How to get better guiding

galactique

The Fine Guidance Sensor: (I want one)

“To ensure Webb stays locked on its celestial targets, the FGS measures the exact position of a guide star in its field of view 16 times per second and sends adjustments to the telescope’s fine steering mirror about three times per second. In addition to its speed, the FGS also needs to be incredibly precise. The degree of precision with which it can detect changes in the pointing to a celestial object is the equivalent of a person in New York City being able to see the eye motion of someone blinking at the Canadian border 500 kilometers (311 miles) away!

“Webb’s 18 primary mirror segments are not yet aligned, so each star appears as 18 duplicate images. On Feb. 13, FGS successfully locked onto and tracked one of these star images for the first time. The FGS team was thrilled to see this ‘closed loop guiding’ working! From now on, most of the alignment process of the telescope mirrors will take place with FGS guiding, while NIRCam images provide the diagnostic information for mirror adjustments.”

–René Doyon, principal investigator for FGS/NIRISS, Université de Montréal; and Nathalie Ouellette, Webb outreach scientist, Université de Montréal

https://blogs.nasa.gov/webb/

w7ay

galactique sends adjustments to the telescope’s fine steering mirror about three times per second.

Guiding feedback rates are important, and the James Webb guys and gals know it.

Ever since ASIAIR released the 2x bin back in v1.8, it is now capable of guiding at 2 times per second (frame rate of 2 FPS). That was why I was jumping for joy earlier, and posted about it, when I could get 2 FPS with 0.5 second guide exposures.

Unfortunately, with the v1.9 Beta (on iOS TestFlight, latest is Build 10, Feb 18, 2022), ZWO screwed something up with 2x Bin star detection in multi-star guiding, and I could no longer use 2x bin with a ASI290MM camera and 250mm/f4.5 guide scope. I had posted about that bug earlier.

I had the opportunity since then to test with a 300mm/f5.6 guide scope (same camera), I was able to get a maximum of perhaps 5 stars with a near full Moon. Without more than 5 or 6 stars in multi-star guiding, "seeing" causes too much measurement error, and we have to go back to 1 FPS or slower guiding.

Both FPS (to help with feedback rate) and multi-star guiding (to help with poor "seeing") are important for good guiding. Right now in v1.9 beta, you cannot get both.

One step forward, two steps back. As usual.

Chen

astrosatch

w7ay One step forward, two steps back. As usual.

As usual. Why talking about october 2022 and neglecting problems at right this moment?