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38576Re: M42 in Near IR, [SII] and Halpha

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  • rdcrisp
    Feb 1, 2005
      --- In ccd-newastro@yahoogroups.com, "Rick Wiggins"
      <rickwiggins@e...> wrote:
      >
      > Richard,
      > There are no wrongs here (other than I have not won the Lottery
      > yet!). I do think my proposed theory is correct. Idon't think you
      are
      > gaining much after 10 to 20 exposures to average down the noise. As
      > far as building up the signal, I don't think that happens. Signals
      > that are so weak as to vary sow up on one frame and not another are
      > probably interpreted as noise. Signals that do show up in all


      I think the signal does build up, slowly.

      i forgot the equation but I believe the signal adds in an rms
      fashion. and the noise adds too but there is a net win in SNR as you
      take more data from what i remember last time stan went through this

      i need to look at my references a bit and get back.

      no question that the 100 frame data is better than 20 of them. I
      already looked at that.



      frames
      > but vary, will be recognized as signal and will be improved. Try
      > using the technique that I suggested if you have time. That's the
      > same as some use for LRBG of M42. They shoot 10 secs, 5 mins,etc.
      > They use the sweet spot out of each set with layers and masks. This
      > should work for you as well. I just don't think that super high
      > numbers of frames buys you much in this situation. I hope that Stan
      > or one of the SNR Gurus chimes in on this one.
      > Tonight I will be continuing on my light box project.
      > Thanks, Rick
      >
      > --- In ccd-newastro@yahoogroups.com, "rdcrisp" <rdcrisp@s...> wrote:
      > >
      > >
      > > here is why i did what i did, right or wrong
      > >
      > > i wanted to image the m42 object without blooming the trapezium
      > stars
      > > and using my nir longpass filter.
      > >
      > > i could expose for about 15 seconds without blooming too badly.
      > that
      > > is it. so my maxiumum exposure time was limited to 15 seconds.
      > >
      > > with 15 seconds my ability to capture the faint nebulosity is
      > > challenged to say the least. so to get as much as I could i
      decided
      > > to take a lot of exposures in hopes that the faint signal would
      > > accumulate.
      > >
      > > this is a situation where low read noise is nice because in 15
      > > seconds there is a part of the nebula where the accumulated
      signal
      > is
      > > just above the noise floor set by the read noise. so the lower
      the
      > > read noise, the better I capture the faint nebulosity.
      > >
      > > So while the maximum exposure time was limited by avoiding
      > blooming,
      > > the faint nebulosity detection/capture was limited by the noise
      > floor
      > > of the camera and the read noise is the biggest part of that as I
      > > understand.
      > >
      > > That was the "why" that I did what I did, right or wrong.
      > >
      > > still the image is noiser than I'd like. I may fiddle with it
      again
      > > tonight.
      > >
      > > rdc
      > >
      > >
      > >
      > > --- In ccd-newastro@yahoogroups.com, "Rick Wiggins"
      > > <rickwiggins@e...> wrote:
      > > >
      > > > Richard,
      > > > The shot is very interesting and quite well done. I am curious
      > > about
      > > > the "100" frames. I have seen this done by several others as
      > well.
      > > I
      > > > must be missing something. ROn and Stan please chime in if you
      > > > agree/disagree/or can shed some light on my thoery here. The
      > basic
      > > > exposure of 15 secs captures all the signal for the bright
      > portions
      > > > of the frame (probably near 50% saturation levels in the
      > brightest
      > > > sections). The series of exposures does not increase the signal
      > of
      > > > the bright areas, but serves to randomize the noise allowing
      for
      > > > reduction of noise in an averaged (or similarly processed)
      > combined
      > > > image. The combination of the captured signal and
      > reduced/averaged
      > > > noise gives higher SNR and a higher quality image. So, after
      you
      > > have
      > > > captured enough images to drive down noise (probably 10-20),
      the
      > > > subsequent frames don't buy you much; they do reduce noise, but
      > > only
      > > > marginally so. Wouldn't it be better to take 10 more frames at
      > > longer
      > > > exposures; say 10 at 1 minute, and 10 at 10 minutes, and then
      > > > composite these combined frames to use each combination's best
      > SNR
      > > > areas of the frame? If the blooming would kill you, then I
      > > understand
      > > > why you wouldn't do this with this camera; but if that's the
      > case,
      > > > what about using a ABG camera to take the long exposures for
      the
      > > > dimmer sections and then combine. It just seems to me that
      after
      > 20
      > > > frames or so, there is marginal (and probably invisible to our
      > eye)
      > > > improvement. Is there something else going on when stacking
      > images,
      > > I
      > > > mean other then noise averaging?
      > > > Thanks for listening to my thoeries.
      > > > Rick
      > > > --- In ccd-newastro@yahoogroups.com, "Richard Crisp \(SBC\)"
      > > > <rdcrisp@s...> wrote:
      > > > > this is a project I've been meaning to start for some time.
      It
      > is
      > > a
      > > > widefield using Near IR longpass, [SII] and Halpha
      > > > >
      > > > > the bright stars in the Trapezium really severely limited my
      > > > exposure time in Near IR, otherwise the sensor would bloom
      > > terribly.
      > > > I ended up taking 15 second exposures. I had to take 100 of
      those
      > > > subexposures to get 25 minutes total exposure.
      > > > >
      > > > > the Halpha and [SII] data were 25 exposures of 60 seconds
      each.
      > > > >
      > > > > each exposure with this camera, IMG6303, is 12.3MB. So I had
      a
      > > lot
      > > > of raw data to crunch: 150 exposures of 12.3MB each.
      > > > >
      > > > > Even so, the data is a bit grainy so I may shoot more data
      > > another
      > > > night soon. Who'd have thought that 150 exposures would be too
      > few?
      > > > >
      > > > >
      http://www.narrowbandimaging.com/m42_ap180_6303_NirS2Ha_page.htm
      > > > >
      > > > > it takes about one minute to cycle through a 15 second
      exposure
      > > > roughly, just as a data point.
      > > > >
      > > > >
      > > > >
      > > > > [Non-text portions of this message have been removed]
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