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Re: low magnification astronomy

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  • John
    Ali: Actually, I do quite a bit of observing at 37.5X -- which in my 8 Dob that has a 1200mm focal length, using a 32mm plossl is a field of view that is 70.5
    Message 1 of 8 , Sep 26 1:15 AM
      Ali:

      Actually, I do quite a bit of observing at 37.5X -- which in my 8" Dob
      that has a 1200mm focal length, using a 32mm plossl is a field of view
      that is 70.5 arc-minutes, or a little over 1 degree.

      I find that this is a great magnification to use with my 8" Dob, and
      the field of view is perfect for most deep sky objects. The field of
      view is a little restricted for finding some objects. Also, it is a
      little restrictive for the Andromeda galaxy. To solve those problems I
      have a 38mm Erfle eyepiece (2" eyepiece), which is 31X magnification,
      with a field of view that is 114 arc-minutes, or almost 2 degrees,
      which makes for very comfortable viewing from an 8" scope.

      I rarely go much over 50X for any of my deep-space observing. I will
      creep the magnification up to 200X on some objects, but this is rarely
      warranted. I live in an area that has a limiting magnitude of over +7
      on almost any moonless night, and very steady seeing, so I can push
      the magnification on my 8" past 250X on objects like M57, M27, M51,
      and almost any of the globular clusters, but again, I rarely do. 99%
      of my viewing is done at under 40X.

      John, east central Nevada
    • ali
      John: Your reply was most informative. I was thinking of making a small refractor working at these magnifications. Your account has made me thinking that a f6
      Message 2 of 8 , Sep 26 3:45 PM
        John:
         
        Your reply was most informative.
        I was thinking of making a small refractor working at these magnifications.
        Your account has made me thinking that a f6 8" scope is a great instrument.
        Over +7 limiting mag means you have great skies!
        Do you take photographs too from your scope?
         
         
        Ali, Lahore Pakistan
         
         
      • John
        Ali, Not a problem. I am limited on the pictures that I can take with my scope -- it is only on a Dobsonian mount -- however, I have taken some good pics of
        Message 3 of 8 , Sep 28 8:07 AM
          Ali,

          Not a problem. I am limited on the pictures that I can take with my
          scope -- it is only on a Dobsonian mount -- however, I have taken some
          good pics of the moon. In my opinion, the 8" Dobsonian is one of the
          best scopes to start doing serious visual observations. I will say,
          however, that even an 8" reflector is limited by sky conditions more
          so than even a 6" reflector. I am a major proponent of the reflector
          telescope. I may be a little bias. When you look at the fact that an
          8" reflector here is states sells for around $300 dollars and will
          give views as good as a 6"-7" refractor that sells for 10X as much,
          and without the problems of chromatic aberration that refractors suffer.


          Yes, Ali, I am blessed with very clear skies here. I am in the high
          desert of Nevada. Where I live is at over 7000' above sea level, my
          main observing site is at over 9600' above sea level. My humidity is
          almost always below 15% and it does not rain here often (less than 10"
          per year). The only problem is that once winter comes, which is very
          soon here, I will not be able to get to my main observing site. That
          is alright though, since my back yard almost always has a limiting
          magnitude of around +6. The only problem is that we see minus 40-50
          below zero Fahrenheit here in the winter, but there is such a
          temperature swing from day to night that we may see 20 degrees above
          zero in the day. I have seen as much as an 80 degree temperature
          difference from day to night here, and as much as a 30 degree
          difference in one hour after sunrise or sunset! Crazy, I know!


          There is one thing that I was going to add, but forgot: When trying to
          figure out what your lowest magnification should be for the telescope
          that you want to use; you simply divide your eyepiece focal length by
          the F ratio of the telescope, this will give you the exit pupil
          diameter. For an adult over about 30 years-old I would go with an exit
          pupil of about 6-6.5, since this is about the maximum diameter that
          your dark-adapted pupil will dilate to anyway.

          John
        • ali
          John, Please send one of your moon pictures if possible. Regards Ali ... From: John To: wannabeastronomers@yahoogroups.com Sent: Sunday, September 28, 2008 9:7
          Message 4 of 8 , Sep 29 6:22 AM
            John,
             
            Please send one of your moon pictures if possible.
             
             
            Regards
             
            Ali
             
            ----- Original Message -----
            From: John
            Sent: Sunday, September 28, 2008 9:7 p
            Subject: [Wanna be Astronomers] Re: low magnification astronomy

            Ali,

            Not a problem. I am limited on the pictures that I can take with my
            scope -- it is only on a Dobsonian mount -- however, I have taken some
            good pics of the moon. In my opinion, the 8" Dobsonian is one of the
            best scopes to start doing serious visual observations. I will say,
            however, that even an 8" reflector is limited by sky conditions more
            so than even a 6" reflector. I am a major proponent of the reflector
            telescope. I may be a little bias. When you look at the fact that an
            8" reflector here is states sells for around $300 dollars and will
            give views as good as a 6"-7" refractor that sells for 10X as much,
            and without the problems of chromatic aberration that refractors suffer.

            Yes, Ali, I am blessed with very clear skies here. I am in the high
            desert of Nevada. Where I live is at over 7000' above sea level, my
            main observing site is at over 9600' above sea level. My humidity is
            almost always below 15% and it does not rain here often (less than 10"
            per year). The only problem is that once winter comes, which is very
            soon here, I will not be able to get to my main observing site. That
            is alright though, since my back yard almost always has a limiting
            magnitude of around +6. The only problem is that we see minus 40-50
            below zero Fahrenheit here in the winter, but there is such a
            temperature swing from day to night that we may see 20 degrees above
            zero in the day. I have seen as much as an 80 degree temperature
            difference from day to night here, and as much as a 30 degree
            difference in one hour after sunrise or sunset! Crazy, I know!

            There is one thing that I was going to add, but forgot: When trying to
            figure out what your lowest magnification should be for the telescope
            that you want to use; you simply divide your eyepiece focal length by
            the F ratio of the telescope, this will give you the exit pupil
            diameter. For an adult over about 30 years-old I would go with an exit
            pupil of about 6-6.5, since this is about the maximum diameter that
            your dark-adapted pupil will dilate to anyway.

            John

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