Re: Back Focus Troubles #Absolute_Encoders


On Jun 1, 2021, at 12:35 PM, Andrew Jones <andjones132@...> wrote:

My second question is, why does Back Focus spacing matter? When using a refractor with a Crayford style focuser, it is not clear why getting precise Back Focus spacing is so important when the camera sensor will be moving in or out to achieve focus anyway.

My understanding is the flattner goes on the focuser drawtube and the camera attaches to the flattner.  At least that is the how the AP and just about every other flattner I’ve seen works… it threads on to the draw tube.  

Ie.  path of the photons is:
Front lens -> OTA -> focuser body -> focuser draw tube -> flattner -> filter wheel -> camera -> sensor. 

The focuser focuses incoming light on the flattner , similar to an eyepiece.  The space between the flattner and the sensor is critical and should be fixed .. and likely/hopefully  not affected by temperature changes. 

The FSQ 106n (which I’ve used) has elements fixed inside the OTA which did not move with the focuser but this was a different optical design than the typical doublet or triplet refractor. 

Flattners and reducers afaik will not deliver consistent FOV over focus changes which would be required as temperature changes. 


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