Backfocus calculations for: RC12 - Integra85 focusing rotator - AP CCDT67 reducer - SX OAG - ASI filter wheel - ASI1600 camera. 1) Backfocus and reducer position The GSRC12Truss backfocus is 285 mm from the M117 thread on the tube backside. Source: https://www.teleskop-express.de/shop/product_info.php/language/en/products_id/6512 # tech tab Source: https://www.teleskop-express.de/en/telescopes-4/rc-ritchey-chretien-telescopes-75/ts-optics-gso-12-inch-f-8-ritchey-chretien-astrograph-carbon-truss-tube-6512 # tech tab I assume they mean the 285 mm starts at the beginning of the thread on the OTA side so that if you screw a tube onto it fully the optical distance of the threads is 0. The M117 thread is part of a tip/tilt system. Its outer ring with the M117 thread connects to the OTA with 3xM4 pull bolts, and 3xM4 push bolts. The M117 thread length is 10mm. 3xM4 M117,m,10 -------- --------------- | Tip | | Camera with | | Tilt | | Sensor | -------- --------------- | | ------------ 285 mm The CCDT67 Astro Physics 0.67x Focal Reducer Optimum working distance from M48x0.75 connection thread to camera sensor is 85 mm I assume they mean the 85 mm starts at the beginning of the thread housing. Source: https://www.teleskop-express.de/shop/product_info.php?products_id=4955 # tech tab The focal length of the CCDT67 CCD Telecompressor is 305 mm, a distance of 101 mm gives the stated 0.67x compression (305 - 101) / 305 = 0.67x . Of this 101 mm 16 mm is taken up by the CCDT67 itself, 101 - 16 = 85 mm. Source: https://www.astro-physics.info/tech_support/accessories/photo_acc/telecompressor-techdata.pdf The CCDT67 optical path length is 30 mm. The nodal lies at 30 - 16 = 14 mm as seen from the OTA side. The OTA side is female M48x0.75, the camera side male M48x0.75 . Source: https://www.astro-physics.info/tech_support/accessories/photo_acc/ccdt67.pdf 1/F(reducer) = 1/F(optimal_reducer_backfocus) + 1/F(backfocus_without_reducer) echo "1/(1/(305) - 1/(101))"|bc -l # -151.0 mm, backfocus_without_reducer, measured at the nodal. Subtract the 16 mm that is taken up by the CCDT67 itself, 151.0 - 16 = 135 mm. So at 285 - 135 = 150 mm the backfocus of a 1x reducer would start. With the CCDT67 optical path length of 30 mm the CCDT67 should start at 150 - 30 = 120 mm. 3xM4 M117,m M48,f M48,m ------- ---------- ---------- | Tip | | Reducer| | Camera | | Tilt| | CCDT67 | | | ------- ---------- ---------- | 120 mm | 30 mm | 85mm | | | -------------------------------- 285 mm reduced to 235 mm 2) OTA-Reducer part The Integra85 focuser has 20 mm of male M68x1 threading on the camera side. And 3xM5 screws with hollow collimating bolts or M94x1 female thread on the OTA side. The optical path length is 1 + 59 + 1 + 5 = 66 mm with one 5 mm adapter plate and its 1 mm spacing and 1 mm collimation bolt included, but without the 20mm of male M68x1 thread. In total 66 + 20 = 86 mm. The focus travel is 10 mm. So from 81 - 91 mm. Source: http://www.geminitelescope.com/Manuals/Integra85_manual.pdf The M68a-M48a adapter ring from Baader has 5 mm optical path length distance, + an estimated 7 mm of M68 thread. 