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Any point in dithering if you're not auto-guiding?

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#1 belliott4488

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Posted 25 February 2021 - 03:16 PM

Newbie question: If I'm not auto-guiding (I'm not), is there any benefit to dithering my exposures, or will slight inaccuracies in tracking likely produce the same effect?



#2 qswat72

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Posted 25 February 2021 - 03:19 PM

I would say yes. I dithered on the SA before I started autoguiding, and never had issues. I don’t think that tracking inaccuracies would be enough to do what actual dithering would.
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#3 imtl

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Posted 25 February 2021 - 03:21 PM

Dithering has nothing to do with guiding or tracking. Mount inaccuracies accumulate during the sub. Dithering is shifting between subs.


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#4 Faris

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Posted 25 February 2021 - 03:21 PM

Random dithering will improve your images significantly. You will find the noise is reduced and can get better images that way.


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#5 jonnybravo0311

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Posted 25 February 2021 - 03:30 PM

OK, I'll piggyback and ask a newbie question of my own. How are you dithering if you aren't auto-guiding? Are you using the hand controller or software to manually adjust the RA/Dec every so often?


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#6 imtl

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Posted 25 February 2021 - 03:36 PM

Dithering has noting to do with auto-guiding. You can directly do dithering with many image capturing programs. SGP, APT etc

If and when using an auto-guiding program like PHD2 for example, the only thing it does is change the expected position of the guide star randomly in RA and DEC. And then sends a guide pulse to put it back in center. Thus dithering.


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#7 belliott4488

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Posted 25 February 2021 - 03:38 PM

Thanks for all the prior responses - it sounds like dithering typically involves greater shifts of the target object in the field of view than you'd be likely to see between subs due to tracking inaccuracies - correct? 

 

That leads me to jonnybravo0311's question - is it sufficient just to give the mount a little poke with the hand controller between subs?



#8 imtl

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Posted 25 February 2021 - 03:40 PM

Thanks for all the prior responses - it sounds like dithering typically involves greater shifts of the target object in the field of view than you'd be likely to see between subs due to tracking inaccuracies - correct? 

 

That leads me to jonnybravo0311's question - is it sufficient just to give the mount a little poke with the hand controller between subs?

That is going to be a very inefficient way of doing it. And how are you planning to randomize it? If you are baby sitting your setup and are bored then sure you can do it. I think you will drop it quite fast.


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#9 Alex McConahay

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Posted 25 February 2021 - 03:41 PM

Dithering is moving the scope around randomly. Most (what you call) "slight inaccuracies in tracking" are not. They are the result of something that tends to repeat itself. 

 

Random hot spots can be pretty well eliminated by dithering. Those that follow a pattern (what you get when the slight inaccuracies repeat) might well leave a pattern. This looks like "walking noise" or other patterns in your stacked images. 

 

As for a little poke, Yes, this will help. But, it makes more sense to dither randomly. Unless you are completely without computer managed sessions, just do it. 

 

Alex


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#10 jonnybravo0311

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Posted 25 February 2021 - 03:43 PM

Dithering has noting to do with auto-guiding. You can directly do dithering with many image capturing programs. SGP, APT etc

If and when using an auto-guiding program like PHD2 for example, the only thing it does is change the expected position of the guide star randomly in RA and DEC. And then sends a guide pulse to put it back in center. Thus dithering.

 

Thanks for all the prior responses - it sounds like dithering typically involves greater shifts of the target object in the field of view than you'd be likely to see between subs due to tracking inaccuracies - correct? 

 

That leads me to jonnybravo0311's question - is it sufficient just to give the mount a little poke with the hand controller between subs?

I think Eyal just answered it :). I just assumed dithering was managed by PHD2 and all the imaging program did was say, "Hey, PHD2, do a random dither of X pixels. Thanks!"

 

What it sounds like is the imaging program itself can dither. Always learning in this hobby!



#11 imtl

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Posted 25 February 2021 - 03:46 PM

I think Eyal just answered it smile.gif. I just assumed dithering was managed by PHD2 and all the imaging program did was say, "Hey, PHD2, do a random dither of X pixels. Thanks!"

 

What it sounds like is the imaging program itself can dither. Always learning in this hobby!

If you use PHD2 that's exactly what they tell it to do. But you don't have to use PHD2 or any other guiding software to dither. You can send random commands to the mount directly.



#12 qswat72

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Posted 25 February 2021 - 04:05 PM

OK, I'll piggyback and ask a newbie question of my own. How are you dithering if you aren't auto-guiding? Are you using the hand controller or software to manually adjust the RA/Dec every so often?

