Ascom Dithering

[photon-noize]

I tried Ascom dithering during two night, but without much success. My idea was to use dithering without guiding for wide fields with a camera lens. My mount is an AZ EQ6, using the Ascom Celestron driver (not Eqmod!).

I enabled Ascom dithering in BYE, which happily shows the ditthering between exposures. But the result is that the mount moves in somewhat constant steps of ~+30 ... +40s in RA. See below for an excerpt of the log file. The "Dither settled to ..." values don't seem to be correct, the new mount position as shown on the control display and in BYE is the one shown in the log as "Current ..." of the next dithering step.

Same thing in 3.1.8 and 3.1.11 (.11 BTW is *way* slower in starting the program!). Any ideas?

Martin

2017-06-17 00:03:40,160 [5] INFO  - ASCOM dithering...
2017-06-17 00:03:40,160 [5] INFO  - ASCOM Dither is starting...
2017-06-17 00:03:40,338 [5] INFO  - Current RA(18h42m28s) DEC(38°56'19")
[...]
2017-06-17 00:03:50,055 [5] INFO  - Dither settled to RA(18h42m20s) DEC(38°55'17")
2017-06-17 00:03:50,071 [5] INFO  - Difference of RA(-0h0m8s) DEC(-00°01'02")
[...]
2017-06-17 00:06:11,743 [5] INFO  - ASCOM dithering...
2017-06-17 00:06:11,743 [5] INFO  - ASCOM Dither is starting...
2017-06-17 00:06:13,564 [5] INFO  - Current RA(18h42m59s) DEC(38°56'22")
[...]
2017-06-17 00:06:23,413 [5] INFO  - Dither settled to RA(18h42m59s) DEC(38°56'39")
2017-06-17 00:06:23,429 [5] INFO  - Difference of RA(+0h0m0s) DEC(+00°00'17")
[...]
2017-06-17 00:08:45,651 [5] INFO  - ASCOM dithering...
2017-06-17 00:08:45,651 [5] INFO  - ASCOM Dither is starting...
2017-06-17 00:08:45,967 [5] INFO  - Current RA(18h43m37s) DEC(38°57'44")
[...]
2017-06-17 00:08:55,448 [5] INFO  - Dither settled to RA(18h43m28s) DEC(38°58'04")
2017-06-17 00:08:55,464 [5] INFO  - Difference of RA(-0h0m9s) DEC(+00°00'20")
[...]
2017-06-17 00:11:15,734 [5] INFO  - ASCOM dithering...
2017-06-17 00:11:15,734 [5] INFO  - ASCOM Dither is starting...
2017-06-17 00:11:15,987 [5] INFO  - Current RA(18h44m8s) DEC(38°59'10")
[...]
2017-06-17 00:11:25,744 [5] INFO  - Dither settled to RA(18h44m9s) DEC(38°59'14")
2017-06-17 00:11:25,759 [5] INFO  - Difference of RA(+0h0m1s) DEC(+00°00'04")
[...]
2017-06-17 00:13:45,808 [5] INFO  - ASCOM dithering...
2017-06-17 00:13:45,808 [5] INFO  - ASCOM Dither is starting...
2017-06-17 00:13:46,078 [5] INFO  - Current RA(18h44m48s) DEC(39°00'21")
[...]
2017-06-17 00:13:55,923 [5] INFO  - Dither settled to RA(18h44m46s) DEC(38°58'43")
2017-06-17 00:13:55,938 [5] INFO  - Difference of RA(-0h0m2s) DEC(-00°01'38")
[...]
2017-06-17 00:16:46,637 [10] INFO  - ASCOM dithering...
2017-06-17 00:16:46,637 [10] INFO  - ASCOM Dither is starting...
2017-06-17 00:16:46,890 [10] INFO  - Current RA(18h45m25s) DEC(38°59'51")
[...]
2017-06-17 00:16:56,445 [10] INFO  - Dither settled to RA(18h45m24s) DEC(38°59'58")
2017-06-17 00:16:56,461 [10] INFO  - Difference of RA(-0h0m1s) DEC(+00°00'07")
[...]
2017-06-17 00:18:22,785 [10] INFO  - ASCOM dithering...
2017-06-17 00:18:22,785 [10] INFO  - ASCOM Dither is starting...
2017-06-17 00:18:23,054 [10] INFO  - Current RA(18h46m4s) DEC(39°01'07")
[...]
2017-06-17 00:18:32,730 [10] INFO  - Dither settled to RA(18h46m6s) DEC(39°01'06")
2017-06-17 00:18:32,730 [10] INFO  - Difference of RA(+0h0m2s) DEC(-00°00'01")

 

[photon-noize]

After some time I came back to this problem and did some more test. Actually the problem seems to be related to using the Celestron driver with the AZ EQ6. I installed Eqmod/EqAscom (using the handbox' PCdirect mode) and tried again. This time the ASCOM dithering dry run with BYE seems to be ok, see below for an excerpt from the log file. BYE's ASCOM dithering seems to be a bit biased towards positive movements in RA and DEC, though?

