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Camera focus with Fastar

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  • Jack Patterson
    Ron, Your book says that short focal ratio scopes require precise focus and more effort to achieve the best focus, and the NGF-S focuser is recommended for
    Message 1 of 2 , Jul 4, 2000
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      Ron,
      Your book says that short focal ratio scopes require precise focus and more effort to achieve the best focus, and the NGF-S focuser is recommended for fine focusing control on a SCT.  It seems like the f2 Fastar would need critical focus and the NGF-S is probably not useable since the central obstruction would probably increase and the 2+" space for the focuser may not be there.  How did you focus the Fastar with the ST237 camera for the images at your site or what would you recommend for Fastar fine focus control?
      Also, have you used your STV on the Fastar and if you have, how did its imaging compare to your ST237/Fastar results?  Do you plan on doing a STV review sometime?
      Thanks,
      Jack Patterson
    • Ron Wodaski
      You are correct; the NGF-S is of no help in the Fastar configuration. The only way to focus in Fastar mode is with the moving primary mirror. I experienced all
      Message 2 of 2 , Jul 4, 2000
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        You are correct; the NGF-S is of no help in the Fastar configuration. The only way to focus in Fastar mode is with the moving primary mirror. I experienced all the usual problems trying to do this -- I would see focus change on me if the telescope was moved any significant distance, for example; and if I had to change directions with the focuser while focusing on a small area of the chip, the star I was focusing on would typically jump right off of that small area.
         
        As far as focusing changing when you move the scope, this is something you have to live with on an SCT unless you actually lock the mirror down (which, of course, you cannot do in Fastar mode). As far as dealing with reversing focusing direction, it pays to develop a sense of where best focus is so that you don't have to switch directions. This takes some experience, however. Because of the way the mirror moves, "down" occurs when you are turning the focusing knob counterclockwise, so I try to make my focus changes moving the knob counterclockwise. I will deliberately move far enough out of focus on the clockwise side, so that when I move the knob counterclockwise to adjust focus, I start out slightly out of focus and gradually get closer to focus turning the knob counterclockwise.
         
        The first one or two times I am adjusting focus, I will take the time to go past focus turning counterclockwise to get a sense of how good a peak pixel value of FWHM I can get. I then turn clockwise to back past focus, then I give the focusing knob a small turn counterclockwise again to put my star back on the chip. I then move back to focus, trying to achieve the best focus without going past. If I do go past, I bite the bullet and start over, and approach again using counterclockwise turns. This keeps the star on the chip for focusing, and I arrive at focus with the mirror reasonably settled.
         
        I haven't yet tried the STV on the Fastar. The STV in normal mode is binned 2x2, and when binned 1x1 you only get the central 320x240 of the chip. So I prefer to use the ST-237 on the Fastar setup.

        Ron Wodaski
        http://www.newastro.com

        -----Original Message-----
        From: Jack Patterson [mailto:jpatters@...]
        Sent: Tuesday, July 04, 2000 8:05 PM
        To: ccd-newastro@egroups.com
        Subject: [ccd-newastro] Camera focus with Fastar

        Ron,
        Your book says that short focal ratio scopes require precise focus and more effort to achieve the best focus, and the NGF-S focuser is recommended for fine focusing control on a SCT.  It seems like the f2 Fastar would need critical focus and the NGF-S is probably not useable since the central obstruction would probably increase and the 2+" space for the focuser may not be there.  How did you focus the Fastar with the ST237 camera for the images at your site or what would you recommend for Fastar fine focus control?
        Also, have you used your STV on the Fastar and if you have, how did its imaging compare to your ST237/Fastar results?  Do you plan on doing a STV review sometime?
        Thanks,
        Jack Patterson
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