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Re: [ccd-newastro] Mirror shift or other flexure?

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  • Ron Wodaski
    Even if the guide scope were mounted with perfect rigidity, there are still things inside the optical systems that can flex and move. A lens might sag a bit in
    Message 1 of 8 , Apr 27, 2013
      Even if the guide scope were mounted with perfect rigidity, there are still things inside the optical systems that can flex and move. A lens might sag a bit in a cell, for example. As Mike pointed out, it can be quite a difficult process to figure out where flexure is. In the end, it's best to stiffen up anything you can think of right from the start, and see what that does for you.

      Sometimes you have to be quite clever to find a movement. Sometimes, the movement is easy to find - and quite a challenge to fix. :-)

      Ron Wodaski



      On Apr 27, 2013, at 8:27 PM, Pete Su <pete.su@...> wrote:

      > Thanks for all the thoughts. The guide camera is one of the little Orion SSAGs, which does not really weigh anything. And, it is attached with T-threads on to the Borg focuser.
      >
      > It was interesting to me that the drift was exactly in line with the RA axis of the mount. I had more expected that if there were trouble that the motion would be more random. But who knows.
      >
      > Anyway, I like the *idea* of having the big wide field of guide stars to pick from, but I am not really the mechanical type and would probably be willing to just put up with an OAG before taking the telescope tube apart.
      >
      > Borg sells some more rigid tube holders for their small guide scopes, so I'll probably try that next while also sticking to what has been working … I got the used ST-2000XM in the first place because it seemed like the most straightfoward way to get good guiding. Except for occasionally not being able to find a well placed star to guide on this has proven to be a pretty good move.
      >
      > Thanks again
      > Pete
      >
      >
      >
      >
      > ------------------------------------
      >
      >
      >
      >
    • Pete Su
      Yeah, I realize that. For me this is a game of trying the straightforward things and seeing if I get lucky. If so, great. If not, I already have a scheme that
      Message 2 of 8 , Apr 28, 2013
        Yeah, I realize that. For me this is a game of trying the straightforward things and seeing if I get lucky. If so, great. If not, I already have a scheme that is working fairly well, so I will just stick to that.

        Pete

        On Apr 28, 2013, at 12:01 AM, Ron Wodaski <yahoo@...> wrote:

        > Even if the guide scope were mounted with perfect rigidity, there are still things inside the optical systems that can flex and move. A lens might sag a bit in a cell, for example. As Mike pointed out, it can be quite a difficult process to figure out where flexure is. In the end, it's best to stiffen up anything you can think of right from the start, and see what that does for you.
        >
        > Sometimes you have to be quite clever to find a movement. Sometimes, the movement is easy to find - and quite a challenge to fix. :-)
        >
        > Ron Wodaski
        >
        >
        >
        > On Apr 27, 2013, at 8:27 PM, Pete Su <pete.su@...> wrote:
        >
        >> Thanks for all the thoughts. The guide camera is one of the little Orion SSAGs, which does not really weigh anything. And, it is attached with T-threads on to the Borg focuser.
        >>
        >> It was interesting to me that the drift was exactly in line with the RA axis of the mount. I had more expected that if there were trouble that the motion would be more random. But who knows.
        >>
        >> Anyway, I like the *idea* of having the big wide field of guide stars to pick from, but I am not really the mechanical type and would probably be willing to just put up with an OAG before taking the telescope tube apart.
        >>
        >> Borg sells some more rigid tube holders for their small guide scopes, so I'll probably try that next while also sticking to what has been working … I got the used ST-2000XM in the first place because it seemed like the most straightfoward way to get good guiding. Except for occasionally not being able to find a well placed star to guide on this has proven to be a pretty good move.
        >>
        >> Thanks again
        >> Pete
        >>
        >>
        >>
        >>
        >> ------------------------------------
        >
        >
        >
        > ------------------------------------
        >
        >
        >
        >
      • Tim Lahey
        Hi Pete, I had a similar problem with my C8 on an EQ6-Pro mount, using a KWIQ-Guider (from KW-Telescope). The KWIQ-Guider is only mounted with a simple
        Message 3 of 8 , Apr 28, 2013
          Hi Pete,

          I had a similar problem with my C8 on an EQ6-Pro mount, using a KWIQ-Guider (from KW-Telescope). The KWIQ-Guider is only mounted with a simple "stalk" type of holder, although it is one with two rings. The focal length of the little scope is about 175mm. The CCD is a QHY5 mono.

          The stars were always elongated, even with exposures of only a few minutes. I had tried both MaxImDL and PHD.

          Recently I tried again.

          This time, I used PHD, but I reduced the "Min Mo" (Minimum Movement) on the Tracking Graph from a value of something like 0.20 down to 0.05 (I had been using the higher value when autoguiding my TeleVue NP-101).

          I have now been able to achieve round stars with exposures of 10 minutes. I tried pushing it to 20 minutes, and got a bit of elongation.


          If you haven't experimented with the different settings in PHD, I would suggest at least trying the Minimum Movement. You didn't mention any settings in your original post. The fact that your elongation is strictly in the E-W direction suggests to me that the problem isn't flexure or mirror flop (as you also thought, due to the apparently non-random alignment).


          I also found PHD worked better for me than MaxImDL, because it continues to guide during image download, whereas MaxImDL suspends guiding. On my mount, this causes the guide star to wander off enough that it takes several seconds for things to stabilize once guiding resumes. If you have programmed a sequence of images, you have to put in a delay between each, to allow the guider to get back on target.


          I was out last night, and was getting RMS guide errors around 0.08 to 0.12, on targets M51 and M101. The stars were nice and round. FWHM values were around 3 arc-seconds (I don't think the seeing was very good).

          Hope this helps.

          Regards
          Tim Lahey
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