Strange Dither behaviour, or Something...

[slickrock]

Hi Guylain,

A couple nights ago I tried to image M13 with a Nikon 1 V3 on a 400mm Lens.  I was using BYN 2.1.3 RC3 and had set up an imaging run of 140 x 30s images with dithering (ASCOM with an AP Mach2 mount) every 3 images.  The imaging run started out good (I had plate solved on M13 and so M13 was centred on the image) and was good for 31 images.  The image centre RA was at 16 41 42.945, and DEC was at 36 25 23.75.  Then, after a dither, M13 jumped and the image centre was at RA 16 42 05.272 and DEC was at 36 22 00.03.  The imaging run continued until the end (another 109 images) all with the image centre at the new RA and DEC.  I had gone to bed and let BYN do it's work and when I got up in the morning the mount was nicely parked.  Any idea what may have caused this?

I've attached two images, # 304 and #305.  304 shows M13 centred and 305 shows M13 shifted.  I've also attached an excerpt of the log file from a dither at image # 302 until the dither just before image # 305.

Thanks,

Terry

BYN-2022-05-07.txt

[astroman133]

It looks like there was a meridian flip.

[slickrock]

No, it was no where near the meridian. It was 2 hours and something East.

Terry

[astroman133]

OK, but the two images that you provided are flipped relative to each other. Also, mounts can be configured to flip either early or late.

If the mount is configured to flip early and BYN uses a slew to adjust the position during direct dithering then the mount could have flipped.

[slickrock]

Ha!  I didn't even notice that the images were flipped.  Stupid me!  I'll have to look into how my mount is configured; I don't ever recall setting the mount up to flip early.

A nice feature that could be added to BYN would be to have an automatic plate solve after a meridian flip or maybe a plate solve after some number of images.

Thanks, I'll look deeper into my mount settings.

Terry

[admin]

I looked at the log file, and everything is fine.... the flip occurred outside BYE.

Look at your mount limits, maybe it played a role in the flip?

[slickrock]

Thanks.  I checked the mount meridian delay settings and they are set for zero, so no advanced or delayed flips.  I need to look deeper.

Terry

[admin]

It may not part of the meridian flip settings. On my 10 Micron mount, I have a slew tolerance setting. If I perform a slew to an object within the tolerance setting of the meridian, the mount will automatically do a flip.

Are you using Direct Mount dithering setting in BYE? In this case, BYE will issue a small slew command equal to the calculated dither. If you have a slew tolerance on your AP mount, like my 10u has, the direct mount dither (via a small slew command) may trigger the internal mount flip procedure.

 

[astroman133]

The AstroPhysics controller has a Meridian Delay setting that can be set negative or positive to do an early or late flip. The OP would have had to set that value to -2 hours to flip as early as he said it did. If I were him I would try this experiment, without using the computer at all. Set the mount to a bit more than 2 hours east of the meridian and let it track for 30 minutes or so and slew to the same RA and Dec where he already is. If the mount flips then he knows that there is a mount setting that is causing the early flip.

[slickrock]

I am using the Direct Mount dithering setting.

Thanks Rick.  I will try your suggestion and see what happens.

Terry

[slickrock]

Well, I don't know....

I set up the mount to do some testing today.  I pointed the mount to DEC 35 51 04 (pretty close to where it was the other night) and 2h 6m East of the meridian.  I let the mount run for over an hour and nothing extraordinary happened; it just tracked.

I then tried the same setup with the camera connected and direct mount dithering turned on.  Everything worked flawlessly.

Oh well....

Terry