How long should I image the Crescent Nebula with the Seestar S50 from a Bortle 6 sky?

About 2 h 49 min of total integration for a solid image, or 11 h for a showpiece. A quick look takes 42 min.

How many subs is that on the Seestar S50?

At the default 10 s sub length, 1,016 subs for a solid image from Bortle 6 and 692 from Bortle 3.

Does a dual-band filter help on the Crescent Nebula?

Yes. It is an emission-line object; from Bortle 6 the filter cuts the solid-image time from 4 h 37 min to 2 h 49 min.

How long to image the Crescent Nebula with the ZWO Seestar S50

From a Bortle 6 suburban sky, the ZWO Seestar S50 needs about 2 h 49 min (1,016 subs of 10 s with the built-in dual-band (LP filter) filter) for a solid image of the Crescent Nebula. A quick look takes 42 min; a showpiece 11 h. Under a Bortle 3 rural sky the solid figure drops to 1 h 55 min. At Bortle 6 the limit is a mix of skyglow and read noise. In this scope's field it fills 42% of the frame’s short side.

Crescent Nebula with the ZWO Seestar S50: 10 s subs with the built-in dual-band (LP filter) filter.
Sky	Quick lookTarget obvious, still noisy	SolidClean enough to share	ShowpieceSmooth faint detail, print-worthy
Bortle 1Excellent dark site	27 min165 subs	1 h 50 min660 subs	7 h 20 min2,639 subs
Bortle 2Typical dark site	28 min168 subs	1 h 52 min671 subs	7 h 27 min2,683 subs
Bortle 3Rural	29 min173 subs	1 h 55 min692 subs	7 h 41 min2,765 subs
Bortle 4Rural / suburban transition	31 min186 subs	2 h 04 min742 subs	8 h 14 min2,966 subs
Bortle 5Suburban	34 min206 subs	2 h 17 min821 subs	9 h 07 min3,283 subs
Bortle 6Bright suburban	42 min254 subs	2 h 49 min1,016 subs	11 h4,062 subs
Bortle 7Suburban / urban transition	58 min347 subs	3 h 51 min1,387 subs	15 h5,545 subs
Bortle 8City	1 h 22 min491 subs	5 h 27 min1,963 subs	22 h7,849 subs
Bortle 9Inner city	2 h 13 min798 subs	8 h 52 min3,191 subs	35 h12,762 subs

Adjust in the calculator

Power and kit for this session

Allowing for slewing, focusing and rejected subs, that is about 3 h 40 min of run time. The Seestar S50’s battery (rated about 6 h) should cover it; a power bank still keeps the phone running the app alive. (Bortle 6, solid image; we allow 30% over the integration time.)

USB-C power bank 20000mAh 5V 3AThe S50 draws 5 V 3 A over USB-C and runs while charging; no PD needed.
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Seestar S50 dew shieldThe built-in heater copes with most nights; a shield helps in heavy dew and blocks stray light.
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red LED head torchRed light keeps your night vision; white light costs you twenty minutes of dark adaptation every time.
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How it frames

The Crescent Nebula is 18′ × 12′ across. The Seestar S50 frames 1.28° × 0.72° at 2.39″ per pixel, so it fills 42% of the frame’s short side.

Longer subs

10, 20 and 30 s in Alt-Az; 60 s only in EQ mode (app 2.5 and later). Time for a solid image by sub length:

Sub length	Bortle 3	Bortle 5	Bortle 7
10 s	1 h 55 min692 subs	2 h 17 min821 subs	3 h 51 min1,387 subs
20 s	1 h 22 min245 subs	1 h 43 min310 subs	3 h 17 min593 subs
30 s	1 h 10 min141 subs	1 h 32 min184 subs	3 h 06 min373 subs
60 s	59 min60 subs	1 h 21 min81 subs	2 h 55 min175 subs

With and without the dual-band filter

Solid quality, default sub length.
Filter	Bortle 3	Bortle 5	Bortle 7
UV/IR-cut (no light-pollution filter)	1 h 40 min602 subs	2 h 51 min1,025 subs	7 h 59 min2,873 subs
dual-band (LP filter)	1 h 55 min692 subs	2 h 17 min821 subs	3 h 51 min1,387 subs

About the Crescent Nebula

A shell blown by a Wolf-Rayet star, bright in H-alpha with a faint OIII skin. Small, so the bigger scopes do it justice.

NGC 6888, Caldwell 27 · Emission nebula in Cygnus · magnitude 7.4 · 18′ × 12′ · best in Jul, Aug, Sep. Reachable from mid-northern latitudes (UK, northern US, central Europe) and from the south.

Emission nebulae glow in a few narrow lines, mostly hydrogen-alpha (deep red) and doubly ionised oxygen (teal). A dual-band filter passes those lines and blocks most skyglow, which is why it cuts the time needed from a suburban sky so dramatically. Use it. From a genuinely dark site the gain is smaller because there is less skyglow to block, and read noise becomes the limit, which is where longer subs (EQ mode, or Expert Mode on a Vespera) earn their keep.

Stars lose colour through a dual-band filter; a short broadband stack blended in fixes that if you care.

About the ZWO Seestar S50

The scope that made smart astrophotography mainstream. 50 mm triplet, 2 MP sensor, dual-band filter on a wheel. Alt-Az stacking at 10 s, EQ mode unlocks 60 s. Out of production since 2026 and replaced by the S50 Pro, but there are a great many in use.

10, 20 and 30 s in Alt-Az; 60 s only in EQ mode (app 2.5 and later). Built-in dual-band light-pollution filter, switchable in the app.

Longer subs matter most under dark skies, where read noise rather than skyglow sets the floor. From a city the difference between 10 s and 60 s subs is small; from a rural site it can halve the total time, especially with a dual-band filter in place.

Stack across nights. Every smart scope app can continue a stack or you can combine the saved frames on a computer; the figures here are totals, not single-session limits.

Where to buy the Seestar S50ZWO store Direct Amazon UK Amazon USThe ZWO store link is a plain link; the others are affiliate links. As an Amazon Associate I earn from qualifying purchases, at no cost to you.

Imaged the Crescent Nebula with your Seestar S50? Did the solid figure match?Anonymous, one tap. It is how the model gets calibrated.

Have an image of the Crescent Nebula from a Seestar S50? Email it with your total time and sky to [email protected] and it goes here, credited.

Same target, other telescopes

All telescopes for the Crescent Nebula · All targets for the Seestar S50