Messier 57 - stars inside

[Uwe Glahn]

Last month I had the opportunity to use the powerful 2400 square inches of Jimi's Barbella, thanks Jimi for this "cosmic adventure" :)

One of the target was the famous Ringnebula Messier 57 but not with a "normal" filter to enhance the emissions of the nebula itself. We use a "central star filter" which works opposing all these famous filters. Reiner Vogel measured some spectra of this filter (aborptions (http://www.reinervogel.net/Filter/spectra/Astroelectronic_ZS_a_UG.jpg) and transmission (http://www.reinervogel.net/Filter/spectra/Astroelectronic_ZS_t_UG.jpg)) The goal is to disable the nebula which outshines faint stars. Famous target is for example the PN NGC 6210.

Without the filter both central stars were visible easily and with direct vision. Magnification was around 814x (Jimi correct me when my memory is wrong), Seeing was average. When we put the filter in, the filter effect on the two central stars was not prominent. But at the outer ring areas the filter showed a very good response. The nebula was visible just very faint and some stars glimpsed out very well. I tried to sketch the roughly positions of the stars (see sketch below) at the few minutes on the ladder and detected 7 stars. All stars but the star at 11.30 (CS middle, north/up 12.00) were easily visible. I don't noticed which was direct/averted vision. Some more stars should be visible when the seeing is better and you more time on the ladder.

My reworking and investigations were much harder than I expected. Perhaps we were the first observers how saw this stars visually. To bring out the exact position was also very hard, even on the HST images some stars were totally out-shined. I tried to compile some very good images. But it is nearly impossible to get the visual positions and relative magnitudes because of the different spectra sensitivities. I think an photographer or a Mallincam with a CS filter has to imagine the PN to get a good card of these stars.

It would be a very exciting project to get these stars, not only within M57 and not only with 2400 square inches


sketch: 48", 814x, CS filter, seeing III, NELM 7m+
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HST color image
- stars seen are labeled with solid line
- stars which could be in reach with broken line
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HST WFPC2 with 469N (He II) filter (2MASS on)
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HST WFPC2 with 547M (strömgen y) filter (2MASS on)
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amateur image - Spiegelteam - Capella Observatory/Crete
24"; LRGB (Ha/[OIII]/[OIII]), Wendel, Binnewies, Pöpsel
RGB imgage (http://www.spiegelteam.de/m57.htm)

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UCLA Infrared Laboratory
infrared image (http://irlab.astro.ucla.edu/images/gallery/flitecam/RingNebula_M57_J.jpg)

[Marko]

Interesting project. I would be concerned that the filter takes out quite a bit more than required in an area close to max human perception just above the O3 lines. I also notice as a smaller point that no Ha is removed and even though the human eye is very poor at Ha/NII area there is some eye response. I bet like many PNs it is mostly OIII but could have Ha and/or NII. I attach a chart found in link stated at the top and I don't know if 0.1 scale to the left is significant for detection. This is simply mentioned here as our perception is what this thread is all about in the limiting cases such as this project.

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At one point I was excited about just managing the central star (no filter) in an 18" after many attempts on assorted nights till a great night of seeing way up on the meridian. Then I seem to recall somebody told me the REAL central star is not the more obvious one highlighted in your color pic with a central star shown. Can somebody comment on if the 'real' central star is indeed the brightest star visible dead center in the ring? Thanks.

[Howard B]

The best I've done with my 28 is to see the two brighter stars inside M57 during excellent seeing, and the second star was still pretty tough to see well. Through the Bok 90 inch on Kitt Peak I've seen these two stars, the one that's seen through/superimposed on the end of the ring and three fainter one's just outside the ring on the same end. The outer halo was seen quite well too, but my biggest surprise was that there weren't more stars visible inside the Ring through such a large scope. The ring itself was extremely bright, had an exceedingly smooth, milky texture and didn't show any hints of finer detail, which was also somewhat of a surprise. But then this was at the very beginning of morning twilight so a darker sky may have revealed more. Still the brightness of the Ring probably hid the fainter stars Uwe shows in his sketch.

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Are central star filters available for purchase somewhere or is this a one-off item?

[Uwe Glahn]

Hi Marko and Howard,

@Marko

I would be concerned that the filter takes out quite a bit more than required in an area close to max human perception just above the O3 lines.
Exactly. The filter only helps to black bright emissions out - like here the brighter parts of the ring for the stars within the ring. For the stars within the middle of the (both "central stars") it brings no advantage. The emission has to be a minimum brightness for a positive effect of the filter.


Can somebody comment on if the 'real' central star is indeed the brightest star visible dead center in the ring?Yes it is. Several, also modern paper refer the central star as the star in the middle. In my understanding the central star has always to be in the middle, when the structure around the star is regular or more precisely symmetrical.

@Howard
Thanks for posting your sketch. My experience with seeing the central star is pretty the same. An interesting fact is, that the third star is also visible with 16". When a sketched the Nebula years ago from one of the best seeing places in Europe - the "Edelweißspitze" I noticed after 2 hours of observing a stellar knot at the SW end of the ring. Of course it was not a knot but the third star, which position I doesn't know during the observation.

16", 720x, Seeing I-II, NELM 7m+
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Are central star filters available for purchase somewhere or is this a one-off item?
Michael Koch, a german retailer produced a small number of filters in the past. The filters are now outsold but with enough inquiries he make a new charge of filters.

[Marko]

Thanks for info on central star. What I was wondering is if the 'real' central star was much lower magnitude at this point in time and if the one we see was just in the right place to make us suspect it was the PN central star or it hides the lower mag real star. Not likely so I guess that was misinformation I had heard after all. Thanks.

[Howard B]

Thanks for info Uwe, next time I have steady seeing I'll have to try for the third star in my scope, and if the central star becomes available again I'd be interested in buying one.

[hajuem]

Hallo Uwe

Fine Sketches and observations with 48"!! A dream has come true!!
It`s a small cluster in M 57;)

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I hope , i see you soon ...

Lg Hajü