Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

How often do you dither with ASI1600MM?

  • Please log in to reply
45 replies to this topic

#26 Der_Pit

Der_Pit

    Apollo

  • *****
  • Posts: 1,105
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 17 August 2019 - 11:41 AM

Yeah, unity is freakin high for LRGB  man. You should use 76 at most, but I would try Gain 0. You just don't have the dynamic range you need for LRGB. Unity is fine for NB, but you are indeed going to saturate FAST with LRGB filters.

 

I've used Gain 0, and swamped the read noise squared 50, 70, in some cases 100x. Swamp factor, assuming you at least get enough, becomes largely meaningless with LRGB even under moderate LP. You just need long-enough exposures that you aren't having to get a ton of them. If you are getting 30-60 second subs right now at unity, you should be at 120-240 seconds at Gain 0. 

Yes, I know it's high (gain).  But I shied back from losing faint objects in the noise and/or posterization. Also ran a test of 30s gain 139 vs. 60s gain 76.  But honestly liked the 30s result better.  But I'll re-do that, this thread made me suspicious that the 60s one might indeed have had too little dither positions.  Might then as well do a 120s gain 0 run :D



#27 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 24,030
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 17 August 2019 - 12:08 PM

Yes, I know it's high (gain).  But I shied back from losing faint objects in the noise and/or posterization. Also ran a test of 30s gain 139 vs. 60s gain 76.  But honestly liked the 30s result better.  But I'll re-do that, this thread made me suspicious that the 60s one might indeed have had too little dither positions.  Might then as well do a 120s gain 0 run laugh.gif

You wouldn't lose faint objects. You are compensating with the longer exposures. Plus, you have quite a bit more  analog DR at gain 0...12.54 stops vs. 11.36 stops. So you can use even longer exposures if you feel you need to. But, therein lies the point...longer exposures. And keep in mind, if you were using 60s subs at unity for RGB, you could be using at least 240s subs at Gain 0....but possibly more, maybe 300s or so. 


  • Der_Pit likes this

#28 stickygerbil

stickygerbil

    Lift Off

  • -----
  • Posts: 11
  • Joined: 04 Feb 2017
  • Loc: Seattle, WA

Posted 17 August 2019 - 12:17 PM

It really depends on how I am imaging that night. I live fairly north so in the summer I only get so much night (2.5 hours true dark in July). Winter I get upwards of 12 hours. So during summer where imaging time is at a premium, I only dither on the fourth frame. Normally after I've shifted through all my filters back to my lum frame. In winter time where I focus on one channel first, before heading to the next, I dither about every 3rd frame. One of my buddies does it every frame but that's him. 


Edited by stickygerbil, 17 August 2019 - 12:18 PM.


#29 GiorgioF

GiorgioF

    Explorer 1

  • -----
  • Posts: 73
  • Joined: 15 Jan 2019

Posted 17 August 2019 - 01:23 PM

You wouldn't lose faint objects. You are compensating with the longer exposures. Plus, you have quite a bit more analog DR at gain 0...12.54 stops vs. 11.36 stops. So you can use even longer exposures if you feel you need to. But, therein lies the point...longer exposures. And keep in mind, if you were using 60s subs at unity for RGB, you could be using at least 240s subs at Gain 0....but possibly more, maybe 300s or so.

Hi John, a quick and dirty question.
In case of so long exposure, even with low gain, how to deal with LP?
Example: I'm under a Bortle 8 to 9, gain say 76 or even 0, long exposure in LRGB like 120 are not to much? 300 sounds a huge..... Sorry if stupid but just for understand.

#30 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 7,354
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 17 August 2019 - 01:45 PM

Not even remotely close to enough. No more than every 3 frames. If you are stacking 100 or less, every 2 frames. With exposures 5 minutes or longer, you might as well dither every frame, the overhead is not that high and the more  you dither the better your results will be. 

