Ocular Sensor Angle for Alt/Az Mounts

[dts350z]

When you setup an Ocular Sensor and Telescope for Alt/Az, the FOV is displayed correctly on the sky, however the angle indicator, displayed on the FOV, remains fixed at whatever you set it for in the Sensors dialog (like it would for an EQ mount).

I think this behavior is wrong, for Alt/Az. The angle value needs to be calculated to match the displayed FOV. This is causing problems for mosaic planning tools, as the angle shown in Stellarium doesn't agree with the angle control in the planning tools (they give different orientations of the FOV on the sky).

Can this be fixed?

[gzotti]

Which angle indicator? Please make a screenshot and mark what bothers you.

[dts350z]

Angle Correct (EQ Mount):

Angle Incorrect (Alt/Az Mount):

Again, the FOV is DISPLAYED at the correct angle, it's just the printed angle that is incorrect.

Is there another place where that angle is shown (correctly)?

[alex-w]

Why you think that second image has incorrect angle? Please enable visualization of horizontal coordinates system (Alt-Az) and compare the behavior.

[10110111]

The angle is correct—if taken with respect to the equatorial grid. Of course, you can't expect it to be correct both relative to the equatorial grid and to the alt-azimuthal one.

[dts350z]

For the purposes of Mosaic Planning one needs to know the "position angle on the sky" which is in the Equatorial Grid. I was hoping to use Stellarium to give me that angle to then populate it in mosaic planning tools and also warn me if the field rotation was deviating too far in certain mosaic frames, given the time of acquisition.

Short of trying to plate solve a screen shot of Stellarium, or measuring the angle of the displayed FOV to the Equatorial Grid on the screen, is there a better way? I've been googling for a formula but no luck yet.

I thought if the angle displayed on the FOV always reflected the "Position Angle On the Sky" it would be very convenient.

[alex-w]

Top right corner has rotation angle, not a position angle

[dts350z]

I guess it's the "rotation angle, in the coordinate system of the telescope".

OK so you are resistant to changing it (to rotation angle in the EQ coordinate system), and the search for some tool or combination of tools for mosaic planning with alt/az mount goes on.

[10110111]

I'm not quite sure what exactly equipment you have and what exactly number you want. Let me guess: telescope roll angle with respect to horizon while having the FoV track the equatorial grid?

[dts350z]

I have lots and lots of gear, but I was trying to put together instructions for people to plan mosaics with the ZWO Seastar S50, which is Alt/Az.

The difficulty comes in that all the planning tools assume Equatorial Mounts, and want to know the rotation angle of the camera (in the EQ Grid). They then display the mosaic "Frames", and give you the RA and Dec Coordinates (and rotation angle) you need to target. e.g. from N.I.N.A. (pulling the target in from Stellarium with one click):

where you need to input the Rotation Angle.

I hadn't yet thought about taking the Ra &Dec coordinates back to the Seestar UI, but I'm assuming you can enter RA and DEC for targets, vs. just searching for an object. I got hung up on how to get that needed Rotation Angle, from Stellarium.

[10110111]

From your picture the angle looks correct. But the size of the footprint is smaller in NINA than in Stellarium. Something is wrong with your camera parameters.

[dts350z]

Screen shots were from points in time where I may not have had the same parameters in both. The angles were matched because at that point I had an eq mount configured camera in Stellarium. My point was just to show that the Framing UI wanted a (EQ) rotation input. Everything works, as long as you are using an EQ mount.

[10110111]

Well, can you then specify your conditions exactly, preferably with screenshots that demonstrate what you actually see so that I could understand what's wrong?

[dts350z]

OK, when you are using an EQ telescope, in Stellarium, this rotation angle can be used in other tools, expecting rotations in EQ coordinates:

That means that you can display an ocular FOV in Stellarium, and use it as a planning tool for framing observations, and it will agree with other tools like this:

and the FOVs line up.

However, If I use a Alt/Az scope in Stellarium (uncheck Equilateral Mount in Ocular Telescope settings) :

I'm told, in this thread, that the Rotation Angle is now in the Azimuthal reference, so I can't use it in other tools (as they are expecting EQ). If I do:

You can see that the FOVs are at different angles, and I'd have to measure the angle, between the FOV frame and the EQ gridlines in Stellarium, , to know what angle to put in the other tools.

So, it would be convenient if there was a way to get the Rotation Angle in the EQ frame of reference, even when you have Equilateral Mount in Ocular Telescope settings unchecked. Maybe it would need to be an option, or a way to see both, to also keep the current behaviour?

Thanks for taking a look.

[gzotti]

Stellarium's behaviour seems to be what all developers have intended to provide. The rotation angle is relative to the current mount system, be it equatorial (not "equilateral") or azimuthal. Probably because in the last 120 years (or so) mosaics would have been planned in EQ mode, so this is hard-coded in our brains. It seems you just need the parallactic angle which is given in the top-left info for all extended objects (DSO, planets).

[alex-w]

All screenshots from N.I.N.A. are showing objects in equatorial coordinates.

[dts350z]

That can work, but I need to flip the sign. By inspection:

Thanks!

[gzotti]

OK. We might be able to overfreight the GUI and label corners further by optionally adding parallactic angle of image centre (or the equivalent image rotation against E-Q-U-A-T-O-R-I-A-L coordinates) in azimuthal mode.