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A Witch's Broom

Discussion in 'Exhibition Lounge' started by DaveS, Sep 7, 2017.

  1. DaveS

    DaveS Well-Known Member

    Formally NGC 6960, also known as the Western Veil, part of the Cygnus Loop, a huge old supernova remnant, currently 3 degrees across. This is only a small part of the whole. North is to the left, the bright star is 52 Cygni.

    Photographed with 2 hours each of nitrogen, hydrogen and oxygen narrow-band filters for a total of 6 hours over several nights at the back end of August.. A bit short time, but we've been clouded out lately, I doubt I'll get more data this season. The sky conditions were pretty dire too, with haze reducing the transparency and exacerbating the light pollution.

    Imaged with my 130mm f/7 apo triplet and 0.75x reducer and cooled CCD camera. The filters were 3nm bandwidth Astrodons

    6 hour DDP shrunk.jpg

    The exposures were broken down into 10 min subs and stacked in AstroArt 5, a dedicated astro imaging program. The processing was long and involved to bring out the faint tendrils without either burning out the highlights or bringing up the background noise. More data would have helped enormously.

    The nitrogen (Technically [NII]) was assigned to red, hydrogen (HII), to green, and oxygen ([OIII]) to blue, similar to the familiar Hubble Space Telescope palette.

    The original PNG is 27MB, a bit big to post!
  2. EightBitTony

    EightBitTony Well-Known Member

    I read it all, but you had me at Western Veil. Amazing shot.
    DaveS likes this.
  3. steveandthedogs

    steveandthedogs Well-Known Member

    I think I agree with all of Tony's post!

    DaveS likes this.
  4. PeteRob

    PeteRob Well-Known Member

    Gosh! Amazing.
    DaveS likes this.
  5. Fishboy

    Fishboy Well-Known Member


    Cheers, Jeff
    DaveS likes this.
  6. beatnik69

    beatnik69 Well-Known Member

    Is there a 'translate' function on the forum? :D Fascinating image.
    DaveS likes this.
  7. Geren

    Geren Well-Known Member

    Wow, that's amazing! Thank you for sharing it.
    DaveS likes this.
  8. DaveS

    DaveS Well-Known Member

    Thanks guys,

    I wasn't sure how much info I should put up, looks like too much! The "full fat" 27MB file I put on SGL for a competition (It won't win, the standard is sky-high) had far more nit-picking detail for the nit-picking astro-imagers.
    Geren likes this.
  9. Geren

    Geren Well-Known Member

    Well most of the information went right over my head - but I didn't even know such things were possible. It's fascinating!
  10. Fishboy

    Fishboy Well-Known Member

    I agree. Whilst I don't claim to understand all the detail it was still really interesting to hear how the image was created.

    There was obviously a lot of time, specialist equipment and dedicated software involved - but more importantly the skill and knowledge of how to put them all together and an 'eye' for how the final picture would appear.

    I imagine that if Dave ever finds himself wondering why he's standing in the freezing cold inky blackness waiting for one of his ten minute exposures to end it's results like this one that make it all worthwhile.

    Cheers, Jeff
    DaveS likes this.
  11. SXH

    SXH Well-Known Member

    Being a watcher of Stargazing Live and occasional episodes of The Sky At Night, and having read a bit about the subject, years ago*, I've got a rough idea what it all means.

    As a matter of interest, do the filters only pass elemental(?) light, or do they show ions too, -OH and so on?

    * so long ago that I knew about cooling film to make it more sensitive. Didn't know it worked for CCDs as well.
  12. steveandthedogs

    steveandthedogs Well-Known Member

    The more info the better. The fact that some of it goes over my head is my problem - but always ready to learn.

