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Re: Webpage on aperture, f-ratio, and focal length issues...

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  • smvictor53
    WOW - I think I get it! Thansk to Ron for the concept, Steve for the math, and Adrian and Dean for the practical advise. I image in very bright skys and now I
    Message 1 of 54 , Mar 2, 2006
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      WOW - I think I get it!

      Thansk to Ron for the concept, Steve for the math, and Adrian and
      Dean for the practical advise. I image in very bright skys and now I
      understand why binning never seemed to add that much for me even thou
      I see 'everyone' doing it.

      I contantly amazes me how accessible truly expert advise is on this
      group. Thanks for all and I'll go back to lurking now.

      Steve


      --- In ccd-newastro@yahoogroups.com, "Bud" <budguinn@...> wrote:
      >
      > Steve, thanks for the nice writeup/explanation.....very easy to
      > understand.
      >
      > Warmest regards,
      >
      > Bud Guinn
      > http://www.budguinn.com
      >
      > --- In ccd-newastro@yahoogroups.com, "sc02492" <sc02492@> wrote:
      > >
      > > Steve, binning helps you increase the signal to noise ratio, as
      Ron
      > > said. Binning 2x2 means that your camera now considers 4 pixels
      as
      > > one unit, thereby creating a functional "superpixel." Read noise
      > > occurs when the electron contents of a pixel are being converted
      into
      > > digital units (ADU conversion). Normally the electrons in 4
      separate
      > > pixels (i.e., unbinned) would be subjected to 4 separate read
      noise
      > > events, since each pixel is being read separately. But with
      binning,
      > > the superpixel is now considered the unit, and it is subjected to
      only
      > > one read noise event.
      > >
      > > Compare the results of 4 pixels binned 2x2, versus those same 4
      pixels
      > > unbinned. The signal, S, is the same in both groups, since the
      same
      > > amount of light is falling on the same number of pixels (4 pixels
      in
      > > either case). Let's call the read noise per functional pixel
      unit "R"
      > > (in electron RMS). By functional pixel unit, I'm referring to the
      > > fact that the read noise is R for each pixel in the unbinned
      example,
      > > and it would also be R for the entire "superpixel" in the binned
      case.
      > >
      > > Ignoring for a moment the contribution of dark signal and sky
      flux to
      > > the noise (i.e., assume a relatively short exposure at a dark
      site),
      > > the noise in the binned example would be sqrt(R^2), or R, and the
      > > signal to noise would be S/R (binned case). Now, compare this to
      the
      > > unbinned case. The signal collected by 4 pixels is the same, S,
      but
      > > there will be 4 read noise events (for the 4 individual pixels)
      > > instead of 1. Since noise adds in quadrature, it would be sqrt
      (R^2 +
      > > R^2 + R^2 +R^2), or sqrt(4 x R^2), for the unbinned group. This
      means
      > > that the signal to noise unbinned would be S/2R (i.e., this is
      bad,
      > > since the signal to noise ratio has been cut in half compared to
      the
      > > binned 2x2 example). Put it another way, from the standpoint of
      read
      > > noise, binning 2x2 allows you to double your S/N ratio, binning
      3x3
      > > allows you to triple the S/N ratio, etc., compared to the unbinned
      > > case. Increasing the signal to noise ratio means that you will
      need
      > > less subexposure time to become photon limited (i.e., to have sky
      > > noise overwhelm read noise), so binning can be very helpful at
      dark
      > > sites, where the sky flux is very low, in order to avoid
      excessively
      > > long subs. Conversely, as the contribution from sky noise
      increases
      > > at light polluted sites, this same reasoning dilutes the signal to
      > > noise advantage of binning (remember that the assumption above was
      > > that we could ignore noise from sky glow, which isn't the case at
      a
      > > light polluted site).
      > >
      > > There are downsides to binning, the most obvious one being that
      your
      > > image scale will proportionally increase, because the size of the
      > > functional pixel unit has increased. So binning 2x2 means that
      you
      > > have doubled your image scale, for instance. This can lead to
      > > decreased resolution (if you are already undersampled), which may
      be
      > > acceptable for RGB images, but would be less acceptable for
      luminance.
      > > That's why many people bin color, and try to avoid binning
      luminance.
      > > That said, if your unbinned image scale is .4 arcsec/pixel (i.e.,
      you
      > > might be oversampled), and it increases to .8 arcsec/pixel with
      2x2
      > > binning, this is still represents very respectable sampling at
      most
      > > imaging sites. In that case binning can cut down on subexposure
      time,
      > > especially at a dark site, with minimal loss in resolution.
      > >
      > > I image at an image scale of between 1.7 and 2.2 arcsec/pixel, so
