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[William Optics] Re: GTF-81 Questions

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  • underdriven255
    Ray, Thanks for the clarification. I m not quite sure what to make of what Agena has told me, as it is at odds with your experience... In any case, it seems
    Message 1 of 15 , May 11, 2013
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      Ray,

      Thanks for the clarification. I'm not quite sure what to make of what Agena has told me, as it is at odds with your experience...

      In any case, it seems that the GTF-81 has been designed for AP since the focus point is relatively close to the tube (based on the focal length and the tube length). My calculations are 48mm for the GTF-81 as opposed to 135mm for the Stellarvue SVR90T (in order to accomodate the diagonal). Have you tried using eyepieces and a diagonal with the GTF-81? It would seem that there isn't enough backfocus for that, unless my calculations are wrong (or the specifications are incorrect)...

      Thanks,

      Keith

      > My scope is a little over a month old.
      > It came with the new DDG focu
      > It would hold the weight but there was a lot of play in the barrel and It
      > had a lot of image shift.
      > I expressed this on the forum and William responded to the email and told me
      > to send the old one back
      > and he mailed me a non DDG focuser.
      >
    • Timm B
      Hi, The GTF102 is a larger version of the 5 element design with the built in field flattener. When I bought it, William advised me that I would need a 1.25
      Message 2 of 15 , May 12, 2013
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        Hi,

        The GTF102 is a larger version of the 5 element design with the built in field flattener. When I bought it, William advised me that I would need a 1.25" diagonal for most eyepieces. I didn't have one, so rather than getting the bundle with the 2" photographic tube and Canon T-Ring (which I had already) he gave me a WO 1.25" dielectric diagonal instead (which is very nice).

        All of the eyepieces I have reach focus without problem using the 1.25" diagonal. I have the WO UWANs in 16mm, 7mm, and 4mm, as well as the WO SPLs in 3mm, 6mm, and 12mm. I also have the WO Zoom II and it reaches focus as well. The only 2" eyepiece I have is the WO 33mm SWAN, and it reaches focus in my 2" diagonal.

        This would lead me to conclude that most other eyepieces will reach focus in my scope. I would think that the GTF81 may have been designed in a similar way, but if anyone has one and has experience trying eyepieces, please post a reply.

        The new R&P design of the WO focuser is very nice.

        Thanks,

        Timm

        --- In William-Optics@yahoogroups.com, "underdriven255" <underdriven255@...> wrote:
        >
        > Ray,
        >
        > Thanks for the clarification. I'm not quite sure what to make of what Agena has told me, as it is at odds with your experience...
        >
        > In any case, it seems that the GTF-81 has been designed for AP since the focus point is relatively close to the tube (based on the focal length and the tube length). My calculations are 48mm for the GTF-81 as opposed to 135mm for the Stellarvue SVR90T (in order to accomodate the diagonal). Have you tried using eyepieces and a diagonal with the GTF-81? It would seem that there isn't enough backfocus for that, unless my calculations are wrong (or the specifications are incorrect)...
        >
        > Thanks,
        >
        > Keith
        >
        > > My scope is a little over a month old.
        > > It came with the new DDG focu
        > > It would hold the weight but there was a lot of play in the barrel and It
        > > had a lot of image shift.
        > > I expressed this on the forum and William responded to the email and told me
        > > to send the old one back
        > > and he mailed me a non DDG focuser.
        > >
        >
      • Ray Hurst
        Keith, I have used a 2 inch diagonal with eyepieces from 4mm to 20 mm with no problem. I am not sure how to calculate distance with the built in flattener . I
        Message 3 of 15 , May 12, 2013
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          Keith,

          I have used a 2 inch diagonal with  eyepieces from 4mm to 20 mm with no problem.

          I am not sure how to calculate distance with the built in flattener .

          I am used to making sure the distance from the flattener to image plane is a specified distance.

          I tried a Canon DSLR ,an SBIG8300M and a DMK and all focus just fine with a perfectly flat image.

          I did have to use an extension when I used a barlow but that is usually the case with most telescopes.

          Ray

           

          From: William-Optics@yahoogroups.com [mailto:William-Optics@yahoogroups.com] On Behalf Of underdriven255
          Sent: Sunday, May 12, 2013 12:51 AM
          To: William-Optics@yahoogroups.com
          Subject: [William Optics] Re: GTF-81 Questions

           

           

          Ray,

          Thanks for the clarification. I'm not quite sure what to make of what Agena has told me, as it is at odds with your experience...

