Head to head
The two telescopes most often recommended as a first deep-sky astrograph. One is a 72 mm ED doublet that needs a flattener; the other is a 51 mm Petzval with the flattener already inside it. The price difference is real and so is the reason for it.
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.
The most sensible entry into wide-field deep-sky imaging, provided you budget for the flattener it does not include.
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 We do not publish prices, because a price we recorded last month is worse than no price at all.
A tiny, expensive, almost foolproof wide-field astrograph. You are paying for the fact that nothing about the optical train can go wrong.
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 We do not publish prices, because a price we recorded last month is worse than no price at all.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 removes the two things beginners most often get wrong — corrector choice and spacing — and covers a full-frame sensor doing it. The Evostar gives you twice the light-gathering area and a working eyepiece for less money, provided you are willing to buy and set up a flattener.
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.
Sky-Watcher Evostar 72ED (OTA) leadsWhat matters for visual →
The Evostar takes a diagonal and eyepieces and makes a pleasant, sharp, low-maintenance grab-and-go telescope. 72 mm is a small aperture and it will not compete with a reflector on faint objects, but it frames the Pleiades beautifully and shows the Orion Nebula's shape well. The RedCat has no visual back and no eyepiece position at all — this is a property of the Petzval astrograph design as implemented, not a missing accessory.
Derived comparison: Sky-Watcher Evostar 72ED (OTA) leads by 5.7 points. The editorial verdict above takes precedence over the arithmetic.
Visual observing
5.7 / 10
Confidence: medium
Sharp and colour-clean for its size, but 72 mm is a small aperture and this is a supplementary visual instrument rather than a main one.
| Factor | Score | Weight |
|---|---|---|
| Aperture | 2.3 | 30% |
| Optical qualityeditorial | 7.5 | 20% |
| Ergonomicseditorial | 7.0 | 15% |
| Portability | 9.7 | 15% |
| Mount usabilitySold without a mount — the mount is a separate purchase and a separate decision. | 2.0 | 10% |
| Valueeditorial | 8.5 | 10% |
Weighted total: 5.7. Read the full methodology.
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.
| Specification | Sky-Watcher Evostar 72ED (OTA) | William Optics RedCat 51 |
|---|---|---|
| Optics | ||
| Aperture | 72 mm (2.8")Leads here | 51 mm (2") |
| Aperture sets how much you can see. Nothing else substitutes for it. | ||
| Optical design | Air-spaced ED doublet | Four-element Petzval with integrated flattener |
| Focal length | 420 mm (16.5")Depends | 250 mm (9.8")Depends |
| Neither is better — it decides magnification range, not quality. | ||
| Focal ratio | f/5.8Depends | f/4.9Depends |
| Focal ratio does not make a telescope brighter visually; aperture and magnification do. | ||
| Telescope type | ED refractor | Petzval refractor |
| Resolving power (Dawes, calculated) | 1.61"Leads here | 2.27" |
| Imaging compatibility | ||
| Usable visually | YesLeads here | No — camera only |
| 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 | 1.9 kg (4.2 lb) | 1.6 kg (3.5 lb)Leads here |
| Tube length | 335 mm (13.2") | 280 mm (11")Leads here |
| Product | ||
| Price band | Mid-range | Premium |
| We publish bands rather than prices, because a price we scraped last month is worse than no price at all. | ||
| Status | Current | Current |
Effectively equalWhat matters for nebulae →
The RedCat is faster to a correct configuration and covers a larger sensor; the Evostar collects twice the light and reaches a similar effective focal ratio once its reducer/flattener is fitted. At 250 mm against a reduced 357 mm, they frame different but overlapping target lists. Neither is meaningfully better than the other at producing a good wide-field nebula image — the difference is how much of your first six months goes into troubleshooting corner stars.
Derived comparison: William Optics RedCat 51 leads by 1.9 points. The editorial verdict above takes precedence over the arithmetic.
Nebula imaging
6.5 / 10
Confidence: high
Focal length, weight and correction all land in the right place for large nebulae; only the required flattener and modest aperture hold it back.
| Factor | Score | Weight |
|---|---|---|
| Focal ratio | 4.1 | 25% |
| Corrected field | 5.0 | 20% |
| Image circleImage circle not published; rated from stated APS-C support. | 7.0 | 15% |
| Focal length fit | 10.0 | 15% |
| Mount demand | 7.7 | 10% |
| Imaging workfloweditorial | 7.0 | 10% |
| Valueeditorial | 8.5 | 5% |
Weighted total: 6.5. Read the full methodology.
