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

About 4 h 30 min of total integration for a solid image, or 18 h for a showpiece. A quick look takes 1 h 07 min.

How many subs is that on the Seestar S50 Pro?

At the default 10 s sub length, 1,618 subs for a solid image from Bortle 6 and 1,111 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 7 h 14 min to 4 h 30 min.

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

From a Bortle 6 suburban sky, the ZWO Seestar S50 Pro needs about 4 h 30 min (1,618 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 07 min; a showpiece 18 h. Under a Bortle 3 rural sky the solid figure drops to 3 h 05 min. At Bortle 6 the limit is a mix of skyglow and read noise. In this scope's field it fills 60% of the frame’s short side.

Cave Nebula with the ZWO Seestar S50 Pro: 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	44 min266 subs	2 h 57 min1,062 subs	12 h4,246 subs
Bortle 2Typical dark site	45 min270 subs	3 h1,079 subs	12 h4,315 subs
Bortle 3Rural	46 min278 subs	3 h 05 min1,111 subs	12 h4,444 subs
Bortle 4Rural / suburban transition	50 min298 subs	3 h 18 min1,190 subs	13 h4,757 subs
Bortle 5Suburban	55 min329 subs	3 h 39 min1,314 subs	15 h5,253 subs
Bortle 6Bright suburban	1 h 07 min405 subs	4 h 30 min1,618 subs	18 h6,470 subs
Bortle 7Suburban / urban transition	1 h 32 min550 subs	6 h 06 min2,198 subs	24 h8,790 subs
Bortle 8City	2 h 09 min775 subs	8 h 36 min3,098 subs	34 h12,392 subs
Bortle 9Inner city	3 h 29 min1,255 subs	14 h5,019 subs	56 h20,073 subs

Adjust in the calculator

Power and kit for this session

Allowing for slewing, focusing and rejected subs, that is about 5 h 50 min of run time. The Seestar S50 Pro’s battery (rated about 9 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 PD power bank 20000mAh 30WAccepts 12 V 3 A PD, so a 30 W bank tops up the 10,000 mAh cell while imaging.
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Seestar S50 Pro dew shieldDew heater is built in; a shield adds margin on damp nights. Check the listing says S50 Pro.
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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 Pro frames 2.45° × 1.38° at 2.30″ per pixel, so it fills 60% of the frame’s short side.

Longer subs

10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, and 60 s is the ceiling. Time for a solid image by sub length:

Sub length	Bortle 3	Bortle 5	Bortle 7
10 s	3 h 05 min1,111 subs	3 h 39 min1,314 subs	6 h 06 min2,198 subs
20 s	2 h 08 min385 subs	2 h 42 min486 subs	5 h 09 min928 subs
30 s	1 h 49 min219 subs	2 h 23 min286 subs	4 h 50 min581 subs
60 s	1 h 30 min91 subs	2 h 04 min124 subs	4 h 31 min272 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 38 min949 subs	4 h 28 min1,610 subs	12 h4,499 subs
dual-band (LP filter)	3 h 05 min1,111 subs	3 h 39 min1,314 subs	6 h 06 min2,198 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 Pro

The S50 optics at 260 mm on an 8 MP 1/1.2-inch sensor: a wider, taller field, four times the pixels, and 60 s subs in EQ mode. Per pixel it is a whisker slower than the S50 because of the longer focal length; the long subs make it the better dark-site scope.

10, 20 and 30 s in Alt-Az; 60 s only in EQ mode, and 60 s is the ceiling. Built-in UV/IR-cut, dual-band (OIII 30 nm / Hα 20 nm) and dark filters on the telephoto camera.

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 S50 ProZWO 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 Pro? 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 Pro? 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 Pro