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66524Re: What Does Differential Flexure Look Like?

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  • sc02492
    Jan 1, 2010
      Gordon, this is consistent with differential flexure. As the mount moves over the course of your 5 exposures, the direction of stress that is being imposed on the cameras and optical assembly changes, hence the changing direction of the elongation. Although your images suggest a stepwise change, the changing direction is almost certainly gradual and is difficult to appreciate on one sub, but can be seen easily in your time lapse series.

      There is movement between your guide camera and guidescope, between your guidescope and main OTA, between your OTA and imaging camera, or some combination of the above. It could be due to a number of things, including:

      1. guidescope that is moving with respect to the OTA, because of less than ideal mounting, and/or because the guidescope is too heavy in relation to the mounting.
      2. guidescope camera that is moving with respect to the guidescope itself, because of focuser tube play.
      3. main OTA assembly deforming as the mount progresses from east to west, resulting in changing relationships between the various optical elements within the tube, and/or changing relationship between the guidescope and OTA (if your guidescope is mounted directly onto the OTA, for instance).
      4. imaging camera that is moving with respect to the OTA, due to focuser tube play.
      5. internal movement of an optical component like a mirror.

      Common things being common, my guess is that it will be related to #1, #2, or #4. If you show us some photos of your set up, it may be possible to help you troubleshoot this, although more than likely it will be a process of trial and error. Also, what is the focal length of your imaging system, and what kind of scope are you using?

      Steve


      Steve Cannistra
      http://www.starrywonders.com

      --- In ccd-newastro@yahoogroups.com, "GordonM" <gmandell@...> wrote:
      >
      > Friends.
      >
      > I found some interesting data and was hoping it could shed some light on my star elongation problem.
      >
      > Follow this link to a GIF animation that is composed of 5, 1200 second, consecutive, guided light frames obtained using a TOA-130 apo refractor with reducer, Astrodon 3nm OIII filter and Starlight Xpress SXVF-H9 camera. The images were aligned in MaxIm DL.
      >
      > http://tinyurl.com/yle2qsu
      >
      > As you can see, not only are the stars in the individual frames elongated but the angle of elongation also changes between exposures.
      >
      > Any additional comments?
      >
      > Thank you.
      >
      > Gordon Mandell
      >
      > --- In ccd-newastro@yahoogroups.com, "mandellgl" <gmandell@> wrote:
      > >
      > > Hi Steve.
      > >
      > > Thank you for your comments.
      > >
      > > From what you say, differential flexure is probably the cause. I am using a separate guidescope. The guidescope camera and imaging cameras are aligned (as well as possible) with the RA and Dec axes, so if guiding errors were the cause of the elongation, I would see the elongation primarily in the x or y axes. What I see is uniform elongation over the entire field and in a diagonal direction.
      > >
      > > Here is a link to a image showing the star elongation. It is a 1200 sec, guided subexposure.
      > >
      > > http://tinyurl.com/yd9c5ke
      > >
      > > Gordon Mandell
      > >
      > >
      > > --- In ccd-newastro@yahoogroups.com, "sc02492" <sc02492@> wrote:
      > > >
      > > > Gordon,
      > > >
      > > > Field rotation would show an arc-like pattern, worse at the periphery, and centered around the location of the guide star (which could be located outside of your FOV).
      > > >
      > > > Field curvature would show up as poorer focus at the periphery of the field, as opposed to the center (assuming that you used a central star to focus). This may not always reveal itself as "less tight" round stars, however. It may start to show optical aberrations at the periphery of the field (such as coma or astigmatism), that are only revealed by the fact that stars in the periphery are out of focus due to the effects of field curvature. In such cases, the effect would still be more prominent in the periphery and would not be uniform throughout the FOV.
      > > >
      > > > Guide errors can cause uniform elongation throughout the FOV, that would be worse with longer exposures, as in your case. However, the tell-tale sign of such errors is that they would generally be in the direction of the RA axis. I am over-simplifying, because it's possible to get guiding errors in DEC, but assuming that your polar alignment is good, and that you do not have major issues with backlash, then most guiding errors reveal themselves in RA.
      > > >
      > > > Finally, there is differential flexure. This in my experience is the most vexing problem to correct. You would see elongation that is uniform throughout the FOV and worse the longer the exposure, and it would not necessarily conform to the RA or DEC axis. If your elongation is like this (i.e., not conforming to the RA or DEC axes), and if you are using a system prone to differential flexure (for instance, a long focal length with a separate guidescope, especially an SCT with the potential for mirror flop, etc.) then this is a distinct possibility. Tracking down the source of flexure can be very difficult, if not impossible, since you don't need much flexure to ruin a relatively long sub.
      > > >
      > > > One of the advantages of the SBIG self guiding system, with a separate guidechip built into the camera, is that differential flexure is eliminated. The other approach is to use an off axis guider. If you are using either of these guiding systems, then differential flexure is unlikely. Otherwise, it is something to consider.
      > > >
      > > > Steve
      > > >
      > > >
      > > > Steve Cannistra
      > > > http://www.starrywonders.com
      > > >
      > > >
      > > >
      > > > --- In ccd-newastro@yahoogroups.com, "GordonM" <gmandell@> wrote:
      > > > >
      > > > > Hello Friends.
      > > > >
      > > > > What does differential flexure look like compared to guiding errors, field rotation from poor polar alignment or field curvature?
      > > > >
      > > > > I am getting star elongation that is uniform over the entire field, although the angle of elongation changes from one exposure to the next. It decreases in magnitude as the exposure times decrease. Any guided exposures less than 720 seconds are fine.
      > > > >
      > > > > Any help would be appreciated.
      > > > >
      > > > > Thanks in advance.
      > > > >
      > > > > Gordon Mandell
      > > > >
      > > >
      > >
      >
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