NGC6543 Cat's Eye Nebula in OIII - shells or artifact?

[Tommohawk]

Hi all.

Currently having a bash at the Cat's Eye PN - it's very nicely placed in my limited sky window! It's obviously going to be a nightmare to process, but I've got some ideas on how to do it by combining RGB with OIII.

One odd thing though - I did the OIII with 6 minute subs using my ASI1600MM at high gain (300) and have some interesting "shells" around the nebula, diameter I guess about 30'. Now this could be gas shells, but this seems a bit far fetched with my humble gear, and TBH is more likely to be an artifact. BUT it isn't to do with calibration weirdness - which I've definitely had before - because it persists even without darks/flats. I haven't had this with previous OIII shots though previous stuff was done with my 300mm camera lens, not the SW200 which I used this time. 

The Hubble shot shows numerous inner shells, but I've seen reference to further outer shells - as in the text accompanying this.

I'll do a proper job with all the data at some point, but for now just want to know whats going on with the OIII.

So what's the vote please... ground-breaking stuff... or just a few reflections??

[Ibbo!]

I am on the same target and have not got anything like that

They have to be calibrtaed as yet

O3

 

doing short subs at the moment to try get the inner shell.

[Davey-T]

Hi Tom, looks like reflections of some sort, what O111 filter are you using / some of them are prone to reflections / haloes on bright things.

Dave

Something similar round the bright star below it in the full size version.

[Ibbo!]

maybe a bit of dew ??

[Tommohawk]

9 hours ago, Ibbo! said:

I am on the same target and have not got anything like that

They have to be calibrtaed as yet

O3

 

doing short subs at the moment to try get the inner shell.

Hi and thanks for that. I've certainly not seen anything like my "shells" on any other image, though 6 mins with high gain is pretty deep. If I was a gambling man I'd go for reflections though! But not seen before - small halos round stars yes, but not big halos like these. 

9 hours ago, Davey-T said:

Hi Tom, looks like reflections of some sort, what O111 filter are you using / some of them are prone to reflections / haloes on bright things.

Dave

Something similar round the bright star below it in the full size version.

Hi Dave - yes agreed the star lower down does show a halo and I've certainly has this before - but much smaller.

9 hours ago, Ibbo! said:

maybe a bit of dew ??

Pretty sure its not dew - I had the secondary heater running heater on higher setting than normal (new power set-up) and it never allowed dew before. And it wasnt a v dewey night.

Would be good to do some test OIII shots with an other bright star, but if its an artifact it my be peculiar to the main "halo" so may not show on a star. 

It's a ZWO OIII filter BTW.

[Demonperformer]

The cat's eye does have an outer shell and it looks like you caught it.

[Dragon_Astro]

I'd say reflections of some sort, the rings in it all look very symetrical.

[iansmith]

I've never seen any reference to further shells outside the irregular shaped one you've captured. They look very symmetrical and circular and you can see other halos round the brighter stars. I suspect it's an optical effect, but you never know.

You could try this: measure the size of the halo in pixels and then look for the CCD filter reflections equation on this website: http://www.wilmslowastro.com/software/formulae.htm

See if you have anything in the optical train at that distance.

Cheers, Ian

[Tommohawk]

9 hours ago, Dragon_Astro said:

I'd say reflections of some sort, the rings in it all look very symetrical.

True - but then so do the pressure rings.

 

10 hours ago, Demonperformer said:

The cat's eye does have an outer shell and it looks like you caught it.

Thanks - but I think the outer shell mentioned in that link is at 6 minutes diameter ie is the more obvious one beyond the very bright centre core. the series of faint outer shells are the ones I'm puzzling over. The link I put in the first post seems to suggest there is a shell beyond this one, though only one. Another link here suggests similar, but again only a single additional shell is mentioned.

 

8 hours ago, iansmith said:

I've never seen any reference to further shells outside the irregular shaped one you've captured. They look very symmetrical and circular and you can see other halos round the brighter stars. I suspect it's an optical effect, but you never know.

You could try this: measure the size of the halo in pixels and then look for the CCD filter reflections equation on this website: http://www.wilmslowastro.com/software/formulae.htm

See if you have anything in the optical train at that distance.

Cheers, Ian

Thanks Ian. Well I'd seen that resource before but not that section.... interesting!!

Trouble is, there are so many concentric rings its difficult to know which to use. The outermost one corresponds to ~1600 pixels, or 30.4mm to the source of the reflections from the sensor. The innermost one suggests a distance of 11.4mm. The latter is certainly close to the separation of the filter from the sensor.

I'm pretty sure that if these are artifacts, the light source causing them must be the central bright star, not the main halo because whilst this looks pretty bright its been stretched to blazes. That being so, if I have a boring night with clear sky and nothing to do (is this likely?) I could find a similar star and do identical exposures and see what I get.

BTW I cant see the faint concentric rings on the individual subs, probably because its too faint. Although this could just mean its a stacking specific artifact. I'll probably end up processing these outer rings out I suppose though its tempting to try and feature them!

Any further thoughts gratefully received!

[Demonperformer]

3 minutes ago, Tommohawk said:

the series of faint outer shells are the ones I'm puzzling over.

Sorry ... really must stop looking at posts while on my phone - they did not show up at all on that.

Now I see what you mean, and I agree that they are not part of the PN.

[Freddie]

The first set of rings being referred to in your links are the ones hidden in your over exposed core. The second set referred to in the links are the irregular ones you have captured. So definitely reflections, not ground breaking, sorry.

[iansmith]

I have one other suggestion for you. A few years ago I  got a large circular halo round NGC7662 in my O3 subs. I was using a (relatively) cheap Baader O3 filter at the time and the halo turned out to be a reflection of near IR light leaking through the filter. I subsequently read that the cheaper filters can leak light that's far away from the original bandpass. I solved it by putting an IR/UV blocking filter just before the camera. If you have one it might be worth trying.

A simple test for a cloudy night would be to shine a IR remote control into the scope with the O3 filter in place and see if the camera sees it.

Cheers, Ian

[Tommohawk]

22 hours ago, Demonperformer said:

Sorry ... really must stop looking at posts while on my phone - they did not show up at all on that.

Now I see what you mean, and I agree that they are not part of the PN.

 

21 hours ago, Freddie said:

The first set of rings being referred to in your links are the ones hidden in your over exposed core. The second set referred to in the links are the irregular ones you have captured. So definitely reflections, not ground breaking, sorry.

OK guys thanks....  for spoiling my day!

12 hours ago, iansmith said:

I have one other suggestion for you. A few years ago I  got a large circular halo round NGC7662 in my O3 subs. I was using a (relatively) cheap Baader O3 filter at the time and the halo turned out to be a reflection of near IR light leaking through the filter. I subsequently read that the cheaper filters can leak light that's far away from the original bandpass. I solved it by putting an IR/UV blocking filter just before the camera. If you have one it might be worth trying.

A simple test for a cloudy night would be to shine a IR remote control into the scope with the O3 filter in place and see if the camera sees it.

Cheers, Ian

That's interesting. I wouldn't call the ZWO filter cheap - but all relative as you say -  and I do wonder if they made different versions which improved over time. I'll try the trick with the IR source, though I'm not sure how I'd get an IR filter in the train.

I'll get on with trying to process this all best possible and lose the artifacts.

Thanks all for the input.