AsiAir star detection metrics?

erhad

When I use the star detection feature in my AsiAir Pro, I get two numbers, the second one seems to be in arc-seconds. What are the metrics and what exactly does this tell me? Am I right to assume that the second number (in arcseconds) is the FWHM? If so, what is the first number? Thanks.

PMTeam@ZWO

Hi, the first number is HFD (unit: px). The second is star size in the Celestial coordinate system.

erhad

Hi, and thanks for taking the time to reply. Could you please elaborate on the second one (Celestial coordinate system)? I don’t quite get it because it varies from frame to frame (dependent on seeing and guiding). Sounds like a measurement based on the Celestial Coordinate System would not be affected by environmental conditions?

Thanks.

SteveZ

I have this question also. What exactly does it mean, "star size in the Celestial coordinate system" and how can I use it? Thanks in advance!

Steve

surfinash

Bueller.. Bueller... Bueller..... Bueller.....

w7ay

surfinash

The first is the HFD in units of pixels on the sensor. The second is HFD in units of arc seconds in the sky.

Just compute the plate scale of your OTA/camera combination (arc seconds per pixel), and you can see that ZWO had simply multiplied the first number by the plate scale to get the second number, based on the pixel size of the camera, and the focal length that you gave to ASIAIR (or the focal length is last plate solved).

Chen