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Debunking dithering!


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What is Dithering?
Dithering is the process of moving the image’s location across images.  The result is that the noise generated by the camera will be in different location for ALL of your images. When you stack your images, the noise will be taken out more efficiently because it's never at the same location.  Without dithering, you end up stacking your noise and dark subtraction can only do so much.

BackyardEOS/BackyardNIKON sends a signal to PHD to tell it to move the “lock position” (the position of the crosshairs in PHD) by a small, random amount.  Once PHD has moved the star and reestablished stable guiding in the new location, BackyardEOS/BackyardNIKON will resume imaging.

Please see this thread on how to configure dithering.


What are all those numbers from 0 to 2.55 for?
The number you see in the progress wheel during dithering is the distance PHD is reporting and is indicative of where PHD is from its new location (chosen at random by PHD). The values are from 2.55 to 0.00. At 2.55 PHD is reporting that it is still at its original location of the guide star… at 0.00 PHD is at the new location.

In reality however, PHD will almost never hit the new intended target dead-on… this is where the "Settle dither at" variable comes into play. If set at 0.20, the dithering process will terminate successfully when the new location reaches a distance of 0.20 away from the new location. Because everything here is random anyway, the fact that PHD rarely settle to its absolute intended new location is irrelevant… because the selection of the new location was random too ~ so all is good here.

PHD moves the guide star once per interval, so if you are guiding at 0.5 versus 2 or 3 second interval then the amount of time it takes to dither will vary greatly; 1 minute is not abnormal at all.


Can I change some Dither settings?
Yes you can.  See the setting dialog in BackyardEOS/BackyardNIKON

PHD dither on start-up: This parameter will automatically turn ON (or OFF) the BackyardEOS/BackyardNIKON dither feature each time you start BackyardEOS/BackyardNIKON.  You still need to check the ‘Enable Server’ in PHD.

Dither aggressiveness: The dither sent will be a random number of pixels in X and Y, scaled by the level you set here.  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 image at, this will usually have a much larger effect in your main images.

Settle dither at: This parameter specifies how far off the star can be from the lock position before Dithering is considered a success.  Typical value is between 0.10 and 0.30.

Calm down period: A set number of seconds to pause between dithering and the next image capture.  This is particularly useful to let PHD stabilize properly.
 

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The common advice posted for DSLR Imaging is to use large / aggressive Dithering (3-5 on the "Dither Aggressiveness" value - which PHD uses as a multiplier against the "Dither Scale" setting on the "PHD Brain" Global tab).  This large Dither will better randomize the Noise recorded during each Exposure while allowing the Dark Frames to remove the Hot/Cold Pixels (or randomizing them such that they get "suppressed" even if no Darks are used). (or so the "theory" goes...)

 

If Aggressive Dithers are specified, then Longer Calm-Down Delays will be required as there is greater chance to "upset" your Mount's Tracking - usually by aggravating an existing Backlash Issue. (If you have such issues in DEC, you can Enable PHD's "Dither in RA Only" setting.)  This does tend to chew into your Imaging time, unfortunately.

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Hello, I put a post on here last night about Dithering and a USB Port.  I spoke again with my friend about this and gathered a little more info.  On his version of BYE 3.0.1, he has a place under settings that says 3rd Party Intergrations and a place to insert his port number that he is using for BYE to Communicate with the auto guider to let the guider software know when each picture is finished so it can dither, and in return PHD will let BYE know that its finished dithering and to wait a set amount of seconds before the next picture......Whew....With all this being said, my version 3.0.3 does not have a 3rd party intergration under the settings tab?  Does this mean that the newer version 3.0.3 automatically finds this port or am I still out in left field somewhere.   I need a clear night to get out and play with this.....Thanks for your help...I hope I am not confusing anyone, BYE is the greatest software that I've come across very easy to use but I will admit, I am confused a bit about this......Glenn

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Hello, I put a post on here last night about Dithering and a USB Port.  I spoke again with my friend about this and gathered a little more info.  On his version of BYE 3.0.1, he has a place under settings that says 3rd Party Intergrations and a place to insert his port number that he is using for BYE to Communicate with the auto guider to let the guider software know when each picture is finished so it can dither, and in return PHD will let BYE know that its finished dithering and to wait a set amount of seconds before the next picture......Whew....With all this being said, my version 3.0.3 does not have a 3rd party intergration under the settings tab?  Does this mean that the newer version 3.0.3 automatically finds this port or am I still out in left field somewhere.   I need a clear night to get out and play with this.....Thanks for your help...I hope I am not confusing anyone, BYE is the greatest software that I've come across very easy to use but I will admit, I am confused a bit about this......Glenn

 

 

3rd Party integration is a Premium feature.  If you do not see it and you have BYE 3.0 or above it means you have the Classic Edition.