 

BTW, if dithering is taking as long as an exposure, you have something misconfigured. Dithering should take 5-10 seconds. It will vary a bit, as each dither is different, a big one takes longer to settle than a small one. But it should never take as long as an exposure (unless your exposures are just 10 seconds long). You should not be trying to settle below your guide RMS. If you are, that is an exercise in true futility, and you are  in effect just timing out and the settling operation will just give up after a while. If your guide RMS is say 1.4 pixels, and you are trying to settle at 0.25 pixels, you are never gonna settle properly. You need to settle at 1.4 pixels in this example. 

 

Jon,
There isn't much of a time penalty for dithering so why wouldn't you dither more often than 2-3 frames--like every frame?  Dithering allows spatially averaging out FPN so the important thing is to make sure that the mean dither distance is greater than autocorrelation distance of the FPN. I believe that going from dithering every 2 frames to every frame should further reduce the uncorrelated spatial noise by sqrt(2).

 

John



#31 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 9,156
  • Joined: 12 Oct 2007

Posted 17 August 2019 - 05:38 PM

If you have a given amount of FPN, sigma, in each frame then an averaged non-dithered stack will have sigma pattern noise in it - along with other noise terms.

 

If you dither every frame it will have roughly sigma/sqrt(N) pattern noise.

 

If you dither every other frame it will have sigma/sqrt(N/2) pattern noise.

 

So the pattern noise is roughly reduced by an additional factor of sqrt(2) when you dither every frame instead of every other frame - but that is a sqrt(2) factor on something that has already been reduced to a tiny amount.

 

If the pattern noise in the stack is already tiny then that extra factor is negligible because other noise terms dominate - and they aren't affected by dithering.

 

Whether it is worth doing every frame will - as usual - depend on many factors, including exposure time, number of frames, time to dither - etc.

 

All these factors are included in my StackSNR tool so you can estimate the trade off of added exposure time vs. greater dithering.  It accounts for the added time to dither, dithering every N frames, and the sqrt(NDither) reduction of noise.

 

https://freestar8n.s...ps.io/StackSNR/

 

Frank


Edited by freestar8n, 17 August 2019 - 05:50 PM.

  • happylimpet and NuclearRoy like this

#32 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 24,030
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 17 August 2019 - 07:26 PM

Jon,
There isn't much of a time penalty for dithering so why wouldn't you dither more often than 2-3 frames--like every frame?  Dithering allows spatially averaging out FPN so the important thing is to make sure that the mean dither distance is greater than autocorrelation distance of the FPN. I believe that going from dithering every 2 frames to every frame should further reduce the uncorrelated spatial noise by sqrt(2).

 

John

There is a time penalty if your exposures are short. The individuals in this thread are using exposures as short as 30 seconds with 20+ seconds of dithering overhead...so, it is a severe penalty. 


  • happylimpet likes this

#33 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 24,030
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 17 August 2019 - 07:28 PM

Hi John, a quick and dirty question.
In case of so long exposure, even with low gain, how to deal with LP?
Example: I'm under a Bortle 8 to 9, gain say 76 or even 0, long exposure in LRGB like 120 are not to much? 300 sounds a huge..... Sorry if stupid but just for understand.

I am just going by his current exposures at Gain 139 (unity), and factoring in the FWC difference (20000/4095) or 4.88. With that, given 60s RGB subs at unity, he should be able to get 60*4.88 or 293s subs at Gain 0. With the same clipping. For less clipping, I would go with 240s. 



#34 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 9,156
  • Joined: 12 Oct 2007

Posted 17 August 2019 - 08:21 PM

For people who are waiting for the guidestar to settle after a dither - and who are using guide exposures of 15 seconds or something - even the guide exposure time will play a role in the duty cycle because if you check for settling based on guide images after dither, the delay will be in multiples of that guide exposure time - i.e. 15s, 30s etc. - and it will never be 0.