2017-07-20 23:37:42,781 [13] INFO  - ASCOM dithering...
2017-07-20 23:37:42,828 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:37:43,013 [13] INFO  - Current RA(18h37m33s) DEC(38°44'29")
2017-07-20 23:37:45,187 [13] INFO  - Dither settled to RA(18h37m34s) DEC(38°44'09")
2017-07-20 23:37:45,202 [13] INFO  - Difference of RA(+0h0m1s) DEC(-00°00'20")
2017-07-20 23:38:16,875 [13] INFO  - ASCOM dithering...
2017-07-20 23:38:16,891 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:38:18,779 [13] INFO  - Current RA(18h37m35s) DEC(38°44'09")
2017-07-20 23:38:20,774 [13] INFO  - Dither settled to RA(18h37m38s) DEC(38°44'20")
2017-07-20 23:38:20,790 [13] INFO  - Difference of RA(+0h0m3s) DEC(+00°00'11")
2017-07-20 23:38:53,661 [13] INFO  - ASCOM dithering...
2017-07-20 23:38:53,661 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:38:55,549 [13] INFO  - Current RA(18h37m38s) DEC(38°44'20")
2017-07-20 23:38:57,733 [13] INFO  - Dither settled to RA(18h37m45s) DEC(38°44'25")
2017-07-20 23:38:57,740 [13] INFO  - Difference of RA(+0h0m7s) DEC(+00°00'05")
2017-07-20 23:39:30,754 [13] INFO  - ASCOM dithering...
2017-07-20 23:39:30,754 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:39:32,666 [13] INFO  - Current RA(18h37m46s) DEC(38°44'25")
2017-07-20 23:39:34,753 [13] INFO  - Dither settled to RA(18h37m45s) DEC(38°45'53")
2017-07-20 23:39:34,764 [13] INFO  - Difference of RA(-0h0m0s) DEC(+00°01'28")
2017-07-20 23:40:07,753 [13] INFO  - ASCOM dithering...
2017-07-20 23:40:07,800 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:40:09,680 [13] INFO  - Current RA(18h37m46s) DEC(38°45'53")
2017-07-20 23:40:11,727 [13] INFO  - Dither settled to RA(18h37m47s) DEC(38°46'29")
2017-07-20 23:40:11,738 [13] INFO  - Difference of RA(+0h0m2s) DEC(+00°00'36")
2017-07-20 23:40:43,488 [13] INFO  - ASCOM dithering...
2017-07-20 23:40:43,488 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:40:45,301 [13] INFO  - Current RA(18h37m48s) DEC(38°46'29")
2017-07-20 23:40:47,558 [13] INFO  - Dither settled to RA(18h37m51s) DEC(38°48'07")
2017-07-20 23:40:47,558 [13] INFO  - Difference of RA(+0h0m3s) DEC(+00°01'38")
2017-07-20 23:41:20,091 [13] INFO  - ASCOM dithering...
2017-07-20 23:41:20,138 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:41:21,960 [13] INFO  - Current RA(18h37m51s) DEC(38°48'07")
2017-07-20 23:41:24,018 [13] INFO  - Dither settled to RA(18h37m54s) DEC(38°46'31")
2017-07-20 23:41:24,218 [13] INFO  - Difference of RA(+0h0m3s) DEC(-00°01'36")
2017-07-20 23:42:01,332 [13] INFO  - ASCOM dithering...
2017-07-20 23:42:01,332 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:42:03,273 [13] INFO  - Current RA(18h37m55s) DEC(38°46'31")
2017-07-20 23:42:05,350 [13] INFO  - Dither settled to RA(18h37m54s) DEC(38°46'55")
2017-07-20 23:42:05,372 [13] INFO  - Difference of RA(-0h0m1s) DEC(+00°00'24")
2017-07-20 23:43:29,472 [13] INFO  - ASCOM dithering...
2017-07-20 23:43:29,487 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:43:29,904 [13] INFO  - Current RA(18h37m54s) DEC(38°46'55")
2017-07-20 23:43:31,976 [13] INFO  - Dither settled to RA(18h38m4s) DEC(38°48'13")
2017-07-20 23:43:31,992 [13] INFO  - Difference of RA(+0h0m9s) DEC(+00°01'18")
2017-07-20 23:44:03,644 [13] INFO  - ASCOM dithering...
2017-07-20 23:44:03,726 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:44:05,542 [13] INFO  - Current RA(18h38m4s) DEC(38°48'13")
2017-07-20 23:44:07,567 [13] INFO  - Dither settled to RA(18h38m10s) DEC(38°48'53")
2017-07-20 23:44:07,583 [13] INFO  - Difference of RA(+0h0m6s) DEC(+00°00'40")
2017-07-20 23:44:40,538 [13] INFO  - ASCOM dithering...
2017-07-20 23:44:40,589 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:44:42,473 [13] INFO  - Current RA(18h38m10s) DEC(38°48'53")
2017-07-20 23:44:44,476 [13] INFO  - Dither settled to RA(18h38m18s) DEC(38°49'57")
2017-07-20 23:44:44,491 [13] INFO  - Difference of RA(+0h0m7s) DEC(+00°01'04")
2017-07-20 23:45:16,199 [13] INFO  - ASCOM dithering...
2017-07-20 23:45:16,252 [13] INFO  - ASCOM Dither is starting...
2017-07-20 23:45:18,218 [13] INFO  - Current RA(18h38m18s) DEC(38°49'57")
2017-07-20 23:45:20,289 [13] INFO  - Dither settled to RA(18h38m19s) DEC(38°51'15")
2017-07-20 23:45:20,305 [13] INFO  - Difference of RA(+0h0m1s) DEC(+00°01'18")

