Q. Does effective length include threads?
A. One of the benefit of using Build-An-Adapter is there is no need to consider threads or other connection features to calculate a custom adapter effective length. Build-An-Adapter does take care of these details and creates a part with appropriate dimensions such that the finished adapter optical length ends up exactly as specified. The 2D adapter drawing displayed on the Build page shows precisely where the adapter effective length is measured from.
If one wishes to dig deeper into the matter, here are the ugly details: conventional male features (typically threads) do not contribute to effective length while conventional female features are an integral part of the effective length.
But unfortunately there are exceptions to this seemingly simple rule, most notably for SCT rear cells which present a male thread to mount a visual back or an imaging system. The length of this SCT thread is NOT included in the length of any adapter designed for one of these telescope rear cell. Or in other words, the back-focus on a SCT is always measured from the TOP of the male thread at the rear of the scope. This is the reference point SCT manufacturers use to measure focal distance. The above rule is also applicable to Meade ACF and Celestron EdgeHD instruments, as well as a few other, less common, optical devices. Although this is clearly not the conventional way to measure back-focus, Build-An-Adapter is smart enough to take these peculiarities into account when calculating an adapter actual length, so you do not need to.
The bottom line is that ONLY the desired NET adapter optical length needs to be specified in Build-An-Adapter. This is precisely the definition of the term Effective Length.
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A. One of the benefit of using Build-An-Adapter is there is no need to consider threads or other connection features to calculate a custom adapter effective length. Build-An-Adapter does take care of these details and creates a part with appropriate dimensions such that the finished adapter optical length ends up exactly as specified. The 2D adapter drawing displayed on the Build page shows precisely where the adapter effective length is measured from.
If one wishes to dig deeper into the matter, here are the ugly details: conventional male features (typically threads) do not contribute to effective length while conventional female features are an integral part of the effective length.
But unfortunately there are exceptions to this seemingly simple rule, most notably for SCT rear cells which present a male thread to mount a visual back or an imaging system. The length of this SCT thread is NOT included in the length of any adapter designed for one of these telescope rear cell. Or in other words, the back-focus on a SCT is always measured from the TOP of the male thread at the rear of the scope. This is the reference point SCT manufacturers use to measure focal distance. The above rule is also applicable to Meade ACF and Celestron EdgeHD instruments, as well as a few other, less common, optical devices. Although this is clearly not the conventional way to measure back-focus, Build-An-Adapter is smart enough to take these peculiarities into account when calculating an adapter actual length, so you do not need to.
The bottom line is that ONLY the desired NET adapter optical length needs to be specified in Build-An-Adapter. This is precisely the definition of the term Effective Length.
show full Table of Content