Starship Asterisk*

Thanks Noel.

- Karthik

Chances are very good that Rob combined multiple separate photos to create that image. That kind of thing is done all the time in astrophotography. Rob usually orients his spikes the same way in all his images that go into mosaics, but that's not always possible if the photos are taken with different hardware configurations. His famous recent M31 image has spikes with slightly different orientations across the image as well.

-Noel

interesting photo, most diffraction spikes in the image form an "X" patters but some form a "+" pattern.

[quote="Karthik"]Noel,
Any idea why they don't appear on images of distant/dim objects (like galaxies etc.)? Is the light not sufficient?
[/quote]
Yes, it's all in the light levels as the diffraction spikes are, in a well-designed optical system, an effect that's relatively small by comparison to the directly focused light. In short, you won't see them unless the light source is quite bright - well beyond the imager's dynamic range. If you're seeing spikes around one blob of overexposed pixels, and no spikes (or dimmer spikes) around another blob of overexposed pixels, the first blob was MORE overexposed than the second. There are usually other diffraction effects, such as a general flare, around bright point light sources, caused by the aperture of the telescope itself.

Also, in the APOD link describing diffraction spikes, there's yet another effect being shown - that of CCD photosite well overflow into adjacent wells. That can be seen as a jagged looking smear heading straight upward in the orangish negative image that they show. It's a shortcoming of electronic imagers, and it's not related to diffraction spikes.

Last but not least there's one more useful thing that can be gained from diffraction spikes, and it's most easily seen in images with multiple bright stars very close to one another. The stars can be exactly located by interpolating the crossing of the spikes, even though that point may be in a large blob of undifferentiated overexposed pixels. Keep in mind that even the brightest/closest stars are point light sources; we're not seeing the actual disk of the star in images, but rather optical flare and adjacent photosite effects in some cases. This image, by Rob Gendler, does a good job of illustrating that spikes can be used to determine that two stars are actually blended into one blob, and exactly where they really are. Note the close double star just to the lower-left of the M81 galaxy:

http://robgendlerastropics.com/M81NMM.html

Notably our own eyes produce some diffraction effects/spikes. Just look at a bright street light at night. This is why spikes in images imply to our brains that they are around a brilliant light source.

-Noel

Harry,

In the following page

http://www.aao.gov.au/images/captions/aat098.html

there is a link in the second sentence (of the description below the image) that leads to a "very deep image" which is supposed to show the other galaxy.

I am unable to make out which one is the other deforming galaxy.

Need to do more searching.

Thanks

Karthik

Noel,

Thanks for the explanation. I found an APOD explaining this.

http://antwrp.gsfc.nasa.gov/apod/ap010415.html

You had mentioned

[quote]Diffraction spikes can help differentiate strong point light sources (e.g., nearby bright stars) from distant/dim points of light in a given image,[/quote]

Any idea why they don't appear on images of distant/dim objects (like galaxies etc.)? Is the light not sufficient?

I found another APOD which shows some quasars with diffraction spikes.

http://antwrp.gsfc.nasa.gov/apod/ap060524.html

Thanks

Karthik

Hello Karthik

Thank you for the link

http://www.spiral-galaxies.com/NGC/NGC-2442.html
NGC 2442 = NGC 2443

I was hoping to see what other galaxy maybe deforming
NGC 2442: Galaxy in Volans
http://antwrp.gsfc.nasa.gov/apod/ap070315.html

Could the galaxy deform itself.

Diffraction spikes, such as the 4 point ones seen in the SSRO image and Hubble images, are generated when light passes by the straight supports used to locate a secondary mirror in the telescope's light path. You can see them in the upper-left of the image of the SSRO Ritchey-Cretien telescope: http://www.rcopticalsystems.com/16inchtruss.html

Not all telescopes have secondary mirror supports, or indeed secondary mirrors. Refractors - telescopes with lenses rather than mirrors, such as the Astro-Physics AP155EDFS used to take the image in your link - do not make diffraction spikes, unless the photographer stretches string across the aperture (which is often done for artistic reasons).

Diffraction spikes can help differentiate strong point light sources (e.g., nearby bright stars) from distant/dim points of light in a given image, as they convey brightness information not attainable from an overexposed pixel or group of pixels. It is less useful to compare spikes between different telescopes and different exposures, however.

-Noel

Not sure if this is what you want

http://www.spiral-galaxies.com/NGC/NGC-2442.html

One interesting thing I notice is that the two objects displaying diffraction spikes in the APOD image do not have any spikes in this image.

I don't know much about diffraction spikes. Perhaps someone can explain why there are no spikes in this second photo.

Thanks

Karthik

It seems as if the spiral arms are not all in the same plane. This is emphasised by the appearance of a fainter 'Northern' arm which suggests it is pointing away from us. Is this an illusion or can we measure the 3D shape?

Hello All

Has anybody got an image of the surrounding galaxies.

NGC 2442: Galaxy in Volans
http://antwrp.gsfc.nasa.gov/apod/ap070315.html

[quote]The distorted structure is likely the result of a close encounter with a smaller galaxy located just outside this telescopic field of view[/quote]


http://arxiv.org/abs/astro-ph/9701015

[quote]Using imaging Fabry-Perot data, we study the star-forming properties and kinematics of the nearby barred spiral galaxy NGC 2442. The Halpha emission is very localized along the strong spiral arms of the galaxy, and shows a marked asymmetry between the sharp, well-defined northern tidal arm and the weaker southern arm. The velocity field appears highly distorted, with a rapidly rotating nuclear component. We find evidence for strong non-circular motions along the northern arm, coincident with the pronounced dust lane and regions of intense star formation. The strong asymmetries, disturbed velocity field, and presence of a perturbed companion suggest that we are witnessing a strong kinematic response to a close interaction, which has redistributed the star formation activity throughout the disk of NGC 2442. Dynamical modeling supports this hypothesis, and suggests that the regions of strongest star formation are coincident with strong shocks occurring along the perturbed northern arm. Despite this redistribution of gas on small scales, this galaxy does not show a significant departure from the Tully-Fisher relation, nor does it appear to be experiencing any strong starburst. Moreover, our models predict that in a few x 10^8 years, NGC 2442 will have recovered from this encounter and will experience another passage -- and ultimately a merger -- in a few Gyr. Given the environment of many disk galaxies, this tidal encounter cycle seems likely to be a normal phase of disk galaxy evolution. [/quote]