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Old 23-02-2019, 06:21 AM
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john guy (John)
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info on reducers and off axis guiding

If I was to add a 6.3 focal reducer to an off axis guider, how do I calculate the focal length of the guide scope that would be the main scope that has a focal length of 3556mm f10as as in phd it requests this
reading to be added. all advice much appreciated
regards J G

Last edited by h0ughy; 24-02-2019 at 07:08 AM. Reason: more descriptive
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Old 23-02-2019, 01:15 PM
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billdan (Bill)
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Hi John,

I don't think that strategy will work. The guide cam sensor and the imaging camera sensor must be the same distance from the OAG prism for both cameras to achieve focus.
If you inserted the 0.63 reducer before the guide cam it will push the sensor too far away from the prism and you will not get it into focus.
In theory if you added a spacer in front of the imaging camera that is the same length as the focal reducer you may be able to get both cameras to come into focus. All depends on how much inward focus travel your focuser has.

As to your question the focal length will be 3556mm x 0.63 = 2240.28mm

Cheers
Bill

Last edited by billdan; 23-02-2019 at 01:29 PM.
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Old 23-02-2019, 05:25 PM
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Merlin66 (Ken)
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If the OAG is before the reducer then no change...
If after then work on (0.63 x 3556)
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Old 23-02-2019, 05:33 PM
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billdan (Bill)
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I think I mis-read the question, I thought he only wanted the guide cam behind the reducer and still operate the main imaging camera at F10.
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Old 24-02-2019, 06:29 AM
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john guy (John)
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thanks everyone, that just what I wanted to know
I appreciated your advise
regards J G
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