Back Focus Troubles #Absolute_Encoders

Andrew J

I could use some help and guidance to better understand Back Focus.


The first question relates to the impact Filters have on Back Focus. For several years I have been trying to get a definitive answer on the how adding filters to an imaging chain affects the Back Focus calculation and in turn how this impacts the length of the spacers required to achieve the recommended Back Focus for a particular scope configuration. For example, I have TEC140ED with Field Flattener that requires 85mm of Back Focus. I also have Chroma filters, that requires an additional 1mm of Back Focus per the manufacture. To keep the math easy, let’s assume my Camera and Filter Wheel (without filters) consume 35mm of the Back Focus. When I add the Chroma Filters to the equation, do I need a 49mm (85 – 35 – 1) or 51mm (85 – 35 + 1) spacer to achieve optimal Back Focus? This would seem like an easy thing to answer, but I just got off the phone with telescope and eyepiece manufacturer, and depending on who I talked to I got completely opposite answers.


My understanding is that when they say the filter adds 1mm, it means just that, it ADDS 1mm to the Total Back Focus between the OTA and the camera sensor. Sticking with the example above and the 85mm of native Back Focus, adding Chroma filters to the optical path would changes the Total Back Focus from 85mm to 86mm. If correct, then the answer to the question above would mean I need a 51mm spacer (85mm native BF + 1mm for filter = 86mm Total BF; 86mm Total BF – 35mm Cam/FW = 51mm spacer). Hoping the knowledgeable experts here can confirm if my understanding is correct.



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. Referring back to the example above, assume instead of the 51mm spacer I went completely nuts and added a 41mm spacer instead so that my Back Focus spacing was 10mm short. Assuming I had 10mm of travel available on my focuser, what would be the difference between having the 51mm spacer and the focuser racked to say 49mm to achieve focus vs. having a 41mm spacer and the focuser racked to 59mm to achieve focus? It is the same 100mm distance from the back of the OTA to the camera sensor either way. So why does it matter which spacer is use? I have spent a fair amount of money over the years with Precise Parts in an attempt to precisely achieve the Back Focus recommendations for various scope configurations. I never really stopped and asked why this is necessary. Hopefully someone here who understands optics can reassure me that the money spent on getting the “optimal” back focus was a worthwhile cause.


Thanks in advance for the education.


Andrew J

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