problem, not a quality problem.
| Factor | Score | Weight |
|---|---|---|
| Focal length fit | 0.8 | 25% |
| Resolving power | 0.7 | 20% |
| Corrected field | 9.5 | 15% |
| Optical qualityeditorial | 8.5 | 15% |
| Mount demand | 10.0 | 10% |
| Imaging workfloweditorial | 9.5 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 5.3. Read the full methodology.
Galaxy imaging
6.6 / 10
Confidence: medium
Sharp optics and reasonable focal length make larger galaxies workable, but 102 mm limits how faint you can reach in practice.
| Factor | Score | Weight |
|---|---|---|
| Focal length fit | 8.3 | 25% |
| Resolving power | 4.6 | 20% |
| Corrected field | 5.0 | 15% |
| Optical qualityeditorial | 9.0 | 15% |
| Mount demand | 5.4 | 10% |
| Imaging workfloweditorial | 7.0 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 6.6. Read the full methodology.
| Specification | William Optics RedCat 51 | Explore Scientific ED102 FCD-100 (OTA) |
|---|---|---|
| Optics | ||
| Focal length | 250 mm (9.8") | 714 mm (28.1")Leads here |
| Scored on fit to small galaxies (roughly 900-2000 mm). | ||
| Aperture | 51 mm (2") | 102 mm (4")Leads here |
| Resolving power (Dawes, calculated) | 2.27" | 1.14"Leads here |
| Focal ratio | f/4.9Depends | f/7Depends |
| Optical design | Four-element Petzval with integrated flattener | Air-spaced apochromatic triplet |
| Telescope type | Petzval refractor | Apochromatic refractor |
| Imaging compatibility | ||
| Field correction built in | YesLeads here | No |
| Corrected image circle | 44 mm | — |
| Largest sensor supported | Full frameLeads here | APS-C |
| Corrector required for imaging | NoLeads here | Yes |
| Backfocus | 55 mm | 55 mm |
| Reducer available | No | Yes (×0.7) |
| Usable visually | No — camera only | Yes |
| Mount and tracking | ||
| Tracking | Depends on your mount | Depends on your mount |
| Mount included | No — OTA onlyDepends | No — OTA onlyDepends |
| GoTo | Depends on your mountDepends | Depends on your mountDepends |
| Size and weight | ||
| OTA weight | 1.6 kg (3.5 lb)Leads here | 4.5 kg (9.9 lb) |
| Tube length | 280 mm (11")Leads here | 730 mm (28.7") |
| Product | ||
| Price band | Premium | Premium |
| Status | Current | Current |
Explore Scientific ED102 FCD-100 (OTA) leadsWhat matters for planetary →
Planetary imaging needs aperture and long focal length. The ED102 has 102 mm and 714 mm — modest on both counts, but unobstructed optics give it a genuine contrast advantage and it produces respectable lunar and planetary results with a Barlow. The RedCat's 51 mm resolves to about 2.3 arcseconds, worse than typical seeing, and its 250 mm focal length would need roughly tenfold amplification. Neither is a planetary instrument, but only one is in the conversation.
Derived comparison: Explore Scientific ED102 FCD-100 (OTA) leads by 1.7 points. The editorial verdict above takes precedence over the arithmetic.
Planetary imaging
3.8 / 10
Confidence: high
Tiny aperture and very short focal length make planetary imaging pointless with this instrument.
| Factor | Score | Weight |
|---|---|---|
| Aperture | 1.0 | 30% |
| Focal length fit | 0.0 | 25% |
| Optical qualityeditorial | 8.5 | 20% |
| Collimation & thermals | 10.0 | 10% |
| TrackingNo mount supplied — tracking quality depends entirely on the mount you pair it with. | 5.0 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 3.8. Read the full methodology.
Planetary imaging
5.5 / 10
Confidence: medium
Optically excellent and thermally quick, but a four-inch aperture caps planetary resolution well below what a large SCT reaches.
| Factor | Score | Weight |
|---|---|---|
| Aperture | 3.7 | 30% |
| Focal length fit | 3.3 | 25% |
| Optical qualityeditorial | 9.0 | 20% |
| Collimation & thermals | 10.0 | 10% |
| TrackingNo mount supplied — tracking quality depends entirely on the mount you pair it with. | 5.0 | 10% |
| Valueeditorial | 6.0 | 5% |
Weighted total: 5.5. Read the full methodology.