12 mm in total. Source: https://www.baader-planetarium.com/en/accessories/adapters-imaging-accessories/m68-zeiss/m68a-to-m48a-reducing-ring.html https://www.teleskop-express.de/shop/product_info.php/info/p7071_TS-Optics-Adapter-with-male-M68x1-thread-and-male-M48x0-75-thread.html The M68i-M68i continuous M68x1 female thread allows the M68a-M48a to connect to the male M68x1 thread of the focuser. Source: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p6940_TS-Optics-M68-System---M68-Umkehr-Adapter-mit-durchgaengigem-M68x1-Innengewinde.html Source: https://www.apm-telescopes.de/en/mechanical-accessories/adapters/foto-adapters/ts-optics-m68-system-conversion-adapter-continuous-m68x1-female-thread.html The length is listed as 12 mm. I assume the optical length is 0 mm if the focuser and M68a-M48a meet. 3xM4 M117,m M117,f 3xM5 3xM5 M68x1,m M68,f M68,m M68,f M68,f M68,m M48,m M48,f ------- --------- ------------ ----------- ----------- ----------- ---------- | Tip | |Adapter| | Focuser | |Extension| | Ring | | Ring | | Reducer| | Tilt| | | | Integra85| |Rings | |M68i-M68i| |M68a-M48a| | CCDT67 | ------- --------- ------------ ----------- ----------- ----------- ---------- | | ?mm | |81-91mm (86) | ?mm | | 0mm | | 12mm | | | | -------------------------------------------------------------------------------- 120 mm TODO: Adapter of unknown optical path length plus extension rings of 22 mm in total to get to 120 mm. There are M68 extension rings of 5, 8, 15 mm at APM. Source: https://www.apm-telescopes.de/en/mechanical-accessories/adapters/foto-adapters.html and of 7.5, 10, 20 mm at Baader. Source: https://www.baader-planetarium.com/en/accessories/adapters-imaging-accessories/m68-zeiss.html?filter_adapters_type=237&p=1&___SID=U&limit=36 3) Reducer-Camera part the SXV-AO-LF optical length is 70 mm. Together with 20 mm of filter wheel this is clearly no option as we have only 85 mm to work with. So the idea is to split the SXV-AO-LF into the AO part and the OAG part, and not use the AO part. The OAG piece of the SXV-AO-LF connects to the AO unit with what looks to be a S70 male connection. Source: diagram on https://www.sxccd.com/product/active-optics-guiding-system-with-usb/ SX has no adapters available (confirmed by Terry), this OAG is intended to only be used with the AO unit. The OAG barrel has no specified optical length. I measure it at 36mm. TS may have an adapter of their Universal Filter Changer line that could fit: Baader UFC S70 M48 Telescope Adapter, female Source: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p9376_Baader-UFC-S70-M48-Teleskopadapter--Innengewinde.html Its optical tube length is 1 mm. If this does not work then I either have to get a custom adapter ring manufactured, or buy a different OAG. I run the ASI1600 camera with an 11mm ring attached. Its combined optical length to the sensor is 17.5 mm. Source: https://astronomy-imaging-camera.com/tutorials/best-back-focus-length-solutions-55mm.html I have another conversion ring between the EFW and the camera+ring of 2mm. The Electronic Filter Wheel (EFW) optical length is 20mm. This leaves 85 - 17.5 - 2 - 20 - 36 - 1 = 8.5 mm of extender rings. 8 mm Extender https://www.teleskop-express.de/shop/product_info.php/info/p10785_TS-Optics-8mm-Extension-with-M48---2--Filter-Thread-and-2--Diameter.html 0.5 mm Fine Tuning ring https://www.teleskop-express.de/shop/product_info.php/info/p6672_TS-Optics-0-5mm-Stainless-Steel-Fine-Tuning-Ring-for-M48-filterthread.html M48,m M48,f M48,m M48,f S70,m S70,f ---------- ------------ --------- -------- --------- -------- -------- ----------- | Reducer| | Extender | | S70 | | SX | | ZWO | | | | 11mm | | Camera | | CCDT67 | | Ring +ft | | ring | | OAG | | EFW | | ring | | ring | | ASI1600 | ---------- ------------ --------- -------- --------- -------- -------- ----------- | | 8.5mm | | 1mm | | 36mm | | 20mm | | 2mm | | 17.5mm | | | -------------------------------------------------------------------------------------- 85mm