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Help in identifying artefact


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Hi all,

Whilst capturing data over the last clear nights on my latest object, NGC 7822, I noticed a star with an odd profile in the OIII subs but put that down to cosmic rays or some other weird artefact. But after my usual preprocessing, I’m still seeing this strange star in all of the OIII subs and much clearer in the stacked image.

The OIII data is a stack of 54 300s subs making for a total of 4.5 hours and similar integration times for Ha and SII. What’s strange is that this only shows up in the OIII data and not the other channels. As can be seen, Ha and SII look completely normal.

At this point I’m trying to understand what might cause this behaviour only in one channel. I have thought about things such as collimation, tilt, debris on the corrector plate, tracking/guiding errors, dew, shooting through branches and filter reflections. However, none of these fit as the star in question is roughly in the center of the FOV and there are other stars around it similar in size and brightness (judging from the other two channels) which logically should show the effects as well.

These were captured using a Mesu 200, RASA 11”, Atik 414EX and Baader F2 High Speed filters. The subs were dithered and 2X Drizzle integration was used. I also rotate my exposures through the filters every imaging session. No other processing done apart from the usual preprocessing steps and an STF curve.

Having a look at the Simbad, 2MASS, Gaia DR2 catalogues reveals the star has designation [D75] 7p (Simbad) and is close in magnitude to the star at the bottom of the images.

http://simbad.u-strasbg.fr/simbad/sim-basic?Ident=[D75]+7p&submit=SIMBAD+search

Can anyone shed some light on what might be happening here? Is this an artefact due to some component in my imaging train or something that I’m missing entirely?

As this doesn’t show in any of my other stars in this image, or any other image I’ve taken with this setup before, I’m not concerned about this being an issue with my equipment at this point. I’d simply like to find out what might have caused this out of pure curiosity.

HT.PNG.4d390ee3e5b5ecf694bcf405e1de4c97.PNG

DCV_Sharp.PNG.27ee3b4fbfcd8069272f005fef9f061d.PNG

Sub.PNG.1fa3b0263132d2846b4fa19c9f9b5d92.PNG

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Could be the filter, it may be susceptible to halos at just that specific wavelength the star emits.

I got one that doubles in size with Sii filter, all other stars stay the same.

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12 hours ago, MarkAR said:

Could be the filter, it may be susceptible to halos at just that specific wavelength the star emits.

I got one that doubles in size with Sii filter, all other stars stay the same.

Thanks for the reply Mark

I must admit I hadn’t thought of that but now that you mention it, it makes sense. I know I read somewhere that NB filters nowadays use several coatings sometimes more than ten per filter and the idea that one or multiple of these coatings are reacting differently to light emitted from this particular star is indeed interesting.

Learn something new everyday...

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