Four questions in this hobby have arithmetic behind them.
Most gear advice is somebody's preference with a confident tone. These four aren't — they're calculations you can check, run against your own numbers, in the browser, free, no email required.
Every figure a calculator uses is named where it is used, with the source it came from. Where the hobby genuinely disagrees — how far to derate a mount's rated payload is the clearest case — you get both positions and the reasoning behind each, not one of them presented as a manufacturer spec.
Mount payload calculator
Add up what actually sits on the saddle, then see it measured against two different standards: the manufacturer's rated capacity, and the 50% rule the forums apply to it. They disagree, and which one you follow is a judgement call — so the tool shows you both instead of picking for you.
23.5 lb on the Sky-Watcher EQ6-R Pro: 53.4% of the rated figure, past the community 50% point but inside the two-thirds upper end.
The same load, measured two ways
These are two different standards, not a headline and a footnote. They disagree by design.
The most-quoted version of the rule puts the ceiling at 22 lb (half of 44 lb). Stated as a range it is 14.67–29.33 lb (one-third to two-thirds). You are at 23.5 lb — 1.5 lb over it.
Cloudy Nights; Stargazers Lounge A community heuristic — no manufacturer publishes it.
Rated at 44 lb / 20 kg. You are at 23.5 lb, or 53.4% of it — 20.5 lb of stated headroom.
Sky-Watcher EQ6-R Pro skywatcher.com states a 44 lb weight capacity for the EQ6-R ("can handily support an optical tube, imaging accessories and camera"); corroborated by skywatcherusa.com and High Point Scientific.
Agena Astro A third figure, separately attributed — neither the manufacturer's nor the forum's.
(). A published, guaranteed PE figure is the strongest evidence on the “modern mounts are already imaging-rated” side of the argument — it is an imaging spec, made by the manufacturer, with a number attached. It is not a statement about payload, and it does not license loading to the rating.
A review at Optical Mechanics states the 44 lb figure is "for photographic use". Verification on 2026-08-02 found that is the reviewer's own unsourced assertion — no such qualifier appears on skywatcher.com or skywatcherusa.com. Treat it as reviewer opinion, not a Sky-Watcher statement.
This tool adds up mass. It cannot model moment arm — how far the weight sits from the mount's axes, which is mostly a function of tube length and how the rig is balanced. A 20 lb short refractor and a 20 lb long Newtonian are the same number here and are not the same load on the gears or in wind. This is the one point the community and the publisher-side accounts of derating actually agree on Cloudy Nights, so treat the percentages above as one input to the decision, not the decision.
Why there are two answers
The rule most often given on the forums is to use no more than 50% of the rated payload for imaging, sometimes stated as a range of one-third to two-thirds. The reasoning is that a visual rating describes what the mount can hold steady enough to look through, and imaging asks it to hold that steady for minutes at a time while tracking. Notably, the rule is not unanimous even where it originates: some members describe the strict 50% version as “more of an upselling tactic than reality.”
Cloudy Nights — “Payload capacity of mount”, “Half payload rule of thumb legit?”; Stargazers Lounge Forum consensus. No manufacturer in our canon states this rule.
Astronomy.com's guide to a first imaging rig states that “these days, the listed payload capacity of many mounts is its true astrophotography capacity; for some manufacturers and for older mounts, a rule of thumb of using 50 percent of the capacity for astrophotography applies.” On this reading the 50% rule is a correction for older worm-drive designs rather than a universal law. The structural evidence sits with the newer mounts: ZWO and Rainbow Astro publish guaranteed periodic error figures for their harmonic mounts — an imaging-oriented spec with a number attached, which is not how a purely visual rating is normally sold.
Astronomy.com; ZWO; Rainbow Astro Publisher position plus manufacturer spec behaviour. One reviewer (Optical Mechanics) claims the EQ6-R's 44 lb is already photographic; we checked, and that qualifier does not appear on Sky-Watcher's own pages.
Moment arm beats raw weight. A long tube, a heavy camera train hanging off the back, or a rig balanced badly will misbehave well under any percentage figure — and a compact, well-balanced rig will often behave above one. Nobody in this argument disputes that, which is why the percentage is a starting point rather than a verdict.
Every mount in this tool
The halved column is the community rule of thumb applied to the rated figure. It is arithmetic on someone else's heuristic, not a specification.
| Mount | Rated capacity, as published | 50% of rated (community rule — not a spec) | Retailer recommendation | Published periodic error |
|---|---|---|---|---|
| Sky-Watcher EQ6-R Pro | 44 lb / 20 kg | 22 lb | — | — |
| Sky-Watcher HEQ5-R Pro | 33 lb / 15 kg | 16.5 lb | — | — |
| Sky-Watcher EQ8-R Pro | 110 lb / 50 kg | 55 lb | — | — |
| ZWO AM5N | 33 lb / 15 kg without counterweight 44 lb / 20 kg with counterweight |
16.5 lb 22 lb with counterweight |
— | ±10 arcsec, guaranteed |
| ZWO AM3N | 17.6 lb / 8 kg without counterweight 28.7 lb / 13 kg with counterweight |
8.8 lb 14.35 lb with counterweight |
— | — |
| iOptron CEM40 | 40 lb / 18 kg | 20 lb | Under ~25 lb (Agena) | — |
| iOptron GEM45 | 45 lb / 20.4 kg | 22.5 lb | — | — |
| iOptron CEM70 | 70 lb (kilogram figure not published in our canon) | 35 lb | Under ~40 lb (Agena) | — |
| Rainbow Astro RST-135 / 135E | 13.5 kg / ~30 lb without counterweight 18 kg (≈39.7 lb) with counterweight |
≈14.9 lb ≈19.85 lb with counterweight |
— | ±2.5 arcsec on the 135E |
Sources
- Sky-Watcher EQ6-R Pro — skywatcher.com states a 44 lb weight capacity for the EQ6-R ("can handily support an optical tube, imaging accessories and camera"); corroborated by skywatcherusa.com and High Point Scientific.
- Sky-Watcher HEQ5-R Pro — High Point Scientific and Ontario Telescope list 33 lb / 15 kg for the current belt-drive HEQ5-R.
- Sky-Watcher EQ8-R Pro — AstroBackyard and High Point Scientific list 110 lb / 50 kg.
- ZWO AM5N — Agena Astro and High Point Scientific list 33 lb / 15 kg without a counterweight and 44 lb / 20 kg with one.
- ZWO AM3N — ZWO's own kg figures (8 kg / 13 kg) converted at 2.20462 lb/kg. Verified 2026-08-04: Agena's page rounds to 17.5/28.5 lb, ~0.1 lb below a straight conversion; we use the conversion so this figure matches ZWO's own published kilograms exactly.
- iOptron CEM40 — ioptron.com spec sheet: 40 lb / 18 kg.
- iOptron GEM45 — ioptron.com's GEM45/GEM45EC product pages and the GEM45EC Instruction Manual, Appendix A: "Max payload 45 lbs (20 kg), exclude counterweight." Verified 2026-08-04 (previously logged as retailer-only).
- iOptron CEM70 — iOptron rates the CEM70 at 70 lb. Only the pound figure is published in our canon.
- Rainbow Astro RST-135 / 135E — rainbowastro.com, BBC Sky at Night Magazine and Sidereal Trading: 13.5 kg without a counterweight, 18 kg maximum with one. Mount head 3.3–3.4 kg.
- The 50% derating rule — Cloudy Nights threads “Payload capacity of mount” and “Half payload rule of thumb legit?”; Stargazers Lounge. Community consensus, cited as consensus, not as a specification.
- The counter-position — Astronomy.com, “Finding your first astroimaging rig”: listed capacity is the true astrophotography capacity for many modern mounts, with the 50% rule applying to some manufacturers and to older mounts.
