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Re: Grainy image from horsehead nebula

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  • Don Goldman
    Hi, Guys, Interesting thread. Yes, our 5 nm H-a transmits 90% at the emission wavelength at 656 nm but only over that ~ 5nm window. The luminance filter
    Message 1 of 20 , Jan 24, 2013
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      Hi, Guys,

      Interesting thread.

      Yes, our 5 nm H-a transmits >90% at the emission wavelength at 656 nm but only over that ~ 5nm window. The luminance filter transmits >90% from 400 - 700 nm, or let's say over 300 nm. So, ignoring QE differences for the moment, the H-a filter would provide a signal 5/300 or about 1.6% as strong as a luminance, or maybe 5% of a Blue filter. Also, as pointed out, the fluorescence-shifted white LED flat panels are dimmer at 656 nm. That, combined with the <5% H-a relative signal means that a 4s exposure for 30,000ADU for a blue filter on an LED flat panel may take 2 min for an H-a filter on an LED flat panel. This is why 5 min NB exposures are generally way too short, and 15-45 min are typical. Unless you are imaging from a city light dome, you generally don't reach the sky noise limit in that time. IF you do, you could use a 3 nm filter that still has >90%T but will decrease the background by 40-50%. Lastly, I take automated sky flats with CCDAutoPilot. The Lum, and RGB are taken first when the sun is further down below the horizon, and the NB filters next as it gets brighter, when there is sufficient light to take the same short exposures to get all flats ~~30k ADU for my camera system.

      Don Goldman
      Astrodon

      --- In ccd-newastro@yahoogroups.com, "mark_manner_spot_obsrv" wrote:
      >
      > Mike the ratio of light through the Ha filter to the L and RGB filters won't be as significant with a real sky source as compared to the flat panel, but my point in response to yours is still valid.
      > Best,
      > Mark
      >
      >
      > --- In ccd-newastro@yahoogroups.com, Mike Dodd wrote:
      > >
      > > mark_manner_spot_obsrv wrote:
      > > > Mike, regarding you comment to Joe below if you are saying that the
      > > > same amount of light gets through a narrow band and broad band
      > > > filter, that isn't correct. Perhaps I am missing your point, so
      > > > pardon me if I am. Although the transmission percentage for narrow
      > > > and wide band filters is close to the same, you are only transmitting
      > > > that percentage of photons with the wavelength in the band of the
      > > > filter. So, a 90% Ha filter and a 90% L or R will be quite different
      > > > (assuming a source that emits light over the wavelengths in
      > > > question). As a quick test, to get the same number of counts in an
      > > > image taken with an STL11K just now pointing at an alnitak flat panel
      > > > (that puts out light in rgb(and some nir), I had to use 30 sec for
      > > > the Ha filter, 2.5 sec for the R, and 1 sec for the L. Mark
      > >
      > > Sure, because the EL flat panel probably puts out only 1/30 of its light
      > > in at 656nm.
      > >
      > > But we care about that only for flat frames. Imaging is different.
      > >
      > > When imaging in NB, I'm interested in the specific wavelength emitted by
      > > that target. As the Astrodon quotation shows, NB filters pass between
      > > 71% and 97% of the light at their peak wavelength. Yes, stars and
      > > objects emitting other wavelengths will be greatly attenuated. I would
      > > expect a reflection nebula to be dim through an H-a filter.
      > >
      > > I was trying to make the point that the light from a NB target is not
      > > attenuated very much through a NB filter, and so does not require longer
      > > exposures. in other words, if I were to image the Horsehead at the same
      > > exposure time unfiltered and through an Astrodon H-a filter, both images
      > > would have approximately the same ADU for the HH areas, ignoring other
      > > possible light pollution.
      > >
      > > As I wrote last night, longer exposures always improve S/N, so images
      > > benefit from using a NB filter that _allows_ very long exposures. It's
      > > great to be able to expose for 20 or 30 minutes without the image being
      > > ruined by sky glow.
      > >
      > > --
      > > Mike
      > >
      > > Mike Dodd
      > > http://astronomy.mdodd.com
      > > Louisa County, Virginia USA N37.58.23 W77.56.24
      > >
      >
    • Mark Striebeck
      I m hesitant to say thanks for all the replies - because that might this very educational thread. But seriously: thanks everybody for your replies. I just
      Message 2 of 20 , Jan 24, 2013
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        I'm hesitant to say thanks for all the replies - because that might this
        very educational thread. But seriously: thanks everybody for your replies.
        I just totally underestimated how big the difference between DSLR and CCD
        is. And because I went with a monochrome CCD, the additional complexity of
        using filters (I just wish that my filter wheel would come in soon :-(

