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Re: Newbie looking for advice/uber oops

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  • ubergroovin
    ... Ok, I was thinking like Geoff also, most people think of the olympus as a film camera, as OM-1 s are very popular among astrophotographers. I am happy
    Message 1 of 58 , May 31, 2005
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      --- In telescopes@yahoogroups.com, "Geoffrey Ogden"
      <cheekybunsbakery@h...> wrote:
      > --- In telescopes@yahoogroups.com, "ubergroovin" <ubernator@h...>
      > wrote:
      > > Since you mention that you are a pro 35mm film guy,
      >
      > But I'm a professional photographer and want to
      > > dabble with my
      > > > Olympus E-1 on well founded optics.
      >
      > Uber oops, Keith. The Olympus E-1 is a 4.9 MP camera.
      >
      > CS
      > Geoffrey

      Ok, I was thinking like Geoff also, most people think of the olympus
      as a film camera, as OM-1's are very popular among
      astrophotographers. I am happy with nikon manual bodies myself, even
      without a mirror lock.

      I still think the c9.25 is a winner for imageing, and the chip of a
      DSLR is almost as big as 35mm, and bigger than many CCD cameras, so
      you still get the benifits of the improvements of the c9.25.

      The construction of the c9.25 is much more rugged than the OTA of the
      RCX400, and there is alot less than can go wrong (the RCX has all
      kinds of electronics inside the OTA even).

      The RCX is not yet proven, and the c9.25 is... and I also think you
      would be happier with a gem than you would be with a big huge wedge
      mounted fork.

      A C11 is another consideration, but it does not have the same optical
      design of the c9.25, the unique overacheiver of the SCT type.

      Correction to my earlier post, the c9.25 does not have a smaller
      obstruction than other SCTS, but from the looks of things, it is
      smaller than the obstruction of the RCX.

      A C925 is about $2000 with XLT coatings, and $6000 could buy one hell
      of a mount, or a $3000 CGE mount (or save and get the CGE925 package)
      and alot of needed accessories, such as focal reducers, LP filters,
      IR filters, UV filters, premium eyepieces for visual use, a CCD/DSI
      for autoguiding etc... It is the accessories that add up... a $4000
      CGE925 setup could come to $8000 when all is said and done =)

      Keith
    • Jim
      ... Well there are a couple of different ways to go about it - you are correct for one way. The first scenario is pretty straight forward. If you know before
      Message 58 of 58 , Jun 3, 2005
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        --- In telescopes@yahoogroups.com, Bruce MacDonald <brumac@g...>
        wrote:
        > I'd be interested in knowing how a numeric value in a file can
        > reveal how it has been changed to some other numeric value.
        > Do you have any references?

        > Perhaps it is done statistically, but however it is done,
        > I'm fascinated.

        Well there are a couple of different ways to go about it - you are
        correct for one way.

        The first scenario is pretty straight forward. If you know before
        hand that the digital file (doesn't matter if it is a picture, a
        movie, or a M$ Word document) requires authentication, there are
        plenty of well understood and defined methods for watermarking,
        digitally signing, etc a file. At a later date, the file can easily
        be assessed to determine if the watermark or digital signature has
        been altered - pretty straight forward and common practice. It does
        require you to know before hand that you want to authenticate the
        file at a later date and have the schema set up in advance.

        But I don't think that is where you were heading. I think you are
        more concerned about when Bubba submits a picture of green aliens
        landing in his back yard and claiming "See, ET visits me every
        thursday to watch Friends and drink beer" is that a valid picture.

        For that, you are on the correct track - statistical testing can be
        done to detect modifications to digital pictures.

        I'll preface some of this first: I don't believe that if a single
        pixel had its shading changed in a 6MP file that these methods would
        detect it. Having said that, neither would your eyes. When we are
        talking about manipulating files, it is of the magnitude that the
        human eye can actually perceive the change (otherwise, why would you
        make the change, right?) Doing something like RBT stated - removing
        a person in front of a white wall or making someone appear to be
        doing something in a picture that wasn't actually done at the time it
        was taken. Fair enough?

        Here is a link to a LARGE (long and big file size) that talks about
        several different statistical methods that can be used to detect
        modifications to pictures. Different tools work better for different
        types of edits, but the gist is the same.

        http://tinyurl.com/8up5k

        There are plenty of other references as well - I just picked one that
        gave a couple of different spins on it.

        Also - this is just the stuff that is in public domain. I'd bet a
        years salary that the folks at Ft Meade, Langely, etc have even more
        sofisticated tools/methods that they've been using for years.

        Jim

        PS - I suppose we should probably continue this thread offline -
        other than the Alien in the backyard, we've probably strayed far
        enough from astonomy that it is no longer germain to this group...
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