Trifid Nebula

M20, NGC 6514

Image exposure:
53 x 90s = 79.5 minutes
Image field of view:
38.7 x 25.6 arcmin
Image date:

The Trifid is a complex region, roughly the same size as the Moon. It is a glorious nebula in Sagittarius.

Visual magnitude: +6.3
Apparent diameter: 29 x 27 arc-min
Actual diameter: 44 light years
Distance: 5,200 light years

It includes:

  • a red emission nebula trisected with laneways of dark nebulae with a bright double star system in the centre (HD 164492) surrounded by a young open star cluster; and
  • a blue reflection nebula which is very conspicuous at the top but much fainter around the outside edges of the red emission nebulosity.

An emission nebula glows because it is excited by ionising ultra-violet radiation from hot young stars embedded in the nebula; whilst a reflection nebula is illuminated from the full spectrum of visible light from local stars.

The blue reflection nebula is illuminated by the broad light spectrum emitted from the type A supergiant star in it’s centre (HD 164514).

I’ve imaged M20 before – it is such an amazing object – but not with my LX-90 203mm Schmidt-Cassegrain reflector telescope:

2021-10-26 LX-90 Deforked and mounted on EQ6 equatorial mount.

This was an eighty minute exposure, consisting of 53 sub-exposures of 90 seconds each, live-stacked as each new sub arrived.

Here is a clickable screen shot of my laptop workspace, taken while the image subs were being acquired and pre-processed:

The telescope navigation is selected in Cartes du Ciel (sky map lower left of screenshot); and managed by EQMOD software (upper right). Images are centred, exposed, captured, stacked and pre-processed with SharpCap software (upper left and centre); and the telescope is kept tightly on target by PHD guiding software (graph lower right).

I pay a modest annual licence fee for the SharpCap software, the other three are freeware, as is the GIMP image processing software which I use afterwards to post-process the final image.

Telescope:Meade LX-90 200mm Schmidt-Cassegrain
(deforked); 2000 mm f/l @ f/10.
Optics:Astronomik light pollution filter.
Mount & Guiding:SkyWatcher EQ6-R Pro mount.
Imaging camera:ZWO ASI 071 MC cooled.

Finally, here’s one I took earlier with my refracting telescope:

2019-09-03 M20 Trifid Neb Stack_63frames_7303s 2

Images © Roger Powell

Previous Posts

ABOVE US ONLY SKY : amateur astronomy in australia



  1. Impressive photo and impressive equipment setup . . . and software.

    . . . so, that’s where the Trifids came from. I hope someone is keeping an eye on that to make sure they don’t return.

    Liked by 1 person

    1. An extra “f” but thanks for the reminder. I read The Day of the Triffids when I wore a young man’s shoes.
      I’ve just downloaded it to read again soon.


    1. I dropped Photoshop for GIMP a long time ago. It’s open source freeware that does most of what Photoshop does. However, there are a lot of commands which I don’t use – or even understand – but that’s down to me not being inquisitive enough to read the manual. I just use a few basic commands.


      Liked by 1 person

      1. I’ve always felt that is GIMP’s biggest drawback – it doesn’t feel intuitive if you are coming from another of the popular paint programs. I do hope to try to learn it though.

        Liked by 1 person

Leave a comment or ask a question . . . .

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s