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Hunting for the Soap Bubble (getting started with narrow band filters)

Still waiting for the coma corretor, so using period of white nights, I decided you check about capabilities of narrow band filtering.
Narrowband filtering seems to be the ultimate solution to fight against street light (except shuting down..) if you like to image nebulae like H-II regions as well planetary nebulae. So I decided the try to hunt for a more difficult nebula, the Cygnus Soap Bubble which was relative lately detected in 2007 (!). When you look about the images available in the web you getting pretty fast an impression why it was so lately discovered. A realy faint shell in the middle of a star crowded field.

My first attempt was using the existing 35nm H-Alpha filter.

Cygnus Bubble ?, June 06th, 2016
STL11000M@ 510/2150mm Newton (currently no corrector)
300s, H-Alpha 35nm, bin=2
Orientation: North->up, West->right.
(klick on image for larger version - shown in a separate tab)

With support from Michael Adrean Observatory / Astronomy Stiftung Trebur I got the opportunity to use a small band filterset O-III, S-II and H-Alpha from ASTRONOMIK with a band with of 6nm and 1.25 Inch diameter. This filters have been purchased for a ST10 where 1.25 Inch diameter is sufficient but I was aware that this would create in my setup a significant vignetting. However it's a good opportunitiy to get an impression what can be achieved.
In the last days certain fair weather conditions allowed first tests. So I tried Cyngnus Bubble again. First in O-III and in a second night in H-Alpha (now having a smaller bandwith).

Cygnus Bubble !, July 07 & 09th, 2016
STL11000M@ 510/2150mm Newton (currently no corrector)
Left: 5*600s, O-III ASTRONOMIK 6nm, bin=4, right: 5*600s H-Alpha ASTRONOMIK 6nm, bin=2
Orientation: North->up, West->right.
(klick on image for larger version - shown in a separate tab)

The H-Alpha image looks VERY faint. There are some reasons for this:

  1. bin=2, OIII is made @ bin=4, so essentially we would need ~ 1200s exporetime to achieve a similar S/N (it's not 2400s because of the special characteristic of the CCD-chip used in the STL-11000).
  2. on different pictures the bubble looks more blue like. From this, we would expected anyhow to get better signal @ O-III wavelengths (500.7nm).
  3. the chip sensitivity as function of wavelength. QE@500nm ~ 0.5, QE@650nm ~ 0.3
  4. looking on the background: it looks like there is also a H-II region. This emits mainly @ H-Alpha. So the constrast of the bubble against the H-Alpha emitting H-II region is low. At O-III wavelengths situation is different: keeping in mind the origin of the radiation of the bubble - pumping by collision - due to shock wave, makes clear why planetary nebula emitts radition @ 500.7 nm while a typical H-II region wan't do this. So contrast versus background radation from a H-II region should be a little better. This is confirmed by the fact, that on the left image (O-III) you see very much less structure from the H-II region in the neighborhood.