          In any case, it seems that the GTF-81 has been designed for AP since the focus point is relatively close to the tube (based on the focal length and the tube length). My calculations are 48mm for the GTF-81 as opposed to 135mm for the Stellarvue SVR90T (in order to accomodate the diagonal). Have you tried using eyepieces and a diagonal with the GTF-81? It would seem that there isn't enough backfocus for that, unless my calculations are wrong (or the specifications are incorrect)...

          Thanks,

          Keith

          > My scope is a little over a month old.
          > It came with the new DDG focu
          > It would hold the weight but there was a lot of play in the barrel and It
          > had a lot of image shift.
          > I expressed this on the forum and William responded to the email and told me
          > to send the old one back
          > and he mailed me a non DDG focuser.
          >

        • underdriven255
          Hi Ray, ... Interesting. How far is the focuser extended when you use a camera? I would think it must be fairly far back if you have enough back focus to use
          Message 4 of 15 , May 12, 2013
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            Hi Ray,

            > I have used a 2 inch diagonal with eyepieces from 4mm to 20 mm
            > with no problem.
            >
            Interesting. How far is the focuser extended when you use a camera? I would think it must be fairly far back if you have enough back focus to use a 2" diagonal...

            > I am used to making sure the distance from the flattener to image
            > plane is a specified distance.
            >
            Yes, I tend to assume that using a reducer or extender is simple--like attaching a teleconverter to a DSLR. But in reality the spacing constraints seem to be pretty tight in order to achieve maximum performance...

            Thanks, Keith
          • underdriven255
            Hi Timm, ... I am familiar with GTF102--it s a bit too big for me, although it is an impressive unit. Looking at the focal length vs. the tube length, the
            Message 5 of 15 , May 12, 2013
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              Hi Timm,

              > The GTF102 is a larger version of the 5 element design with the
              > built in field flattener. When I bought it, William advised me
              > that I would need a 1.25" diagonal for most eyepieces.
              >
              I am familiar with GTF102--it's a bit too big for me, although it is an impressive unit. Looking at the focal length vs. the tube length, the GTF102 seems to have less backfocus than the GTF81, so I'm not surprised that a 2" diagonal isn't supported. It seems to be very oriented to AP...

              Have you tried any reducers with the GTF102? I'm just curious if you have found any that work well with the flat field of the GTF scopes...

              Thanks for the eyepiece info...

              > The new R&P design of the WO focuser is very nice.
              >
              Well, this is something of an enigma to me since Ray has indicated his DDG focuser had to be replaced, and he was sent a non-DDG unit. I have also seen several WO scopes on eBay recently (listed as new) which also have non-DDG focusers. A GTF-81 from Agena would have a DDG focuser, and I'm not sure if I would be getting the best version of the focuser or not. I'm sure WO would take care of it in any case, but I don't like to have doubts when I buy something...

              Thanks, Keith
            • underdriven255
              ... I should have looked closer, as this is actually an Optec unit and therefore is available here in the US at a lower cost: $275 (from their price list).
              Message 6 of 15 , May 12, 2013
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                --- In William-Optics@yahoogroups.com, "underdriven255" <underdriven255@...> wrote:
                >
                > http://www.teleskop-express.de/shop/product_info.php/info/p2181_Optec-NexGen-Ultra-Wide-Field-0-7XL--NGUW--reducer.html
                >
                I should have looked closer, as this is actually an Optec unit and therefore is available here in the US at a lower cost: $275 (from their price list).

                Keith
              • Timm B
                Hi I don t have a flattener to try. Regarding the focusers, the DDG part is an electronic piece, and I have an original one on my 80FD which is a Crayford
                Message 7 of 15 , May 12, 2013
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                  Hi

                  I don't have a flattener to try.

                  Regarding the focusers, the DDG part is an electronic piece, and I have an original one on my 80FD which is a Crayford style. I think the newest R&P design, whether with DDG or without is a better design.

