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Dithering and distance



I'm trying to use the DSLR processing procedures presented by Tony Hallas to the SCAE and recorded for Youtube.  To do this, dithering should move "two to three star diameters".  I'm not sure how to accomplish this.  Is this something that can be done in BYEOS or should I be doing it in PHD2?



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We'll take your Calculations as Accurate (they sound right)...


Yes, that is a rather large Dither Motion, but then you are getting your "Rule of Thumb" from a Tony Hallas presentation based on a much Narrower-FOV / Deeper-Space / Higher-Resolution Imaging Session.


OK, so...

The actual BYE-PHD2 Dither Interaction is described in this How-Tos Forum Thread (and also decently illustrated on this webpage)

Assuming you wish to keep to the 2-3 Star Diameters, or rather 4' of Dither-Motion - PHD2 Dither Operations are going to be based on the Resolution displayed by the Guider:

1) Using your stated desire of 4' of Dither size: (4' / 8.55"per-Guider-pixel) = (240" / 8.55"pp) = 28.07 Guider-Pixels for desired Dither

2) PHD2 Dither Motion is based on an internal Randomizer variable (0.0-1.0) times the "Dither Scale" setting (0.01-100.0) in the "PHD Brain" / "Global" Tab times the "Dither Aggressiveness" setting (1-5 in a PHD Lookup Table) in the BYE Dither Settings

3) Suggest:  PHD2 "Dither Scale" of "10.0" and a BYE "Dither Aggr." of "4" (results in 3x PHD "Dither Scale" randomized result) --> this should result in randomized Dithers of "up to 30 Guider Pixels"


You will also need to adjust BYE's "Dither Settle At" and "Calm Down Period" parameters.

1) Given the Ratio between your Imager and Guider Resolutions, you could well do a "Dither Settle At" of 0.5 (Guider Pixels) without any remaining "Settling" being seen as Star Trailing

2) The "Calm Down Period" setting should be based on the general performance of your Mount and your overall Guiding - select a time that doesn't lose you too much Imaging Time but is sufficient to usually allow the PHD2 Guiding Graph to settle into its Normal Rhythm


Of course, all of this is "General Guidance", and your Results will Vary based on factors of your Mount and Guiding and Seeing and...

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Wow, that' a lot to process but I think I have the picture, or least a good starting point.  Thank you very much for your detailed response.  Hopefully our rainy season will be over in a couple of weeks and I can get out and give this a try.  The learning curve never levels out does it?



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My best advice to you is don't try to do too much from the get go.


Try dithering with the default settings first.  And as you get more comfortable change some settings.


Tonny Hallas is a king in the industry... but any dithering is better than no dithering... and I don't really agree that you need 2 to 3 star diameter to get excellent result.  Just a few pixels will work wonders too.


Baby steps, start with the default setting, get to know dithering / PHD / BYE, then adjust some settings and because you went slowly in changing settings you will be able to compare with the results you had with the default setting... then you really get the feel of the impact of each settings.


Hope this helps,

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Uh, I'm seeing a number of different ways to look at this.  I settled on plugging values into Wodaski's CCD Calculator and got 5.31 arc-seconds/pixel for my Canon 450D with a 200mm lens and 8.55 arc-seconds/pixel for the Lodestar guider on the Antares 7x50 finder (yes I do a lot of wide field lately).  I zoomed in on an image taken previously and a medium sized star seems to be about 10 pixels across.  That would be about 53.1 arc-sec for one star diameter or a little over 2 1/2 arc-minutes for 3 star diameters.  My first thought is Wow that seems like a lot of distance for a dither, or have I done something wrong.  I would need to dither for about 4 arc-minutes with the Lodestar.  Assuming all my assumptions and arithmetic are correct I have no idea what parameters to use in PHD2 to make this happen.

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