 

When you combine dec. backlash with short target exposure times and long guide exposure times - dithering could have a strong impact on how many frames you can expose in a given time.  It's another reason to use a bright guidestar and short guide exposure times, so that as soon as the mount settles on the guidestar you can start the next exposure.

 

Frank



#35 OldManSky

OldManSky

    Mercury-Atlas

  • *****
  • Posts: 2,601
  • Joined: 03 Jan 2019
  • Loc: Valley Center, CA USA

Posted 17 August 2019 - 08:25 PM

Jon,

 

To your point but not to drift off topic although this is all related, my mount is actually having difficulties settling (it failed to settle in most instances las imaging session) with the following settings:

 

- settle pixel tolerance: 1.5

 

- minimum settle time: 10 sec

 

My RMS for that oscillated between 0.7 and 1.6"

 

Do you have any recommendations? 

 

Thanks

I have one: deal in the same units.

Above your settle tolerance is in pixels, your RMS error is in arcsecs...


  • Jon Rista likes this

#36 jdupton

jdupton

    Vanguard

  • *****
  • Posts: 2,171
  • Joined: 21 Nov 2010
  • Loc: Central Texas, USA

Posted 17 August 2019 - 09:08 PM

Frank,

 

For people who are waiting for the guidestar to settle after a dither - and who are using guide exposures of 15 seconds or something - even the guide exposure time will play a role in the duty cycle because if you check for settling based on guide images after dither, the delay will be in multiples of that guide exposure time - i.e. 15s, 30s etc. - and it will never be 0.

 

When you combine dec. backlash with short target exposure times and long guide exposure times - dithering could have a strong impact on how many frames you can expose in a given time.  It's another reason to use a bright guidestar and short guide exposure times, so that as soon as the mount settles on the guidestar you can start the next exposure.

 

   It is an interesting coincidence that you mention this.

 

   Lately, one feature I wish guide software offered (that I am not aware of) is to take exposures of length E but either stack and accumulate or just use the latest E*N for guiding where I can pick the number N. Thus with 2 second exposures you could settle in a reasonable time but only issue a guide command every Nth exposure time. I would find this useful for helping with speeding up Dither Settle time but don't need to guide my mount every 2 seconds. I would rather have my guiding adjustment every 6 to 10 seconds as my mount mount tracks very well. I don't need to use use such short exposures but am forced to do so anyway for the very reasons you state.

 

 

John



#37 freestar8n

freestar8n

    Vendor - MetaGuide

  • *****
  • Vendors
  • Posts: 9,156
  • Joined: 12 Oct 2007

Posted 17 August 2019 - 09:37 PM

Frank,

 

 

   It is an interesting coincidence that you mention this.

 

   Lately, one feature I wish guide software offered (that I am not aware of) is to take exposures of length E but either stack and accumulate or just use the latest E*N for guiding where I can pick the number N. Thus with 2 second exposures you could settle in a reasonable time but only issue a guide command every Nth exposure time. I would find this useful for helping with speeding up Dither Settle time but don't need to guide my mount every 2 seconds. I would rather have my guiding adjustment every 6 to 10 seconds as my mount mount tracks very well. I don't need to use use such short exposures but am forced to do so anyway for the very reasons you state.

 

 

John

Hah - well that is how MetaGuide works.  It is constantly receiving images from the guide camera as a video stream and it always has a live view of the star.  Every time I measure the centroid it is based on the previous N video frames.  For me with OAG and a good guidestar I am usually viewing at 10 frames per second - and I make guide corrections every 1 second.  I could go ahead and calculate the controid in the live stream so it is always "fresh" and based on the very latest stream of images - but 1 second granularity is fine for watching the dither settle.  

 

As far as I know all other guide software takes each guide exposure on demand - then analyzes it and waits until the next guide period and exposes again.  I assume they could add a "watch the dither settle" mode where they don't make corrections and instead just calculate the centroid - but I think it might be hard to work that in.  And if people really are using 10-20 second guide exposures - and not just making corrections at that rate with shorter exposures - there wouldn't be much benefit anyway.

