ESATTO Visual Backs and Extension Tubes: Spacing Explained
By Cloud Break Optics | Published: 2026-11-04
Category: Handleidingen
Learn how to choose and space ESATTO visual backs and 2-inch extension tubes for visual or camera use, with a step-by-step check to avoid thread and spacing mistakes.
Quick answer
Choose the visual back that matches your focuser's thread and accessory nosepiece, then add a 2-inch extension tube only when you need extra back-focus. Check the adapter's optical thickness and the extension's net spacing before you order.
Threads decide the fit, not the label
ESATTO visual backs attach to specific drawtube threads. The 3-inch visual back for ESATTO 3.5-inch LP uses the M98 thread, while the 2-inch visual back for ESATTO 2-inch and ARCO 2-inch uses the M56 (2-inch) or M81 (3-inch) thread. Buying by the visual back's name alone can leave you with an adapter that will not start on your drawtube.
The same rule applies to the ARCO 2 visual back: confirm it is the M56/M81 version for your ESATTO 2-inch or ARCO 2-inch focuser before you plan the rest of the train.
Spacing adds up faster than most observers expect
Every adapter and extension consumes back-focus. The 3-inch visual back for ESATTO 3.5-inch LP has an optical thickness of 12.5 mm, and the 2-inch extension with 1.8-inch net spacing adds 45.7 mm. Stacking both without checking can push a camera or diagonal past the point where your focuser can still reach focus.
For visual use, the 2-inch visual back is usually the simpler choice because it holds a 2-inch diagonal or eyepiece directly. For imaging, you may need the extension to clear a filter wheel or rotator, so measure the whole train rather than guessing from the adapter alone.
When a 2-inch extension tube is the right tool
A 2-inch extension tube slips into a 2-inch opening and accepts 2-inch accessories or a 1.25-inch adapter. The ADA202 version has a brass locking ring, an overall length of 3 inches, 48 mm filter threads, and 1.8 inches (45.7 mm) of net spacing. That makes it useful for reaching focus with certain eyepieces or cameras, but it is not a universal fix for a focuser that is already too short or too long.
If your current visual back already holds accessories securely and you can reach focus, adding an extension usually only increases leverage and the chance of flex. Add one only when you have measured a shortfall.
Step-by-step: fit and space your ESATTO visual back
- Identify your drawtube thread: Look up your focuser model and note whether the rear thread is M98, M56, or M81. Match it to the visual back: M98 for the 3-inch ESATTO 3.5-inch LP version, and M56 or M81 for the 2-inch ESATTO 2-inch and ARCO 2-inch version. Do not order by the visual back's inch label alone; the thread is the deciding specification.
- Choose the accessory side: Decide whether you will insert a 2-inch diagonal, a 2-inch eyepiece, or a camera nosepiece. The 2-inch visual back accepts 2-inch nosepieces and uses a brass compression ring with three locking screws; the 3-inch version is for 3-inch nosepieces on the ESATTO 3.5-inch LP. Check that your heaviest accessory has a nosepiece that matches the visual back's opening.
- Add up optical thickness: Write down the optical thickness of each part in the train. The 3-inch visual back adds 12.5 mm; the 2-inch extension adds 45.7 mm of net spacing. Include diagonals, adapters, and camera back-focus if you are imaging. Keep the total below your focuser's available travel, or you may run out of inward or outward focus.
- Test the stack before tightening: Thread the visual back on by hand, insert the accessory, and confirm the locking screws meet the brass ring rather than the nosepiece. If you added the extension, check that it seats fully in the 2-inch opening and that the brass locking ring grips the accessory. A nosepiece that bottoms out before the screws tighten is a sign of a mismatched adapter, not a spacing problem.
- Recheck focus at the eyepiece or camera: Point at a distant target and run the focuser through its travel. If you cannot reach focus, remove the extension first, then reassess whether a different visual back or a shorter train is needed. Change one part at a time so you know which component caused the focus shift.
Product facts that affect your spacing
- 3" visual back for ESATTO 3.5” LP: The 3-inch visual back for ESATTO 3.5-inch LP threads onto the M98 thread and has an optical thickness of 12.5 mm. That 12.5 mm comes out of your available back-focus, so include it when you calculate whether a camera will still reach focus.
3" visual back for ESATTO 3.5” LP - 2" Visual Back for ESATTO 2" and ARCO 2": The 2-inch visual back for ESATTO 2-inch and ARCO 2-inch fits the M56 (2-inch) or M81 (3-inch) thread and uses a brass compression ring with three locking screws. The brass ring protects a 2-inch nosepiece from scratches, while the three screws help hold heavy cameras or accessories without relying on a single thumbscrew.
2" Visual Back for ESATTO 2" and ARCO 2" - 2" Extension - Length 1.8" (ADA202): The 2-inch extension with 1.8-inch net spacing has an overall length of 3 inches and is threaded for 48 mm filters. The 45.7 mm of net spacing is the number to use in your focus calculation, and the filter thread lets you place a 48 mm filter without another adapter.
