Dithering Questions

[BenKolt]

Greetings!

I recently realized that I benefit from using larger dithering steps.  This has illuminated to me that I know very little about PHD2's dithering options and algorithm.  My acquisition software is SGP, which communicates dithering and settling information to and from PHD2.

First, I have set random dithering.  Does each dither step originate from the current guide lock position and then move a random direction by a random distance?  Or, is this random number applied to the original location?  By my observation it's the former, and this would mean in principle that if the dithering is large, the frame could eventually walk out so far that the target of interest is no longer in the frame.  Is this possible?

My next question may be an SGP one, but perhaps folks here know the answer.  When setting the dithering amount to minimal, medium, high, etc. (sorry I don't have the exact options in front of me just now), how much are these quantitatively in number of pixels?  I am also aware of the setting in PHD2 to multiply whatever these amounts are by a set number (1 by default) in order to control this even more.

Finally, in PHD2 there is a spiral setting.  Here, does the lock position spiral out continuously?  Here again I am concerned that in principle the guide position could walk itself out of the frame of interest.  Is this possible?

Thank you for your help understanding this.

Best Regards,

Ben

[bw_msgboard]

From: open-phd...@googlegroups.com [mailto:open-phd...@googlegroups.com] On Behalf Of BenKolt
Sent: Tuesday, September 04, 2018 11:36 AM
To: Open PHD Guiding
Subject: [open-phd-guiding] Dithering Questions

Greetings!

 

I recently realized that I benefit from using larger dithering steps.  This has illuminated to me that I know very little about PHD2's dithering options and algorithm.  My acquisition software is SGP, which communicates dithering and settling information to and from PHD2.

 

First, I have set random dithering.  Does each dither step originate from the current guide lock position and then move a random direction by a random distance?  Or, is this random number applied to the original location?  By my observation it's the former, and this would mean in principle that if the dithering is large, the frame could eventually walk out so far that the target of interest is no longer in the frame.  Is this possible?

 

The incremental dither is always relative to the current lock position – PHD2 doesn’t keep an internal history of lock positions.

 

My next question may be an SGP one, but perhaps folks here know the answer.  When setting the dithering amount to minimal, medium, high, etc. (sorry I don't have the exact options in front of me just now), how much are these quantitatively in number of pixels?  I am also aware of the setting in PHD2 to multiply whatever these amounts are by a set number (1 by default) in order to control this even more.

 

Finally, in PHD2 there is a spiral setting.  Here, does the lock position spiral out continuously?  Here again I am concerned that in principle the guide position could walk itself out of the frame of interest.  Is this possible?

 

I think in practice it would be hard to encounter the problem you’re worried about.  PHD2 won’t do a dither that moves the guide star beyond the edge of the guide frame.  For most users, the guide frame is considerably smaller than the main imaging sensor so even if they’re using an OAG, the field of view is usually much smaller.  Since PHD2 doesn’t know anything about the size or centering position of the main sensor relative to the guide frame, I guess no guarantees can be made – but I can’t really picture a practical scenario where you’d encounter this problem and I’ve never heard of one on the forum.

 

Hope this helps,

Bruce

 

Thank you for your help understanding this.

 

Best Regards,

Ben

 

 

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[Andy Galasso]

Hi Ben,

The dither amounts for Random dither are: +/- 0.5, 1.0, 2.0, 3.0, 5.0 guide camera pixels (multiplied by the dither scale factor on the Global tab in the brain).

For random dither PHD2 will move the lock position by a random amount on each axis (RA, Dec) relative to the current position.  It will never allow the lock position to move too close or off of the guide camera frame.  This dither method produces a random walk, with an average expected total distance from the starting position of [dither_size * sqrt(2 * num_dithers / pi)], IOW, the expected distance is proportional to the square root of the number of dithers.

In spiral dither mode, each dither follows a square spiral path relative to the starting position, with each step along the path being the exact dither amount (no randomness.) The total distance there too increases with the square root of the number of dithers; the distance at the corner of the spiral would be [dither_size * sqrt(num_dithers / 2)]

So in spiral dither mode if you did 64 dithers (that would be a lot of subs!) with amount = 1 and scale = 1, you'd be no further than 5.7 pixels from the starting position.

In random dither mode you could expect to be about 6.4 pixels from the starting point on average with 64 1-pixel dithers @ scale 1.0.

Andy

[BenKolt]

Thank you, Bruce and Andy!  Your replies were very helpful.  I understand the dithering options and behavior much better now.

Ben