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Thread: PHD2 and Dithering

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    Default PHD2 and Dithering



    Hi Everyone,


    As a DigicamControl user, I just got some great information about dithering with the software from Peter on the OpenPHD Guiding group.
    Something that I am very curious about is how PHD reacts to a dither.

    If anyone on the forum could send me an image of what a dither looks like from a guide log and how the guiding settles down post dither, I would be very grateful. I have some good info to go on and I have played around with the app PHDMax and see the dither command in the PHD guide graph, but I really do not see a change in guiding at all. A normal graphic of a dither would be very helpful to me.

    Thank you very much for your help!

    Jim
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    Default Re: PHD2 and Dithering

    Here is a section of a recent guide log including a dither:
    48,208.067,"Mount",0.125,-0.042,-0.131,0.007,-0.092,0.000,107,E,0,,,,1323,25.53,0
    49,212.259,"Mount",0.040,0.041,-0.028,-0.050,0.000,0.000,0,,0,,,,1336,25.65,0
    50,216.576,"Mount",0.102,-0.089,-0.122,0.058,-0.086,0.000,100,E,0,,,,1333,25.65,0
    INFO: SET LOCK POSITION, new lock pos = 217.182, 215.125
    INFO: DITHER by 0.013, 0.000, new lock pos = 217.170, 215.129
    INFO: SETTLING STATE CHANGE, Settling started
    INFO: SETTLING STATE CHANGE, Settling complete
    51,220.807,"Mount",0.012,-0.003,-0.013,0.000,-0.013,0.000,15,E,0,,,,1337,25.70,0
    52,225.223,"Mount",0.277,-0.013,-0.270,-0.063,-0.189,0.000,220,E,0,,,,1349,25.81,0
    53,229.401,"Mount",0.045,0.030,-0.036,-0.041,0.000,0.000,0,,0,,,,1325,25.55,0
    54,233.710,"Mount",0.106,0.183,-0.052,-0.205,-0.037,-0.072,42,E,36,N,,,1328,25.59,0
    55,238.009,"Mount",0.089,0.009,-0.084,-0.033,-0.058,0.000,68,E,0,,,,1340,25.72,0
    56,242.387,"Mount",0.153,0.042,-0.136,-0.082,-0.095,-0.029,111,E,15,N,,,1328,25.63,0
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    Default Re: PHD2 and Dithering

    Thank you Kathy!

    So, your log looks very similar to mine. If my calculations are correct, my imaging scale is 6.6 arcsec/pixel for my guider and 1.1 arcsec/pixel for my imagine camera/scope. If I set my imaging software to dither at 3 pixels, would that not equate to 18 pixels on my imaging camera?

    This is what may PHD graphic looks like. Does this look anywhere near what a normal dither looks like?
    This dither was created with the PHDMax/PHD2 a few nights ago. I plan to try Digicamcontrol/PHD2 the next time that I am out. I just want to make sure that what I come up with looks like I am effectively dithering.



    Thanks a bunch!

    Jim
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    Default Re: PHD2 and Dithering

    I don't know what "normal" is for dithering. I use "medium dither" in SGP, with a scale factor of 0.1 in PHD2. I don't remember what that translates to in pixels, but I know that I wanted to keep the size of the dither small in order to minimize the settling time. There may be a good reason to use larger dithers, but it seemed to me that, if you are dithering for noise reduction, one pixel should be sufficient. I am quite happy with my results.
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    Default Re: PHD2 and Dithering

    Thank you Kathy.

    I definitely understand the desire to get the lowest settling time to get the most out of the imaging time.
    I have read in several sources that when using a dslr, dither at least 12 pixels in a non random way.
    I am just trying to get an idea what that might look like in a PHD graphic.
    Looks like the PHDMax app that I was using my last session was definitely creating a dither in PHD. I have been using a 60 second cool down or “pause” step in between 120 second exposures, so it looks like the settling did occur within that pause time.
    Thanks for sharing your thoughts and experience!

    Jim
    Scopes: Explore Scientific ED102 Triplet APO, Celestron Nexstar 130 SLT.
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    Default Re: PHD2 and Dithering

    For my DSLR images, I use a "dither scale" of 2, set to "random" with PHD2. With APT and BYEOS I used a dither distance of 2 which corresponds to () 1.0 pixel. These settings seem to work well with both my Nikon D5300 and Canons.

