SharpCap sequencer - stopping X degs before the meridian

[BucketDave]

Robin,

I have been trialling my sequencer routines for imaging either side of a meridian flip and I hve found what appears to be a 'bug' or 'inaccurate calculation' in SharpCap. The bug involves the sequencer command :

- Stop running these steps when X degrees from the meridian

I am including it is as part of a larger sequence:

- Stop running these steps when X degrees from the meridian
- Capture 999 images with guiding equals True

- Run the sequence stored in 'Meridian flip'

I have set X to -1 degs so that the mount will go 1 degs past the meridian, at which point the sequencer should terminate the image capture step and move to the next step (which executes the meridian flip). Except that it doesn't happen at the correct azimuth. Examination of the SC log shows that the image capture is being terminated precisely 4 minutes after the meridian is crossed, rather than at 181 degs azimuth.

Having just spent an hour playing about in Stellarium (at 51.5 degs N), I have analysed the motion of three objects at different declinations and concluded that 4 minutes per degree is very much an approximation. It depends where-abouts in the sky you are looking!

At -11 degs DEC, the mount takes 219 s to move 1 deg at the meridian,
At +35 degs DEC, the mount takes just 99s to move 1 deg at the meridian,
At +72 degs DEC, the mount takes 305s to move 1 deg at the meridian (this object being circumpolar).

So the object at -11 degs DEC will actually be 1.1 deg past the meridian when the flip is triggered (which is fine). However, the object at +35 degs DEC will be 2.4 degs past the meridian when the flip is triggered (which is rather later than the user may expect).

In my case, I now realise that the sequencer WILL initiate the flip but that it might not be exactly when expected (which corresponds to experience outside). The problem I forsee is where a user has set the sequencer to stop 5 degs after the meridian, only to find that the mount has stopped tracking after 15 minutes (having gone 10 degs past the meridian because that was their user-defined behaviour in the mount settings).

As the sequence command specifies an angle, it would be nice if it respected that angle, rather than a nominally equivalent time. However, I can appreciate that fixing this 'bug' may require some complex celestial geometry.

[admin]

Hi Dave,

what you are seeing is expected behaviour, since the 'degrees past meridian' is being measured against the RA of the equatorial mount (24 h = 360 degrees = full circle) rather than the azimuth angle. I'm pretty sure that the mounts that have settings for tracking past the meridian will also be working in terms of the angle measured in RA too, rather than in Azimuth.

The reasoning behind this is that the 'stop at meridian' (or stop at 15 degrees past meridian or whatever) for an EQ mount is necessary because once past the meridian, the mount starts being in a 'weights up, telescope down' position (at the meridian it's horizontal). If you go too far into 'weights up, telescope down' you tend to find that the telescope or something attached to it hits the pier/tripod and then things can go bad very rapidly... The threshold for things hitting is going to depend on the physical rotation of the RA axis, not how far past the meridian you are in azimuth.

cheers,

Robin

[BucketDave]

of course... degs of RA, not degs of azimuth. D'oh!