FOV of the simulator and the actual image slightly shifted with same coordinates


I use Telescope Simulator to get FOV and it’s particularly useful to frame multiple objects. However, when I get the FOV coordinates from the Simulator and enter them to my ASIAIR Plus, I would get the FOV of my camera slightly shifted. So, I have to move the center using AA+ preview. Actually, after I took the subs, when I get the coordinates of a sub from the FITS header and enter those coordinates into the Simulator, the FOV of the Simulator is slightly off, probably around 2 minutes in RA.

So, I’m wondering if I miss some settings or do something wrong. Does the lat/long of the location matter? I got the lat/long from Google Maps and entered them into Telescopius.

Below is an example of SH2-284/282/280. The coordinates are the same between the sub frame and the Simulator. Note that the bright star at the left side of the Simulator FOV is not in the sub and SH2-280 is within the FOV for my sub but it’s partially out of FOV in the Simulator.
By the way, I use Siril to stack subs if that matters.

I use
Nikon 180mm prime lens
ASI533MC Pro


The reason for this is most likely that coordinates on Telescopius are J2000 while ASIAIR uses JNow.

One of the upcoming features on Telescopius is being able to switch between J2000 and JNow throughout the app, but for the time being the workaround I can suggest is using the Toolbox > Calculators and search for the J2000 ↔ JNow conversion calculator.

1 Like

Thank you very much for a quick reply. Yes, that is the answer that I wanted to know.
Last night, I determined FOV using the Simulator, got RA and DEC (J2000), and converted them to JNow using the calculator tool. Then, enter the RA & DEC (JNow) into ASIAIR Plus. The FOV of the subs was what I expected and very close to the Simulator’s!

I’m happy with this workflow but also looking forward to having the selection of the coordinate systems in the Simulator in the future!

1 Like

Great, thanks for letting me know it worked!

For sporadic shooting, this workflow works well. But for multiple targets or big mosaics, having to convert everything manually is quite tiresome. I’ll definitely add the possibility to switch epochs in the near future.

1 Like