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

About 3 h 09 min of total integration for a solid image, or 13 h for a showpiece. A quick look takes 47 min.

How many subs is that on the Draco?

At the default 30 s sub length, 378 subs for a solid image from Bortle 6 and 248 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 5 h 25 min to 3 h 09 min.

How long to image the Cave Nebula with the DwarfLab Draco

From a Bortle 6 suburban sky, the DwarfLab Draco needs about 3 h 09 min (378 subs of 30 s with the built-in dual-narrowband (Hα + OIII) filter) for a solid image of the Cave Nebula. A quick look takes 47 min; a showpiece 13 h. Under a Bortle 3 rural sky the solid figure drops to 2 h 04 min. At Bortle 6 the limit is a mix of skyglow and read noise. In this scope's field it fills 67% of the frame’s short side.

Cave Nebula with the DwarfLab Draco: 30 s subs with the built-in dual-narrowband (Hα + OIII) filter.
Sky	Quick lookTarget obvious, still noisy	SolidClean enough to share	ShowpieceSmooth faint detail, print-worthy
Bortle 1Excellent dark site	29 min59 subs	1 h 58 min236 subs	7 h 50 min941 subs
Bortle 2Typical dark site	30 min60 subs	2 h240 subs	7 h 59 min959 subs
Bortle 3Rural	31 min62 subs	2 h 04 min248 subs	8 h 16 min992 subs
Bortle 4Rural / suburban transition	33 min67 subs	2 h 14 min268 subs	8 h 56 min1,072 subs
Bortle 5Suburban	37 min75 subs	2 h 30 min300 subs	10 h1,200 subs
Bortle 6Bright suburban	47 min95 subs	3 h 09 min378 subs	13 h1,512 subs
Bortle 7Suburban / urban transition	1 h 06 min132 subs	4 h 23 min527 subs	18 h2,108 subs
Bortle 8City	1 h 35 min190 subs	6 h 19 min759 subs	25 h3,033 subs
Bortle 9Inner city	2 h 36 min313 subs	10 h1,252 subs	42 h5,007 subs

Adjust in the calculator

Power and kit for this session

Allowing for slewing, focusing and rejected subs, that is about 4 h 06 min of run time. The Draco’s battery (rated about 5 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 20000mAhRated 5 h on its 10,000 mAh battery. DwarfLab has not published the charging input, so check PD support before relying on it mid-session.
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How it frames

The Cave Nebula is 50′ × 30′ across. The Draco frames 1.66° × 1.24° at 1.46″ per pixel, so it fills 67% of the frame’s short side.

Longer subs

Auto Mode picks the exposure; Pro Mode allows up to 300 s, held by built-in guiding and field derotation. The 30 s default here is our assumption. Time for a solid image by sub length:

Sub length	Bortle 3	Bortle 5	Bortle 7
30 s	2 h 04 min248 subs	2 h 30 min300 subs	4 h 23 min527 subs
60 s	1 h 27 min88 subs	1 h 53 min114 subs	3 h 47 min227 subs
120 s	1 h 09 min35 subs	1 h 35 min48 subs	3 h 29 min105 subs
300 s	2 h 30 min30 subs	2 h 30 min30 subs	3 h 18 min40 subs

With and without the dual-band filter

Solid quality, default sub length.
Filter	Bortle 3	Bortle 5	Bortle 7
No filter	1 h 53 min226 subs	3 h 18 min396 subs	9 h 29 min1,138 subs
dual-narrowband (Hα + OIII)	2 h 04 min248 subs	2 h 30 min300 subs	4 h 23 min527 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 DwarfLab Draco

DwarfLab’s big scope: 90 mm of folded f/3.8 optics, the most aperture of any Seestar or DWARF, a narrow 13 nm dual-band filter and 300 s subs with built-in guiding. A small-target scope; big nebulae need its mosaic mode.

Auto Mode picks the exposure; Pro Mode allows up to 300 s, held by built-in guiding and field derotation. The 30 s default here is our assumption. Built-in Hα + OIII dual-narrowband filter, 13 nm per line; the SHO Edition adds an SII option.

Longer subs matter most under dark skies, where read noise rather than skyglow sets the floor. From a city the difference between 30 s and 300 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.

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

Have an image of the Cave Nebula from a Draco? 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 Draco