I've noticed that a lot of these pictures are like this. Is it something to do with the error in the machining of the lenses?
They are diffraction spikes created by the internal support structure of some telescopes. Some astronomers who don't have such telescopes even tape wires overtop of the scope for artistic flair:I've noticed that a lot of these pictures are like this. Is it something to do with the error in the machining of the lenses?
Depends on the number of supports for the secondary mirror and the brightness of the starAre there strong technical reasons why a telescope should have 4 rather than 6 diffraction rays?
Not total nonsense, I think. The relative brightness of the diffraction pattern vs star will relate to the areas of the spider and mirror. The level of the diffraction pattern will be many orders of magnitude up on the main image. You need a source that's smaller than the width of the diffraction spike and it needs to be bright enough for the (very diffuse) diffraction pattern to be visible.well, that doesn't even begin to make sense
Just bad wording, I think. Planets and galaxies are extended sources so they follow the same visibility argument. The word "foreground" could have been chosen better. "Bright enough stars" would perhaps have been a better term. Most astro images don't seem to have many spikes stars in them (you could show me an exception, I'm sure) because of the vast range of magnitudes. To get more spikes would involve severe burn-out of the brightest star images. That's the power of the logarithm for you.it is, specially this part ......
no argument there ... it's always seen on the brighter lower %'age of the stars in the image and of course as said earlier, by those telescopes with "spider" type mounting of the secondary mirror.. Most astro images don't seem to have many spikes stars in them (you could show me an exception, I'm sure) because of the vast range of magnitudes
Has anyone come up with a design with a plastic membrane supporting the secondary mirror? I guess you would need two layers to stop the mirror tilting and that would involve four surfaces plus Newton's rings from any curvature. I'd bet someone, somewhere has tried it though. It could be cheaper than the optics of SC style optics.generally refractors and reflectors like Schmitt Cassigrain's don't have spikes
because there are no or no need for supporting vanes
Ok diffraction spikes, what is the mechanism in these spots? The TV camera?
Not a fault, usually. You can buy ‘star filters’ for cameras. They have a lattice of lines all over so the diffraction pattern works for bigger and less bright sources ( chosen on aesthetic grounds).For cameras the usual cause is the iris used to stop down the lens. In general any edge that protrudes into the optical path can be the culprit.
That’s an interesting take on things. I can sympathise but what can you do and what subjects do you photograph?To put it mildly I'm in the opposite camp of that particular never ending discussion. ;)
I tried the opposite and bought "filter step down rings" (used to fit a smaller filter to your lens) to be able to step down the aperture of some of my camera lenses (to improve the optical quality) without getting any diffraction spikes. The main reason was that camera lenses usually have many blades (LOTS of spikes) and that some of my premium lenses have slightly curved blades in the iris and thus produce not spikes but fan like shapes.
I like them, they remind me of the 70s, glad it's come full circle and it's nice to have a technical explanation.Not a fault, usually. You can buy ‘star filters’ for cameras. They have a lattice of lines all over so the diffraction pattern works for bigger and less bright sources ( chosen on aesthetic grounds).
At one time photographers did their best to avoid artefacts but you can see them everywhere these days. Effective in some cases, sloppy in others.
alantheastronomer said:in fact it's a way to distinguish foreground stars from extended sources like planets and galaxies!
you're right, I should've said "background stars" for planets, and reserved "foreground stars" for galaxies...daven said:Stars are NOT foreground to planets
I think you're thinking of the very well defined, closeup, extended images of galaxies we are all familiar with. I'm referring to wide field images of clusters of galaxies like this one of NGC 7619 taken by astrophotographer Jim Burnell (jburnell.com) If you click on it you'll see two objects center right that are of similar brightness to some of the stars yet have no diffraction spikes - they are cluster members! Sorry for the confusion.daven said:and galaxies are not point sources so they are totally irrelevant.
I assume the galaxy in question is one of the bright images without the star pattern; it is a distributed source and the star pattern is therefore distributed and not visible.I'm referring to wide field images of clusters of galaxies like this one of NGC 7619 taken by