[admin]

On 6/19/2017 at 2:29 AM, admin said:

Hi,

Yes, starting BYE 3.1.10 might be a little bit slower on slower computer because it now has to initialize 100 plan items, but this should me minimal in most cases.  Other that this there should be no difference.

I don't understand you comment for dithering constant... the log show dithering variation, both positive and negative.  Can you elaborate?

Awesome, thanks for reporting back.

[admin]

Hi,

Yes, starting BYE 3.1.10 might be a little bit slower on slower computer because it now has to initialize 100 plan items, but this should me minimal in most cases.  Other that this there should be no difference.

I don't understand your comment for dithering constant... the log show dithering variation, both positive and negative.  Can you elaborate?

[photon-noize]

Thx for the reply. The problem is that the Ascom dithering is more or less steadily moving the mount in + 30...40s steps along the RA axis (which is quite a huge step each). The "Difference of" shown in the log is wrong, as is the "Dither settled to". Have a look at the sequence of the "Current RA ... DEC ..." values and you'll see what's going wrong.

[astroman133]

I may be wrong here, but I think that you are assuming that the RA and DEC should stay the same between dithers. That may not be correct.

I have a Celestron CGEM and what I see, when doing visual observing and not connected to a computer is that the mount tries to be very sophisticated with respect to its pointing model. It attempts to correct for non-orthogonality of the scope and the mount, based on the results of the alignment/calibration process. The upshot is that in addition tracking in RA to keep the object in the FOV, it also adjusts the RA and DEC by small amounts. I think that is what you may be seeing when you look at the differences between the RA and DEC at the start of each dither, assuming that your AZ EQ6 operates the same way.

This seems to be confirmed by the difference between the reported RA/DEC when a dither has settled and the reported RA/DEC when the next dither starts.

This is easy to test...Do your normal alignment, slew to an object and look at the pointing position. Wait for some time and look again. Is it the same, or has it shifted?

[photon-noize]

Thanks for looking into this. But, well, I do know my way around with dithering, using it sucessfully a couple years with PHD2 and BYE. ;-)

Of course RA/DEC will change with dithering, but only slightly. In my case, the field of view is moving approx. 15 pxl towards celestial north with each frame, with a 50mm lens! Which is something like 6 arcmin (also approx. calculated from +30s RA), way more than I would expect from dithering! With Ascom dithering disabled the field and coordinates stay put.

And the "Dither settled" values are definitely wrong. At the time when BYE logged this message, both BYE's mount window and the mount's display showed the new values logged as "Current" for the next step.

[photon-noize]

BTW: I also tried the aggressiveness setting at both extremes, 1 and 5, this has no impact on my problem.

[photon-noize]

As other people seem to get Ascom dithering to work

https://www.cloudynights.com/topic/535025-m101-white-zone-unguided-ascom-dithering/

probably the Celestron driver together with the AZ EQ6 doesn't handle guiding pulses for the dithering very well? Any way to debug what BYE sends and receives from the Ascom driver?

[astroman133]

ASCOM logs calls to the driver and the Celestron driver logs what it is doing. The ASCOM logging is activated by clicking on the "Trace" link at the upper left of the chooser window. The driver's logging is enabled via a checkbox in the Properties dialog.