×

INDI Library v2.0.6 is Released (02 Feb 2024)

Bi-monthly release with minor bug fixes and improvements

Dither affecting RMS error?

  • Posts: 239
  • Thank you received: 38
I have noticed this as well, I have a minute settle after dithering and I still have this issue as well. It seems the guide star is off a little after dithering and the error goes into the RMS error no matter how long the guiding settle is.
3 years 10 months ago #54718

Please Log in or Create an account to join the conversation.

  • Posts: 1119
  • Thank you received: 182
Yes, I am also seeing the same thing.
3 years 10 months ago #54724

Please Log in or Create an account to join the conversation.

  • Posts: 985
  • Thank you received: 161

Replied by Alfred on topic Dither affecting RMS error?

Me too. This is one of the issues I noticed on a regular basis but for whatever reason I never reported.
The following user(s) said Thank You: Craig
3 years 10 months ago #54745

Please Log in or Create an account to join the conversation.

  • Posts: 133
  • Thank you received: 33

Replied by Brian on topic Dither affecting RMS error?

I see it here too, settling time seems to have little if any effect.
The following user(s) said Thank You: Craig
3 years 10 months ago #54750

Please Log in or Create an account to join the conversation.

  • Posts: 216
  • Thank you received: 120
Looking at gmath.cpp, calc_square_err is looping through all values of the drift array to calculate the RMS error, and I believe performProcessing is adding all movements (including dither movements) to the drift array, but I'm also not familiar enough with the guiding code to jump in and make any changes.
3 years 10 months ago #54751

Please Log in or Create an account to join the conversation.

  • Posts: 985
  • Thank you received: 161

Replied by Alfred on topic Dither affecting RMS error?

I suspect "Dither settle" is not what you're looking for. Dither settle is the time span between dithering being finished and guiding being resumed. I never quite understood what this is supposed to achieve. What you're probably looking for is "Guiding settle" which is (illogically, I might add) placed at Ekos Setup -> Options -> Ekos -> Capture -> Guiding settle. That's the time span between guiding being resumed and the next capture started. This (set to 13s here) provides enough time for guiding to get to a stable state before the next exposure is triggered. I've been advocating for moving this setting to the Capture Tab -> Options before where I think an "average user" would expect it to be.
The following user(s) said Thank You: Craig
Last edit: 3 years 9 months ago by Alfred.
3 years 9 months ago #54819

Please Log in or Create an account to join the conversation.

  • Posts: 239
  • Thank you received: 38
Yes, Herrhausen, you are quite correct, however RMS values are still affected because of the shift in star position after dither. I do run dither settle for about 20 seconds or so, as my stars would appear egg shaped or rather Tadpole shaped because of the residual movement of the guiding action after dither. I think what may need to happen is RMS values should be taken AFTER GUIDING SETTLE time. This would at least eliminate the RMS values from showing incorrectly high values.
The following user(s) said Thank You: Alfred
Last edit: 3 years 9 months ago by Sonny Cavazos.
3 years 9 months ago #54822

Please Log in or Create an account to join the conversation.

  • Posts: 985
  • Thank you received: 161

Replied by Alfred on topic Dither affecting RMS error?

Totally agree!
3 years 9 months ago #54825

Please Log in or Create an account to join the conversation.

  • Posts: 1119
  • Thank you received: 182
The RMS error that is indicated does not affect the performance, even though its calculation is probably not correct as many of us have noticed by now.

What would be more important is to command the guide module to search for a new guide star following each dither event and then restart guiding. From the movements I am seeing on the guide plot, it looks to me like the module is moving the mount BACK to where it was before it started dithering, instead of resetting the position of the green square onto the shifted position of the guide star and then continuing from there. If what I am seeing is correct, that would negate the entire effect of dithering and explain why I still have these streaks in my OSC images, event though I am dithering like crazy.

Alfred, what do you think? Does that match what you are seeing?

Jo
The following user(s) said Thank You: Alfred
3 years 9 months ago #54827

Please Log in or Create an account to join the conversation.

  • Posts: 216
  • Thank you received: 120
Jo, the more I remember the guiding I've done before, the more I think you might be correct. I've noticed in processing that the offsets when aligning are pretty small even over several images, leading me to believe we aren't actually getting much out of the dither.
3 years 9 months ago #54828

Please Log in or Create an account to join the conversation.

  • Posts: 1009
  • Thank you received: 133
Just the quick side note that PHD2 also computes the RMS including the dither settle. So it rather seems 'common practice' :P
The following user(s) said Thank You: Craig
3 years 9 months ago #54830

Please Log in or Create an account to join the conversation.

  • Posts: 985
  • Thank you received: 161

Replied by Alfred on topic Dither affecting RMS error?

Jo,

I agree RMS doesn't have any effect on performance. Once RMS is shown, it should be correct though. With regard to dithering I have to confess it works pretty perfectly for me. What happens during a dither is the guide star as well as the green box are being moved by a certain amount of pixels in a random direction. Once guiding restarts, the guide star is fixed to this new position. The effect can be seen when blinking through the subs that have been taken this way. I've attached a small sample of cropped subs (8,8 pixel dithering, ASI 294MC Pro).

Alfred
The following user(s) said Thank You: Craig
Last edit: 3 years 9 months ago by Alfred.
3 years 9 months ago #54834
Attachments:

Please Log in or Create an account to join the conversation.

Time to create page: 0.706 seconds