Loading ...
Sorry, an error occurred while loading the content.

Still not guiding well enough (advanced newbie)

Expand Messages
  • basscentric
    Hello . . . After doing my best to balance my scope, reduce/ eliminate cable pull, reduce flexure between scope and guider, and twiddling PHD Guiding s
    Message 1 of 8 , Mar 20, 2014
    • 0 Attachment

      Hello . . .

       

      After doing my best to balance my scope, reduce/ eliminate cable pull, reduce flexure between scope and guider, and twiddling PHD Guiding's parameters, I've reduced my tracking error RMS value to the 0.05-0.06 range. With all that, way more often than not, I am unable get perfectly round stars, even with 2 minute exposures. The attached image is a screencap of a Nebulosity/ PHD Guiding session that 1) highlights 3 oval stars and 2) displays PHD’s guiding error graph (which displays the RMS value for the past 50 values and smoothness of the tracking error plot). That image can be found in the album, “Avi’s Images”.

       

      My setup is as follows:

       

      My imaging assembly consists of a Stellarvue triplet APO (80mm aperture, 480mm focal length, f/6), a Stellarvue field flattener, and a SBIG STF8300M imager, whose chip consists of 5.4u pixels. The imaging scale works out to 2.32”/ pixel.

       

      My guiding assembly consists of an Orion guide scope (50mm aperture, 162mm focal length, f/3.2), an Orion StarShoot AutoGuider, whose chip consists of 5.2u pixels. The imaging scale works out to 6.61”/ pixel.

       

      The above are mounted on my Vixen SXD2 mount.

       

      This is where I step on thin ice - An “average” guiding error, as measured by my guiding assembly, of  0.06 pixel, translates to about a 0.17 pixel “average” guiding error on my imaging assembly. That seems to me to be good tracking. Why then am I recoding, way more often than not, non-round stars?

       

      Any help by members of this forum would be greatly appreciated.

       

      - Avi

    • Ron Wodaski
      If you want an accurate picture of how guiding affects your star shapes, take your guiding data and plot it as an X-Y scatter plot instead of as individual
      Message 2 of 8 , Mar 20, 2014
      • 0 Attachment
        If you want an accurate picture of how guiding affects your star shapes, take your guiding data and plot it as an X-Y scatter plot instead of as individual lines for each axis. (Excel can do this for you; the exact method you use depends on the format of your guiding data.)

        That is the place to start the analysis, in my experience. The X-Y plot will show you the effective star shape to expect from the guiding. 

        If the scatter plot is round, but your stars are still elongated, then you typically have differential flexure issues between your main and guide scope. Given that you are using an external guide scope, I suspect that's exactly what's going on. 

        Ron Wodaski






        On Mar 20, 2014, at 4:26 PM, basscentric@... wrote:

         

        Hello . . .

         

        After doing my best to balance my scope, reduce/ eliminate cable pull, reduce flexure between scope and guider, and twiddling PHD Guiding's parameters, I've reduced my tracking error RMS value to the 0.05-0.06 range. With all that, way more often than not, I am unable get perfectly round stars, even with 2 minute exposures. The attached image is a screencap of a Nebulosity/ PHD Guiding session that 1) highlights 3 oval stars and 2) displays PHD’s guiding error graph (which displays the RMS value for the past 50 values and smoothness of the tracking error plot). That image can be found in the album, “Avi’s Images”.

         

        My setup is as follows:

         

        My imaging assembly consists of a Stellarvue triplet APO (80mm aperture, 480mm focal length, f/6), a Stellarvue field flattener, and a SBIG STF8300M imager, whose chip consists of 5.4u pixels. The imaging scale works out to 2.32”/ pixel.

         

        My guiding assembly consists of an Orion guide scope (50mm aperture, 162mm focal length, f/3.2), an Orion StarShoot AutoGuider, whose chip consists of 5.2u pixels. The imaging scale works out to 6.61”/ pixel.

         

        The above are mounted on my Vixen SXD2 mount.

         

        This is where I step on thin ice - An “average” guiding error, as measured by my guiding assembly, of  0.06 pixel, translates to about a 0.17 pixel “average” guiding error on my imaging assembly. That seems to me to be good tracking. Why then am I recoding, way more often than not, non-round stars?

         

        Any help by members of this forum would be greatly appreciated.

         

        - Avi



      • Jerry Barr
        how come there is no mention of polar alignment / jus curious   Jerry  KJ6NTL Skcc# 10843 10-10 # 76401 On Thursday, March 20, 2014 4:26 PM,
        Message 3 of 8 , Mar 20, 2014
        • 0 Attachment
          how come there is no mention of polar alignment / jus curious
           
          Jerry
           KJ6NTL
          Skcc# 10843
          10-10 # 76401


          On Thursday, March 20, 2014 4:26 PM, "basscentric@..." <basscentric@...> wrote:
           
          Hello . . .
           
