Re: Counter weight up slew vs. continuing past the meridian, any real differences?


Ray Gralak
 

Hi Luca,

My observation was that in theory starting from a CW up position saves imaging time, in practice I have found the
opposite because safety slews take longer.
First, the primary purpose of starting or ending in a counterweight-up position is not necessarily to save time but to be able to use the same guide star across the meridian. If you are using an off-axis autoguider without a camera rotator you may not get a good guide star (or any guide star) after a pier flip. But, if you have a camera rotator, using a separate guide scope, or unguided, there is not much advantage to counterweight-up imaging.

The precision I
require for the centering process is 30 points maximum error, which translates to roughly 20 arc-sec. If I am not
running a model on the Mach2 or the AP1100, the "closed loop slew" (to use TSX terminology) to the target will
usually take 2-3 slews. the first gets you with a degree, usually, and the second one gets you perfectly there or
If you are using an APCC pointing model, the declination pointing error should not be much less than the +/- 1/2 degree declination tolerance before a safety slew. If it is, then you probably should redo the pointing model, including adding more counterweight-up points.

If I allow APCC to run counterweight up slews within East limits and if the target is within East limits, when SGP
issues the first slew command to center the target, the mount will go in a CW up position with the scope on the East
side pointing East. When I have been next to the mount I have observed that the next slew or two to refine the
position of the mount and meet the required precision for centering the frame will be safety slews.
Again, a better pointing model should prevent safety slews, as exceeding the +/- 1/2 degree declination tolerance is unlikely unless SGPro is purposely moving the mount away from the target.

-Ray


-----Original Message-----
From: main@ap-gto.groups.io [mailto:main@ap-gto.groups.io] On Behalf Of Luca Marinelli
Sent: Sunday, March 14, 2021 3:17 PM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] Counter weight up slew vs. continuing past the meridian, any real differences?

On Sun, Mar 14, 2021 at 10:07 AM, Ray Gralak wrote:


Hi Luca,



It's not a big deal either way; we are only talking about two or three slews per target. But then again,
neither is a
Meridian flip, which also take a couple of slews per target for plate solving. That was my original point.

Can you clarify? Are you saying that SGPro needs to slew two or three times in the course of platesolving a
new target, and in the process of doing this APCC is doing a pier flip and multiple safe slews for the *same* target?

-Ray


When I set up a target in SGP, I'll enter RA and DEC coordinates for the center of the image, as well as rotation
angle of the frame. I start the evening with a "closed loop slew" to use TSX terminology to the target. The precision I
require for the centering process is 30 points maximum error, which translates to roughly 20 arc-sec. If I am not
running a model on the Mach2 or the AP1100, the "closed loop slew" (to use TSX terminology) to the target will
usually take 2-3 slews. the first gets you with a degree, usually, and the second one gets you perfectly there or
almost. If I am using a model, depending on where the target is in the sky, it will take 1-2 slews to get there.

If I allow APCC to run counterweight up slews within East limits and if the target is within East limits, when SGP
issues the first slew command to center the target, the mount will go in a CW up position with the scope on the East
side pointing East. When I have been next to the mount I have observed that the next slew or two to refine the
position of the mount and meet the required precision for centering the frame will be safety slews.

The same holds for meridian flips. After issuing the slew command that triggers the meridian flip, SGP will run a
platosolve task to confirm the correct position of the target. Typically, it will then issue a second and rarely a third
slew to center the target with 20 arc-sec precision. The slews after a meridan flip will not be safety slews because
they are CW down.

My observation was that in theory starting from a CW up position saves imaging time, in practice I have found the
opposite because safety slews take longer. That's all I was saying. Again, this is an entirely minor difference and
overall minute contribution to overhead compared to time spent focusing, dithering, saving files, etc. If you relax the
precision requirement, obviously it will take fewer slew to get the frame centered. I image targets over the course of
several months and require this precision so I don't have to trim the edge of the frames excessively.

If there is a better way to set up APCC to reduce this time overhead, I am always looking for ways to improve how
the software works together.

Luca

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