For me at least, I had it fairly easy. The SA is able to randomly dither between frames if you connect your camera to the mount.

#13 Ken Sturrock

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Posted 25 February 2021 - 04:30 PM

What it sounds like is the imaging program itself can dither. Always learning in this hobby!

Right. It will depend on the software used. Some software dithers by "guiding the mount" to a new location between frames. Other software just jogs the mount between frames (but has to deal with the overhead of stopping/re-starting guiding if applicable).



#14 belliott4488

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Posted 25 February 2021 - 05:38 PM

I guess I should have made it clear in my OP that I'm just running a DSLR with no computer automation at all during imaging.

 

The only automation I have is my tracking mount and an intervalometer for the camera. So - unless I'm going to sit next to the scope and occasionally give it a random slew, I'm probably not going to be dithering.

 

I'm aware that I'm not going to produce images of the quality one gets with a more sophisticated setup, but I'm okay with that for now.



#15 Peregrinatum

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Posted 25 February 2021 - 07:28 PM

yes, dithering is a simple way to improve your image that requires no effort on your part



#16 Alex McConahay

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Posted 25 February 2021 - 07:55 PM

Okay, Bruce-----Just do what you can do. We have all been there. If you do not have a computer running the session (and dithering the mount), I would suggest you take the following steps:

 

Be sure you have good calibration frames AND USE COSMETIC CORRECTION. This will tend to clean up the outlying pixels. Dithering really helps in this process because it moves those outliers around so that even if they are still around, they are ignored in the stacking. Calibration and Cosmetic Correction correct for the issues, but still leaves some anomaly there. Those traces may not be enough to be ignored. And the traces of them are harder to reject if they are always on the same pixel. 

 

Use aggressive pixel rejection in your stacking. 

 

(And as soon as you can beg, borrow, or acquire an old laptop, even a poopy old one, hook it up to your rig to dither and take pictures.) 

 

Alex



#17 BQ Octantis

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Posted 26 February 2021 - 02:48 AM

The answer is yes.

 

I hate the hardware bloat of autoguiding. But shooting at 200mm f/2.8 on my APS-C DSLR, I can get away with 30 sec subs with just PEC on the mount. Without dithering, the fixed pattern noise (residual calibration error primarily from temperature mismatch between lights and darks) is "walked" across the field in the direction of the pointing drift (principally along the RA axis). But during a 1-4 hour session, if I just do 1-3 manual offsets in pointing of 10's of pixels in a random RA+DEC direction (every 20 minutes or so)—coupled with sigma-reject stacking—the fixed pattern "rain" / "walking" noise is eliminated.


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#18 T~Stew

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Posted 26 February 2021 - 11:36 AM

OK, I'll piggyback and ask a newbie question of my own. How are you dithering if you aren't auto-guiding? Are you using the hand controller or software to manually adjust the RA/Dec every so often?

One neat thing about the current version of the Star Adventurer tracker is it will dither built-in, no need for computer or apps other than its own app that runs on your phone. Of course its just RA only, but helps a little for those just starting out with tracker.

 

Speaking of dithering with just a tracker, if I may piggyback with my own question... so I have my tracker set up with dithering in RA. I haven't tried yet but I could go out and in between subs turn the DEC adjustment just slightly. I don't babysit my setup so wouldn't do it every frame (I only dither RA every 3rd shot anyhow) but what would be the minimum number of shifts required to make a worth while manual dither? Say if I take 30 to 40 5-minute subs, would it be any benefit to dither DEC say 3 times? Or just not worry about it?


Edited by T~Stew, 26 February 2021 - 11:41 AM.


#19 Alex McConahay

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Posted 26 February 2021 - 01:26 PM

>>>>>>>One neat thing about the current version of the Star Adventurer tracker is it will dither built-in, no need for computer or apps other than its own app that runs on your phone.

 

Just curious......How does the ap know when to dither? Is it also controlling the camera exposure?

 

Alex



#20 qswat72

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Posted 26 February 2021 - 03:06 PM

>>>>>>>One neat thing about the current version of the Star Adventurer tracker is it will dither built-in, no need for computer or apps other than its own app that runs on your phone.

Just curious......How does the ap know when to dither? Is it also controlling the camera exposure?

Alex

Yep that’s exactly how it works. You connect your camera to the mount via a shutter release cable, so it doubles as both an intervalometer and a star tracker. Then it just syncs its dithering with the imaging.
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