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73510RE: Re: [ccd-newastro] Guider/Imager Image Scale relationship help needed

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  • w0fly2
    Oct 23, 2013
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      Thanks Ron,

       It very well could be Diff. Flexure and I'll be looking at the sites you mentioned to see if I can determine how much flexure I have. One thing I'm looking forward to trying is balancing the scope more to the east. Right now I have the image setup balanced fairly evenly.

       But, I'm still a little vague about the large Image Scale ratio between my guider and imager whether it is within normal parameters or not. Does the image scale of the guider even matter when compared to the image scale of the imager?


      ---In ccd-newastro@yahoogroups.com, <ccd-newastro@yahoogroups.com> wrote:

      Actually, it sounds like you are seeing differential flexure: the two scopes are flexing slightly differently. That is a common problem with external guidescopes. Sometime, you can reduce it somewhat by reinforcing various parts of the telescopes, but it can be hard sometimes if the flexure is in an unexpected place, or if something is simply loose.

      What happens is that as you track across the sky, the various points of flexure flex, slightly altering the pointing of each of the scopes. But the flexing is different because the scopes are not identical.

      Sometimes you can simply mount the guide scope more firmly, but if the flexure is in the optics, the focuser assemblies, or some other location it can be a lot harder to remedy.

      Some links to bring you up to speed (I did not read these myself, but they look OK at a glance):


      Ron Wodaski

      On Oct 22, 2013, at 5:44 PM, lobo59@... wrote:


      I'm getting a bit frustrated with my guiding and thought I'd see if I could get some help from this group on how the difference between the Image Scales of the Guider chip and Imager chip effect the ease of guiding.

      I'm using the SBIG STF-8300 attached to an AT106 refractor (f/6.5 FL690mm) as the main imager and the SBIG STi with the Guider Lens kit attached (f/2.8 FL100MM) as the guider camera.

      The Images scale of the STi is 15.3 arcsec/pixel and the Image Scale for the STF is 1.60 arcsec/pixel. Is that too great a difference in Image Scales? What would be considered an ideal Image Scale ratio between the guider and imager? Is the short focal rail of the guider too short?

      I've been fighting with this setup using MaximDL and last night PHD and the graph and numbers in the log data look good, not a lot of wandering around. Even watching the actual guider image, the star hardly moves. But, the stars on my imager subs look like footballs which indicates overshooting but I'm not seeing much help when adjusting aggressiveness, guide correction length etc. I know it could be flexure, but the guider camera seems pretty solid.

      I've read in MaximDL's manual the Image Scale ratio should be no more then 10, which is where I'm at, but I'm wondering if that's too much. What looks like a small move in the guider image is actually a much greater movement in the main imager. 

      If someone wouldn't mind giving me a quick tutorial on the the Guider/Imager Image Scale relationship I'd really appreciate it. (Or, point me to a link that explains it!)

      I'm getting ready to pull the STi and the Guider Kit off the OTA and put the STi in a AT 72ED which has a f/6.5 and a FL of 430mm and an Image Scale of 3.5 arcsec/pixel and see how that works. It would be ring mounted and I'm worried about a greater risk of flexure.

      Thanks in advance for helping out a newbie. I'm really trying hard to get this all figured out so I can take a bunch of subs and then start bugging everyone on how to process them!


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