      I'm
      > > already undersampled (my best seeing is about 3 arcsec/pixel,
      which
      > > isn't very good, but this means that I should ideally be sampling
      at
      > > an image scale of 3/3.3 (Nyquist), or .9 arcsec/pixel. So at 2.2
      > > arsec/pixel, I'm undersampled). I really have no room to move
      with
      > > respect to increasing my image scale, without further
      compromising my
      > > sampling. Add to this the fact that my sky glow is signficant.
      Both
      > > of these factors mean that binning affords me very little
      advantages.
      > > But if my image scale were much lower, and if I were at a dark
      site,
      > > then binning would make perfect sense.
      > >
      > > PS- thanks to Ron for teaching me everything that I know about
      binning
      > > <g>!
      > >
      > > Steve Cannistra
      > > http://www.starrywonders.com
      > >
      > >
      > >
      > > --- In ccd-newastro@yahoogroups.com, "smvictor53" <SMVICTOR@>
      wrote:
      > > >
      > > > Sorry to not let this die, but then what is the advantage of
      binning
      > > > 2x2 color? It seems from what you are saying that you would
      get the
      > > > same signal from the object as binned 1x1 with more sky
      gradiants.
      > > >
      > > > Steve
      > > > >
      > > > > Ron....
      > > > >
      > > > > I agree with you and Steve, but object S/N is an important
      concept
      > > > that
      > > > > helps to explain many issues, such as why binning or reducing
      focal
      > > > > ratio doesn't increase the magnitude limit.
      > > > >
      > > > > For example, I see Mike Dodds message (sorry to drag you in
      Mike!)
      > > > that
      > > > > quotes that binning will increase light by 4x and others
      could be
      > > > led to
      > > > > believe that we could see fainter with a lower focal ratio or
      with
      > > > > binning. At a pixel level there is more light per binned
      pixel, but
      > > > no
      > > > > more light will come from the object.
      > > > >
      > > > > Adrian Catterall
      > > > > Amateur Astronomer
      > > > > Herts UK
      > > > > http://www.acatterall.com
      > > > >
      > > > >
      > > > > -----Original Message-----
      > > > > From: ccd-newastro@yahoogroups.com [mailto:ccd-
      > > > newastro@yahoogroups.com]
      > > > > On Behalf Of Wodaski Yahoo
      > > > > Sent: 01 March 2006 03:31
      > > > > To: ccd-newastro@yahoogroups.com
      > > > > Subject: RE: [ccd-newastro] Re: Webpage on aperture, f-ratio,
      and
      > > > focal
      > > > > length issues...
      > > > >
      > > > > Steve Cannistra's comments, posted to the SBIG group, are
      right on
      > > > the
      > > > > money. I am copying them here on the assumption that Steve
      won't
      > > > mind
      > > > > educating a few folks here as well:
      > > > >
      > > > > "Here's the way that I look at this:
      > > > >
      > > > > "1. It is easy to see how the the light collected for a given
      object
      > > > > per unit time is simply related to the collecting surface area
      > > > > (aperture). It can be no other way. Whether the lens or
      mirror
      > > > > requires a longer or shorter distance to bring this light to
      a focus
      > > > > does not change how much light was collected, for a given
      aperture.
      > > > >
      > > > > "2. Focal length determines magnification (among other
      things).
      > > > For a
      > > > > given aperture, whether you magnify the object so that it
      covers the
      > > > > entire chip (i.e., longer focal length, higher f ratio), or
      reduce
      > > > the
      > > > > object so that it covers half of the chip (i.e., shorter
      focal
      > > > length,
      > > > > lower f ratio), the object itself still has the same photon
      flux
      > > > > associated with it. You can't make more photons than what you
      > > > > collected! (Again, speaking from the standpoint of the object
      > > > itself).
      > > > >
      > > > > "3. However, for a given aperture, a longer focal length
      (i.e.,
      > > > higher
      > > > > f ratio), means that the total surface area of the CCD chip
      will be
      > > > > exposed to less total light (since you are now capturing a
      smaller
      > > > > fraction of the collected sky flux). Note that this does not
      mean
      > > > > that the number of photons associated with the object itself
      has
      > > > > changed- it hasn't. You are still capturing the same number
      of
      > > > > object-specific photons (assuming that the entire extent of
      your
      > > > > target is still on the chip). But the narrower FOV with a
      higher
      > > > > focal length will translate into a smaller absolute number of
      sky
      > > > > photons (e.g., from light pollution) hitting the entire
      extent of
      > > > the
      > > > > chip's surface area.
      > > > >
      > > > > "4. And now for the important point <g>: Although the total
      number
      > > > of
      > > > > photons (from the chip's perspective) contributed by sky flux
      will
      > > > be
      > > > > lower (at a higher f ratio, keeping aperture constant), the
