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Re: Star size (apparent)

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  • davidfirebaugh@ymail.com
    Im rural but i could probalby find someone in my old club Stckton Astronomical Society to test the mirror. My zip in 95423 ,the boonies. Though would rather
    Message 1 of 16 , Apr 16 1:08 PM
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      Im rural but i could probalby find someone in my old club Stckton Astronomical Society to test the mirror. My zip in 95423 ,the boonies.

      Though would rather drive than ship my mirror. if i ship it id probalby ask Swaze to test it ,or refigure -then go with enhanced coatings afterwords.I do have back up scopes 13" ect.



      --- In bigdob@yahoogroups.com, "Bratislav" <bratislav3162@...> wrote:
      >
      > > Commnets or suggestions?
      >
      > Have the primary tested.
      >
    • davidfirebaugh@ymail.com
      ... Maybe just coinsidence Ive observed on soggy nights, Thurdsay wee hours 39 degrees- and both primary and secondary dewed up. never saw more than 6 in trap.
      Message 2 of 16 , Apr 16 1:31 PM
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        >>>>In response to your post .Having been a newt-dob guy setting up the 20 obsession got lots of collimation practice. I think the coolings covered . Now conditions , admittedly the 15 gets more use at 1300 ft - than the 20 ever got at 7800 ft .Peddler Hill,Ca- is just so good up there.. Yes the 20 was more effected by seeing so I only used it at higher ,4000 ft plus elevations. Now with the health issue ,its the deck and the 15 most of the time. Im 45 so my vision shouldnt bee that bad. 20-15 as of 6 months ago .
        Maybe just coinsidence Ive observed on soggy nights, Thurdsay wee hours 39 degrees- and both primary and secondary dewed up.

        never saw more than 6 in trap. with my smaller scopes(except the 13" dob) ,but those others were SCT's.

        6 stars in the trap in a 4"?,a refractor I presume.
        My home observation site is very transparent ,but has poor seeing typically . Cant get into the mtns much with the scope anymore .

        Furthermore recent neorlogical issues may play into this fiacsco-

        Have to figure it all out.
        Thanks Dave


        --- In bigdob@yahoogroups.com, "pensack1" <Pensack1@...> wrote:
        >
        > Though the Airy disks are larger in a 15", they are still so small that they should appear as pinpoints. If the focuser allows you to achieve a tight focus (which is more difficult the shorter the f/ratio, remember), then the issue is one or more of the 3 "C"s:
        > Collimation
        > Cooling
        > Conditions.
        > If all 3 of the above are optimum (and 6 stars in the Trapezium is barely a test for a 4", so it makes a poor test for a 15". Any 15" that has trouble seeing 6 stars under heavy light pollution and 8 or 9 in dark skies is not performing up to snuff), then the optics may not be of premium quality. If observations on nights of good seeing with a cooled mirror don't show tiny little pinpoint stars (such as resolving the core in M15), then you should think about having the mirror tested. Many mirror makers will test a mirror for a charge--there is probably one near you.
        >
        > But look at your weather map. If the jet stream is above you, or the pressure isobars are close together, forget about getting good star images.
        >
        > Still, I would have thought your 20" would have been just as impacted by seeing conditions. So it may be mirror quality. 5-10 nights under the stars should tell you more.
        >
        > Don Pensack
        > Los Angeles
        >
        > --- In bigdob@yahoogroups.com, "garyk9rx" <RXDesign@> wrote:
        > >
        > >
        > > David,
        > >
        > > the trap should be easy and is a reasonable test for a 15". In my experience given equal quality optics and the same power the stars should be exactly the same. One common scenario when comparing scopes is that one forgets about the aperture change and compares ep for ep... i.e. put a 20mm ep in the 15... walk it over and put it in the 20...and then forget that in the 20 the magnification is higher and you're no longer comparing apples to apples... but if you did this the other way around the stars should, if anything, appear slighlty better (in a 15 using the same ep) as the power is less.
        > >
        > > Lesson of course is to be sure if comparing side-by-side or if going by memory to use similar powers...
        > >
        > > Although it sounds like it's only one night and more than likely it was just seeing conditions the one thing I'd be concerned about is indeed optics quality. It will take a few nights to come to a conclusion but if you're using it where you would have normally used the 20 and you don't see similar results (as you remember with the 20) then check out the optics.
        > >
        > > g.
        > >
        > >
        > > --- In bigdob@yahoogroups.com, "davidfirebaugh@" <davidfirebaugh@> wrote:
        > > >
        > > > >>> Yes the mirror and ambient temp were measured less tha a degree differential (per primary sensor ect) Was clear, dark and steady , scope well aclimated -4 hours . Observations made near midnite or lil after. Decent enough to show close doubles, just nowhere in comparison size wise to the stars in the 20" id owned previously.
        > > > Seeing was 6 star in trapezium quality. Maybe expecting too much for this aperature . maybe this info helps ,
        > > > Thanks
        > > > Dave
        > > >
        > > > --- In bigdob@yahoogroups.com, Attilla Danko <danko@> wrote:
        > > > >
        > > > > Was your mirror cool when you made the comparison? Look at a star out
        > > > > of focus. If you see slow swirling, your mirror is warm and the tube
        > > > > currents causing the star images to bloat.
        > > > >
        > > > >
        > > > >
        > > > > On Wed, Apr 14, 2010 at 15:45, davidfirebaugh@
        > > > > <davidfirebaugh@> wrote:
        > > > > > I had been obseving with a 20 inch Obsession for about 8 years.
        > > > > >
        > > > > > Health issues made ladder work unsafe impractical ,so I traded a fellow with a 15" Obsession.
        > > > > >
        > > > > > I cant help but notice even on the best nights the stars seem much larger and swollen (compared to the 20" f5). Even with the paracorr dropped in...as the 15" is f4.5
        > > > > >  (which helps much) Stars are not as small and sharp as id like.
        > > > > >
        > > > > > The mirror is clean and does not appear foggy although its an early 15" mirror.  Commnets or suggestions?
        > > > > >
        > > > > > Clear Skies
        > > > > > Dave
        > > > > >
        > > > > >
        > > > > >
        > > > > > ------------------------------------
        > > > > >
        > > > > > Yahoo! Groups Links
        > > > > >
        > > > > >
        > > > > >
        > > > > >
        > > > >
        > > > >
        > > > >
        > > > > --
        > > > > attilla danko, danko@, http://cleardarksky.com/csk
        > > > >
        > > >
        > >
        >
      • scott_beard2000
        Dave, Sorry for being a little off topic. The thread was talking about star image sizes, and the ask.com question was also dealing with star image sizes.
        Message 3 of 16 , Apr 16 1:42 PM
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          Dave,

