They may be used to form new stars and planets. The elements contained within planetary nebulae, such as carbon, oxygen, and nitrogen, are recycled into the surrounding space. The Southern Ring Nebula will dissipate in only thousands of years. They only last for about 10,000 years before the central stars cool so much that the surrounding nebulosity becomes invisible to us. Planetary nebulae are relatively short-lived phenomena. The origin of the term “planetary nebula” is uncertain, but it was used before the 19th century. The nebulae do not, however, have anything to do with planets. They are called planetary nebulae because their round shape vaguely resembles that of a planet when seen through a telescope. The Southern Ring Nebula and other planetary nebulae are in effect the expelled outer layers of evolved stars that reached the end of their lives. The same fate awaits our Sun in about 6 billion years. These shells are ionized by the exposed hot stellar cores, the remnants of the central stars. In the late red giant phase, the stars periodically expel layers of material and ultimately end up surrounded by huge shells of gas and dust. Planetary nebulae are the final stage in the lives of intermediate-mass stars (stars with 1-8 times the mass of the Sun) that evolve away from the main sequence and expand into red giants. Image Credit: National Aeronautics and Space Administration (NASA), European Space Agency (ESA), Canadian Space Agency (CSA), Space Telescope Science Institute (STScI) More important is having nearly identical profiles across the field, and the R/C configuration does a good job of this (especially if a field flattener is added).A James Webb Space Telescope near-infrared image of the NGC 3132 planetary nebula, taken using the NIRCAM instrument. I bet that this is a really minor change, as the eye is really good at seeing extended features like spikes, but that they are low-level and contain little flux. This means that less flux will be included inside of the aperture, and therefore a slightly lower signal/noise will occur for the measurement. So the photometry will be fine.Īs mentioned before, the one exception to this is that visible diffraction spikes means that some light from the usual Gaussian profile has been scattered into the spikes. Even if part of the spikes are excluded for the target, they are also excluded in the comparison star. Diffraction spikes occur on all stars in an image, and so the profile will be identical (within reason). )įor aperture photometry, the important aspect is that you contain the same fraction of a star's profile inside of the aperture, for both the target and the comparison star. I have had to go to considerable trouble to secure the primary mirror and even so I have not fully licked the problem Again though, even trailed images seem to generate valid data, just the S/N ratio is impacted, raising the err range and a small err is a common goal for us all. I have frequently wished I had employeed an RC and may eventually change, the reason being mirror flop problems with the SCT. The scope selection concern I have seen regularly is that of matching the focal length to the pixel size on your camera - minimum of two pixels, preferably three or more per average FWHM image. My impression is the RC is probably thought of as the optimal design for photometry, lacking any lenses (which can filter certain colors differentially) and being well corrected across the field, but in the end nearly any scope you can put the instrument on seems to be useful. In any event, if the stars are sufficiently separated to do differential photometry on in the first place, it is hard to image that a distant refraction spike could involve enough energy to make a material impact. The only way I an concieve of defraction spikes impacting photometry would be if a comp star was quite near the subject star and the spike from one happened to fall on the image of the other, but again there would be at least a partial wash as the reverse would be true, spikes being reciprocal features. In part this may be because if your inner annulus is a multiple of your FWHM, so even some coma becomes trivial in the scheme of things and most people try and keep the comps near the subject star anyway. I use a focal reducer that is also a field flattener on my SCT system, but I have never seen any discussion about coma being a big concern. Some defects, like coma can vary across the field. Most imperfection on the subject star is reproduced on the comps stars, so it washes, though any defect that spreads the light reduces the S?N ratio and none of us like that. But I have been surprised how little difference image quality in general makes in photometry. I will be interested to see what response you get to this, as I am a relative novice myself.
0 Comments
Leave a Reply. |
Details
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |