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Some flexure perhaps?

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  • dan_kordella
    Using my GTF81 with the focuser hefting an Atik 383l+ mono camera, starlight Xpress filter wheel. The total weight is about 3.5 pounds. When I m focused, the
    Message 1 of 7 , Dec 18, 2013
      Using my GTF81 with the focuser hefting an Atik 383l+ mono camera, starlight Xpress filter wheel. The total weight is about 3.5 pounds. When I'm focused, the draw tube is racked out about 7 cm and I've got some egg-y stars in a set of 10-minute exposures. The are egg-y in the horizontal axis - does this sound like flexure? Any suggestions? I'm not using an OAG at the moment, but I do have one. I just haven't had decent enough weather to try out any possible fixes.
    • drollain
      I had the same issue with an 81GTF on my first light with that scope and a QHY12 OSC... had eggy stars in a 5 min exposures with tracking dead on... ended up
      Message 2 of 7 , Dec 18, 2013
        I had the same issue with an 81GTF on my first light with that scope and a QHY12 OSC... had eggy stars in a 5 min exposures with tracking dead on...  ended up reducing my exposure to just a few seconds to check, and noticed they were still eggy...   I ended up using a straightening ring that came with the camera and that took care of it...  2nd time I had the scope and camera out I did it again without the ring,  but made sure everything was in good,  and tight.  I had no issues the 2nd time.  I chalked it up to the camera adapter not being tight enough the first time.  (rookie mistake for me) 
      • dan_kordella
        Thanks for the response! I m using the nose-piece for the Atik camera inserted into the focuser, you think it s maybe drooping a bit? It is kind of short and I
        Message 3 of 7 , Dec 19, 2013
          Thanks for the response! I'm using the nose-piece for the Atik camera inserted into the focuser, you think it's maybe drooping a bit? It is kind of short and I could look into a beefier T-adapter. But - if it was droop, wouldn't that cause different regions in the frame to be slightly in/out of focus?  
        • drollain
          I use the Hotech SCA... http://www.telescopes.com/telescope-accessories/photographic/hotech2inchscatadapter.cfm I like it a lot and this was the first time I
          Message 4 of 7 , Dec 19, 2013
            I use the Hotech SCA... http://www.telescopes.com/telescope-accessories/photographic/hotech2inchscatadapter.cfmI like it a lot and this was the first time I ever had an issue... mainly due to first time out with equipment... the SCA centers the adapter in the drawtube... just make sure you tighten it good... =)
          • dan_kordella
            Sorry - more confused now. Are you saying you are using a flattener WITH the GTF81? Doesn t that defeat the purpose of the built-in flattener? Or did you
            Message 5 of 7 , Dec 19, 2013

              Sorry - more confused now. Are you saying you are using a flattener WITH the GTF81? Doesn't that defeat the purpose of the built-in flattener? Or did you remove the flattening elements? 


              Additionally, what gear do you have in the optical train? I've run into an issue with spacing when I've tried using other telescopes without built-in flatteners. For example, I've found that with a Televue TRF-2008 the backfocus is supposed to be 56mm, but I only got a flat-field when I had a total of 46mm backfocus (my camera is 17mm and my filter-wheel is 29mm). Ideally I want to fit an OAG into my optical train, and I have a Starlight Xpress thin OAG (with 10mm backfocus), but with the Televue, for example, this produced some very noticeable field curvature (in two different scopes, both ED-doublets from Stellarvue and Sky Watcher, haven't tested in a triplet system but I wouldn't think the results would be that different). 


              Anyway, this is why I went with the GTF81 - to fit an OAG into the train and to not have to worry about spacing requirements for a stand-alone flattener or reducer. But - if you are adding an additional flattener, why use the GTF81?

            • drollain
              Dan, Not to confuse issues with facts... I use to use a Hotech FF with T-Adapter the SCA on a different scope yes... But like you said, the WO 81GTF has
              Message 6 of 7 , Dec 20, 2013

                Dan,


                Not to confuse issues with facts...  I use to use a Hotech FF with T-Adapter the SCA on a different scope yes...   But like you said,  the WO 81GTF has the FF built in.  


                That being said,  because I like the Hotech SCA FF/T-adapter so much,  I purchased the T-Adapter only  (still with the SCA)   So what I am referencing nwith the WO 81GTF is the Hotech SCA T-Adapter only...  NOT THE FF version.  


                To sum it up,  I said that I had eggy stars even at images of just a couple seconds.  I realigned the adapter, tightened down the SCA on the t-adapter, that took care of my issues.



                look at this again...   http://agenaastro.com/hotech-2-sca-t-adapter.html   



                its t-adapter only not the FF version...  


                Kind Regards,

                -Doug



                ---In WILLIAM-OPTICS@{{emailDomain}}, <dkordella@...> wrote:

                Sorry - more confused now. Are you saying you are using a flattener WITH the GTF81? Doesn't that defeat the purpose of the built-in flattener? Or did you remove the flattening elements? 


                Additionally, what gear do you have in the optical train? I've run into an issue with spacing when I've tried using other telescopes without built-in flatteners. For example, I've found that with a Televue TRF-2008 the backfocus is supposed to be 56mm, but I only got a flat-field when I had a total of 46mm backfocus (my camera is 17mm and my filter-wheel is 29mm). Ideally I want to fit an OAG into my optical train, and I have a Starlight Xpress thin OAG (with 10mm backfocus), but with the Televue, for example, this produced some very noticeable field curvature (in two different scopes, both ED-doublets from Stellarvue and Sky Watcher, haven't tested in a triplet system but I wouldn't think the results would be that different). 


                Anyway, this is why I went with the GTF81 - to fit an OAG into the train and to not have to worry about spacing requirements for a stand-alone flattener or reducer. But - if you are adding an additional flattener, why use the GTF81?

              • dan_kordella
                Ah - okay! I ll definitely give that a try, then - thanks!
                Message 7 of 7 , Dec 20, 2013

                  Ah - okay! I'll definitely give that a try, then - thanks!

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