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.
| Specification | Sky-Watcher Evostar 72ED (OTA) | William Optics RedCat 51 |
|---|---|---|
| Optics | ||
| Focal ratio | f/5.8 | f/4.9Leads here |
| A faster system reaches a given signal level on extended targets sooner. | ||
| Focal length | 420 mm (16.5")Equal | 250 mm (9.8")Equal |
| Scored on fit to large nebulae (roughly 200-600 mm), not on raw length. | ||
| Aperture | 72 mm (2.8")Depends | 51 mm (2")Depends |
| For extended nebulosity, focal ratio and field matter more than raw aperture. | ||
| Optical design | Air-spaced ED doublet | Four-element Petzval with integrated flattener |
| Telescope type | ED refractor | Petzval refractor |
| Resolving power (Dawes, calculated) | 1.61"Leads here | 2.27" |
| 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 | No | YesLeads here |
| Built-in correction removes a corrector, a spacer stack and a backfocus problem from the optical train. | ||
| Largest sensor supported | APS-C | Full frameLeads here |
| Corrector required for imaging | Yes | NoLeads here |
| 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 | Yes (×0.85) | No |
| Usable visually | Yes | No — camera only |
| 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 | 1.9 kg (4.2 lb) | 1.6 kg (3.5 lb)Leads here |
| Tube length | 335 mm (13.2") | 280 mm (11")Leads here |
| Product | ||
| Price band | Mid-range | Premium |
| Status | Current | Current |
Sky-Watcher Evostar 72ED (OTA) leadsWhat matters for galaxies →
Neither telescope has a serious claim on galaxy imaging. The Evostar at 420 mm reaches about 1.85 arcseconds per pixel with a typical sensor, so the larger Messier galaxies are workable if unspectacular; the RedCat at 250 mm gives about 3.1 arcseconds per pixel, which reduces nearly every galaxy to a compact smudge. If galaxies matter to you, both of these are the wrong class of telescope.
Derived comparison: the two score within a rounding error of each other. The editorial verdict above takes precedence over the arithmetic.
Galaxy imaging
5.4 / 10
Confidence: high
At 420 mm the image scale is far too coarse for anything but the largest nearby galaxies.
| Factor | Score | Weight |
|---|---|---|
| Focal length fit | 4.5 | 25% |
| Resolving power | 2.3 | 20% |
| Corrected field | 5.0 | 15% |
| Optical qualityeditorial | 7.5 | 15% |
| Mount demand | 7.7 | 10% |
| Imaging workfloweditorial | 7.0 | 10% |
| Valueeditorial | 8.5 | 5% |
Weighted total: 5.4. Read the full methodology.
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.
| Specification | Sky-Watcher Evostar 72ED (OTA) | William Optics RedCat 51 |
|---|---|---|
| Optics | ||
| Focal length | 420 mm (16.5")Leads here | 250 mm (9.8") |
| Scored on fit to small galaxies (roughly 900-2000 mm). | ||
| Aperture | 72 mm (2.8")Leads here | 51 mm (2") |
| Resolving power (Dawes, calculated) | 1.61"Leads here | 2.27" |
| Focal ratio | f/5.8Depends | f/4.9Depends |
| Optical design | Air-spaced ED doublet | Four-element Petzval with integrated flattener |
| Telescope type | ED refractor | Petzval refractor |
| Imaging compatibility | ||
| Field correction built in | No | YesLeads here |
| Corrected image circle | — | 44 mm |
| Largest sensor supported | APS-C | Full frameLeads here |
| Corrector required for imaging | Yes | NoLeads here |
| Backfocus | 55 mm | 55 mm |
| Reducer available | Yes (×0.85) | No |
| Usable visually | Yes | No — camera only |
| 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 | 1.9 kg (4.2 lb) | 1.6 kg (3.5 lb)Leads here |
| Tube length | 335 mm (13.2") | 280 mm (11")Leads here |
| Product | ||
| Price band | Mid-range | Premium |
| Status | Current | Current |
Sky-Watcher Evostar 72ED (OTA) leadsWhat matters for planetary →
Planetary imaging needs aperture and long focal length, and these have 72 mm at 420 mm and 51 mm at 250 mm respectively. The Evostar's larger aperture resolves to about 1.6 arcseconds against the RedCat's 2.3, both of which are worse than or comparable to ordinary seeing. There is no configuration in which either produces a competitive planetary image, and a modest Maksutov will beat both comprehensively.
Derived comparison: Sky-Watcher Evostar 72ED (OTA) leads by 0.3 points. The editorial verdict above takes precedence over the arithmetic.
Planetary imaging
4.1 / 10
Confidence: high
Aperture and focal length are both far below what planetary work needs, and no amount of amplification substitutes for either.
| Factor | Score | Weight |
|---|---|---|
| Aperture | 2.3 | 30% |
| Focal length fit | 0.0 | 25% |
| Optical qualityeditorial | 7.5 | 20% |
| Collimation & thermals | 10.0 | 10% |
| TrackingNo mount supplied — tracking quality depends entirely on the mount you pair it with. | 5.0 | 10% |
| Valueeditorial | 8.5 | 5% |
Weighted total: 4.1. Read the full methodology.
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.
| Specification | Sky-Watcher Evostar 72ED (OTA) | William Optics RedCat 51 |
|---|---|---|
| Optics | ||
| Aperture | 72 mm (2.8")Leads here | 51 mm (2") |
| Aperture sets the resolution ceiling before seeing takes over. | ||
| Focal length | 420 mm (16.5")Equal | 250 mm (9.8")Equal |
| Long native focal length reduces how much amplification you have to add. | ||
| Resolving power (Dawes, calculated) | 1.61"Leads here | 2.27" |
| Calculated from aperture. Real nights are usually limited by seeing well before this. | ||
| Focal ratio | f/5.8Depends | f/4.9Depends |
| Optical design | Air-spaced ED doublet | Four-element Petzval with integrated flattener |
| Telescope type | ED refractor | Petzval refractor |
| Imaging compatibility | ||
| Corrected image circle | — | 44 mm |
| Field correction built in | No | YesLeads here |
| Backfocus | 55 mm | 55 mm |
| Largest sensor supported | APS-C | Full frameLeads here |
| Corrector required for imaging | Yes | NoLeads here |
| Reducer available | Yes (×0.85) | No |
| Usable visually | Yes | No — camera only |
| 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 | 1.9 kg (4.2 lb) | 1.6 kg (3.5 lb)Leads here |
| Tube length | 335 mm (13.2") | 280 mm (11")Leads here |
| Product | ||
| Price band | Mid-range | Premium |
| Status | Current | Current |
| Aspect | Sky-Watcher Evostar 72ED (OTA) | William Optics RedCat 51 |
|---|---|---|
| Corrected image circle | APS-C with the matched flattener | 44 mm — covers full frameLeads here |
| Only matters if you own or plan to own a full-frame camera, but if you do, it is decisive. | ||
| Total accessories needed before first light | Flattener plus spacers plus adapters | One 55 mm backfocus adapterLeads here |
| The most common reason a first imaging project stalls is an optical train that has not been spaced correctly. | ||
| Light gathering | About 2× the RedCatLeads here | Baseline |
| 72 mm against 51 mm. Roughly halves the integration time for the same signal, all else equal. | ||
| Usable visually | Yes, with a diagonalLeads here | No — camera only |
| A genuine capability difference rather than a matter of degree. | ||
Both telescopes will produce a good wide-field nebula image. Anyone telling you one is clearly superior is comparing specifications rather than outcomes.
The question is where you want your difficulty to sit. The Evostar puts it in the optical train: choose the right flattener, get the spacing right, verify corner stars, adjust. The RedCat puts it in the price: pay more, attach camera, focus, shoot.
72 mm has about twice the collecting area of 51 mm. That is roughly a halving of integration time for the same signal-to-noise, which over a season of imaging is a lot of clear-sky hours.
Against that, the RedCat covers a 44 mm image circle against the Evostar’s APS-C-with-flattener, and it does so with no possibility of a spacing error. For a full-frame camera owner that is not a convenience, it is a capability.
The Evostar’s matched flattener sets a required distance — typically 55 mm — between its rear shoulder and the camera’s sensor plane. Your camera contributes some of that distance, your filter drawer or adapter contributes more, and you make up the remainder with precision spacer rings.
Get it right and the corners are clean. Get it wrong by two or three millimetres and the corner stars elongate radially or tangentially depending on the direction of the error. Diagnosing which way to move is the single most common first-year frustration in deep-sky imaging.
It is not hard. It is a measurement, done once. But it is a step, and the RedCat does not have it.
1.9 kg against 1.6 kg sounds like nothing, and by itself it is. Add rings, a guide scope, a camera and a dew heater and the Evostar package reaches a point where a small star tracker starts to struggle; the RedCat package usually does not.
Since the mount is normally the most expensive part of a beginner’s imaging setup, this can quietly be the deciding factor.
If you already own a small equatorial mount and an APS-C camera, buy the Evostar and the matched reducer/flattener together. You get more aperture, an eyepiece option, and change left over.
If you are starting from a camera and a tripod, or you shoot full frame, buy the RedCat. What you are paying for is the absence of an entire category of problem — and for a first astrograph, that is worth real money.
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.
Apochromatic refractorOTA only
The classic do-everything four-inch apo: excellent visually, capable on nebulae with a flattener, and never the cheapest way to any single goal.
Achromatic refractorOTA only
A cheap, light, genuinely fun rich-field refractor for sweeping the Milky Way, badly miscast as a planetary or imaging telescope.
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