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elrico

F6.3 FR

12 posts in this topic

Does anyone know the correct spacing to get 6.3 using the Celestron 6.3 FR , 9.25 Celestron evo and the Atik Infinity Colour

thanks 

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a good start would be to position the CCD chip 110mm behind the reducer.

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The attached Pdf. is written around the MallinCam-line of video-cams, but the principles it touches upon hold true across-the-board:

Focal Reduction For Dummies.pdf

Have a good read and take it slow - you'll get there.

Dave

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Roughly 110 mm

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Elrico,

Here's a handy calculator that will give you a starting point for focal reducer spacing.  For the 9.25 SCT use the moving mirror equation.

http://www.wilmslowastro.com/software/formulae.htm#FR

I get 105mm for no spacing between the FR and scope.  If the FR is spaced out from the scope, the effective focal length changes because the mirror moves for focusing.  You can also try different spacings to get different focal reductions.  The 105mm should give you f6.3.

Don

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Don,

I've raised the issue before with WilmslowAstro...their calculator doesn't effectively take into account the changing focal length/ focal ratio when the reducer is "moved".

I have a spreadsheet which does include this factor.

 

reducers_V2b.zip

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52 minutes ago, Merlin66 said:

Don,

I've raised the issue before with WilmslowAstro...their calculator doesn't effectively take into account the changing focal length/ focal ratio when the reducer is "moved".

I have a spreadsheet which does include this factor.

 

reducers_V2b.zip

The calculator has an input for scope to FR spacing.  Are you saying it's not accurate or has the calculator been updated?

Don

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Don,

As the back focus distance changes so does the focal length of a SCT. This means that the final focal ratio will change.

The Wimslow formula did not accurately take this into account.

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Is the back focus of the infinity 13mm deducted from the  suggested spacing, ie: remove Visual back, screw on 6.3 FR, T adaptor spacer, infinity.

eg 105mm from FR to infinity Chip or deduct 13mm from the 105mm ??

Just asking haha

Many thanks

Edited by elrico

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Elrico,

I think what's important is that you get the FOV and image scale that you want.  That will be a function of your sensor size (11mm diagonal for the Infinity) and effective focal length.  Try to get 105mm from the back of the focal reducer to the sensor.  With an SCT, you can screw the FR directly to the rear threads of the scope.  Test it out and put the image in astrometry.net and get the FOV.  Once you get that, there are programs that will give you the focal length you have based on the sensor size.  In any case, if it gives you the image scale you want then you have a winning setup.  If you want something wider (shorter FL), you need more spacing, and if narrower (longer FL), less spacing.  The wider you go, the more likely you will run into aberrations and vignetting.

Hope this helps.

Don

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