- Moment arm outweighs raw weight — Cloudy Nights, and accepted on both sides of the derating argument.
- Guaranteed periodic error — ZWO (AM5N, ±10 arcsec, via Agena Astro) and Rainbow Astro (RST-135E, ±2.5 arcsec, Renishaw RA encoder; rainbowastro.com, BBC Sky at Night Magazine, Sidereal Trading).
Figures verified 2026-08-02. Where a capacity is published in only one unit, the converted value is marked “≈” and is ours, not the manufacturer's.
This calculator is arithmetic over published figures. It does not know your sky, your tube's length, your balance, or your wind. Every capacity it uses is named and dated above, and where the hobby disagrees you get both positions rather than ours.
No prices anywhere in this tool. Several mount brands enforce minimum advertised pricing, and a figure we scraped once would be wrong within weeks — so we don't print one. Nothing here is ranked by what it would pay us.
Pixel scale & sampling matcher
Pixel scale in arcseconds per pixel is 206.265 × pixel size (µm) ÷ focal length (mm) Diffraction Limited. Under typical seeing most deep-sky rigs want to land roughly 1–2″/px Stellarnomads 2026; AstroBackyard. Whether it is better to miss that window high or low is genuinely disputed — both positions are below, and both are applied to whatever number you compute.
Two presets, because two sensors are fully sourced in our canon. Both use 3.76 µm pixels, so at the same focal length they give the same pixel scale — what changes is how much sky the chip covers. Both now carry a field-of-view figure: the IMX571's wide APS-C rectangle against the IMX533's smaller, square 1″ frame.
Editing this switches the camera to Custom.
The bare focal length of the OTA. Reducers and Barlows go in the next field.
0.8 for a 0.8× reducer, 2 for a 2× Barlow. Leave at 1 for a bare OTA.
Most non-alpine sites sit at 2.5–3.5″; 2″ is a good night; sub-1″ happens only at premier sites such as Mauna Kea Diffraction Limited. Leave blank and the verdict is judged against the 1–2″/px practical band instead.
Well sampled — 1.46″/px, inside the 1–2″/px practical band. Band from Stellarnomads (2026) and AstroBackyard. Enter a seeing figure for a site-specific window.
- Effective focal length
- 530 mm
- 2×2 binned
- 2.92″/px
- Field of view
- —
- Pixels across the seeing disc
- —
Inside the window, so the classic preference for erring fine is not in play.
Astronomical SolutionsInside the window, which is where the modern guidance points too.
Stellarnomads 2026Binning N×N multiplies pixel scale by N — 2×2 here gives 2.92″/px.
Where the sampling advice splits
| Position | What it holds | Attributed to |
|---|---|---|
| Classic | Oversampling is the safer error — "over-sampled is still better than under." | Astronomical Solutions |
| Modern dissent | Err toward slight undersampling, and rely on dithering and drizzle to recover detail. | Stellarnomads, 2026 |
| Not in dispute | Seeing sets the ceiling. Diffraction Limited frames it via Nyquist — about 2–3 pixels across the seeing FWHM, so the practical maximum scale is seeing÷2 to seeing÷3. | Diffraction Limited |
If in doubt, err toward slight undersampling: a well-dithered, drizzled 2″/px dataset beats a noisy 0.7″/px one on almost every night you will actually get.Stellarnomads, 2026 — quoted in full because paraphrasing it softens it
This is a live disagreement between practitioners, not a settled question with one right answer, so the calculator reports both readings of your number and stops there. If your rig lands outside 1–2″/px, which direction to move depends on your site, your guiding and whether you dither — and that is a judgement, not arithmetic.
