2x Barlow

Why it matters more than it looks

The telescope resolves 1.53" (the Dawes limit, set by the 76 mm aperture). Natively the pixels are 0.853" wide, so that limit spanned only 1.8 px — the sensor was recording less detail than the optics were delivering. The Barlow halves the scale to 0.426"/px, so the same limit now spans 3.6 px and the detail actually survives to the file.

It is not magnification for its own sake. It stops throwing away detail already collected.

Cross-check from a different direction: the measured 0.853"/px implies a 1.45 µm pixel (scale = 206265 x pixel_mm / focal_mm). The planetary rule of thumb wants roughly f/5 x pixel-size-in-microns, so f/7.2 is ideal here. Native f/4.6 is under-sampled; f/9.2 is slightly over, which is the right side to err on. Two independent routes, same conclusion.

Costs

• Field halves to 23.2' x 15.4'. Aiming is already the hardest part of a session (see the field note about there being no finder), and this makes it harder. Acquire without the Barlow, centre, then slide it in.
• Exposure x4 — f/4.6 to f/9.2 is a quarter of the light. 1 ms becomes about 4 ms. No effect on frame rate; 4 ms is nothing against a 66 ms frame interval at 15 fps.
• Refocus needed. A Barlow pushes focus outward, which on a Newtonian is usually the helpful direction.
• The Moon no longer fits at all — 4377 px across on a 3264 px axis. That is a feature for terminator close-ups, not a problem.

Config

AstroDMx equipment profile "NT76-350 + 2x Barlow" — 700.0 mm / 76.0 mm, created and activated 30/08/2026. The effective focal length is stored directly rather than using the Barlow field, because the dropdown's value tokens are not recoverable from the binary and a wrong token would either be ignored or double-counted. Aperture stays 76 mm, so the Dawes limit AstroDMx reports remains correct at 1.53". Switch back to "NG Reflecot" when the Barlow comes out.