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13446Re: [ETXASTRO] Entering seconds for custom object autostar 497

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  • Ron
    Aug 16 10:47 AM
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        This must have been routed through the US Postal System.  It is about 24 hours old.
       
      Ron
       
      From: Ron
      Sent: Thursday, August 15, 2013 2:36 PM
      Subject: Re: [ETXASTRO] Entering seconds for custom object autostar 497
       


      OK Chris
       
        First Camera lens and telescope lens are measured differently.  If you had a 10 mm telescope, the front lens or objective would measure 10mm in diameter.  There may be exceptions but I doubt they are in the realm we operate in.
       
        Again I don’t know enough to give this as a solid statement but I think the xx mm camera lens is closer to the focal length.  I don’t know where or how it is measured and it probably has a lot to do with the element make up.  I have a 70 mm to 200 mm zoom.  The physical length of the lens doesn’t change when I go from 70 mm to 200 mm.  The lens stays the same length.  On the other hand my 100 mm – 400 mm lens does extend out.  I haven’t gotten into short camera lens yet but I have used my wife’s 18 mm – 135 mm lens to shoot parts of the Milky Way and can see some possibilities.
       
        So all in all a camera lens is a horse of a different color.  While they are used in astro photography the best place to get answers for questions related to them would probably be on a photography group.
       
        Telescopes, particularly reflectors,  can have long focal lengths in short spaces.  the Meade LX200 series secondary mirror also magnifies the focal length (image) by 5, I think.  Also the ETX 125 which is also a reflector must have a magnifying factor associated with the secondary. The F15 times the 125 objective diameter comes out to 1875 mm or in inches to (15 X 5) 75.
       
        Optics are a diverse field.  The subject  can be quite confusing.  This discussion has pushed me to look for and find answers to questions I had but ignored since I didn’t understand the any answers I had read previously.  In particular, if I had a lens with an advertised 60 degree apparent (???) field, why was the moon so big in it.
       
        Brian, I have a root beer in my fridge just waiting to meet up with a couple of scoops of ice cream.
       
        Thanks to all who contributed to this discussion.
       
      Ron
       
       
       
      Sent: Thursday, August 15, 2013 1:44 PM
      Subject: Re: [ETXASTRO] Entering seconds for custom object autostar 497
       


      I'm doing my best to get my head around this .. but also when applied to photography lenses and the angle of view, initially I thought a 10mm focal length lens was also telling you its apparent field of view, but when it comes to fish-eye or wide-angle lenses this isn't the case. I was considering the cheaper Nikkor 10mm lens as an alternative to the expensive 10mm wide-angle lens - but it turns out they have totally different fields of view, and the cheaper one does not get me a fish-eye lens, damn . ..
      This page has some very clever calculators; but I'm not claiming to fully understand it: www.tawbaware.com/maxlyons/calc.htm

      I still don't understand why Stellarium is showing equatorial lines spaced by 5mins; thus Pleiades looks like it is 5 arc mins across, yet SEDS says its 110 arc mins across .. ? 5 mins makes sense. What is SEDS on about exactly? I have Googled/Wikiepedia'd this but I'm travelling in a 'circle of confusion' (not good if you've had a beer).




      On 15/08/2013 19:03, Ron wrote:
       
        Sometimes discussions stimulate me.
       
        Here is an explanation of the difference of Apparent and True FOVs.  I will probably forget it in a few hours .
       
       
      Other times discussions make me believe in something.  I believe I will go have a beer.
       
      Ron
       
      From: Ron
      Sent: Thursday, August 15, 2013 12:50 PM
      Subject: Re: [ETXASTRO] Entering seconds for custom object autostar 497
       


      I meant to say has NOT reduced my enjoyment
       
      Ron
       
      From: Ron
      Sent: Thursday, August 15, 2013 12:27 PM
      Subject: Re: [ETXASTRO] Entering seconds for custom object autostar 497
       


        Similar if not the same information is in the back of the users manual for the LX200R and LXD75 under optional accessories with the Eyepieces.   It gives Power and Actual Field.  The ETX manual only gives the power.
       
        Not knowing the difference between Actual and apparent and other items about the FOV has reduced my observation enjoyment.
       
        Dr. Clay’s site provides a lot of helpful information and he uses ‘plain’ english.
       
      Ron
       
      Sent: Thursday, August 15, 2013 12:06 PM
      Subject: Re: [ETXASTRO] Entering seconds for custom object autostar 497
       


      Hi,
       
      I got the info from Dr. Clay's website where he lists ETX scopes and FOV for various FL eyepieces (very useful info).  For the ETX 125 and a 26mm eyepiece, his table reads:
      26mm - 73X / (42')

      The site can be found here:
       
      All the Best,
       
      Brian
       
      Brian
       
      Brian Straight, Ph.D.
      The Walnut Ridge Observatory
      40.389452/-87.044881
      West Point, Indiana


      On Thu, Aug 15, 2013 at 12:57 PM, Chris Harvey <chris@...> wrote:
       

      Ahh yea I see .. sorry .. just looking in Stellarium; it looks like the Plaiedes is aprox 5 arc mins square, for context.
      But SEDS says the Plaiedes has: Apparent Dimension: 110.0 (arc min)
      What have I got wrong?





      On 15/08/2013 17:39, Ron wrote:
       
        I am pretty sure he was saying the Field Of View of the 26mm is 40 odd ArcMinutes.
       
        That is how I read it.
       
      Ron
       
      Sent: Thursday, August 15, 2013 10:09 AM
      Subject: Re: [ETXASTRO] Entering seconds for custom object autostar 497
       


      Erm, are you saying a star takes 40mins to get accross the FoV on an ETX125 with 26mm? That canny be right... more like 4mins, if that ?!
      Sorry if I'm misunderstanding you . .. .
      (I have read the Weasner news, will read again properly later to understand further, you don't miss somet 'till its gone ..  . . .. . )

      Nothing to do with ETX, but check out my Perseids including an animation of the smoke from a fireball dying away over 14mins:
      www.flitemedia.com/photography/night-sky.php
      Cheers,
      Chris


      On 15/08/2013 15:56, Brian Straight wrote:
       
      ...and with a 40-odd-minute FOV at 26mm, not necessary...
       
      Brian
       
      Brian Straight, Ph.D.
      The Walnut Ridge Observatory
      40.389452/-87.044881
      West Point, Indiana


      On Thu, Aug 15, 2013 at 10:37 AM, BrianS <briansxx@...> wrote:
       

      OK--I'm sure this is a "dumb" question, but I'll ask it any way. I want to use my 125 to take a look at the new nova in Delphinus. I've created a user object, and I need to enter an RA of 20h:23m,30s, but the seconds only allows for single-digit entry, which seems rather strange. Maybe seconds is more resolution that is needed for the ETX, given the field of view, but any thoughts or help appreciated.

      Thanks and Best,

      Brian

       


       

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