Reference: pixel scale at 3.76 µm
Both sensors in our canon use 3.76 µm pixels, so this one table covers the IMX571 and the IMX533. Verdicts are against the 1–2″/px practical band Stellarnomads 2026; AstroBackyard, not against any particular night's seeing. The binned column is twice the printed scale, so every row multiplies on the page — see the display-rounding note in the method block.
| Focal length | Pixel scale | 2×2 binned | Against 1–2″/px |
|---|---|---|---|
| 200 mm | 3.88″ | 7.76″ | Undersampled |
| 250 mm | 3.10″ | 6.20″ | Undersampled |
| 300 mm | 2.59″ | 5.18″ | Undersampled |
| 400 mm | 1.94″ | 3.88″ | In band |
| 480 mm | 1.62″ | 3.24″ | In band |
| 530 mm | 1.46″ | 2.92″ | In band |
| 600 mm | 1.29″ | 2.58″ | In band |
| 700 mm | 1.11″ | 2.22″ | In band |
| 800 mm | 0.97″ | 1.94″ | Oversampled |
| 1000 mm | 0.78″ | 1.56″ | Oversampled |
| 1200 mm | 0.65″ | 1.30″ | Oversampled |
| 1500 mm | 0.52″ | 1.04″ | Oversampled |
| 2000 mm | 0.39″ | 0.78″ | Oversampled |
The two sensors this tool knows
| Spec | IMX571 | IMX533 |
|---|---|---|
| Format | APS-C BSI CMOS | 1″ square (1:1) BSI CMOS |
| Resolution | 6248 × 4176 · 26.09 MP | 3008 × 3008 · 9.05 MP |
| Pixel size | 3.76 µm | 3.76 µm |
| ADC† | 16-bit | 14-bit |
| Full well | ~50 Ke− standard; extended to 73 Ke− at gain −25 ZWO ASI2600 manual | ~51 Ke− standard; up to ~73 Ke− in extended modes Atik technical guide |
| Peak QE | 91% mono / 80% colour ZWO ASI2600 manual; "exceeds 80% in the green channel" Atik | >80% green Atik technical guide |
| Read noise | ~1.2 e− high gain to ~3.6 e− low gain Atik | Not in our canon — we will not print a figure we have not pulled |
| Amp glow | Native zero | Native zero Atik technical guide |
| Cameras | ASI2600 family | ASI533 family |
Same sensor, different vendor implementations: ToupTek's ATR2600M datasheet reports read noise as low as 0.883 e− and full well up to 100 Ke− in a high-full-well mode. Sensor canon is not camera canon.
† 16-bit against 14-bit: debated, not settled
The difference is a real published spec, not an inference: the IMX571 digitises at 16 bits and the IMX533 at 14 ZWO ASI2600 manual; Atik technical guide. Four times as many tonal steps per sub is the reason the deeper converter is usually presented as the better one.
At minimum gain the IMX571's read-noise-in-ADU advantage is marginal SharpCap forum — on that reading the extra bits are digitising noise rather than recovering signal, and the spec gap overstates the difference in a finished stack.
The bit depth is in the table because it is a real published spec. Whether it is a reason to choose one sensor over the other is a live disagreement in our canon, so we report both readings and stop — the same treatment the oversampling argument gets above.
What pixel scale should I actually aim for?
Roughly 1–2″/px under typical seeing. Diffraction Limited gets to a similar place from Nyquist: about 2–3 pixels across the seeing FWHM, which puts the practical maximum scale at seeing÷2 to seeing÷3. On a 3″ night that is 1.00–1.50″/px. Stellarnomads 2026 · AstroBackyard · Diffraction Limited
Do the IMX571 and IMX533 sample differently?
No. Both have 3.76 µm pixels, so on the same telescope they produce identical pixel scale. What differs is coverage: the APS-C IMX571's 6248 × 4176 array frames about 2.54° × 1.70° at 530 mm, while the IMX533's smaller 3008 × 3008 square array frames about 1.22° square at the same focal length — roughly half the IMX571's frame width, in a 1:1 rather than 3:2 shape. Choose between them on framing and file size, not on sampling. Both fields of view derived from ZWO's own published array dimensions using the same 206.265 formula
Does binning fix an oversampled rig?