          Sorry for being a little off topic.
          The thread was talking about star image sizes,
          and the "ask.com" question was also dealing with star image sizes.
          That was the only connection with the thread.

          I wish I had downloaded the "answer" before navigating away from the ad, because I'm sure I would have gotten a good laugh. I put in the post, hoping that the question itself was enough that everyone would enjoy a good laugh at the end of a long week. I know that it has been a long week for me.

          My best guess is that the person that was asking the question was having trouble undestanding focusing. A lot of first-timers don't realize that the star gets "smaller" when it comes into focus. They try and make it bigger, thinking that when the image gets larger, they are zooming-in.

          This is way, way, below everyone's ability on this forum. I'm sorry if I bothered anyone. I was just trying to inject humor in a long week, and hopefully lighten the angst of the fellow with the 15" who is comparing star image sizes.

          I'll shut up, and go back to lurking.

          Sincerely,
          Scott Beard


          --- In bigdob@yahoogroups.com, "davidfirebaugh@..." <davidfirebaugh@...> wrote:
          >
          > Scott ,I dont understand the question or how it applies to newtonians.
          > Can you give more info?
          > Dave
          >
          > --- In bigdob@yahoogroups.com, "scott_beard2000" <indianola@> wrote:
          > >
          > >
          > > Funny timing on this. I was web surfing, and an ad for Ask.Com came up, with a list of astronomical questions that they were providing answers for.
          > >
          > > One of the questions was "Should the star image size through a telescope be larger than the lens at the front of the telescope?"
          > >
          > > -Scott
          > >
          >
        • Duane Balch
          Google on John Lightholder for Lightholder Optics.  He can test your mirror and refigure it if needed.  He s out of South Lake Tahoe, so reasonably close if
          Message 4 of 16 , Apr 16 6:09 PM
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            Google on John Lightholder for Lightholder Optics.  He can test your mirror and refigure it if needed.  He's out of South Lake Tahoe, so reasonably close if you drove there.  Give them a call!
            Duane
            Sacramento




            ________________________________
            From: "davidfirebaugh@..." <davidfirebaugh@...>
            To: bigdob@yahoogroups.com
            Sent: Fri, April 16, 2010 1:08:59 PM
            Subject: [bigdob] Re: Star size (apparent)

             
            Im rural but i could probalby find someone in my old club Stckton Astronomical Society to test the mirror. My zip in 95423 ,the boonies.

            Though would rather drive than ship my mirror. if i ship it id probalby ask Swaze to test it ,or refigure -then go with enhanced coatings afterwords.I do have back up scopes 13" ect.

