EdgeHD 8″ telescope comes standard with an internal focuser and, if you want to use it for astrophotography, using an external focuser allows you to lock the primary mirror (by using the locking screws provided in telescope rear cell) and prevent mirror shift during astrophotography sessions. More, by adding a rotator allows you to better frame the object and to use this telescope also with alt-azi mounts for deep-sky astrophotography. In this article we’ll see how to use a rotating focuser to EdgeHD 8” also if you want to use it with the dedicated 0.7x reducer.
Our setup is composed of an EdgeHD 8″ telescope with 0.7x reducer: after the reducer, we installed the PL3600542 adapter, then the ESATTO 2″ LP focuser, the PL3600538 adapter, the ARCO 2″ rotator and the PL3600216 camera adapter after the ARCO 2″. This allows us to have, after the ARCO 2″, a M48 male thread, since we’re connecting an imaging train composed of cooled camera, filter drawer and off-axis guider that has, on telescope side, a female M48 thread.
EdgeHD 8″ telescope has a backfocus requirement of 133mm but, when you add the dedicated 0.7x reducer, backfocus is only 105mm from the end of the reducer thread and, considering that the male thread on reducer is 12mm deep and that the PL3600542 adapter fully threads in (it means that the camera side of the reducer “enters” in the ESATTO 2″ LP focuser), it means that we have 12mm backfocus more that we can use in order to accomodate all our instruments.
Since we’re using a cooled camera with 17.5mm distance from sensor to front camera thread, this is our calculation:
PL3600542 adapter: 2.5mm optical thickness
ESATTO 2″ LP focuser with ARCO 2″ rotator: 54.2mm optical thickness
PL3600216 adapter: 4mm optical thickness
OAG, filter drawer and adapters: 37.6mm
Cooled camera: 17.5mm optical thickness
Total optical thickness: 115.8mm
Since we have 105mm + 12mm = 117mm optimal backfocus distance, we can just extract the ESATTO 2″ LP draw tube by the remaining 1.2mm (117 – 115.9) in order to achieve the best backfocus distance.
This allows us to add an ESATTO 2″ LP focuser and ARCO 2″ rotator after the 0.7x reducer of EdgeHD 8″ telescope and then connect an imaging train that may include an off-axis guider, a filter drawer and a cooled camera. Just note that the above calculation is valid for this specific example and, if you have different accessories and/or a different camera, you will have to adjust the backfocus calculation accordingly.