 

There are many advantages to having a constant stream of guidestar images coming in live - and handled continuously in a separate thread so you can grab the latest measurement any time you want - and it is fresh.  It is also helpful to be able to watch the dither happen live and in realtime because you can see when dec. backlash is limiting things - and how it behaves.  I will try to post videos of this stuff sometime.

 

Frank


  • jdupton likes this

#38 georgian82

georgian82

    Viking 1

  • *****
  • topic starter
  • Posts: 619
  • Joined: 16 Jan 2016
  • Loc: Healdsburg, California

Posted 17 August 2019 - 09:47 PM

I have one: deal in the same units.
Above your settle tolerance is in pixels, your RMS error is in arcsecs...


Thanks for pointing that out...is there an equation to change arcsec to pixels?

Thanks

#39 Stelios

Stelios

    Cosmos

  • *****
  • Moderators
  • Posts: 8,217
  • Joined: 04 Oct 2003
  • Loc: West Hills, CA

Posted 18 August 2019 - 12:27 AM

Thanks for pointing that out...is there an equation to change arcsec to pixels?

Thanks

Yes, divide by guider image scale. (Image scale: 206.3*pixelsize_in_um/focallength_in_mm).



#40 GiorgioF

GiorgioF

    Explorer 1

  • -----
  • Posts: 73
  • Joined: 15 Jan 2019

Posted 18 August 2019 - 12:49 AM

I am just going by his current exposures at Gain 139 (unity), and factoring in the FWC difference (20000/4095) or 4.88. With that, given 60s RGB subs at unity, he should be able to get 60*4.88 or 293s subs at Gain 0. With the same clipping. For less clipping, I would go with 240s.


Ooook... I suppose I misunderstood.
Now is clear (almost).
Thanks for the patient.

#41 Der_Pit

Der_Pit

    Apollo

  • *****
  • Posts: 1,105
  • Joined: 07 Jul 2018
  • Loc: La Palma

Posted 18 August 2019 - 04:57 AM



When you combine dec. backlash with short target exposure times and long guide exposure times - dithering could have a strong impact on how many frames you can expose in a given time.  It's another reason to use a bright guidestar and short guide exposure times, so that as soon as the mount settles on the guidestar you can start the next exposure.

Unfortunately, there's those of us who happen to have an iOptron EC mount that excludes short exposures (or rather, rapid guide pulses) frown.gif

 



   Lately, one feature I wish guide software offered (that I am not aware of) is to take exposures of length E but either stack and accumulate or just use the latest E*N for guiding where I can pick the number N. Thus with 2 second exposures you could settle in a reasonable time but only issue a guide command every Nth exposure time. I would find this useful for helping with speeding up Dither Settle time but don't need to guide my mount every 2 seconds. I would rather have my guiding adjustment every 6 to 10 seconds as my mount mount tracks very well. I don't need to use use such short exposures but am forced to do so anyway for the very reasons you state.

I had thought that the Z-filter algorithm of PHD2 was supposed to do this, but when I tried, it didn't do what I expected.  Maybe user error, I don't know.  Have you tried it?

But of course such a software then also would have to have a dither-settle specific part where it would correct after every frame, or you wouldn't gain much speed. 


  • jdupton likes this

#42 jhayes_tucson

jhayes_tucson

    Fly Me to the Moon

  • *****
  • Posts: 7,354
  • Joined: 26 Aug 2012
  • Loc: Bend, OR

Posted 18 August 2019 - 09:07 AM

There is a time penalty if your exposures are short. The individuals in this thread are using exposures as short as 30 seconds with 20+ seconds of dithering overhead...so, it is a severe penalty. 

