Starship Asterisk*

Occasionally you come across an astroimage showing a part of the sky that you have rarely or never seen photographed before. This happened to me when I happened across this image of van den Bergh 18, a reflection nebula, and NGC 1342, an open cluster. I found the picture incredibly beautiful, so I can't help it, I've got to post it here.


Click on image to enlarge

Vdb 18 and NGC 1342 (Perseus)
ASA 10" STL 11000. LRGB, About 13.5 hrs in four nights.
© Gimmi Ratto, Collecting Photons
October - November 2009

Ann

I realize that lens flare is an artifact of the equipment but it I will admit that I've always loved its effect on astronomical images..

Orca wrote:I realize that lens flare is an artifact of the equipment but it I will admit that I've always loved its effect on astronomical images..

There is no evidence of lens flare in this image. Are you perhaps talking about diffraction spikes?

I assumed 'lens flare' was part of the same thing. Did some reading, apparently the spikes are caused by diffraction while 'flare' is unwanted the scattering of light inside the device.

In this example, we see the diffraction spikes as well as sharp halos around the most of brighter stars. Is the halo considered a flare, or is it another effect entirely?

Orca, there's lens flare in this image (and if I'm wrong about this, I certainly hope someone corrects me!): (That a submission from here by Salvatore Grasso.)

Orca, I think that what you are seeing is the blue reflection nebula van den Bergh 18. You can see it above and below the blue star on the left. The reflection nebula is for real, and it is not a camera effect.

A reflection nebula is created when light from the star is reflected in a dust cloud. A famous yellow reflection nebula exists around red supergiant star Antares. Most reflection nebulae are blue, however. The most famous blue reflection nebula is the one surrounding the Pleiades.



In this picture, you can see a large yellow-orange reflection nebula surrounding red supergiant star Antares. There is also a blue reflection nebula surrounding blue B-type star Rho Ophiuchi. There is a red emission nebula to the right of hot star Sigma Scorpii. Sigma Scorpii is hot enough to ionize the hydrogen gas surrounding it, so that the gas glows by its own red light, rather than just reflecting the color of the star illuminating the cloud of (hydrogen) gas and dust.

Ann

I appreciate the response Ann; I understand the various types of nebulae. Chris was correct; I was referring to the refraction spikes in the stars which happened to be prominent in the image you shared with us. The problem: I was under the impression any and all artifacts of the equipment fell under a broad description of "lens flare." I wasn't aware that the term in fact refers to a specific type of scattering of light inside the optical device.

Chris Peterson wrote:

Orca wrote:I realize that lens flare is an artifact of the equipment but it I will admit that I've always loved its effect on astronomical images..

There is no evidence of lens flare in this image. Are you perhaps talking about diffraction spikes?

Wikipedia actually lists diffraction spikes as a type of lens flare. I don't know enough about the two terms to really say if it needs correction, though.

geckzilla wrote:Wikipedia actually lists diffraction spikes as a type of lens flare. I don't know enough about the two terms to really say if it needs correction, though.

I'd say they are using a poor definition. I've worked around optics my entire career, and I've never heard diffraction confounded with lens flare. In an optical system, light can end up where you don't intend it, due to multiple effects. Diffraction is always present, and is typically defined by fundamental physics, meaning little can be done about it. Scatter is light that bounces around in the system due to non-specular reflections. Lens flare is light that bounces around in the system due to specular reflections. Diffraction, scatter, and flare are all different things.

Orca wrote:In this example, we see the diffraction spikes as well as sharp halos around the most of brighter stars. Is the halo considered a flare, or is it another effect entirely?

Halos and bloated stars can come from multiple sources, and frequently more than one are acting together. Diffraction increases the size of stars from points to discs, and the size of the visible disc in an image is related to intensity. So bright stars appear larger. Most mirrors are not perfectly specular- they have a degree of surface roughness that scatters light, and that shows up as a halo. And there are flat surfaces in the optical path, usually near the sensor- filters, chamber windows, and CCD cover slips are the main ones- which produce internal reflections that typically show as halos.

In astronomical images (those made with lenses- most are not) true lens flare tends to show up as ghost images of bright stars some distance from the stars themselves- sometimes even for stars that are outside the field of view.