  13. El_Sid

    El_Sid Well-Known Member

    Patience pays off... big time...:cool:
  14. Craig20264

    Craig20264 Well-Known Member

    I think he's made it all up. He's doodled that in Microsoft Paint..........:cool:
    I've looked everywhere in the East Anglian sky and I can't see that thing.
    EightBitTony likes this.
  15. beatnik69

    beatnik69 Well-Known Member

    I don't think cooling the CCD makes it more sensitive. It just stops it from overheating and not working.
  16. SXH

    SXH Well-Known Member

    Fair enough! I did say it was a long time ago - they probably didn't have easily available CCDs in them days.
  17. DaveS

    DaveS Well-Known Member

    OK, fasten your seat belts, it's gonna be a bumpy ride :D.

    The telescope was mounted on a direct drive (No gears, belts or wheels!) German Equatorial mount made by ASA in Austria, equipped with shaft encoders giving 0.02 arc sec per "tick". The software makes a "model" of the sky, comparing where the telescope should be with where it actually is. It does this by taking a series of short images and comparing the positions of the stars in the image with their positions in a catalogue. In addition, when I start a run the mount will do a local model, making several exposures along the path the 'scope should be taking, so it can correct any residual errors.
    Because the object wouldn't fit into the field of view of the sensor (Only 10x12mm) I added an optical device to reduce the focal length by 0.75x, which also flattened the field and corrected any minor aberrations. The CCD camera (Made in the UK by Starlight Xpress) was cooled to -20 deg C to reduce thermal noise. Come winter I'll be able to get it colder, maybe -35 or -40.

    OK, on to the image itself.

    It was made through filters transmitting only the light from singly ionised hydrogen, singly ionised nitrogen, and doubly ionised oxygen. The latter two are "forbidden" (Hence the square brackets), and are only seen in very rarefied gasses, eg in the aurorae and nebulae. In order to separate the lines of hydrogen and nitrogen I had to use very narrow bandwidth filters, only 3nm wide.
    I collected the data in 12 x 10 min exposures (Subs) to make a total of 2 hours each.
    Each set of 12 subs was added up (Stacked) using software (AstroArt 5) that can reject artefacts, eg aircraft and satellite trails, while aligning the wanted image. I also calibrated each frame using "Bias" exposures, taken with as short an exposure as possibly to eliminate errors in the camera.
    The resultant stacks each had an adaptive gradient removal to get rid of vignetting and any light-pollution gradients.
    The three stacks were combined with nitrogen in the red channel, hydrogen in the green and oxygen in the blue, following the example of the "Hubble Palette", then colour balanced. There were borders that had to be cropped out due to minor alignment errors, then the resultant colour image was given another (Slightly different) gradient removal before the heavy processing.
    Most of the heavy lifting was done by a DDP (Dynamic Digital Process) algorithm that mimics the non-linearity of film. This was done slowly, checking the screen brightness range, in several iterations, in order to preserve the delicate tendrils on hydrogen emission (Green) while avoiding burning out the highlights (One or two may be close to clipping) or bringing up the noise level too much.
    After I'd finished there was still a little colour noise, so I used a Butterworth FFT noise reduction routine to smooth it out a little while preserving detail in the image, before giving it a touch of curves to push the noise into the black after, another gradient removal "just in case".

    The image posted is a scaled JPEG, as the original PNG is too big in both dimension and size to post here.

    Phew! If you've made it this far, well done, you probably deserve a medal! :).
    steveandthedogs and Fishboy like this.
  18. Craig20264

    Craig20264 Well-Known Member

    Hmmmm, And I thought I fussed over what aperture to use when taking pictures of the cat!!

    (Good work again Dave ;))
    I take it you'll be up against Sara in the SGL comp!!
    EightBitTony and DaveS like this.
  19. DaveS

    DaveS Well-Known Member

    Oh, gosh I hope not :eek:. Have you seen her new dual imaging rig? Scary!
    Sara or Olly? Hope they have other fish to fry!
    EightBitTony and Craig20264 like this.
  20. EightBitTony

    EightBitTony Well-Known Member

    No! Please don't misunderstand, I just meant the image is amazing and speaks for itself, even without the explanation. But I find that fascinating and interesting as well.
    DaveS likes this.

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