Trifid Nebula
M20 · Emission nebula · Sgr
Never clears your horizon from this site — 18° at best, well under your 20° minimum.
- Magnitude
- 6.3
- Angular size
- 28′
- Surface brightness
- 22.2 mag/arcsec²
- Altitude now
- 15°
- Azimuth now
- 202° SSW
- Moon separation
- 160°
- Rise
- —
- Transit
- 20:52 (18°)
- Set
- 01:04
- Best window
- —–— CEST
- Tonight score
- 30 · Not realistic with 200 mm at Bortle 8
Tonight's sky
- Trifid Nebula
Through your instrument
40× · TFOV 1.70° · exit pupil 5.00 mm · f/6.0
The eye's colour receptors need far more photons than a camera sensor collects over minutes, so faint deep-sky objects genuinely look grey at the eyepiece.
200 mm f/6 Dobsonian at 40×
Newtonian: image rotated 180°
Your optics, in numbers
- Aperture
- 200 mm
- Focal ratio
- f/6.0
- Magnification
- 40×
- True field
- 1.70°
- Exit pupil
- 5.00 mm
- Dawes limit
- 0.58″
- Rayleigh limit
- 0.69″
- Max useful power
- 250×
- Limiting magnitude
- 11.6
This target peaks below 20° tonight — atmospheric extinction costs roughly 0.5 magnitudes.
Recommended settings
Low power: 30 mm (40×, exit pupil 5.0 mm)
A 5–7 mm exit pupil gives the darkest sky background per unit of object brightness — best for large faint nebulae and star clouds.
High power: 6 mm (200×)
Planets, the Moon and double stars take up to about 2× the aperture in mm, capped at your 250× seeing limit.
Aperture verdict
Surface brightness 22.2 mag/arcsec² is faint; 200 mm needs a genuinely dark night.
Filters
A UHC filter lifts contrast noticeably here; an OIII is even stronger on planetary nebulae, at the cost of some star field.
Technique
Dark-adapt for 20–30 minutes, use averted vision on faint targets, tap the tube gently to catch motion, and wait for the object to reach transit rather than shooting over rooftops.
Seeing vs transparency
Planets need steady air (seeing); faint deep-sky objects need clean, transparent air. A hazy but still night is a planet night.
Gear for this object
Affiliate links — we may earn a commission at no cost to you.
- UHC narrowband filter (1.25″) Affiliate link
- OIII filter (1.25″) Affiliate link
- 32 mm wide-field eyepiece Affiliate link