| Specification | William Optics RedCat 51 | Explore Scientific ED102 FCD-100 (OTA) |
|---|---|---|
| Optics | ||
| Aperture | 51 mm (2") | 102 mm (4")Leads here |
| Aperture sets the resolution ceiling before seeing takes over. | ||
| Focal length | 250 mm (9.8") | 714 mm (28.1")Leads here |
| Long native focal length reduces how much amplification you have to add. | ||
| Resolving power (Dawes, calculated) | 2.27" | 1.14"Leads here |
| Calculated from aperture. Real nights are usually limited by seeing well before this. | ||
| Focal ratio | f/4.9Depends | f/7Depends |
| Optical design | Four-element Petzval with integrated flattener | Air-spaced apochromatic triplet |
| Telescope type | Petzval refractor | Apochromatic refractor |
| Imaging compatibility | ||
| Corrected image circle | 44 mm | — |
| Field correction built in | YesLeads here | No |
| Backfocus | 55 mm | 55 mm |
| Largest sensor supported | Full frameLeads here | APS-C |
| Corrector required for imaging | NoLeads here | Yes |
| Reducer available | No | Yes (×0.7) |
| Usable visually | No — camera only | Yes |
| Mount and tracking | ||
| Tracking | Depends on your mount | Depends on your mount |
| Mount included | No — OTA onlyDepends | No — OTA onlyDepends |
| GoTo | Depends on your mountDepends | Depends on your mountDepends |
| Size and weight | ||
| OTA weight | 1.6 kg (3.5 lb)Leads here | 4.5 kg (9.9 lb) |
| Tube length | 280 mm (11")Leads here | 730 mm (28.7") |
| Product | ||
| Price band | Premium | Premium |
| Status | Current | Current |
| Aspect | William Optics RedCat 51 | Explore Scientific ED102 FCD-100 (OTA) |
|---|---|---|
| Flattener | Built into the optical designLeads here | Sold separately, spacing must be set correctly |
| The single biggest practical difference for a first-time imager. One telescope has an imaging configuration; the other has an imaging project. | ||
| Realistic minimum mount | A battery-powered star trackerLeads here | A mid-size equatorial mount |
| Mount cost usually dominates an imaging budget, and 1.6 kg versus 4.5 kg changes which mounts are viable by a wide margin. | ||
| Light gathering | Baseline | About 4× the RedCatLeads here |
| Aperture area scales as the square of diameter. 102 mm collects four times what 51 mm does, which partially offsets the slower focal ratio. | ||
Written as specifications these look like neighbours: small refractors, similar money, both with good glass. In use they have almost no overlap at all.
The RedCat is a 250 mm f/4.9 camera lens in a telescope tube. It has one job, one configuration and one accessory requirement, and it is very good at that job. The ED102 is a 714 mm f/7 apochromat that observes, images, splits doubles and shows planetary detail, and does none of those things as well as a specialist would.
The RedCat’s flattener is inside the optical design. Attach a camera at 55 mm backfocus and you are done; there is no spacing to iterate, no third-party adapter introducing tilt, and no corner-star troubleshooting.
The ED102 needs a matched flattener or the 0.7× reducer/flattener, and getting the spacing right is a real task the first time. It is not difficult — it is measured once and then forgotten — but it is the stage where a meaningful number of beginners give up.
If this is your first imaging telescope, that difference is worth more than any specification on the sheet.
The ED102 collects four times the light. That sounds decisive until you notice it also frames a sky area roughly eight times smaller.
For a target that fits both fields, the ED102 wins on signal. For a target that only fits the RedCat’s field, the ED102 cannot photograph it at all without mosaicking. The Veil complex, the North America and Pelican pair, the Rho Ophiuchi region — these are RedCat subjects, and no amount of aperture changes that.
RedCat: 1.6 kg with an Arca-Swiss compatible ring. It goes on a star tracker on a photo tripod in a field, runs off a power bank, and packs in a camera bag. Setup is minutes. There is nothing to collimate and nothing to decide.
ED102: 4.5 kg on a 730 mm lever. It needs a real mount, and a real mount needs a real tripod. Setup is longer. In exchange you can pull the camera off, put a diagonal in, and spend an hour on the Moon — which the RedCat can never do.
If you are buying your first telescope and wide-field astrophotography is why, buy the RedCat and put the saved effort into the mount.
If you want a telescope rather than an astrograph — something to look through, something that handles several kinds of night — buy the ED102, budget for the flattener at the same time, and accept that your nebula fields will be narrower.
Required, not included
Worth adding
Required, not included
Worth adding
Sections on optical design, mount demands, sensor compatibility and thermal behaviour are covered in the pair-specific analysis above and on each product page.
ED refractorOTA only
The most sensible entry into wide-field deep-sky imaging, provided you budget for the flattener it does not include.
Maksutov-CassegrainOTA only
The best lunar and planetary view available from a telescope this small, bought at the cost of a very narrow field and a long cooldown.
Scores are derived from a fixed weighting of measurable specifications plus four declared editorial ratings, applied identically to every telescope on this site. Verdicts are written per pair and per purpose, and they override the arithmetic when the arithmetic misses something. We have not tested these telescopes side by side and do not claim to have.
Read the full methodology · Report an error in this comparison
Last updated · first published