Re: Back Focus Troubles #Absolute_Encoders

Andrew J

Hi Jeff.

You are right, I picked a bad example. I remembered after my initial post that the FF on the TEC140ED moves with the focuser and because it is the last correcting element before the sensor means that the distance between this element and the sensor will not change when the focuser is repositioned. So, again bad example.  My understanding is that the back focus is determined by the LAST correcting element in the light path before the camera sensor. Therefore, I should have used the example of the TEC140ED without the FF which I think has a recommended BF of 170mm.

Another example I am familiar with is the Celestron EDGE HD scopes that have a correcting element in the baffle tube of the scope. Celestron states in their Edge HD White Paper that back focus for the EDGE HD 925, 1100, 1400 is 146.05mm. However, it is fairly common for people to add Crayford focusers to the back of the EDGE HD scopes and lock the primary mirrors. In this case the last correcting element does not move with the focuser and distance between the last correcting element and the sensor will change as the focuser is repositioned. In examples where the last correcting element does not move, why is dialing in a specific back focus distance using spacers so critical if the distance is going to change depending on the position of the focuser?


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