        MarkS


        On Thu, Jan 24, 2013 at 11:50 AM, Don Goldman <donclearview@...>wrote:

        > **
        >
        >
        > Hi, Guys,
        >
        > Interesting thread.
        >
        > Yes, our 5 nm H-a transmits >90% at the emission wavelength at 656 nm but
        > only over that ~ 5nm window. The luminance filter transmits >90% from 400 -
        > 700 nm, or let's say over 300 nm. So, ignoring QE differences for the
        > moment, the H-a filter would provide a signal 5/300 or about 1.6% as strong
        > as a luminance, or maybe 5% of a Blue filter. Also, as pointed out, the
        > fluorescence-shifted white LED flat panels are dimmer at 656 nm. That,
        > combined with the <5% H-a relative signal means that a 4s exposure for
        > 30,000ADU for a blue filter on an LED flat panel may take 2 min for an H-a
        > filter on an LED flat panel. This is why 5 min NB exposures are generally
        > way too short, and 15-45 min are typical. Unless you are imaging from a
        > city light dome, you generally don't reach the sky noise limit in that
        > time. IF you do, you could use a 3 nm filter that still has >90%T but will
        > decrease the background by 40-50%. Lastly, I take automated sky flats with
        > CCDAutoPilot. The Lum, and RGB are taken first when the sun is further down
        > below the horizon, and the NB filters next as it gets brighter, when there
        > is sufficient light to take the same short exposures to get all flats ~~30k
        > ADU for my camera system.
        >
        > Don Goldman
        > Astrodon
        >
        > --- In ccd-newastro@yahoogroups.com, "mark_manner_spot_obsrv" wrote:
        > >
        > > Mike the ratio of light through the Ha filter to the L and RGB filters
        > won't be as significant with a real sky source as compared to the flat
        > panel, but my point in response to yours is still valid.
        > > Best,
        > > Mark
        > >
        > >
        > > --- In ccd-newastro@yahoogroups.com, Mike Dodd wrote:
        > > >
        > > > mark_manner_spot_obsrv wrote:
        > > > > Mike, regarding you comment to Joe below if you are saying that the
        > > > > same amount of light gets through a narrow band and broad band
        > > > > filter, that isn't correct. Perhaps I am missing your point, so
        > > > > pardon me if I am. Although the transmission percentage for narrow
        > > > > and wide band filters is close to the same, you are only transmitting
        > > > > that percentage of photons with the wavelength in the band of the
        > > > > filter. So, a 90% Ha filter and a 90% L or R will be quite different
        > > > > (assuming a source that emits light over the wavelengths in
        > > > > question). As a quick test, to get the same number of counts in an
        > > > > image taken with an STL11K just now pointing at an alnitak flat panel
        > > > > (that puts out light in rgb(and some nir), I had to use 30 sec for
        > > > > the Ha filter, 2.5 sec for the R, and 1 sec for the L. Mark
        > > >
        > > > Sure, because the EL flat panel probably puts out only 1/30 of its
        > light
        > > > in at 656nm.
        > > >
        > > > But we care about that only for flat frames. Imaging is different.
        > > >
        > > > When imaging in NB, I'm interested in the specific wavelength emitted
        > by
        > > > that target. As the Astrodon quotation shows, NB filters pass between
        > > > 71% and 97% of the light at their peak wavelength. Yes, stars and
        > > > objects emitting other wavelengths will be greatly attenuated. I would
        > > > expect a reflection nebula to be dim through an H-a filter.
        > > >
        > > > I was trying to make the point that the light from a NB target is not
        > > > attenuated very much through a NB filter, and so does not require
        > longer
        > > > exposures. in other words, if I were to image the Horsehead at the
        > same
        > > > exposure time unfiltered and through an Astrodon H-a filter, both
        > images
        > > > would have approximately the same ADU for the HH areas, ignoring other
        > > > possible light pollution.
        > > >
        > > > As I wrote last night, longer exposures always improve S/N, so images
        > > > benefit from using a NB filter that _allows_ very long exposures. It's
        > > > great to be able to expose for 20 or 30 minutes without the image
        > being
        > > > ruined by sky glow.
        > > >
        > > > --
        > > > Mike
        > > >
        > > > Mike Dodd
        > > > http://astronomy.mdodd.com
        > > > Louisa County, Virginia USA N37.58.23 W77.56.24
        > > >
        > >
        >
        >
        >