Arithmetically, yes: N×N binning multiplies pixel scale by N, so 0.65″/px binned 2×2 becomes 1.30″/px. What binning does to read noise on a CMOS sensor is a separate question, and our canon holds no measured figure for it — so this tool reports the scale change and claims nothing else. Arithmetic only · no sourced CMOS binning-noise figure in Part 3
Why 206.265 and not 206?
There are 206,265 arcseconds in a radian, which is where the constant comes from Diffraction Limited; MaxIm DL help. We use the exact value. You will meet the formula written with a rounded 206 in places; at 3.76 µm on 530 mm the two differ by about 0.002″/px — enough to make two answers look different, never enough to change a verdict. We do not name who rounds it, because our canon records the constant and not that. Diffraction Limited · MaxIm DL help
Method. One constant and four inputs: pixel scale = 206.265 × pixel size (µm) ÷ (focal length × reducer/Barlow). Every figure on this page traces to a named source — the formula and seeing figures to Diffraction Limited, the 1–2″/px band to Stellarnomads (2026) and AstroBackyard, the sensor specs to the ZWO ASI2600 manual and Atik's technical guides. Nothing is estimated. Figures derived from the pixel scale — the binned column, the reference table — are computed from the scale as printed, so the arithmetic works on the page rather than only behind it.
What is deliberately missing. Only two sensor presets, because only two sensors are fully sourced in our canon. The IMX294 is absent because its amp-glow behaviour is community characterisation rather than measurement; the IMX462 is absent because we have not pulled its figures to a manufacturer datasheet. The custom pixel-size field covers both without us asserting a spec we cannot stand behind. This tool carries no affiliate links and prints no prices — manufacturers set minimum advertised prices and street prices move, so we band prices in articles and send you to the retailer for the actual number.
Imaging-train back-focus builder
Add up what is already in the train, add the filter's share, and the number left over is the spacer stack you have to buy. The filter step is the one most people miss, so it gets its own line rather than being folded into the total.
Your train
What to fill
The live total needs JavaScript. The arithmetic is short enough to do on paper — worked here against the target in the field above, with a 3 mm filter as the example:
| Target back focus | 55.00 mm |
| Filter compensation, 3 mm filter ÷ 3 | + 1.00 mm |
| Distance you actually need | 56.00 mm |
| Minus everything in your stack | − your total |
| Spacers to add | = the gap |
Come up short and the sensor sits too close to the corrector, which then under-corrects the field; overshoot and it over-corrects. Either way the centre of the frame stays sharp while the corner stars stretch, and the two stretch along axes at right angles to each other — which is why "my corners are bad" on its own never tells you which way to move.
What we will not tell you: which elongation direction belongs to which error. Sources contradict each other on the mapping and it is not in our verified canon, so rather than pick one, change the spacing by a known amount, re-shoot, and let the corners give you the sign. We publish no tolerance figure either — how far off you can be before it shows depends on your corrector and focal ratio, and that number belongs to your corrector's documentation, not to us.
The order components go in
The order is a separate question from the arithmetic. Get the order wrong and no amount of correct spacing saves the image; get the spacing wrong and a correctly ordered train still will not come to focus. This is the order; the panel above is the distance.
- 01 Telescope Whatever the focuser presents — the train starts where the OTA ends.
- 02 Field flattener or reducerMeasuring starts here The corrector's shoulder is the datum. The 55 mm standard is measured from this face to the sensor, so everything below this line is what the calculator counts Celestron knowledgebase; ZWO.
- 03 Off-axis guider Normally upstream of the filter wheel, so the guide camera's pick-off prism is not looking through a narrowband filter — behind one, the guide star can be too faint to hold. The reverse arrangement exists where clearance forces it. This is an assembly convention, not a manufacturer specification, and no figure in this tool depends on it.
- 04 Filter wheel Count the wheel's mechanical thickness in the stack. The filter inside it is accounted for separately — it changes the distance you need, not the distance you have.
