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Old 03-12-2007, 04:14 PM
jase (Jason)
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Join Date: Sep 2006
Location: Melbourne, Victoria
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Quote:
Originally Posted by g__day View Post
As to mirror shift and mirror flop - I have (maybe very ignorantly) ignored these because I only need to re-focus my SCT about once every 3 months. I have a Meade motor focus so I never adjust the mirror position with the knob on the back of the SCT. Focus on a carbon fibre tube seems so consistent I have read this as the mirror isn't moving at all so I have no worries... Does not having to re-focus indicate very little mirror shift or flop? Or asked the other way how do I best tell how much mirror shift / mirror flop I have and how to be compensate for it?
There is still focus shift with a carbon fibre tube, be it minimal. This shift typically does not affect the primary mirror riding on the internal baffle. Thus, if you have a secondary focuser (which you do), it is unlikely that this will be contributing to the mirror movement. Its important to note that the mirror can shift/flop by very small amounts depending on where you are pointing the telescope. For example, the shift maybe ~10 pixels when pointing east and ~4 pixels in the west. This of course can impact your pointing model as the flop is not always consistent. During guiding, I'd imagine that there would be minimal (I'd like to say, almost non existent, but in reality...)

How I combated mirror movement (the easy way) was to rack the course focuser all the way out. Thus the primary mirror was snug at the end of the baffle at the rear of the OTA. I then built the imaging train around this with the appropriate spacers so I could reach focus. As luck would have it, I could use the Lumicon GEG, JMI NGF with Robofocus, a 10mm spacer, then finally the STL11k. This combination reached focus exceptionally well. Though I could only operate the GEG in a few focal reducing modes. I think F/4 and F/5.5 if I recall correctly. Now the hard way of reducing movement is drilling holes into the OTA rear cell and inserting three threaded plastic or Teflon screws to push against the mirror. You need to make sure you turn each screw in a methodical fashion otherwise problems will arise (as you can imagine).

Of course, you can not worry about either of these and try auto guiding on axis (SBIG style) or off axis through pick of prism/mirror. The latter seems the most logical step in my opinion. Just connect your current guide camera up to an off axis guider. I'd be interested to see what you experience then. You've already mentioned PE reductions. Make sure you also focus on this. The lower the PE, the less guiding corrections will need to be made.

I think you mentioned focusing. Yes. This is another important piece of the puzzle, though not as much as one would think. Software like MaximDL can find the star centoid through different algorithms (predominately HFD), even if focus is soft. If you have a desire to calibrate manually - you can do this through a MaximDL plugin - http://winfij.homeip.net/maximdl/manualcalibration.html though I doubt you'll need to go this deep. Its important to get a good S/N ratio on your guider output. Hence taking darks for your guider can assist. This will also allow you to guide on much fainter stars.

You'll soon obtain pin point stars once you get all components working harmoniously together. As you put it. Understanding the basics is the easy part, but to make it work efficiently and consistently is difficult.
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