          After doing my best to balance my scope, reduce/ eliminate cable pull, reduce flexure between scope and guider, and twiddling PHD Guiding's parameters, I've reduced my tracking error RMS value to the 0.05-0.06 range. With all that, way more often than not, I am unable get perfectly round stars, even with 2 minute exposures. The attached image is a screencap of a Nebulosity/ PHD Guiding session that 1) highlights 3 oval stars and 2) displays PHD’s guiding error graph (which displays the RMS value for the past 50 values and smoothness of the tracking error plot). That image can be found in the album, “Avi’s Images”.
           
          My setup is as follows:
           
          My imaging assembly consists of a Stellarvue triplet APO (80mm aperture, 480mm focal length, f/6), a Stellarvue field flattener, and a SBIG STF8300M imager, whose chip consists of 5.4u pixels. The imaging scale works out to 2.32”/ pixel.
           
          My guiding assembly consists of an Orion guide scope (50mm aperture, 162mm focal length, f/3.2), an Orion StarShoot AutoGuider, whose chip consists of 5.2u pixels. The imaging scale works out to 6.61”/ pixel.
           
          The above are mounted on my Vixen SXD2 mount.
           
          This is where I step on thin ice - An “average” guiding error, as measured by my guiding assembly, of  0.06 pixel, translates to about a 0.17 pixel “average” guiding error on my imaging assembly. That seems to me to be good tracking. Why then am I recoding, way more often than not, non-round stars?
           
          Any help by members of this forum would be greatly appreciated.
           
          - Avi


        • echesak@flash.net
          Sounds to me like your guiding might be OK. What kind of focuser is on your scope? The stock SV focusers are not up to the task of imaging (speaking from
          Message 4 of 8 , Mar 21, 2014
          • 0 Attachment
            Sounds to me like your guiding might be OK. What kind of focuser is on your scope?   The stock SV focusers are not up to the task of imaging (speaking from experience).  It's possible that your focuser is sagging (as mine was), which can be causing the star problems.  This is even an occasional problem with the 4" focuser on my FSQ-106ED.  If you have a FT or Moonlight focuser, then you might look at slippage or sagging of the camera into the scope interface.  Using a nosepiece is OK, but threaded connections are best.  Does a single sub show the ovaled stars?  Are the stars at the start of the imaging session ovaled the same direction as at the end of the session?  All these should help you ID the problem. As the mount rotates, the camera hangs in a different direction, and the stars are ovaled in a different direction.  This should have been a clue to me, but it was early in my imaging days. 

            Just thought I'd mention that, in case you haven't looked into that.

            Eric
          • basscentric
            Thanks Ron. I ll do the analysis the next chance I get to image. In addition, on the chance that my issue is the result of differential flexure between main
            Message 5 of 8 , Mar 21, 2014
            • 0 Attachment

              Thanks Ron. I'll do the analysis the next chance I get to image.

              In addition, on the chance that my issue is the result of differential flexure between main and guide scope, and because I've always felt it was the weakest component of my setup, today I invested a modest sum of money in upgrading my guide scope and it's rings.

            • basscentric
              I do perform polar alignment using my mount s polar scope. I figure I normally do not err by more than a handful of arc minutes. That should not result in
              Message 6 of 8 , Mar 21, 2014
              • 0 Attachment

                I do perform polar alignment using my mount's polar scope. I figure I normally do not err by more than a handful of arc minutes. That should not result in visible star elongations when taking 2 minute exposures. In any event, until very recently, I had my setup correct for declination drifts.

                 

                Thanks for your interest, Jerry.

                 

                Avi

              • basscentric
                Eric . . . Your sagging camera suggestion sound very promising to me. I think I will quickly test it by taking a few images of an overhead target and then of a
                Message 7 of 8 , Mar 21, 2014
                • 0 Attachment

                  Eric . . . Your sagging camera suggestion sound very promising to me. I think I will quickly test it by taking a few images of an overhead target and then of a target near the horizon. If its the root cause of my problem, the former should show little or no star elongations, whereas the latter should show prominent star elongations. Of course to reach that conclusion, I will also have to make sure that the quality of my guiding remains about the same as I image targets at different parts of the sky.

                   

                  BTW, my scope has the 2.5" Feathertouch focuser and the camera threads into a series of threaded extension tubes.

                   

                  Thank you for your help.

                   

                  - Avi

                • vandoren.robert
                  Hi to all. I m a little late on the subject but my autoguider seems to wander a little when I use a brighter star to guide on, try using a dimmer star in the
                  Message 8 of 8 , Mar 21, 2014
                  • 0 Attachment

                    Hi to all. I'm a little late on the subject but my autoguider seems to wander a little when I use a brighter star to guide on, try using a dimmer star in the field to guide on. Hopefully it's a quick fix that cost nothing.

                    bob

                  Your message has been successfully submitted and would be delivered to recipients shortly.