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RE: [ccd-newastro] Re: Webpage on aperture, f-ratio, and focal length issues...

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  • Adrian Catterall
    Sorry, I meant to say on Richards site. I tried your resource too and again came up with a very short exposure of 1 minute. I live with around Mag 5 skies..
    Message 1 of 54 , Mar 2, 2006
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      Sorry, I meant to say on Richards site.
      I tried your resource too and again came up with a very short exposure
      of 1 minute. I live with around Mag 5 skies..

      For info, I put in:
      F=0.92
      Sky ADU 6130
      Exposure time 10 minutes
      Gain 1.3 (ST10XME)
      Read noise 8.8

      Seems very short for a sub exposure?

      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 sc02492
      Sent: 02 March 2006 12:53
      To: ccd-newastro@yahoogroups.com
      Subject: [ccd-newastro] Re: Webpage on aperture, f-ratio, and focal
      length issues...

      The link that you provided is for John Smith's subexposure calculator,
      which works quiet well. I have provided another analysis for
      determining subexposure duration on my website, with an excel
      spreadsheet at the end:

      http://www.starrywonders.com/snr.html

      The premise is somewhat different, as is the final equation, but the
      results are very comparable to John's.

      Steve Cannistra
      http://www.starrywonders.com

      --- In ccd-newastro@yahoogroups.com, "Adrian Catterall" <adrian@...>
      wrote:
      >
      >
      > I should have added Richards link for the sub-exposure calculator!
      > http://www.ccdware.com/resources/subexposure.cfm
      >
      > Actually, I put some figures in and came up with really quite short
      sub
      > exposures, even from a dark site?
      >
      > 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 Adrian Catterall
      > Sent: 02 March 2006 11:53
      > To: ccd-newastro@yahoogroups.com
      > Subject: RE: [ccd-newastro] Re: Webpage on aperture, f-ratio, and
      focal
      > length issues...
      >
      > Steve...
      >
      > I see that you have received a few replies, all of which are correct,
      in
      > that the advantage of binning reduces readout noise. However, if the
      > contribution of readout noise is negligible, you will notice no effect
      > from binning your colour exposures. The readout noise will be
      negligible
      > when other sources of noise predominate, particularly sky background
      > noise. Unfortunately most of us live with light pollution! Also you
      need
      > to factor the exposure duration and if sufficiently long enough will
      > also drown out readout noise. So if you live in a moderately light
      > polluted sky and use sub exposures of reasonable length (around 15
      > minutes), then binning will not significantly improve S/N.
      >
      > OTOH, if you image with narrowband filters or from a good dark sky,
      then
      > readout noise will be significant and binning will result in a
      > noticeable beneficial effect.
      >
      > 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 smvictor53
      > Sent: 02 March 2006 03:17
      > To: ccd-newastro@yahoogroups.com
      > Subject: [ccd-newastro] Re: Webpage on aperture, f-ratio, and focal
      > length issues...
      >
      > 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
      > >
      >
      >
      >
      >
      >
      >
      >
      >
      > Yahoo! Groups Links
      >
      >
      >
      >
      >
      >
      >
      >
      >
      >
      >
      >
      > Yahoo! Groups Links
      >








      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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