M27 Dumbbell Nebula with the Optolong L Ultimate
A field decision guide for imaging the Dumbbell from Shinfield in June. Covers when the L Ultimate earns its place in the train, why the summer twilight makes it essential rather than optional, and the capture settings that make it work on the ASI585MC.
The verdict first
M27 is a planetary nebula, a dying star throwing off its outer shell. Almost all of its light arrives in two narrow emission lines: hydrogen alpha at 656nm and doubly ionised oxygen at 500nm. The L Ultimate passes exactly those two lines at 3nm bandwidth and rejects everything else.
Yes. Fit the L Ultimate. M27 emits almost nothing the filter blocks, and in June the filter blocks almost everything the sky throws at you.
This is the rare case where a narrowband filter costs you essentially nothing on the target while removing nearly all of the noise floor. Broadband subs on M27 from a suburban site in midsummer would be washed flat.
Why June makes it non negotiable
At 51 degrees north the sun never drops far enough below the horizon in mid June for astronomical darkness. The sky sits in permanent nautical twilight all night, a broad blue grey glow that ruins broadband contrast even before Shinfield's light pollution joins in.
Twilight glow and sodium and LED streetlight emission are broadband sources. A 3nm dual band filter rejects the overwhelming majority of both while M27's Ha and OIII sail straight through. The signal to noise advantage tonight is enormous.
Capture setup
The session workflow
Fit the filter and refocus
The L Ultimate shifts the focal plane slightly compared with unfiltered imaging. Run autofocus after fitting it, on a bright star, and again if the temperature drops more than a couple of degrees through the night.
Wait for the darkest window
In mid June the deepest twilight runs roughly midnight to 2am BST. M27 rises through the east into good altitude across that window. Start subs earlier if you like, but expect the first hour to grade noticeably worse.
Expose for the lines, not the sky
Set gain 252 on the 585MC for the high conversion gain mode, subs at 180 to 300s, and check the histogram peak sits clear of the left edge by roughly a tenth to a fifth of full scale. If it hugs the left wall, lengthen the sub.
Dither and guide conservatively
Dither every 2 to 3 subs to break up walking noise, which narrowband stacks show mercilessly. With 300s subs on the AM5N keep guiding aggression modest and let the mount's encoders do their job.
Bank star colour for later
Stars through a 3nm filter come out tight but colourless. On a darker autumn night, grab 30 to 60 minutes of short unfiltered RGB subs of the same field and blend the natural star colour back in during processing with StarXTerminator separation.
Quick reference
| Setting | Value | Why |
|---|---|---|
| Filter | Optolong L Ultimate | M27 emits almost entirely in Ha and OIII |
| Gain | 252 | High conversion gain, lowest read noise on the 585MC |
| Sub length | 180 to 300s | 3nm bandpass starves the sensor, longer subs beat read noise |
| Dither | Every 2 to 3 subs | Narrowband stacks reveal walking noise badly |
| Best window | Midnight to 2am BST | Deepest twilight in mid June at 51N |
| Moon tonight | 10 percent waning crescent | Negligible, rises after midnight |
| Star colour | Separate RGB run later | Blend natural stars in via StarXTerminator |