- 05 Spacers and adapters The only adjustable element in the train, which is why they go last before the camera: everything else has a fixed depth, so the spacers are what you tune to close the gap.
- 06 CameraMeasuring ends here The measurement ends at the sensor plane, not at the camera's front face. The camera's own flange-to-sensor depth is a component in the stack like any other.
Spacing arithmetic is a different job from assembly order. The panel above does the distance; this card does the sequence.
Where these two numbers come from — and how strong each one is. The 55 mm figure is documented by Celestron's knowledgebase and by ZWO, measured from the corrector's shoulder to the sensor plane Celestron knowledgebase; ZWO. That is a Tier 1 source in our map. The one-third rule is weaker evidence and we would rather say so: glass in a converging beam pushes focus back by roughly a third of the filter's thickness, so a 3 mm filter adds about 1 mm to the distance you need — but that ratio is a widely repeated rule of thumb which our source pass has not traced to a specific Celestron or ZWO page Rule of thumb — no manufacturer page in our canon. It is the step most often skipped, and skipping it is a common reason a train that measures 55 mm on paper still will not come to focus; treat the 1 mm as an order of magnitude, and your corrector's own documentation as the authority.
What we do not supply. Component depths. We publish figures we can trace to a manufacturer, and a generic table of camera and filter-wheel depths is not one — models and revisions differ. Every depth above is yours to enter from your own spec sheets.
Power & battery calculator
Add up what your rig actually draws, and see the battery size that answer implies at two very different usable-capacity assumptions — lead-acid/AGM and LiFePO4 are not close, so the tool never collapses them into one number.
| Mount, as published | 48 W |
| Cooled camera, at your assumed duty | 36 W |
| Anti-dew heater, sensor window | 0 W |
| Guide camera | 2 W |
| Focuser | 2 W |
| Filter wheel | 1 W |
| Laptop / mini-PC / ASIAIR | 15 W |
| Dew heater, duty-cycled average | 6 W |
| Average draw | 110 W |
| × session hours | 6 |
| Watt-hours required | 660 Wh |
| + 25% planning margin | + 165 Wh |
| Watt-hours with margin | 825 Wh |
110 W average on the Sky-Watcher EQ6-R Pro, for 6 hours: 660 Wh required, 825 Wh with the 25% margin this site applies.
Two chemistries, not one number
Lead-acid/AGM and LiFePO4 give back very different shares of their rated capacity. Both figures below are real answers to "how big a battery do I need" — for two different batteries.
1650 Wh of RATED capacity — roughly 137.5 Ah at 12V nominal. AGM is "typically limited to about 50% depth of discharge if you want to preserve battery life" — a 100Ah AGM battery gives you roughly 40–60Ah you feel good about using, not the full 100.
Battle Born Manufacturer educational content, not a spec sheet.
916.67 Wh of RATED capacity — roughly 76.4 Ah at 12V nominal, using 90% usable as a single representative figure within the 80–100% range Battle Born states ("a 100Ah LiFePO4 can realistically give you close to the full 100Ah"). Your real number could land anywhere in that range depending on the pack.
Battle Born Manufacturer educational content, not a spec sheet.
Checked against five named power stations
Compared to the Wh-with-margin figure above, DC-direct (this site's stated preference, not through the AC inverter). The AGM and LiFePO4 percentages above are not applied to these rows: those figures answer "how big a raw deep-cycle battery do I need", where you choose the chemistry and how deeply to discharge it. A sealed station manages its own pack behind a BMS and publishes one capacity number, so the comparison below is against that published figure as-is.