    It's worth noting that PHD2 accepts different dither levels from different control software. For example to quote "The dither sent from BYEOS will be a random number of pixels in X and Y, scaled by the level you set in the dither aggressiveness. In the lowest level, the random numbers will vary from -0.5 to 0.5 pixels and in the highest they will vary from -1.5 to 1.5 pixels in the guide frame. Since people typically guide at shorter focal lengths than they might image at, this will usually have a much larger effect in your main images. which can then be scaled with a multiplier in PHD2"...Ref https://www.otelescope.com/forums/to...ing-dithering/

    However, with APT to quote "The dither distance defines how big to be the move. This is the upper limit. By definition, dithering is random by direction and by distance. For PHD(2) if the distance is too small you can use the Brain button (Advanced Parameters) to enter "Dither scale". For each value, here is how much the dither size would be on the guiding chip (assuming you keep dither scale = 1.0):

    Dither distance/level = pixels
    1 ()0.5
    2 ()1.0
    3 ()2.0
    4 ()3.0
    5 ()5.0
    .....


    In PHD2 set the graph in pixels.

    For APT Dithering the value defines the max distance in pixels of the imaging camera

    For APT Pulse Dithering the value * 100 gives the max guiding pulse in milliseconds to execute

    As such the dither scale in PHD2 should be set to 1 and the required distance set in the APT control software." end quote.

    Ref... https://ideiki.com/astro/usersguide/...nd_guiding.htm

    My guide camera is running at 1.84 "/pixel (QHY5LII + 420mm Guide scope) and my imaging camera is running at 1.67 "/pixel (Nikon D5300 + 480mm scope), giving a guide-ratio of 1.1:1. Occasionally I'll drift align if I have time. For the most part, however, I'll use the EQMOD polar alignment tool. I can usually sit at lower than subpixel guiding quite comfortably using this method; typically <0.5px.

    I usually set my settling time for around 0.9px and leave the camera to pause for 60 seconds between shots. I leave the "settle" value high as I know that after an aggressive dither 60s is more than enough time for the guiding to settle completely as well as allowing the camera to cool a little, so no need to worry about the settling time.

    Typically, with my setup when dithering, I'll get a peak that will reach a maximum of 2px (usually lower). I believe the PHD2 will dither 0 to <=(dither level x dither scale). With both APT and BYEOS I have the dithering level set to 2 which is () 1.0 pixel according to the data provided above, multiply that by a dither scale of 2, my PHD2 scaling value, and we have () 2.0 pixels in terms what I'd expect to see on the graph when the software is dithering.

    Of course, if you increase the dither level in the control software or change the multiplying level in the PHD2 dither scale settings then you'll have to work out just what you'd expect to see on the graph.

    The above isn't exhaustive or extensive but is just how my setup works and what I'd typically expect to see.

    After writing this I think I'm going to try a higher value with my Canons.

    I hope this helps.

    YMMV

    G
    Last edited by Gmetric; 04-16-2019 at 05:16 PM.
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    Default Re: PHD2 and Dithering

    Hi G,

    It helps a tremendous amount. So much for what I have read on the internet that when imaging with a dslr, dither at least 12 pixels.
    At least I know that I can dither using PHDMax. I recently got some information from Peter on the OpenPHD Guiding group to set up my imaging software, DigicamControl, to dither with PHD.
    I need to go back and look at my imaging scale calculations. I must be off somewhere. I calculated guider/ imaging scope as I listed in a post above.

    Thanks G for the very helpful information.

    Thanks,
    Jim
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    Default Re: PHD2 and Dithering

    Quote Originally Posted by Juno16 View Post
    Hi G,

    It helps a tremendous amount. So much for what I have read on the internet that when imaging with a dslr, dither at least 12 pixels.
    At least I know that I can dither using PHDMax. I recently got some information from Peter on the OpenPHD Guiding group to set up my imaging software, DigicamControl, to dither with PHD.
    I need to go back and look at my imaging scale calculations. I must be off somewhere. I calculated guider/ imaging scope as I listed in a post above.

    Thanks G for the very helpful information.

    Thanks,
    Jim
    I'm glad that it helped a little, Jim.

    DSLR astrophotographers tend to use more aggressive dithering to compensate for the noise generated by hot sensors. TBH, I've never tried anything higher than 2 in both dithering distance and scale. As Kathy mentioned, if you want more time imaging then you'll need to find the balance that suits you in terms of imaging time vs processing time vs image quality. If you don't wait between exposures then you'll need to set your settling time appropriately, if you allow a cooldown time between exposures then you don't need to worry so much with the settling time as long as the guiding settles within the cooldown time.

    I noticed that my Canon T6I is very noisy and has a walking pattern in the noise. This usually means that more aggressive dithering is required. Something I need to try with it.
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    Default Re: PHD2 and Dithering

    Thanks a bunch G.

    My outing with M101 last week was the first time that I tried out dithering. I did not know that DigicamControl supported dithering, so I used PHDMax and it seemed to do the job. PHDMax sends a dither to PHD when it sees a file save. I can only shoot at 120 seconds with iso 800 due to my lp. I do use a 60 second cool down step. It looked like guiding settled down well within the cool down step.
    Thanks again for your insight and experience G.

    Take care,
    Jim
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