Loading ...
Sorry, an error occurred while loading the content.

RE: [MetRec] Failing to see stars - Lux sensitivity?

Expand Messages
  • sebtor@astronomo.org
    images are shown as out of focus . + But AllSky cams in polluted sky , maybe not a good idea , ... the backlights ( and surface objects ) grow to about the
    Message 1 of 3 , Oct 24, 2012
    • 0 Attachment
      images are shown as out of focus.

      +

      But "AllSky cams" in polluted sky, maybe not a good idea, ... the backlights (and surface objects) grow to about the inverse square of the focal length, but the stars (and specific objects) in turn maintain the brightness per pixel, ... only depends on the diameter of the actual size of the lens

      In places with light pollution, you have to choose the lens focal more mm. Focal if he wants increased detection stars, although lower impact on field of view.


      -------- Original Message --------
      Subject: [MetRec] Failing to see stars - Lux sensitivity?
      From: "black_bart_data" <mccartney@...>
      Date: Sat, October 20, 2012 2:03 pm
      To: metrec@yahoogroups.com

       
      I built my camera and system 2 years ago, but got frustrated then distracted.

      Now I've fixed my focus problem, but am not seeing stars in Metrec, but clearly I am getting images to the system. You can see in this blog post..

      http://texasmeteoritelab.com/building-a-skycam-for-central-texas

      My skies are 3.5 mag skies here in Austin, TX. So I can't really see many stars normally, but my camera should be able to see at least sirus.

      How sensitive are other peoples cameras? Mine is supposedly 0.001 lux.

      I have many bad pixels on this camera so I get white dots that faked me out originally. So look at two photos before you say you can see stars in these shots.
    • sirko@molau.de
      Hmm, I m not fully convinced on the second statement. Typically non-intensified cameras have limiting magnitudes of only three to four mag. So also non-perfect
      Message 2 of 3 , Oct 24, 2012
      • 0 Attachment
        Hmm, I'm not fully convinced on the second statement. Typically
        non-intensified cameras have limiting magnitudes of only three to four
        mag. So also non-perfect four to five mag skies should have no impact,
        because they still are beyond the sensitivity of the camera.

        I'm always amazed when I stop visual observations because of strong
        twilight and then look at the screen of my cameras only to see, that
        they have just started to notice that the sky is getting brighter. But a
        little later, the limiting magnitude of the camera totally breaks down
        in only five minutes or so.

        Best,
        Sirko

        --
        ************************************************************
        * Sirko Molau * *
        * Abenstalstr. 13b * __ *
        * D-84072 Seysdorf * " 2B v 2B " *
        * Germany * *
        * phone: +49-8752-869437 * Shakespeare *
        * email: sirko@... * *
        * www : www.molau.de * *
        ************************************************************

        Am 24.10.2012 16:40, schrieb sebtor@...:
        >
        >
        > images are shown as out of focus.
        >
        > +
        >
        > But "AllSky cams" in polluted sky, maybe not a good idea, ... the
        > backlights (and surface objects) grow to about the inverse square of the
        > focal length, but the stars (and specific objects) in turn maintain the
        > brightness per pixel, ...only depends on the diameter of the actual size
        > of the lens
        >
        > In places with light pollution, you have to choose the lens focal more
        > mm. Focal if he wants increased detection stars, although lower impact
        > on field of view.
        >
        > -------- Original Message --------
        > Subject: [MetRec] Failing to see stars - Lux sensitivity?
        > From: "black_bart_data" <mccartney@...
        > <mailto:mccartney@...>>
        > Date: Sat, October 20, 2012 2:03 pm
        > To: metrec@yahoogroups.com <mailto:metrec@yahoogroups.com>
        >
        > I built my camera and system 2 years ago, but got frustrated then
        > distracted.
        >
        > Now I've fixed my focus problem, but am not seeing stars in Metrec,
        > but clearly I am getting images to the system. You can see in this
        > blog post..
        >
        > http://texasmeteoritelab.com/building-a-skycam-for-central-texas
        >
        > My skies are 3.5 mag skies here in Austin, TX. So I can't really see
        > many stars normally, but my camera should be able to see at least sirus.
        >
        > How sensitive are other peoples cameras? Mine is supposedly 0.001 lux.
        >
        > I have many bad pixels on this camera so I get white dots that faked
        > me out originally. So look at two photos before you say you can see
        > stars in these shots.
        >
        >
        >
        >
      • Oscar van der Velde
        In my experience visibility of stars is better if there is *some* background sky luminosity (not too much of course). I use a fixed manual gain on the Watec
        Message 3 of 3 , Oct 24, 2012
        • 0 Attachment
          In my experience visibility of stars is better if there is *some* background sky luminosity (not too much of course).
          I use a fixed manual gain on the Watec 902H Ultimate and always widest aperture and fixed exposure. I have observed stars easily under polluted night skies like that of Toulouse. When I moved this camera out to a remote area, with glorious starry sky, I literally could only see a black image with only the brightest stars. I had to switch to automatic gain to see something. Similarly, stars are nicely visible with some moonlight around, instead of drowned between the noise. The detection of a weak optical phenomenon like elves (donut-like emissions at 90 km altitude above far away lightning strikes) is similarly enhanced under moonlight. 

          Why? Because adding a bit of background light just adds photons to the photons of the event and the combined signal becomes better visible above the image noise, and improve S/N ratio. The same could likely be gotten with slightly longer exposure times. For detection purposes, there is of course a limit to the amount of background signal: the difference between event signal to accumulated background sky signal (i.e. contrast) must still be large enough to separate the two, and this difference should remain larger than the variation caused by image noise. In a hypothetical case, if image noise would be zero, you could still detect events if they are 1 RGB value brighter than the background, which you wouldn't be able to detect by eye. For fast-moving very weak events like meteors, this contrast might be reduced quickly under city lights and long exposures.

          For fast-moving or very short-lived events, reducing the exposure time cuts the background light but not the brightness of the event (it stayed the same amount of time on one pixel regardless of the exposure). This is easily noticed when observing with high-speed cameras (e.g. the short-lived recoil leaders in lightning flashes are very bright, but in normal video or stills they do not expose enough compared to the less intense but long-lasting background glow). Of course, using shorter exposures than 1/frame rate means gaps in meteor trails, but also reveals the speed in a visual way.
          regards,
          Oscar
          ~~~
          Photography of Lightning & Sprites
          http://www.lightningwizard.com


          From: "sirko@..." <sirko@...>
          To: metrec@yahoogroups.com
          Sent: Thursday, October 25, 2012 1:17 AM
          Subject: Re: [MetRec] Failing to see stars - Lux sensitivity?

          Hmm, I'm not fully convinced on the second statement. Typically
          non-intensified cameras have limiting magnitudes of only three to four
          mag. So also non-perfect four to five mag skies should have no impact,
          because they still are beyond the sensitivity of the camera.

          I'm always amazed when I stop visual observations because of strong
          twilight and then look at the screen of my cameras only to see, that
          they have just started to notice that the sky is getting brighter. But a
          little later, the limiting magnitude of the camera totally breaks down
          in only five minutes or so.

          Best,
          Sirko
        Your message has been successfully submitted and would be delivered to recipients shortly.