IC443 SW filament

[reiner]

This is an observation from a year ago, together with Matthias Kronberger from the Black Forest in Germany at 1200 m. Conditions were excellent with the Gegenschein being visible. As it is now again the right time for IC 443, I thought I post it here.

Matthias came over from his 15" Dob, suggesting to try the southwestern filaments of IC443. I had tried this one already earlier with my 22" and OIII filter, but without success. Matthias suggested to try the UHC filter, based on similar intensities of this filament in OIII and H beta. And indeed, with UHC at 5.5 mm exit pupil and the conditions given, the filament was not overly difficult. We also observed it in Matthias 15" Dob later on. With OIII filter, the filament vanished.

Here is a DSS image of the filament with four prominent stars framing it

[Marko]

Well it took a year but I finally got to this project! I very much like this object in photos and your post 'tossed in the gauntlet' so to speak but I missed it last year.

I spent some time on this Sat Feb 22 in mag 21.5 sort of skies. This object was observed in my StarMaster 18" f/3.7 dob with Lockwood optics and my trusty and H-beta friendly NPB filter (LOVE that filter). Mostly used 13mm Ethos through ParaCorr for 150x but tried 8mm Delos some of the time. I don't really think OIII is in this object much at all but if present it would have passed through the NPB as well. The Astronomic and Baader UHC are the closest spectrum to the NPB as both let in Ha. Lumicon UHC shows no Ha. There are 'flavors' that vary most on the Ha line but that is another thread. The NPB as you know lets in OIII, Hb and Ha although we have rather poor ability in Ha in general the Hb is basically 1/4 the intensity so I bet that is what we mostly see.

I started out with the main bright ridge to the NE and could see texture as it trailed off to the non-distinct SW very long edge. The sharp cutoff of the main lobe or top of the 'jellyfish' was quite apparent of course. I did not jot down or even try to explain all the filiments and texture on that less distinct edge but it was all over the place trailing off in the SW direction.

As for the tentacles Here is what I found (I like to think of this object as 'Jellyfish Nebula' as the common name).

The most apparent one was the farthest south one which in your picture is mid-bottom. While looking at this I used the very thin keystone of 4 stars who's skinny end points NW and is seen 1/3 of the way from the left edge in your picture where the bright star there is on the longer edge of the keystone. What I found as kind of interesting is this keystone points off to the SE two apparent doubles of fairly close spacing that are co linear on a line pointing to nu Gem. These doubles are about 4/10 of the way from bottom in your picture and dead center. So if those two 'doubles' are at two corners to form the base an isosceles triangle of 2x height then the vertex or 'high point' of the triangle lies about at the center of a line of glow heading SSW from the center of the base. The glow had enough strength to form a fair sized line heading SW to end at the brightest knot in this tentacle.

The 'middle' tentacle or the one you have highlighted I felt was more difficult but was observed only in moments we had better transparency as it could just disappear for minutes at a time being of marginal intensity. I felt that this was at my limits but a brighter short segment of this same tentacle was easier and it lies above the upper left star in your 'square' about 1/2 of a side of your square in distance from that corner square. I felt good that I had seen the segment you highlight but it was very difficult and dim then later asked over another observer but at that time he and I were unable to convince ourselves of it's presence. I think transparency had killed the later views.

As for the area near Nu I had to say I was unable to see glow due to the bright Nu so nearby.

My Astrophoto of this object along with M35 and so on is here (if curious) http://astrospotter.zenfolio.com/p31...4a157#h2f4a157

[reiner]

Hi Marko,

great that you picked this up! Your conditions must have been quite good as you could see the filamentary structure extending from the bright NW rim (I see that clearly only during good nights).

The most apparent one was the farthest north one which in your picture is mid-bottom.

That's what I did not understand. Did you intend to write "the farthest south"? Maybe you can mark it in the DSS print! If it is the southern one: I did not look closely at that one, maybe Matthias did.

I also followed the filament that I described above farther NW and I think we saw as well the part extending beyond the four-star asterism. I haven't written up that observation, though.

[Marko]

Hi Reiner,
I have corrected that post as it was the farthest south one and not north. I am apparently easily confused by directions ;-)
I do think that the curious dual double with all 4 stars collinear is an interesting 'base' to identify the field as if those form the base of a 2x to 3x tall triangle with the glow through the center of this triangle and heading SSW from the center of this base.