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Eggs or potato stars with my GTF81

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  • dan_kordella
    Having issues with my GTF81 where I get egg-shaped stars where the long-axis is consistently in the same direction. My imaging gear is an Atik 383l camera, a
    Message 1 of 11 , Dec 30, 2013
    Having issues with my GTF81 where I get egg-shaped stars where the long-axis is consistently in the same direction. My imaging gear is an Atik 383l camera, a Starlight Xpress filter wheel. I'm now using a Hotech SCA t-adapter. With this gear,my draw tube is racked out about 7cm. The mount I'm using is a Celestron CGEM and use the all-star polar alingment routine. I do not get field rotation with long exposures, so I don't suspect polar alignment to be my issue here. 

    I'm trying to get an off-axis guider into the train, but am having some unrelated issues.

    I've attached a couple recent pics with the above gear being used, with crops of the stars. They were taken on different nights, but show pretty consistent results. 

    Any suggestions as to what may be at issue here? Thanks!


  • timm_bottoni
    Hi - looks like periodic error to me. If you aren t guiding at all and all the stars have the long axis blur in the same way (most likely aligned with the RA
    Message 2 of 11 , Dec 30, 2013
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      Hi - looks like periodic error to me.  If you aren't guiding at all and all the stars have the long axis blur in the same way (most likely aligned with the RA axis) then you are seeing typical imperfections that exist in the tracking of the mount.   

      How long were these exposures?

      Timm


    • Nelson Viegas
      Hi there! Not 100% sure, but that really looks like a guiding issue, but in you message is not clear if you re actually guiding or not. Either way, that looks
      Message 3 of 11 , Dec 30, 2013
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        Hi there!

        Not 100% sure, but that really looks like a guiding issue, but in you message is not clear if you're actually guiding or not. Either way, that looks like a guiding issue or, if unguided, a typical effect of the R.A. axle periodic error.

        Best regards,
        Nelson Viegas


        2013/12/30 <dkordella@...>
         
        [Attachment(s) from dkordella@... included below]

        Having issues with my GTF81 where I get egg-shaped stars where the long-axis is consistently in the same direction. My imaging gear is an Atik 383l camera, a Starlight Xpress filter wheel. I'm now using a Hotech SCA t-adapter. With this gear,my draw tube is racked out about 7cm. The mount I'm using is a Celestron CGEM and use the all-star polar alingment routine. I do not get field rotation with long exposures, so I don't suspect polar alignment to be my issue here. 


        I'm trying to get an off-axis guider into the train, but am having some unrelated issues.

        I've attached a couple recent pics with the above gear being used, with crops of the stars. They were taken on different nights, but show pretty consistent results. 

        Any suggestions as to what may be at issue here? Thanks!



      • dan_kordella
        Hi Timm, thanks for getting back to me. These exposures were all 10 minutes, I believe. I ve been using a guidescope for these, is it reasonable to think that
        Message 4 of 11 , Dec 31, 2013
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          Hi Timm, thanks for getting back to me. These exposures were all 10 minutes, I believe. 


          I've been using a guidescope for these, is it reasonable to think that an OAG would cure this if that's all it is? Down the road a hypertune may be in order, as well.

        • dan_kordella
          Sorry if I wasn t clear - these exposures were guided exposures. I am using a QHY5l-II mono camera in a WO 50mm guidescope.
          Message 5 of 11 , Dec 31, 2013
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            Sorry if I wasn't clear - these exposures were guided exposures. I am using a QHY5l-II mono camera in a WO 50mm guidescope. 

          • timm_bottoni
            Hi Dan, Thanks for clarifying. Well you have a good setup, and they look like mine, to be honest, and I have been using a WO 50mm guidescope with an old Meade
            Message 6 of 11 , Dec 31, 2013
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              Hi Dan,


              Thanks for clarifying.  Well you have a good setup, and they look like mine, to be honest, and I have been using a WO 50mm guidescope with an old Meade DSI as my auto guiding camera, on my GTF102.  I need to experiment and try different settings in PHD, but from what I have read it is more art than science to get all the settings correct.  I have an iOptron iEQ45 and when it warms up again (its a bit too cold in Chicago in Winter for me to get the laptop outside) I'm going to spend time just trying different settings until I get my stars round.  I believe my settings that are over correcting, so I'm, going to scale back some of the options to see if I can come up with the right choices.  


              What software are you using to autoguide?   There is a really good Yahoo group called "stark-labs-astronomy" and there is another called "autoguiding", plus I'm sure there is a group or two for your mount.  If you orient your camera so that it had the up-down direction exactly in line with the RA movement and then look at your pictures, you will likely just see that the movement is in the RA direction.  If it't not in that direction, then you have a different problem.


              Hope that help, and Happy New Year!


              Timm

            • drollain
              Dan, if you scale back you exposure to a couple seconds, and you still have eggy stars, its probably in the alignment of your optical train... just my 2¢
              Message 7 of 11 , Jan 1, 2014
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                Dan, 

                if you scale back you exposure to a couple seconds,  and you still have eggy stars,  its probably in the  alignment of your optical train...

                just my 2¢
              • drollain
                also shut you mount down... if star trails go in one direction, and are eggy in another direction, its alignment for sure...
                Message 8 of 11 , Jan 1, 2014
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                  also shut you mount down...  if star trails go in one direction,  and are eggy in another direction, its alignment for sure...
                • dan_kordella
                  Thanks all for the suggestions. I am using PHD to autoguide, mostly out of habit. I use MaximDL to capture and do some processing, but haven t yet tried using
                  Message 9 of 11 , Jan 1, 2014
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                    Thanks all for the suggestions. I am using PHD to autoguide, mostly out of habit. I use MaximDL to capture and do some processing, but haven't yet tried using it to guide. 


                    Next clear night I get I'll try out some of the suggestions here and post an update. 

                  • jubeis_astro
                    Dan, All that looks like flexure. I experienced the same problems until I switched to an OAG. If you have a sensitive autoguider, go for an OAG, you will not
                    Message 10 of 11 , Jan 9, 2014
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                      Dan,

                      All that looks like flexure. I experienced the same problems until I switched to an OAG. If you have a sensitive autoguider, go for an OAG, you will not regret it.

                      Juergen
                    • dan_kordella
                      I ve recently purchased a new OAG/wheel combo (Atik to match my camera), and am also in the process of getting my mount hypertuned. If it s flexure or periodic
                      Message 11 of 11 , Jan 9, 2014
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                        I've recently purchased a new OAG/wheel combo (Atik to match my camera), and am also in the process of getting my mount hypertuned. If it's flexure or periodic error, one way or another I'll get this thing fixed!


                        In the meantime I've decided to bin all my images - and the reduced pixel scale has eliminated the egg-shaped stars. With a KAF-8300 sensor, I've got pixels to spare...

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