I wouldn't mind some thoughts on where I am going with this one. I am collecting data on the Abell 35 PN which is very faint at the best of times. Some information even suggests that it is not a PN but that doesn't change the fact that it is there and I am imaging it. Depending on the source of information it is somewhere between mag 12 and 13 with a diameter of about 10'.
I am well on the way to collecting the Ha and OIII subs that I need. My target for each channel is to collect 30 x 30 minute sub each. I am going to need the extra data to try and pull out the information from the background and I will probably also collect some RGB data as well for the star field.
The thing that I am pondering is whether to bother with collecting photons for SII. I have attached a screen shot of the Pixinsight desktop with one of each of the Ha, OIII and SII subs open so that you can see how relatively faint the SII data is. Using the readout tool in PI for the SII sub, it is showing background ADUs in the order of 350 with the strongest signal in the actual object itself being around 430 ADU and much of it barely above the background noise.
So, my question to my learned collegues on this forum is - would you bother with the SII data collection or is it likely to be swamped by the the other signal anyway?. I would be interested to know what others think on this one.
An adventurous undertaking. The H-alpha is looking promising.
A bit like RCW 104 in Norma, the problem is not so much pulling out the OIII and SII from the noise, which you are already doing; it is seeing it against the stars, which become brighter and more intrusive with further exposure.
When doing RCW 104, we could see something in the SII image, but it looked to be exactly and precisely the same as the H-alpha image, making us wonder whether what we were seeing was just leakage of H-alpha through the SII filter. We gave up on the SII, not convinced that we were really photographing SII. All we had really shown was that if there was any, it was a ridiculously faint ghost.
Perhaps in view of the difficulty of pulling OIII out from the starry background, you could do what Rick suggested for us, and present the H-alpha and OIII as two quite separate images to be compared side by side rather than trying to combine into a colour image. I would just skip the SII.
It is very good to see difficult targets being investigated.
It seems typical for both PN's and some WR's for the OIII to be less extensive than the H-alpha, because the sufficiently hard UV from the central star has been used up before the softer UV that stimulates H-alpha emission has been used up. Similarly the OIII emission seems to be often more featureless, perhaps because the gas that is emitting H-alpha was emitted earlier, has gone out further, and is colliding with pre-existing material, causing visual detail due to shock fronts. You are seeing both those things happening here, as we did with RCW 104.
Thanks very much for that Mike. I did notice the similarity between the faint SII data and the Ha which made me more inclined to bypass the SII. The OIII certainly looks interesting as it has captured what looks to be a shock wave around the core star. In terms of managing star brightness, this should be ok to manage with some of the masking techniques in Pixinsight. Thanks again for your comments Mike.
I'd also agree with skipping the Sii if it merely mirrors the Ha.
I find the technique of removing the stars before doing colour processing (shamelessly stolen from JPM's tone mapping) really helps with dim, noisy NB data.
I'd also agree with skipping the Sii if it merely mirrors the Ha.
I find the technique of removing the stars before doing colour processing (shamelessly stolen from JPM's tone mapping) really helps with dim, noisy NB data.
Cheers,
Rick.
Thanks Rick, I have been aware of that technique for a while but have not used it in anger as yet so worth a shot. At least this field is not too heavily populated by stars so it should be easier to manage from that aspect.
This object is proving an interesting challenge and is becoming a bit of an obsession. Strange seeing that it won't be a particularly stunning visual display but an enigma all the same.
I have completed enough data capture for now to kick off the processing activity in earnest. The data set includes: 42 x OIII @ 1800s; 41 x Ha @ 1800s; 24 Red @ 300s; 24 Gre @ 300s; 24 Blu @ 300s.
The attached screen shots show the Ha and OIII Masters as output from the Pixinsight calibration, cosmetic correction and registration process. Following this I cropped the registered images and then ran them through the PI Image Integration tool. I did it this way as the OIII master showed some gradients around the edge of the field so I felt it better to crop the master first before doing image integration.
I was puzzled by the apparent extended nebulosity at the 9 o'clock position in the Ha master as this is not apparent in other images of this object that I had found previously. A search led me to the Sharpless Catalogue site which has some great information on SH2-313 (Abell 35) and many other Sharpless objects. In this case the information includes an inverted image of the Ha data taken with a TMB 152mm refractor.
I did a quick stretch of my Ha data to compare it to the inverted image found on the Sharpless site and the result is the second screen shot that I have attached. The data that I have captured is present in the image from the Sharpless site which is good. We shall now press on with processing the data.
I take it that the most popular line of thought is that it is a planetary nebula?
Thanks MnT, in responding I must first acknowledge the excellent rendition of this object and discussion started by Shiraz(Ray) in this IIS thread.
The above thread references the discussion by Don Goldman, Astrodon Imaging relating to Abell 35. The Astrodon site references two papers which cast doubt on the classification of this object as a PN:Frew and Parker (PASA, 27, 129, 2010) and Ziegler et al. (A&A, 548, A109 , 2012). Much seems to be made of the observation that there is a binary double at the core of the nebula. Frew and Parker conclude that the object is in fact a "Stromgren sphere". I didn't know what that was either so I had to look it up on Wikipedia.
Anyway, given that the above papers are relatively recent, 2010 and 2012, one would have to accept them as current and unchallenged (to my knowledge) and accept the conclusion that it is not a PN. That doesn't seemed to have budged the object away from the PN catalogues though.