 

Your friend is misleading you.  The 3rd party integration configuration has nothing to do with dithering, it is for Plate Solving with AstroTortilla.  It seams to me like you should concentrate on guiding first with PHD and forget about dithering until you master guiding with PHD and you master BackyardEOS controlling your Canon DSLR independent from one another.

 

Hope this helps.

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I'm using a 820mm refractor as the guide scope with my Canon and 200mm lens piggy back for imaging.  Given what you state above about the focal length for guiding usually being lower than imaging, should I dramatically increase my dither aggressiveness setting?

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The goal with DSLR dithering is to move the scope enough so that the stars are on a different set of 4 pixels which are unrelated, in that they are not combined during de-Bayering.  In your case you should be able use less aggressive dithering because your guiding focal length is greater than your imaging focal length.  I would think that the default aggressiveness parameters in PHD should be fine.

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I don't know about how the settings in BYE that are passed to PHD2 differ between spiral and random dithering, but I believe the point of spiral dithering is to keep PHD from "wandering" away from the original lock point as a result of the random directional changes which can accumulate over a large number of images.

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It does not make a difference for BYE/BYN because PHD has always been 100% in control of the actual dithering.  

 

My only guess is that spiral may try to keep the dithering closer to the starting point where random may eventually move your target further from the center as the night goes on.  This is just a guess on my part.

 

Regards

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PHD2 "Spiral Guiding" is only just making it's debut in a Dev Snapshot release - v2.6.1d4 - not even the "Latest Release" yet.

 

Honestly, it appears to have been added without much of a discussion/description on the Open PHD Google Group.

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You imply above that the maximum dither is + or - 1.5 pixels on the guide camera. Is there any way to increase this further? My guide and imaging focal lengths are very similar. 

 

In the past I have used very large manual dithers (unlock the guide star move the mount and then re-lock the guide star) to get rid of a single large artefact within my imaging DSLR caused by a mono-conversion, flat frames don't quite fully cut it, so up to 50 pixels or so between frames? For me this fully removes the issue when used along side Kappa Sigma Clipping. 

 

Also I am assuming that such a large movement could result in the image moving away from my original framing over 20 or so images?

 

Thanks!

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Adam,

 

I would suggest upping the Dither Scale in PHD2's parameters to increase the dither movement.

Hi Astroman133,I just cant see that setting in PHD2 I am running 2.6.1.  I only see enable server under the tools menu.

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I have read the above on dithering plus on some other forums;  I still have questions;  I am not sure what the dithering scale on BYEOS means and what the multiplier on PhD means.  Here is my example: I have a guide scope imaging at 2arcsec/pixel and in this example imaging scope the same at 2arc sec.  If I want to move my imaging scope pixels with each dither of 4 pixels this implies a 8 arcsec move in both instruments. So what does the scaler in BYEOS  mean. SO does the use of 2 in BYEos mean it will ask phd to move  the guide star by 2 pixels multiplied by the scaler used in phd?  I am only ditheirng in  RA. 

thanks cjacobson

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When you talk about the dithering scale in BYE, I am assume that you mean the Dither Aggressiveness parameter. This is explained in the first post in this thread, which says:

Dither aggressiveness: The dither sent will be a random number of pixels in X and Y, scaled by the level you set here.  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 image at, this will usually have a much larger effect in your main images.

The allowable values range from 1 to 5 where 1 means +- 0.5 and 5 means +- 1.5 pixels. So a value of 2 would be +- 0.75, 3 would be +- 1.0, and 4 would be +- 1.25.

In PHD2, the Dither scale is an optional multiplier used to adjust the maximum-dither amount specified by the image application.

If I am interpreting this correctly, to specify a dither move between -4 pixels and 4 pixels you would want a BYE Dither Aggressiveness of 5 (1.5 pixels) and a Dither Scale in PHD2 of 2.7. Since your guiding resolution and your imaging resolution are the same, this should work for you.

However, for others, if the imaging and guiding resolutions were different, you would want to take that into account. For example, if there are 5 imaging pixels per guiding pixel, you would want to reduce the amount of the max dither move by a factor of 5. You could do that by adjusting the move amount from 4 pixels to 0.8 pixels (a factor of 5). To achieve this you could set the Dither aggressiveness in BYE to 2, and the dither scale in PHD2 to 1.07.

That is, if all works as advertised and I am interpreting it correctly.

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Here's a perhaps interesting question - what if I use BackyardEOS and my ASCOM imaging rig is not actually using PHD or any other autoguiding software?  That is, the mount tracks extremely well by itself - no guiding is necessary.

How can I make BYEOS dither? Just a teeny little nudge in a random direction should suffice, once in a while, between exposures.

 

Grant

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