Worth knowing: none of these manufacturers publishes how much of that rated capacity is actually retrievable at a 12V DC load, and real output falls short of the label once conversion losses and BMS cut-offs are counted. Treat a row that lands close to your requirement as closer than it looks, and lean on the 25% margin already built into the figure above.
| Station | Capacity | Chemistry | 12V DC port | Checked against your night |
|---|---|---|---|---|
| Jackery Explorer 1000 (original) | 1002 Wh | Li-ion NMC | 12V car port rated 10A Reviewer-reported ~13.2V regulated output, not Jackery's own regulation claim | Covers your night |
| Jackery Explorer 1000 v2 | 1070 Wh | LiFePO4 | 1500W AC — no separate 12V DC port rating in our source set Not published in our source set | Covers your night |
| EcoFlow RIVER 2 | 256 Wh | LiFePO4 | 300W AC — no 12V DC port rating in our source set Not published in our source set | Short for this load |
| EcoFlow RIVER 2 Max | 499 Wh | LiFePO4 | 12V car port: 12V/24V, 8A, 100W max. (EcoFlow's separate DC-output block is listed at 12.6V, 10A/3A/3A, 126W max.) EcoFlow does not publish an explicit "regulated" label for this port in our source set | Short for this load |
| Bluetti EB3A | 268 Wh | LiFePO4 | 1× 12V/10A car outlet; 2× 12V/10A DC 5521 "All Regulated", Bluetti's own spec-sheet wording | Short for this load |
Running gear off a station's AC inverter instead of its 12V DC port adds roughly 15% overhead — Jackery's own published runtime formula ("working time = watt-hours × 0.85 ÷ operating power") implies about that much. The table above assumes DC-direct throughout.
Every mount in this tool
The full reference behind the dropdown above — useful if JavaScript is off, or if you just want to check the arithmetic by hand.
| Mount | Figure at 12V, as published | Tier | Source |
|---|---|---|---|
| Sky-Watcher EQ6-R Pro | 12V (4 amp minimum) | Tier 1 | Sky-Watcher USA |
| Sky-Watcher HEQ5 Pro | 11–15V DC, 2A | Tier 2 | telescopes.net, a retailer spec table |
| ZWO AM5 / AM5N | Tracking: 12V, 0.7A | Tier 1 | ZWO's own AM5/AM5N user manual |
Cold changes the two halves of the story differently
Discharging a cold LiFePO4 battery and charging one afterward are governed by different limits — imaging overnight is the discharge half, and it is usually the easier one.
LiFePO4 discharge is possible down to about −20°C, with reduced capacity: roughly 80–90% at 32°F (0°C), 60–70% at 14°F (−10°C), 40–50% at −4°F (−20°C). This is the state your battery is in while you image — usually workable, just with less capacity than the number on the case.
GridWright"When charging lithium iron phosphate batteries below 0°C (32°F), the charge current must be reduced to 0.1C and below −10°C (14°F) it must be reduced to 0.05C. Failure to reduce the current below freezing temperatures can cause irreversible damage." This is the real risk for a cold night out — not the imaging session itself, but plugging a cold battery into a normal charger afterward.
RELiONLead-acid/AGM vs LiFePO4: both positions, attributed
LiFePO4 advocates point to far more usable watt-hours per kilogram, greater than 95% round-trip efficiency, and thousands of cycles (3,000–7,000, against AGM's 500–1,500) Anern; Battle Born. Lead-acid/AGM defenders point to a lower upfront cost, and to the fact that AGM can charge below freezing where LiFePO4 cannot without an added heater Battle Born. Weight tells the same story as the cycle-life figures: a 100Ah LiFePO4 battery runs roughly 24–30 lb against an equivalent AGM's roughly 60–70 lb, industry-consensus figures rather than a single manufacturer's spec sheet TheGreenWatt. Taken together: LiFePO4 wins on usable Wh/kg and on cycle life; AGM keeps a cold-charging edge and an upfront-cost edge. Neither side is wrong.
