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

About 4 h of total integration for a solid image, or 16 h for a showpiece. A quick look takes 1 h.

How many subs is that on the Seestar S50?

At the default 10 s sub length, 1,439 subs for a solid image from Bortle 6 and 970 from Bortle 3.

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

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

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

From a Bortle 6 suburban sky, the ZWO Seestar S50 needs about 4 h (1,439 subs of 10 s with the built-in dual-band (LP filter) filter) for a solid image of the Cave Nebula. A quick look takes 1 h; a showpiece 16 h. Under a Bortle 3 rural sky the solid figure drops to 2 h 42 min. At Bortle 6 the limit is a mix of skyglow and read noise. In this scope's field it is just over the frame at 1.2×.

Cave 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	39 min232 subs	2 h 34 min925 subs	10 h3,697 subs
Bortle 2Typical dark site	39 min235 subs	2 h 37 min940 subs	10 h3,760 subs
Bortle 3Rural	40 min243 subs	2 h 42 min970 subs	11 h3,880 subs
Bortle 4Rural / suburban transition	43 min261 subs	2 h 54 min1,043 subs	12 h4,169 subs
Bortle 5Suburban	48 min290 subs	3 h 13 min1,157 subs	13 h4,628 subs
Bortle 6Bright suburban	1 h360 subs	4 h1,439 subs	16 h5,753 subs
Bortle 7Suburban / urban transition	1 h 22 min494 subs	5 h 29 min1,975 subs	22 h7,898 subs
Bortle 8City	1 h 57 min702 subs	7 h 48 min2,807 subs	31 h11,228 subs
Bortle 9Inner city	3 h 11 min1,146 subs	13 h4,583 subs	51 h18,330 subs

Adjust in the calculator

Power and kit for this session

Allowing for slewing, focusing and rejected subs, that is about 5 h 12 min of run time. The Seestar S50’s battery is rated at about 6 h, which is cutting it fine; take a power bank. (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 Cave Nebula is 50′ × 30′ across. The Seestar S50 frames 1.28° × 0.72° at 2.39″ per pixel, so it is just over the frame at 1.2×. Point at the part you want; the edges will fall outside the frame.

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	2 h 42 min970 subs	3 h 13 min1,157 subs	5 h 29 min1,975 subs
20 s	1 h 53 min339 subs	2 h 24 min433 subs	4 h 40 min842 subs
30 s	1 h 37 min194 subs	2 h 08 min256 subs	4 h 24 min529 subs
60 s	1 h 20 min81 subs	1 h 52 min112 subs	4 h 08 min248 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)	2 h 22 min850 subs	4 h 03 min1,461 subs	11 h4,131 subs
dual-band (LP filter)	2 h 42 min970 subs	3 h 13 min1,157 subs	5 h 29 min1,975 subs

About the Cave Nebula

A faint hydrogen arc with dark dust in Cepheus. Low surface brightness; this one wants dark skies and a long dual-band stack.

Sh2-155, Caldwell 9 · Emission nebula in Cepheus · magnitude 7.7 · 50′ × 30′ · best in Aug, Sep, Oct. 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 Cave 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 Cave 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 Cave Nebula · All targets for the Seestar S50