        [Non-text portions of this message have been removed]
      • Mike Dodd
        ... Yes, but I think you ll be extremely pleased with the results once you get a handle on NB imaging. My suggestion: Put the H-a filter on the camera and go
        Message 3 of 20 , Jan 24, 2013
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          Mark Striebeck wrote:
          > I'm hesitant to say thanks for all the replies - because that might
          > this very educational thread. But seriously: thanks everybody for
          > your replies. I just totally underestimated how big the difference
          > between DSLR and CCD is. And because I went with a monochrome CCD,
          > the additional complexity of using filters....

          Yes, but I think you'll be extremely pleased with the results once you
          get a handle on NB imaging.

          My suggestion: Put the H-a filter on the camera and go out and make a
          dozen 15-minute exposures, dark subtract them, and stack them to see
          what you get. I'll bet you'll really be excited.

          Enjoy!
          --
          Mike

          Mike Dodd
          http://astronomy.mdodd.com
          Louisa County, Virginia USA N37.58.23 W77.56.24
        • Mark Striebeck
          Thanks for the encouragement Mike! My challenge there is that so far I could only manage to guide successfully for 5 minutes. If I go longer I get too many
          Message 4 of 20 , Jan 24, 2013
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            Thanks for the encouragement Mike!

            My challenge there is that so far I could only manage to guide successfully
            for 5 minutes. If I go longer I get too many failures. One of the main
            issues (I suspect) is that I guide my SCT with a guidescope and not an OAG.
            As soon as the filter wheel comes in, I can start using the new OAG (it
            attaches into the filter wheel).

            MarkS


            On Thu, Jan 24, 2013 at 4:49 PM, Mike Dodd <mike@...> wrote:

            > **
            >
            >
            > Mark Striebeck wrote:
            > > I'm hesitant to say thanks for all the replies - because that might
            > > this very educational thread. But seriously: thanks everybody for
            > > your replies. I just totally underestimated how big the difference
            > > between DSLR and CCD is. And because I went with a monochrome CCD,
            > > the additional complexity of using filters....
            >
            > Yes, but I think you'll be extremely pleased with the results once you
            > get a handle on NB imaging.
            >
            > My suggestion: Put the H-a filter on the camera and go out and make a
            > dozen 15-minute exposures, dark subtract them, and stack them to see
            > what you get. I'll bet you'll really be excited.
            >
            > Enjoy!
            >
            > --
            > Mike
            >
            > Mike Dodd
            > http://astronomy.mdodd.com
            > Louisa County, Virginia USA N37.58.23 W77.56.24
            >
            >
            >


            [Non-text portions of this message have been removed]
          • Mike Dodd
            ... Yes, flexure from an external guide scope makes long exposures difficult. The OAG sounds like a solution. Let us know how it works. -- Mike Mike Dodd
            Message 5 of 20 , Jan 24, 2013
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              Mark Striebeck wrote:
              > Thanks for the encouragement Mike!
              >
              > My challenge there is that so far I could only manage to guide successfully
              > for 5 minutes. If I go longer I get too many failures. One of the main
              > issues (I suspect) is that I guide my SCT with a guidescope and not an OAG.
              > As soon as the filter wheel comes in, I can start using the new OAG (it
              > attaches into the filter wheel).

              Yes, flexure from an external guide scope makes long exposures
              difficult. The OAG sounds like a solution. Let us know how it works.
              --
              Mike

              Mike Dodd
              http://astronomy.mdodd.com
              Louisa County, Virginia USA N37.58.23 W77.56.24
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