Camera adapter lengths

It is often useful to introduce additional lenses to the optical path between the telescope and the image sensor. These can match a telescope's focal length to the requirements of a specific target and image sensor. Focal reducers (positive lenses), Barlow lenses (focal expander, negative lenses), and eyepiece projection are commonly used to decrease or increase the focal length of the telescope. The geometric placement of the lenses in the optical path is required to calculate their effect on the focal length.

Vendors often don't provide the dimensions of adapters and extension tubes needed to perform these calculations. When dimensions are given, it is often not clear if a dimension is the total length or the distance added when screwed into an optical train. The table below gives the optical geometric path length of parts I have in hand for the Questar. These lengths exclude the overlap of screwed couplings and are measured to the nearest 1mm. Be advised that over time, as vendors change suppliers and specifications, these dimensions may change.

Optical lengths of optical train parts for the Questar
Questar coupling and extension tubes
Questar swivel coupling 33mm
Lolli non-swivel coupling 16mm
Questar short extension tube 6mm
Questar medium extension tube 14mm
Questar long extension tube 30mm T-thread extension tubes
Very short extension tube 5mm
Short extension tube 10mm
Medium extension tube 15mm
Long extension tube 20mm
Optical lengths of camera adapters and T-Rings
Ultra-slim M42 (P thread) to Sony NEX/E-Mount 3mm flange Adapter to Helicoid Tube eBay 3mm
Questar P to T adapter 11mm
E-Mount short generic T-Ring requires P to T 8 mm
TelescopeAdapters short E-Mount T-minus T-Ring requires P to T 11mm
E-Mount long camera infinity focus T-Ring requires P to T 37mm
Eyepiece and lens measurements
Questar eyepiece focal point offset from threads to focal point 12 mm
Questar Barlow lens total cell length 16mm
Questar Focal Reducer Lens total cell length 16mm
Camera flange to sensor distances
C-mount flange focal distance 17.526 mm
Sony E-mount flange focal distance 18mm
Micro Four Thirds flange focal distance 19.25 mm
Canon EF-mount flange focal distance 44 mm
Nikon F-mount flange focal distance 46.5 mm
T-mount flange focal distance 55 mm

2016-06-30 added more info on E-mount adapters and T-thread extensions

Content created: 2016-06-29




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