                  Timm

                  --- In William-Optics@yahoogroups.com, "underdriven255" <underdriven255@...> wrote:
                  >
                  > Hi Timm,
                  >
                  > > The GTF102 is a larger version of the 5 element design with the
                  > > built in field flattener. When I bought it, William advised me
                  > > that I would need a 1.25" diagonal for most eyepieces.
                  > >
                  > I am familiar with GTF102--it's a bit too big for me, although it is an impressive unit. Looking at the focal length vs. the tube length, the GTF102 seems to have less backfocus than the GTF81, so I'm not surprised that a 2" diagonal isn't supported. It seems to be very oriented to AP...
                  >
                  > Have you tried any reducers with the GTF102? I'm just curious if you have found any that work well with the flat field of the GTF scopes...
                  >
                  > Thanks for the eyepiece info...
                  >
                  > > The new R&P design of the WO focuser is very nice.
                  > >
                  > Well, this is something of an enigma to me since Ray has indicated his DDG focuser had to be replaced, and he was sent a non-DDG unit. I have also seen several WO scopes on eBay recently (listed as new) which also have non-DDG focusers. A GTF-81 from Agena would have a DDG focuser, and I'm not sure if I would be getting the best version of the focuser or not. I'm sure WO would take care of it in any case, but I don't like to have doubts when I buy something...
                  >
                  > Thanks, Keith
                  >
                • underdriven255
                  Hi Timm, I realize that the DDG is the digital readout part of the focuser, however I am trying to use that in order to differentiate between the original R&P
                  Message 8 of 15 , May 12, 2013
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                    Hi Timm,

                    I realize that the DDG is the digital readout part of the focuser, however I am trying to use that in order to differentiate between the original R&P model and the updated one. I read a post from England where an original R&P focuser needed to be replaced and now Ray has had the same issue--in his case he received a non-DDG replacement which he says is much improved. While it is certainly possible that a focuser with a DDG readout could also contain the "new & improved" hardware, it is worth noting that Ray was sent one without the DDG display. Not that he seems to care that the DDG aspect is missing (and I wouldn't either), it is still something he paid for. My guess is that whatever hardware improvements were done to the R&P mechanism caused the cost to go up, and ditching the DDG display was a way to offset that cost increase. Of course, that is just a guess on my part...

                    It's not that I don't appreciate the situation that these small companies are in--they order a production run of X units, and if there is an issue they are faced with a tough decision on how to fix it. They can upgrade all the scopes and lose money or they can send them out as is and issue replacement focusers when people complain. For visual use the original DDG unit is probably fine--it is only for AP that there is a problem. So I would do exactly what they are doing and ship scopes with the original focuser. But from my perspective I'm not thrilled to buy a telescope that I know will require a focuser swap right away. Yes, Ray got a replacement and I probably would as well if mine had the problem, but I have to take that on faith...

                    Cheers, Keith
                  • Ray Hurst
                    Keith, With a 2 inch diagonal and a 6mm eyepiece the focuser is extended about 15 mm . With a 20 mm is about 20 mm. When I use my SBIG ST8300 and QHY filter
                    Message 9 of 15 , May 12, 2013
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                      Keith,

                       

                       

                      With a 2 inch diagonal and a 6mm eyepiece the focuser is extended about 15 mm .

                      With a 20 mm is about 20 mm.

                       

                      When I use my SBIG ST8300 and QHY filter wheel I use a  2 inch extender and no diagonal.

                      The focuser is extended about 20 MM .

                      It will probably work without the extender but I like using the extender by force of habit.

                      With a DMK I use 2 inch extender with no diagonal. The focuser is extended about 50 mm with this camera.

                       

                      I see a GT81 for sale at Agena.

                      This model comes with an external flattener which can probably be replaced with a reducer flattener if desired.

                      May be this model might work better if you want to lower the F number.

                      Maybe William can comment on this.

                       

                      Ray

                       

                       

                       

                       

                       

                      From: William-Optics@yahoogroups.com [mailto:William-Optics@yahoogroups.com] On Behalf Of underdriven255
                      Sent: Sunday, May 12, 2013 2:31 PM
                      To: William-Optics@yahoogroups.com
                      Subject: [William Optics] Re: GTF-81 Questions

                       

                       

                      Hi Ray,

                      > I have used a 2 inch diagonal with eyepieces from 4mm to 20 mm
                      > with no problem.
                      >
                      Interesting. How far is the focuser extended when you use a camera? I would think it must be fairly far back if you have enough back focus to use a 2" diagonal...

                      > I am used to making sure the distance from the flattener to image
                      > plane is a specified distance.
                      >
                      Yes, I tend to assume that using a reducer or extender is simple--like attaching a teleconverter to a DSLR. But in reality the spacing constraints seem to be pretty tight in order to achieve maximum performance...

                      Thanks, Keith

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