M 35 — colour–magnitude diagram from a deep Seestar stack

The open cluster NGC 2168 imaged by ZWO Seestar S50s, co-added into one deep stack and reduced to an HR diagram with isochrones.

1 · The colour–magnitude diagrams

521 cluster members measured on a single deep stack of 8 Seestar S50 frames totalling 1.11 h. Green is the luminosity proxy — the synthetic Seestar green sits within 0.02 mag of Johnson V, so the vertical axis is effectively V — and the three Seestar colours are the temperature proxies.

2 · The deep stack

Frames are resampled through their CrowdSky plate solutions onto one tangent plane at 2.38″/pixel and co-added by inverse-variance weighting, after matching each frame's sky pedestal and flux scale to the deepest one. 8 of 8 offered frames were used; the deepest pixel stacks 8 frames.

Frame	Exposure (s)	SQM	Telescope	Observed
3258	580	21.15	S50_98ac1c17	2026-03-07 20:45:41
3256	560	21.15	S50_98ac1c17	2026-03-07 19:45:33
3255	520	21.15	S50_98ac1c17	2026-03-07 21:15:23
10484	500	21.15	S50_98ac1c17	2026-03-07 20:29:27
10481	480	21.15	S50_98ac1c17	2026-03-07 19:29:27
10482	480	21.15	S50_98ac1c17	2026-03-07 19:59:45
3253	460	21.15	S50_98ac1c17	2026-03-07 20:16:36
3252	420	21.15	S50_98ac1c17	2026-03-07 21:01:07

3 · Isochrones and the age

Published isochrones come in standard photometric systems; none of them is the Seestar's. The reference catalogue bridges that: it carries, for the same stars and from the same Gaia XP spectra, both synthetic Johnson–Kron–Cousins B/V/R and synthetic Seestar b/g/r. Fitting one against the other gives a transformation good to 0.007 mag, well below the photometry's own accuracy, after which any BVRI isochrone can be pushed into the Seestar passbands.

Colour	Best age (Myr)	Range within 10%	E(B−V)	Members used
b − g	16	16–16	0.40	311	⚠ ran to the grid edge
b − r	251	79–398	0.30	311
g − r	200	25–501	0.28	311

b − g reached the edge of the reddening or age grid, so its optimum lies outside the searched range and its quoted age is a bound, not a fit. b−r carries the result: it has the longest colour baseline and so the most temperature leverage.

Over the fitted window the 63 and 251 Myr isochrones differ by at most 1.532 mag in b−r (median 0.033), against a measured colour accuracy near 0.03 mag. That makes the age a real constraint.

4 · How good is the photometry?

Every measured star also has Gaia XP synthetic photometry in the same S50 passbands, so the two can be compared star by star. Any disagreement is the Seestar's error, not a passband mismatch — which is also why the zero point needs no colour term.

g range	Stars	b − g	b − r	g − r
8–11	101	+0.019 ± 0.031	+0.041 ± 0.027	+0.018 ± 0.020
11–12	134	+0.001 ± 0.044	+0.021 ± 0.034	+0.018 ± 0.026
12–13	247	-0.005 ± 0.044	+0.002 ± 0.045	+0.003 ± 0.032
13–14	420	+0.001 ± 0.051	-0.011 ± 0.060	-0.014 ± 0.040
14–15	581	+0.018 ± 0.087	-0.025 ± 0.097	-0.038 ± 0.061
15–16	140	+0.049 ± 0.143	-0.016 ± 0.161	-0.050 ± 0.097

Band	Zero point	Flat rms	With illumination surface	Calibrators
R	23.405	0.055	0.048	774
G	23.446	0.062	0.057	787
B	23.878	0.075	0.071	768

Members are selected on Gaia DR3 proper motion within 0.70 mas/yr of (+2.293, -2.918); the expected field contamination inside that cut is 102 stars. Stars with a comparably bright neighbour within 20″ are rejected — blending is the dominant systematic here and always reads too bright.

5 · Credits — contributing Seestars

This diagram is built from 8 stacks contributed by 1 Seestar unit(s): S50_98ac1c17. Data via the CrowdSky archive; reference photometry from Gaia DR3 and Gaia XP synthetic spectra; isochrones from PARSEC v1.2S (Bressan et al. 2012).

Downloads: the deep stack (m_35_deep_stack.fits) and the measured photometry (photometry.ecsv) are published alongside this page.

Generated 2026-09-20 20:08 UTC · CrowdSci