Quote:
Originally Posted by h0ughy
Can't quite get my head around this backfocus distance for the camera the manual says that the adapter is 17.5mm and the camera sensor is 6.5+4 =28
So do I need a spacer to get 29 or doesn't it need it?
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Yes you will need a spacer or new adapter to get the sensor in to the correct location. The
correct location is defined variously as :
= 29mm (28.73mm?) from the TILT COLLAR SURFACE (shown in RED on the exploded view in the detailed drawing you provided) OR
= 25mm from the front butting surface of the M42 camera adapter mounted on the scope (
according to the EXCERPT form the Celestron manual you provided below)
Quote:
Originally Posted by h0ughy
From the celestron manual
Backfocus– For all flat-field astrographs, including the RASA 8, it is important that the camera’s sensor is positioned at the proper distance from where it mounts to the telescope. Otherwise, performance at the edge of the sensor, especially larger sensors, may suffer. For the RASA 8, this location is 29 mm from the top surface of the lens group cell. A camera’s backfocus, also known as its flange focal distance, is the distance between the camera sensor and where the camera body mechanically connects to a camera lens or telescope. Therefore, cameras with sensors deeply recessed within their body having more than 29 mm of backfocus cannot be used with the RASA 8 as the sensor cannot be placed at the proper location. DSLR cameras cannot be used with the RASA 8 due to their large obstruction and long backfocus. Installing a Camera If using a camera that mounts with M42 threads, the supplied M42 camera adapter can be used. This will likely require utilizing optional M42 extension rings to place the camera sensor at the proper location. When using the M42 camera adapter, the required backfocus distance is 25 mm. So, for example, if the backfocus of a camera is 17 mm, then 8 mm of M42 extension ring(s) between the camera and M42 camera adapter will be needed. For best results with larger sensors, get the backfocus within 1 mm of the required distance. The supplied C-thread camera adapter can be used to attach any camera which mounts with a standard C-thread. If the camera’s sensor spacing meets the C-thread standard (i.e. 17.5 mm backfocus), no additional adapters are required.
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All that being the case, and IF the adapter shown in your picture of your RASA 8 is indeed the Celestron M42 Camera adapter, then you will need to put the sensor 25mm away from its front abutting surface. Since the sensor is recessed 6.5mm in to the body of your camera, then the distance between the camera's front abuttment surface (the red aluminium surface at the base of the front male M42 thread) and the Celestron M42 camera adapter mounting surface requires a spacer/adapter of total length 25-6.5mm = 18.5mm. You can do this in one of two ways since you already have an 11mm M42 spacer ring (black T2 ring) that came with the camera:
1.
Use an additional 7.5mm extender spacer ring with the 11mm spacer ring you already have to get a total length of 18.5mm OR ....
2.
Use an 18.5mm (probably custom made) one piece spacer ring on its own and
NOT use the 11mm spacer ring supplied with the camera.
Best
JA