Ah, ok...I didn't see any reference to short exposures in the OP's question.  Your previous estimate of dithering time "Dithering should take 5-10 seconds."  seems closer to the right answer.  My system rarely takes more than about 5 seconds for a dither cycle--unless I'm using an especially faint guide star.  It seems like a good rule of thumb would be to dither at no more than about 10% of the total exposure time.  That would tell you how often to dither.  For example, if your dither time is 10s and your subs are greater than 100s, then dither every frame. If your subs are ~50 seconds, then dither every other frame.

 

John


  • PilotAstronomy and Der_Pit like this

#43 PilotAstronomy

PilotAstronomy

    Vostok 1

  • *****
  • Posts: 105
  • Joined: 25 Sep 2017

Posted 18 August 2019 - 11:52 AM

Hyperstar in LP means my subs are super short LRGB and even not all that long NB. The balance I have been using with great results is to dither every 3 minutes (rounding down if necessary). If subs are 60 seconds, that’s every 3rd sub. If 2 minutes, I dither every sub. If subs are 30 seconds, it’s every 6 subs. I usually do one filter per night. In the winter that adds up to a TON of frames sometimes. This offsets the dithering less often than 3. I notice zero FPN in my results.

 

(note: this is from my previous location in Las Vegas. I have a lot less light pollution here but in the couple months that I havnen’t had a chance to get out and image a lot)


Edited by PilotAstronomy, 18 August 2019 - 12:12 PM.

  • Der_Pit likes this

#44 Jon Rista

Jon Rista

    ISS

  • *****
  • Posts: 24,030
  • Joined: 10 Jan 2014
  • Loc: Colorado

Posted 18 August 2019 - 12:43 PM

Ah, ok...I didn't see any reference to short exposures in the OP's question.  Your previous estimate of dithering time "Dithering should take 5-10 seconds."  seems closer to the right answer.  My system rarely takes more than about 5 seconds for a dither cycle--unless I'm using an especially faint guide star.  It seems like a good rule of thumb would be to dither at no more than about 10% of the total exposure time.  That would tell you how often to dither.  For example, if your dither time is 10s and your subs are greater than 100s, then dither every frame. If your subs are ~50 seconds, then dither every other frame.

 

John

There were other posts where guide exposures were stated to be around 20s. That seems extremely long for a guide exposure to me, but if it takes 3-4 guide pulses to get the star back to lock and settled, then a 20s guide exposure would mean it takes 60-80 seconds to settle!! :o I have been using 2.5 second guide exposures myself, so I am usually settling on an average of around 10 seconds.



#45 Morefield

Morefield

    Vostok 1

  • -----
  • Posts: 148
  • Joined: 01 Jun 2014
  • Loc: Portland, OR

Posted 18 August 2019 - 01:11 PM

Funny enough I dither every frame and wouldn’t even know how to dither less frequently. The time cost is maybe 5 seconds. If there was a reason to dither less frequently it would be a matter of the number of frames in your channel stack. I tend to use a rejection of > 2.2 STD dev and a max of 10-15% of the stack.

I guess the question is how many uncorrected pixels, cosmic rays hits and airplane trails are going to overlap the same pixel? Certainly an uncorrected pixel will overlap if dithering is not done.

If PHD is having trouble returning to the guide star after the dither, maybe you could dither less? I guide with CCDAP and TSX, so maybe it works differently. My dither is 4 pixels.

#46 georgian82

georgian82

    Viking 1

  • *****
  • topic starter
  • Posts: 619
  • Joined: 16 Jan 2016
  • Loc: Healdsburg, California

Posted 18 August 2019 - 03:18 PM

Yes, divide by guider image scale. (Image scale: 206.3*pixelsize_in_um/focallength_in_mm).


Got it...thanks.

My guider image scale is 4.60. When I divide my RMS by this number I get a value of 0.2 pixel which is way below the tolerance value of 1.5 pixels I have set it at so I am not sure why my mount is not settling. Maybe I need to increase the settling time to 15 or 20 seconds?

Also, is there a window setting in PHD2 where I can input dithering settling data? The tolerance value is set in NINA but not PHD2...

Thanks


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics