The Rainbow Astro RST-135 is a premium, counterweight-free harmonic mount weighing about 3.3 kg yet carrying 13.5 kg — ideal for travel imaging. Its periodic error is characterized (not guaranteed) at about ±30 arcseconds over a 430-second cycle, so autoguiding is essential. The encoder-equipped RST-135E cuts that to ±2.5″ for near-unguided use.
Verdict up front
The RST-135 is a travel-and-portability play first: a harmonic head that carries more than four times its own weight, no counterweight required, at a price of admission that isn’t obvious from the spec sheet — autoguiding isn’t optional, PEC doesn’t exist on the base model, and Rainbow Astro itself declines to guarantee the tracking figure it publishes. That’s not a hidden flaw so much as the honest trade this mount makes for its weight class, and it rewards a spec-literate buyer who already knows what they’re signing up for.
If you’re deciding between this mount and ZWO’s AM3N specifically — the other serious travel-harmonic contender — that head-to-head, including which one wins on bare weight versus which wins on guaranteed tracking, is covered start to finish in our travel harmonic mount guide; this review stays on the RST-135 and RST-135E on their own terms rather than re-running that verdict.
The evidence base behind this review is thinner than our EQ6-R Pro and AM5N deep reviews. Rainbow Astro is a small-batch, direct-import brand: one magazine gave it a qualitative hands-on, two named bloggers measured guiding performance (mostly on the encoder-equipped 135E, at a single focal length each), and the rest of the field evidence sits in a large but anonymous Cloudy Nights thread. Manufacturer specs cover payload and the 135E’s guaranteed PE; there is no manufacturer-published guided-RMS figure for either model, and the base RST-135’s own periodic error is characterized, not guaranteed, by Rainbow Astro’s own choice. Where this review states a number, it says plainly whose number it is.
Specs that matter
The RST-135 weighs 3.3 kg (7.27 lb) at the head, with the encoder-equipped RST-135E weighing marginally more at 3.4 kg (about 7.5 lb) Rainbow Astro; Cloud Break Optics; Astroworld. Both share the same rated payload — 13.5 kg (29.76 lb) without a counterweight, rising to a stated maximum of 18 kg (about 39.7 lb) with a counterweight added Rainbow Astro; Cloud Break Optics; Astroworld.
Periodic error is where the two models actually diverge. Rainbow Astro’s own FAQ is explicit that the base RST-135 carries no guarantee at all:
“We don’t make an announcement or guarantee the figure about periodic error of RST-135. However, it has a periodic error about ±30 arcsec in a 430 seconds cycle. Thus the auto guide is needed for a photography.”Rainbow Astro FAQ, rainbowastro.com
Treat ±30″ as a characterization, never as a spec sheet number — Rainbow Astro is telling you, in its own words, that it won’t stand behind it. The base RST-135 also has no PEC training option at all — see our periodic error & PEC guide for how PEC training works and why it applies poorly to a harmonic drive like this one — and the reason here is load-dependence: Rainbow Astro’s FAQ notes plainly that “the magnitude of periodic error varies depending on the weight of load” Rainbow Astro FAQ, which is also why a strain-wave drive’s error doesn’t give you the same clean, repeatable curve a worm gear does to record and replay — the general mechanism is covered in our harmonic vs. GEM comparison rather than here.
The RST-135E closes that gap with hardware rather than a workaround:
“A high-resolution encoder from Renishaw is built into the RA axis. As a result, a periodic error of only ±2.5 seconds was achieved.”Rainbow Astro, rainbowastro.com
±2.5″ is a manufacturer-guaranteed figure, not a characterization — a genuinely different class of claim than the base model’s ±30″.
Power is a newer addition to Rainbow Astro’s published specs: input runs 12–16V DC, with tracking drawing 0.2A at 12V and slewing drawing 1.0–2.0A at 12V rainbowastro.com spec table, a voltage range BBC Sky at Night’s own hands-on corroborates independently (“Power input: 12V to 16V DC”) BBC Sky at Night. No wattage figure is published, and this review won’t back into an “official” watt rating by multiplying volts by amps — that would dress up an estimate as a manufacturer spec, which is exactly what sourcing every figure on this site is supposed to prevent.
RST-135 vs. RST-135E at a glance
| Spec | RST-135 | RST-135E |
|---|---|---|
| Weight (head only) | 3.3–3.4 kg (about 7.27–7.5 lb) Rainbow Astro | |
| Payload, rated | 13.5 kg (29.76 lb) Rainbow Astro | |
| Payload, maximum (with counterweight) | 18 kg (about 39.7 lb) with a counterweight added Rainbow Astro | |
| Periodic error | ~±30″ over a 430s cycle — characterized, not guaranteed Rainbow Astro FAQ | ±2.5″ — guaranteed, Renishaw RA encoder Rainbow Astro |
| PEC | None | Not applicable — the encoder closes the loop directly |
| Power | 12–16V DC; 0.2A tracking @12V; 1.0–2.0A slew @12V rainbowastro.com | |
| Encoder | No | Yes — Renishaw, RA axis |
The payload calculator on this site takes your actual imaging train — OTA, camera, guide scope, guide camera, filter wheel, and accessories — and checks it against the RST-135’s 13.5 kg rated (no counterweight) and 18 kg maximum (with a counterweight added) figures, alongside the community-heuristic derating band covered in our payload-derating guide.
Guiding performance in practice
Rainbow Astro publishes no guided-RMS figure for either model — only the characterized/guaranteed periodic-error numbers above. What exists instead is a small set of named, rig-specific field reports, plus a much larger body of anonymous forum experience that this review treats as context, not spec.
Andrew Burwell, writing at Mac Observatory, measured the RST-135E’s first-light guiding at 0.64″ total RMS in poor seeing, then settled into a consistent 0.3–0.5″ after Rainbow Astro shipped a firmware fix for a DEC-axis excursion bug he’d been chasing, running an EdgeHD 8 at 1442mm Andrew Burwell, Mac Observatory. In passing, Burwell also notes the base RST-135 held around 0.3″ RMS on some of his setups at a 0.5-second guide exposure rate Andrew Burwell, Mac Observatory — a real number, but a smaller aside inside his 135E testing, not a dedicated base-model review.
Burwell’s firmware-fix report is dated, and this review can’t confirm whether the DEC-excursion bug he hit — or Rainbow Astro’s fix for it — still describes current-production units. Treat it as one documented history, not a standing characteristic of the mount as currently sold.
Ya Ge, publishing a review hosted on ZWO’s own site, reports the RST-135E reliably achieving “0.3+ arc sec’s guiding RMS” across three nights at 530mm Ya Ge, via zwoastro.com. Worth flagging plainly: that review lives on a competitor’s website, not an independent one — though since the figure favors the RST-135E rather than one of ZWO’s own mounts, that placement arguably cuts against bias rather than toward it.
BBC Sky at Night’s Tim Jardine reports successfully autoguiding the base RST-135 through PHD2 in his hands-on review, without publishing a numeric RMS figure at all BBC Sky at Night, Tim Jardine — useful as confirmation the workflow works, not as a data point for how tight.
Beyond the named reviewers, the Cloudy Nights “The ideal imaging mount?” thread — a 100+-page discussion that is itself the top organic result for this mount — contains scattered anonymous owner reports clustering around 0.7″ RMS as a typical figure and roughly 0.40″ cited as a best case Cloudy Nights, unverified forum reports. Treat those as a sense of lived community experience, not as sourced specs; none of the posters are named, verified reviewers, and rig, seeing, and guide camera all vary post to post.
Some marketing and dealer copy for the RST-135 has floated 500mm as workable unguided. A Cloudy Nights contributor pushes back hard on that framing, arguing that a ±2.5″ peak-to-peak figure only supports a meaningfully shorter unguided focal length — on the order of 150mm, not 500mm Cloudy Nights. The catch: that critique borrows the RST-135E’s encoder-corrected ±2.5″ guaranteed figure and applies it to the base RST-135, which Rainbow Astro characterizes at a much looser ~±30″ over a 430-second cycle — not the same mount, not the same number. What both sides would likely agree on: whichever PE figure applies, the base RST-135 without PEC is not a serious unguided platform at deep-sky focal lengths. The real disagreement here is which model a given “unguided” claim was ever describing.
Build quality & the hRST hand controller
The RST-135’s housing is CNC-machined aluminum (6061), controlled through Rainbow Astro’s own hRST hand controller rather than a third-party app Rainbow Astro. Owner support runs mostly through a dedicated Groups.io community rather than a manufacturer forum, and the printed manual is unusually thorough for a mount this size — useful, since BBC Sky at Night’s review is explicit that the RST-135 isn’t an out-of-the-box solution: buyers need to source their own tripod, saddle, and cabling accessories separately, the same way you would for any independent (non-ASIAIR) mount BBC Sky at Night. One convenience the review does credit: the reviewer converted the mount from alt-azimuth to full equatorial tracking in under a minute, including the necessary power cycle BBC Sky at Night.
Specialist-dealer support experience
Cloud Break Optics, First Light Optics, and Tolga Astronomy in Australia are the specialist dealers most commonly named as stockists Rainbow Astro; Cloud Break Optics. There’s no big-box retailer carrying the RST-135, and Cloudy Nights threads describe stock as intermittent rather than reliably available Cloudy Nights, unverified stock reports. Pricing sits in the Observatory tier — Rainbow Astro isn’t one of the four MAP-enforced brands this site tracks, but we band its pricing the same way pending a verified figure, and the mount is sold only through the specialist dealers above rather than the retailers most of this hub links to.
Agena Astro, this site’s default astronomy merchant, does not carry Rainbow Astro at all. No verified affiliate program covers this mount through any merchant we route through, so the two links below go straight to Rainbow Astro’s own spec pages and pay us nothing — we would rather send you to the manufacturer’s real numbers than quietly leave the mount out, or route you to a retailer who doesn’t actually stock it.
Full specifications for the RST-135 and the RST-135E are published directly by Rainbow Astro.
Meridian behavior
Like any equatorially-mounted tracker, the RST-135 tracks its target from east to west and must flip sides at the meridian rather than run its optical tube into its own base on the next slew. Rainbow Astro’s controller lets you configure how far past the meridian to keep tracking before the flip triggers, specified as an angle rather than a time — entering 30° buys roughly two hours of additional past-meridian tracking, by Rainbow Astro’s own conversion Rainbow Astro FAQ. Why any equatorially-tracking mount needs a meridian flip at all, and how to plan a full session around one, is a broader planning question covered start to finish in our meridian flip planning guide; treat the RST-135’s angle-based setting here as one mount’s specific implementation of that problem, not the whole topic.
Who actually needs the E
The RST-135E’s Renishaw encoder buys a manufacturer-guaranteed ±2.5″ figure and near-unguided consistency, but that doesn’t make the base RST-135 the wrong buy for everyone. Our travel-mount roundup already covers this exact trade-off and reaches the same conclusion this review does: if your guiding is already tight enough to absorb ±30″-class raw periodic error, tight guiding substitutes for the encoder, and plenty of travel imagers run exactly that combination successfully see our travel harmonic mount guide. Where the encoder earns its premium is longer subexposures or a longer focal length than a typical wide-field travel setup, where guided correction has less headroom to work with — see our best travel & portable harmonic mounts guide for the full buy/don’t-buy breakdown against the AM3N. This review stays in its own lane: whether the RST-135 family is a good mount, not which specific SKU or competitor to buy.
Where it sits
Two comparisons this review deliberately doesn’t re-run: how the RST-135 stacks up against the ZWO AM3N for travel imaging specifically — covered start to finish in our travel harmonic mount guide — and the general strain-wave-vs-worm-gear technology trade-off, covered in our harmonic vs. German equatorial mounts comparison. Both apply directly to the RST-135; neither needs restating here.
FAQ
What’s the difference between the Rainbow Astro RST-135 and RST-135E?
Payload is identical on both — 13.5 kg rated without a counterweight, 18 kg maximum with a counterweight added Rainbow Astro. The E adds a built-in Renishaw RA-axis encoder that brings periodic error to a manufacturer-guaranteed ±2.5″, versus the base model’s characterized-only (not guaranteed) ~±30″ over a 430-second cycle Rainbow Astro FAQ.
Does the Rainbow Astro RST-135 need autoguiding?
Yes, essentially always for deep-sky work. The base model has no PEC at all, and Rainbow Astro itself states plainly that its characterized ~±30″ periodic error means autoguiding is needed for astrophotography Rainbow Astro FAQ.
Can I buy the RST-135 through Agena Astro or another verified retailer?
No. Agena Astro, this site’s default astronomy merchant, does not stock Rainbow Astro, and no verified affiliate program covers this mount through any merchant we route through. It’s sold through specialist dealers — Cloud Break Optics, First Light Optics, and Tolga Astronomy in Australia are the ones most commonly named — and this article’s manufacturer links pay us nothing.
Who actually needs the RST-135E’s encoder instead of the base RST-135?
Travel imagers already running tight guiding can often absorb the base model’s ~±30″ raw periodic error without issue. The encoder earns its premium mainly at longer subexposures or longer focal lengths, where guided correction has less room to work with see our travel harmonic mount guide.