Re: Continuation of the discussion using APCC CW up within the East Limit slews with SGPro #APCC


Luca, thank you for that logical and brilliant explanation of the interplay

TJF Mobile

On May 1, 2021, at 10:46, Luca Marinelli <photo@...> wrote:

Hi Eric,


I responded to your thread in the SGP forum and shared my configuration in APCC and SGP, which allows CW up imaging and pre-flipping of the mount, if you desire. I have worked out an example and hopefully this will be clearer.


I image unattended and my goal is always to have a belt and suspenders system with redundancy so using “Just Warn” in the meridian limit tab of APCC is out of the question for me. I am never next to the mount and I regard APCC meridian limits as a safety in case anything goes wrong with the imaging run or control of the meridian flip. I use “Stop Tracking” when the meridian limit is reached so the mount cannot track past this point and I have a visual indication that something went wrong in the morning when I shut everything down.


SGP controls the meridian flip. SGP is controlling all imaging equipment and knows when is a good time to take a break and flip the mount. As Ray said, all APCC needs to do, is tell SGP at what time it can start flipping the mount. SGP will request a flip as soon after this time as is compatible with the imaging run. This is what is accomplished with the “Flip Offset” entry and “Send Limit with offset to SGPro” checkbox in the meridian limit tab in APCC.


With the camera I use on this system, at most I take 10 minute frames, so I chose 20 minutes as Flip Offset. I will always have time to complete a frame before hitting the meridian limit (where the mount would stop tracking and the imaging run abort) with this choice of Flip Offset.


So let’s look at an example. It’s 10am here and M31 is in the Eastern sky, just before the meridian, at a declination that is compatible with CW up imaging on my system, so I picked this target and created a sequence in SGP. I ran two scenarios: one with CW Up Slews within East Limits checked and one without checking the box.


Conventional imaging with only West Limits checked (safety limit):


Take a look at the screenshot. Meridian flip is controlled by SGP and APCC is setup to pass the meridian limit with flip offset to SGP. If you compare the timer until meridian flip in APCC (2h 38m 11s) with the timer in SGP (2h 19m 36s) – circled in blue in the screenshot – you’ll see that the difference between the two is exactly the Flip Offset: SGP will request a meridian flip as soon as possible after the timer runs out and it will have 20 minutes before the mount bumps into the actual meridian safety limit set in APCC.


This is my usual mode of imaging. I image multiple targets during the night to make efficient use of dark hours and this mode is safe, works, and is efficient. I launched the sequence in SGP and you can see in the 3D View that the mount is CW down, West pointing East, in a conventional position. APCC has passed the time to flip to SGP and in Meridian Flip Options you see 80 minutes past meridian to flip. That number is set dynamically by SGP based on the information about the meridian with flip offset passed along by APCC. You don’t change this number manually. Try changing the Flip Offset and you will see that this number and the timer at the bottom of the SGP window update automatically.


CW up imaging using East Limits checked (opportunity limit):


The same target M31 is within the East meridian limits and so I can also choose to start imaging with CW up and pre-flip the mount. All I need to do is check the East Limits checkbox in the meridian limit tab in APCC and everything else stays the same. The advantage of this setup is: 1) APCC still acts as the ultimate safety limit – if everything else fails and the mount reaches the meridian limit, the mount will stop tracking. 2) If you have multiple targets in your SGP run and some start the imaging run within East limits, some don’t, the ones that allow a start with CW up will use that configuration, the ones that don’t will start in a standard CW down position. I have used this mode of operation unattended without any adverse consequences.


When I start the imaging run, you can see that the Scope timer in SGP says N/A because no flip will be requested during this run. In the 3D View you can see that the scope is starting the imaging run CW up, because it is possible. Easy peasy.


That’s about it. It’s really not that complicated if you use two guiding principles: 1) the imaging application requests meridian flips because it knows when it can do it without interrupting other operations, 2) APCC is responsible for the safety of the gear.






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