            --- In bigdob@yahoogroups. com, "Bratislav" <bratislav3162@ ...> wrote:
            >
            > > Commnets or suggestions?
            >
            > Have the primary tested.
            >







            [Non-text portions of this message have been removed]
          • John Mahony
            It was a joke. How could a star image size (which can only be measured as an angle) be larger than the aperture? -John ... Can ... Dave
            Message 5 of 16 , Apr 16 11:23 PM
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              It was a joke. How could a star image size (which can only be measured as an angle) be larger than the aperture?


              -John



              ----- Original Message ----
              > From: "davidfirebaugh@..." <davidfirebaugh@...>
              >
              > Scott ,I dont understand the question or how it applies to newtonians.
              Can
              > you give more info?
              Dave


              > "scott_beard2000" <indianola@...> wrote:
              >
              >
              > Funny
              > timing on this. I was web surfing, and an ad for Ask.Com came up, with a
              > list of astronomical questions that they were providing answers for.
              >
              >
              > One of the questions was "Should the star image size through a
              > telescope be larger than the lens at the front of the telescope?"
            • John Lightholder
              Hi Duane, Yup, nestled up in the High Sierra, less than 2 hours from Stockton. David, give me a call and come on up. I can test your mirror. You could sit
              Message 6 of 16 , Apr 21 6:12 AM
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                Hi Duane,

                Yup, nestled up in the High Sierra, less than 2 hours from Stockton.

                David, give me a call and come on up. I can test your mirror.
                You could sit behind the knife edge to have a look too!
                Spring is still coming - dress warm....

                Best regards,

                John Lightholder

                Lightholder Optics
                Website: http://www.lightholderoptics.com
                E-Mail: john@...
                TEL/FAX: (530) 577-4328


                --- In bigdob@yahoogroups.com, Duane Balch <geolgst3807@...> wrote:
                >
                > Google on John Lightholder for Lightholder Optics.  He can test your mirror and refigure it if needed.  He's out of South Lake Tahoe, so reasonably close if you drove there.  Give them a call!
                > Duane
                > Sacramento
                >
                >
                >
                >
                > ________________________________
                > From: "davidfirebaugh@..." <davidfirebaugh@...>
                > To: bigdob@yahoogroups.com
                > Sent: Fri, April 16, 2010 1:08:59 PM
                > Subject: [bigdob] Re: Star size (apparent)
                >
                >  
                > Im rural but i could probalby find someone in my old club Stckton Astronomical Society to test the mirror. My zip in 95423 ,the boonies.
                >
                > Though would rather drive than ship my mirror. if i ship it id probalby ask Swaze to test it ,or refigure -then go with enhanced coatings afterwords.I do have back up scopes 13" ect.
                >
                > --- In bigdob@yahoogroups. com, "Bratislav" <bratislav3162@ ...> wrote:
                > >
                > > > Commnets or suggestions?
                > >
                > > Have the primary tested.
                > >
                >
                >
                >
                >
                >
                >
                >
                > [Non-text portions of this message have been removed]
                >
              • davidfirebaugh@ymail.com
                ... D.
                Message 7 of 16 , Apr 21 7:55 AM
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                  >>>>> Thanks Duane ,I will likely ship it. I will call them ..Thanks for connection
                  D.

                  --- In bigdob@yahoogroups.com, Duane Balch <geolgst3807@...> wrote:
                  >
                  > Google on John Lightholder for Lightholder Optics.  He can test your mirror and refigure it if needed.  He's out of South Lake Tahoe, so reasonably close if you drove there.  Give them a call!
                  > Duane
                  > Sacramento
                  >
                  >
                  >
                  >
                  > ________________________________
                  > From: "davidfirebaugh@..." <davidfirebaugh@...>
                  > To: bigdob@yahoogroups.com
                  > Sent: Fri, April 16, 2010 1:08:59 PM
                  > Subject: [bigdob] Re: Star size (apparent)
                  >
                  >  
                  > Im rural but i could probalby find someone in my old club Stckton Astronomical Society to test the mirror. My zip in 95423 ,the boonies.
                  >
                  > Though would rather drive than ship my mirror. if i ship it id probalby ask Swaze to test it ,or refigure -then go with enhanced coatings afterwords.I do have back up scopes 13" ect.
                  >
                  > --- In bigdob@yahoogroups. com, "Bratislav" <bratislav3162@ ...> wrote:
                  > >
                  > > > Commnets or suggestions?
                  > >
                  > > Have the primary tested.
                  > >
                  >
                  >
                  >
                  >
                  >
                  >
                  >
                  > [Non-text portions of this message have been removed]
                  >
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