Dew-heater draw, full power
Kendrick's own published table, via All-Star Telescope, an authorized dealer — every figure at the 100%-controller-setting / full-power rating.
| Element | Full power | Full-power current |
|---|---|---|
| 1.25 inch eyepiece | 3 W | 0.2 A |
| 2 inch eyepiece | 4 W | 0.3 A |
| 4 inch optic | 11 W | 0.9 A |
| ~8 inch SCT | 20 W | 1.6 A |
| 9/10 inch | 25 W | 2 A |
| 12 inch | 33 W | 2.6 A |
| 14/16 inch | 52 W | 4.1 A |
A controller run at less than 100% draws the full-power figure above only for a fraction of the time. In Kendrick's own words: "if you set your controller to operate at 30%, the heaters will draw their full amperage 30% of the time." 30% is Kendrick's illustrative example, not a measured typical setting — the calculator further up this page lets you set your own duty cycle and shows the full-power and duty-cycled figures side by side.
Sources
- Mount draw — Sky-Watcher USA (EQ6-R Pro, Tier 1); telescopes.net, a retailer spec table (HEQ5 Pro, Tier 2, pending manufacturer confirmation); ZWO's own AM5/AM5N user manual (Tier 1, tracking state only — GOTO spikes not modelled). High Point Scientific publishes a different AM5/AM5N figure set; noted as a retailer variant, not used in the arithmetic.
- Cooled-camera cooler draw — ZWO ASI2600MC Pro manual (Tier 1): 12V @ 3A DC adapter recommended for cooling (36 W), stated as a peak/maximum rating, not a typical running draw; anti-dew heater on the sensor window around 5 W; sensor/logic board up to 4.7 W separately over USB at 5V.
- Dew heater — Kendrick's own published table, via All-Star Telescope, an authorized dealer (Tier 1 spec). The 30% duty-cycle default is Kendrick's own illustrative wording, not a measured typical.
- Usable capacity by chemistry — Battle Born (Tier 2, manufacturer educational content, not a spec sheet): AGM approximately 50% depth of discharge; LiFePO4 80–100% usable, 90% used here as a stated representative mid-range figure.
- Cycle life and round-trip efficiency — Anern (Tier 2): LiFePO4 3,000–7,000 cycles vs AGM 500–1,500; LiFePO4 greater than 95% round-trip efficiency vs AGM approximately 80–85%.
- Weight comparison — TheGreenWatt (Tier 2, industry-consensus figures, not a single manufacturer's spec sheet): 100Ah LiFePO4 approximately 24–30 lb vs equivalent AGM approximately 60–70 lb.
- Cold-weather discharge — GridWright (Tier 2): LiFePO4 discharge to about −20°C with reduced capacity.
- Cold-weather charging — RELiON (Tier 1, manufacturer): reduced charge current required below 0°C and below −10°C, or risk irreversible damage.
- Power stations — Jackery's own spec sheet (Explorer 1000 original, Li-ion NMC; Explorer 1000 v2, LiFePO4 — two different generations, never blended) plus Gnomad Experiences (Tier 2, the original's ~13.2V regulated-output claim) and The Drive; EcoFlow's own spec pages (RIVER 2; RIVER 2 Max, "All Regulated"); Bluetti's own spec page (EB3A, "All Regulated"). Jackery's own runtime formula (working time = Wh × 0.85 ÷ operating power, Tier 1) is the source for the ~15% AC-inverter overhead figure, used only as a footnote since this tool's arithmetic is DC-direct throughout.
- Planning margin — 20–30% is this site's own recommended range, not attributed to an external source; 25% is the single value this tool applies.
Figures current as of 2026-08-08. Where a figure is retailer-published rather than manufacturer-published, or is a manufacturer's educational content rather than a spec sheet, this page says so at every place that figure appears, not only here.
This calculator is arithmetic over published figures, plus whatever you tell it about your own gear. It does not know your camera model, your controller, or your night's temperature. Every figure it uses is named and tiered above, and where the hobby's own sources disagree — lead-acid/AGM against LiFePO4 is the clearest case — you get both positions rather than ours.
No prices anywhere in this tool. Several mount brands enforce minimum advertised pricing, and a figure we scraped once would be wrong within weeks — so we don't print one, for any product, mount or power station alike. Nothing here is ranked by what it would pay us.