Wolf-Rayet Oxygen-III bubble and other lushes H-alpha fields
Here are some lushes narrow band images of some famous nebulas built from many hours of data recorded from suburban Sydney, NSW. Many of the sub-frames had strong light pollution gradients associated with light scattered through thin cloud and smog. Light pollution gradients are especially strong during summer when cloud free nights are rare in Sydney. My strategy was to record as many sub-frames as possible across multiple nights to help average away the gradients, and then apply one instance of the PixInsight automatic gradient removal tool to the final integrated image.
OK, impressive work for urban sydney, obviously you know what your doing. Interesting to compare your bubbles with Andys also now on display on these hallowed pages. The highlights are detailed and smooth, but the slight over processing and high background level (with all its urban induced problems) are a bit distracting and make them loose impact a bit. I suspect just dumping the background would make them pop more?.
Thanks Fred. I share your concern, especially for the extremely faint Sharpless 308 images. At first I thought that the diffuse halo of OIII (with a little H alpha) emission surrounding the inner bubble was an artefact caused by light pollution. No doubt the background emissions are not entirely faithful because of light pollution gradients. However, with each passing night I began to think that at least some of the features in the background emissions were real. The dark halo surrounding the bright inner bubble may be the void blown out by an earlier, more ancient episode of intense stellar wind. Next I plan to do some more OIII runs using a focal reducer to increase the field of view. The idea is to see if the OIII emission near the edge of the field is part of a diffuse secondary bubble, or just a combination of light pollution gradients combined with diffuse OIII emission permeating the region.
... with each passing night I began to think that at least some of the features in the background emissions were real. The dark halo surrounding the bright inner bubble may be the void blown out by an earlier, more ancient episode of intense stellar wind ...
Yes! Someone thinking like an experimental scientist! Not just content with a beautiful image but thinking about what it might mean, and then coming up with a plan to test that hypothesis: moon and cloud gradients won't always be in the same place. Even light pollution gradients will move as one photographs in different parts of the sky.
I don't think you have anything to fix here, but just in the spirit of your investigation: Take another set of raw unprocessed flats, pretend that they're astronomical images, and process them with your existing master flat. Use the same stretches that you used on the Gourd. That will give you a sense of the reliability of the background nebulosity.
Agree with Fred that a tiny bit less noise reduction might produce less posterizing of the shock fronts. Accepting a slightly gritty background and letting the eye extract the detail might be a acceptable trade-off. A difficult call.
Loving the good work. Inspired by your and Andy's efforts, I want to have a shot at this next year, but it's going to be a 4 panel mosaic.
I'm interested in this one....
The "core" WR star, HD 50896 is classified as a WN4-s star.
Normally these Nitrogen WR have no Oxygen emissions, basically Helium and Nitrogen.
The strong HeII at 6560A is commonly mis-identified as Ha.
The strong line at 4686A contributes to a green colour....
I tend to agree with Mike. I think your picture is actually a lot deeper than Marco's. By pushing Marco's to ridiculous levels, the nebulosity you have captured matches his. The JPEG in his link was black clipped anyway.
The "core" WR star, HD 50896 is classified as a WN4-s star. Normally these Nitrogen WR have no Oxygen emissions, basically Helium and Nitrogen. The strong HeII at 6560A is commonly mis-identified as Ha. The strong line at 4686A contributes to a green colour....
Ken, for us mice, does your comment relate to the Gourd Nebula (the great vastness of Murray's image, or only the spectroscopy of the central star?
The spectrum of the star may have little to do with either the chemical composition or the spectrum of the nebula.
I am thinking that the nebula itself comprises shocked material, some of which contains hydrogen and oxygen previously shed by the star, and some of which was pre-existing background material. Hard UV from the central star would produce both H-alpha and OIII emission in this composite material. Even shock energy could contribute to broadening the acceptable excitation wavelengths n the nebula.
Mike, et al,
When the WR star blasts away their final hydrogen shell (velocities >2000 km/s) the chemical consistency of the subsequent shell material is:
Hydrogen/ Helium 98.5%
Nitrogen 0.1%
Carbon 0.35%
Oxygen 1%
There are some terrific ideas in these posts. Having studied the first version of Marco’s image (thanks Jo), I am very confident that most of the diffuse background is real. I know that Marco produces superb images built from raw data recorded under very dark outback skies, and there is a very strong resemblance between his “background” and mine. I also think that I understand what Mike (Placidus) is getting at – process a series of OIII flat fields as light frames calibrating them with the master flat field for the OIII filter. The idea is to test just how good one’s calibration procedures are, determining the extent to which calibration errors can generate artifacts in a naturally flat background. We should all do a test like this from time to time, especially when trying to extract faint nebulosity from the noise. The luminance data of my Sharpless 308 image is based upon about 29 x 15 minute exposures at 2x2 binning through an Astronomik 12 nm OIII filter. A large fraction of the sub frames were contaminated by passing thin clouds (summer in Sydney). I deleted about another dozen sub-frames which I judged too poor because of passing clouds. Thank goodness for narrow band filters and sigma rejection.
Sharpless 308 consists of a bubble of OIII emission surrounding a Wolf-Rayet Star. As seen from Earth, the OIII bubble is displaced off center from the WR driver. There is a dark void beyond the primary OIII bubble. Further beyond, diffuse OIII emission rises in intensity again. It is worth testing whether the outer diffuse OIII emission is the remnant of an even more ancient bubble. For this image, I used a focal reducer to decrease the focal length of the Televue NP 127is from f/5.2=660 mmm to f/4.2=520 mm. Although the outer emission could conceivably be a very faint secondary bubble, this wider field image is suggestive but far from conclusive. Is anyone interesting in doing a deep OIII run working at a shorter focal length or with a large format camera?