      read
      > > > noise
      > > > > will be the same!
      > > > >
      > > > > "5. This means that for a higher f ratio (but same
      aperture),you
      > > > may
      > > > > need to increase your subexposure duration to make certain
      that you
      > > > > are still photon-limited (and to thereby reduce the effects
      of read
      > > > > noise).
      > > > >
      > > > > "So an increase in subexposure duration with a higher f ratio
      may be
      > > > > needed to counter the decreased sky flux hitting the chip's
      surface
      > > > > area, and to thereby ensure that you are still photon-
      limited. In
      > > > > that situation, the increased subexposure duration is not
      needed to
      > > > > improve object-specific signal (which is dependent upon the
      > > > aperture),
      > > > > it's needed to decrease the read noise contribution. So f
      ratio can
      > > > > influence the subexposure duration, but it will not impact on
      > > > > object-specific photon flux. So in a way, I think that both
      sides
      > > > of
      > > > > the argument have their points, but in the end, f ratio
      itself
      > > > isn't a
      > > > > very important consideration compared to aperture, once you
      become
      > > > > photon-limited."
      > > > >
      > > > > Above originally posted to the SBIG group by:
      > > > > Steve Cannistra
      > > > > http://www.starrywonders.com
      > > > >
      > > > >
      > > > > Ron Wodaski
      > > > > http://www.newastro.com
      > > > >
      > > > >
      > > > > -----Original Message-----
      > > > > From: ccd-newastro@yahoogroups.com [mailto:ccd-
      > > > newastro@yahoogroups.com]
      > > > > On
      > > > > Behalf Of Adrian Catterall
      > > > > Sent: Tuesday, February 28, 2006 12:55 PM
      > > > > To: ccd-newastro@yahoogroups.com
      > > > > Subject: RE: [ccd-newastro] Re: Webpage on aperture, f-ratio,
      and
      > > > focal
      > > > > length issues...
      > > > >
      > > > > Dennis...
      > > > >
      > > > > I'm afraid it is not as simple as that and what Ron says has
      an
      > > > element
      > > > > of truth, but that book was written some years ago and our
      > > > understanding
      > > > > has moved on. Rather than re-cite what is being discussed in
      the
      > > > SBIG
      > > > > group, I suggest having a look there.
      > > > >
      > > > > Adrian Catterall
      > > > > Amateur Astronomer
      > > > > Herts UK
      > > > > http://www.acatterall.com
      > > > >
      > > > >
      > > > > -----Original Message-----
      > > > > From: ccd-newastro@yahoogroups.com [mailto:ccd-
      > > > newastro@yahoogroups.com]
      > > > > On Behalf Of Dennis Persyk
      > > > > Sent: 28 February 2006 16:41
      > > > > To: ccd-newastro@yahoogroups.com
      > > > > Subject: [ccd-newastro] Re: Webpage on aperture, f-ratio, and
      focal
      > > > > length issues...
      > > > >
      > > > > I quote from "The New CCD Astronomy", Ron Wodaski, page 27,
      section
      > > > > entitled "Focal Ratio":
      > > > >
      > > > > For example, an f/10 8" SCT has an aperture of 200 mm, and a
      focal
      > > > > length of 2000 mm. An f/6 refractor has an aperture of 100
      mm, and
      > > > a
      > > > > focal length of 600 mm. You might expect that the 8" SCT
      world
      > > > > capture more light, and provide shorter exposure times. In
      fact,
      > > > the
      > > > > opposite is true: the f/6 refractor will require shorter
      exposure
      > > > > times. [End quote.]
      > > > >
      > > > > I believe Ron's book offers expert, proven advice to the
      imager, be
      > > > > he newbie or expert. Note also that authors Suiter,
      Covington, and
      > > > > Sidgwick offer the same information. So we have at least four
      peer-
      > > > > reviewed books by authors who state that numerically-smaller
      > > > > ("faster") focal ratios require shorter imaging times.
      > > > >
      > > > > I read those books, believed them, and bought an f/3.3
      Takahashi
      > > > > astrograph because of what I read. I have not been
      disappointed.
      > > > > Thank you Ron, for your advice - it paid off.
      > > > >
      > > > > Clear skies,
      > > > >
      > > > > Dennis Persyk
      > > > > Igloo Observatory Home Page http://dpersyk.home.att.net
      > > > > Hampshire, IL
      > > > >
      > > > > New Images http://home.att.net/~dpersyk/new.htm
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > > Yahoo! Groups Links
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > > Yahoo! Groups Links
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > > __________ NOD32 1.1421 (20060228) Information __________
      > > > >
      > > > > This message was checked by NOD32 antivirus system.
      > > > > http://www.eset.com
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > >
      > > > > Yahoo! Groups Links
      > > > >
      > > >
      > >
      >
    • Christopher Abissi
      Dave, Thanks for the links and advice. I have been using a DVI connection to my monitor. Is that the best way to go and does that affect the adjustability of
      Message 54 of 54 , Mar 5, 2006
      • 0 Attachment
        Dave,



        Thanks for the links and advice.



        I have been using a DVI connection to my monitor. Is that the best way to go
        and does that affect the adjustability of the display?



        Chris



        _____

        From: ccd-newastro@yahoogroups.com [mailto:ccd-newastro@yahoogroups.com] On
        Behalf Of Dave Weaver
        Sent: Sunday, March 05, 2006 2:41 AM
        To: ccd-newastro@yahoogroups.com
        Subject: [ccd-newastro] Re: Flat Field Problem (Another) Best Monitor



        PC Magazine has a good summary here:
        http://www.pcmag.com/article2/0,1759,1774271,00.asp

        PC Magazine has a good review here:
        http://www.pcmag.com/article2/0,1895,1774272,00.asp

        -Dave

        --- In ccd-newastro@yahoogroups.com, "Dave Bailey" <dcbailey@...> wrote:
        >
        > Is the wide screen effectivly used? Or would the normal 4:3 ratio be more

        > practical?
        > Dave Bailey
        >
        >
        > ----- Original Message -----
        > From: "Dave Weaver" <tak@...>
        > To: <ccd-newastro@yahoogroups.com>
        > Sent: Sunday, March 05, 2006 12:02 AM
        > Subject: [ccd-newastro] Re: Flat Field Problem (Another) Best Monitor
        >
        >
        > >I also have the 2405 and I have been quite pleased with it. The email I
        > >received from Dell
        > > for the sale price of $799 didn't require any coupons. I haven't
        > > purchased one of the
        > > calibration tools yet, but Photoshop seems to find the default
        calibration
        > > file for it.
        > >
        > > http://accessories.us.dell.com/sna/productdetail.aspx?
        > > c=us&l=en&s=dhs&cs=19&sku=24053YR&category_id=4009
        > >
        > > -Dave
        > >
        > > --- In ccd-newastro@yahoogroups.com, "Dean Jacobsen" <deanjacobsen@>
        > > wrote:
        > >>
        > >> Chris, I am using a Dell 2405 and I am extremely pleased with it. I
        > >> just got a notice from Dell yesterday that the 2405 was on sale for
        > >> $800. I believe in an earlier post you said that you have a Spyder
        > >> 2 Pro. The Spyder 2 calibrated my Dell 2405 very nicely.
        > >>
        > >> Check here for two coupons for the Dell 2405 for $799 and the Dell
        > >> 2001 for $458!!
        > >>
        > >> http://www.gotapex.com/
        > >>
        > >> Dean Jacobsen
        > >> www.astrophoto.net
        > >>
        > >> --- In ccd-newastro@yahoogroups.com, "cja2173" <cabissi@> wrote:
        > >> >
        > >> > --- In ccd-newastro@yahoogroups.com, "Wodaski Yahoo" <yahoo@>
        > >> wrote:
        > >> > >
        > >> > > "In terms of image quality, the response on dark-gray shades was
        > >> > weak and
        > >> > > greatly affected by the viewing angle."
        > >> > >
        > >> > > The combination of a 2002 vintage and viewing angle problems
        > >> makes
        > >> > that
        > >> > > particular monitor one that is going to give you some difficulty
        > >> in
        > >> > > producing properly balanced images.
        > >> > >
        > >> > >
        > >> > > Ron Wodaski
        > >> > > http://www.newastro.com
        > >> > >
        > >> > Ron,
        > >> >
        > >> > Thanks for clarifying why I am having so much difficulty. You have
        > >> put
        > >> > me in the market for a new monitor. Do you or anyone have any
        > >> > recommendations of models or specifications that I need to look
        > >> for?
        > >> >
        > >> > From an old thread in the forum I saw the Dell 2405 mentioned. Any
        > >> > feedback greatly appreciated.
        > >> >
        > >> > Having a hard time seeing ....
        > >> > Chris A
        > >> >
        > >>
        > >
        > >
        > >
        > >
        > >
        > >
        > >
        > >
        > >
        > > Yahoo! Groups Links
        > >
        > >
        > >
        > >
        > >
        > >
        > >
        >













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