#1  
Old 25-08-2020, 04:34 AM
pmrid's Avatar
pmrid (Peter)
Ageing badly.

pmrid is offline
 
Join Date: May 2008
Location: Cloudy, light-polluted Bribie Is.
Posts: 3,665
Some imaging help please

Here's a bit of a short stack on NGC346 in Ha (6 x 10 minutes only). They have been stacked without any darks of bias frames to see what the raw data is showing. As you can see, there is a serious artifact there. It is composed of a series of bright spots that seem to correspond to the number of subs taken. So the images, whatever they are are marching across the image. Yet the image itself is stable enough. Which I suppose means my guiding was adequate. But if that's so, why are these dots moving?

Peter
Attached Thumbnails
Click for full-size image (artifact.jpg)
144.9 KB113 views
Reply With Quote
  #2  
Old 25-08-2020, 06:23 AM
Slawomir's Avatar
Slawomir (Suavi)
Registered User

Slawomir is offline
 
Join Date: Sep 2014
Location: North Queensland
Posts: 3,240
Hi Peter,

The bright dots are hot pixels, and there is a slight drift between the subs, indicating that guiding pauses during downloads/refocusing or that dithering pushes the mount to one side, suggesting that either mount is not balanced or polar alignement could be improved.
Reply With Quote
  #3  
Old 25-08-2020, 08:50 AM
pmrid's Avatar
pmrid (Peter)
Ageing badly.

pmrid is offline
 
Join Date: May 2008
Location: Cloudy, light-polluted Bribie Is.
Posts: 3,665
Quote:
Originally Posted by Slawomir View Post
Hi Peter,

The bright dots are hot pixels, and there is a slight drift between the subs, indicating that guiding pauses during downloads/refocusing or that dithering pushes the mount to one side, suggesting that either mount is not balanced or polar alignement could be improved.
Thanks Suavi. I'll redo the PA. I have been using SharpCap for PA and found it pretty good. I'll see if that helps. And I'll reduce the length of guiding subs too. Currently I use 5-second subs but I'll cut that to 1 second and see if that helps too. Always something isn't there?

Peter
Reply With Quote
  #4  
Old 25-08-2020, 09:00 AM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
Not sure what mount you’re using Peter but when I had an EQ6 I found the 1-2s mark to be optimum.

Suavi is correct, they’re hot spots that will largely be removed when you calibrate the data. You’d be best setting up a dithering pattern for the long term though. Although what you have at the moment will statistically remove hot pixels you may end up with a form of PRNU; walking noise. You won’t notice it until you start stacking for longer integrations and it’ll really limit how much stretching you can do for the fainter signals.
Reply With Quote
  #5  
Old 25-08-2020, 11:54 AM
pmrid's Avatar
pmrid (Peter)
Ageing badly.

pmrid is offline
 
Join Date: May 2008
Location: Cloudy, light-polluted Bribie Is.
Posts: 3,665
Quote:
Originally Posted by Atmos View Post
Not sure what mount you’re using Peter but when I had an EQ6 I found the 1-2s mark to be optimum.

Suavi is correct, they’re hot spots that will largely be removed when you calibrate the data. You’d be best setting up a dithering pattern for the long term though. Although what you have at the moment will statistically remove hot pixels you may end up with a form of PRNU; walking noise. You won’t notice it until you start stacking for longer integrations and it’ll really limit how much stretching you can do for the fainter signals.
G'day Colin. Not sure what PRNU is. Another job for Google.
Peter
Reply With Quote
  #6  
Old 25-08-2020, 12:58 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
Quote:
Originally Posted by pmrid View Post
G'day Colin. Not sure what PRNU is. Another job for Google.
Peter
Come to think of it, I think PRNU is the wrong term in this situation. Walking noise is FPN, got my acronyms all mixed up FPN is Fixed Pattern Noise. CMOS sensors show it a lot more than CCDs do and that is mostly due to the lower read noise of CMOS. FPN tends to show itself in lower SNR areas (the background).

A little while ago I did a whole lot of 0.1s exposures on the star Eta Carina to capture the nebulosity surrounding it. Because I didn't dither even the "bright" Eta Carina Nebula was really faint in the image and was destroyed by FPN.
Dark and flat calibration really helps to mitigate it but ultimately you need dithering to average it out entirely.
Reply With Quote
  #7  
Old 25-08-2020, 06:30 PM
LewisM's Avatar
LewisM
Novichok test rabbit

LewisM is offline
 
Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
I don't know which programme(s) you use for image processing, especially integration, but I have found a Median integration to nearly always mitigate or even remove entirely any FPN. Average integration on the other hand, seems to show it (especially on bloody SBIG cameras).
Reply With Quote
  #8  
Old 25-08-2020, 07:56 PM
pmrid's Avatar
pmrid (Peter)
Ageing badly.

pmrid is offline
 
Join Date: May 2008
Location: Cloudy, light-polluted Bribie Is.
Posts: 3,665
Quote:
Originally Posted by LewisM View Post
I don't know which programme(s) you use for image processing, especially integration, but I have found a Median integration to nearly always mitigate or even remove entirely any FPN. Average integration on the other hand, seems to show it (especially on bloody SBIG cameras).
G'day Lewis. I'm using a SX CCD in Maxim to image, DeepSky Stacker to stack and PS5 to process (while I try to get my head around PI).

I'm approaching the solution a step at a time. Tonight's first baby step was to redo the PA. SharpCap told me I managed an "excellent" alignment. I haven't bothered to do a SYnScan 3-star alignment and Polar align (the new version of SynScan offers a good routine for this).

If this artifact persists, I will drop my guiding subs from 5 to 2 seconds. But I'm imaging from a suburban location within a km of the sea so seeing quality is pretty bleah making it difficult to find a guide star most of the time unless you run to longer subs.

If neither of those things produces a result I'll try to find how to dither in Maxim. I can't see it on a quick skim but it's sure to be there somewhere. If not I'll switch guiding to PHD2 but that means running yet another software package and straining the old CPU a heap more than I like.

Anyway, that's the plan. It'll be interesting to whether there are incremental improvements or none at all. I'll try to come back to this thread with a report. Might be a little while though. I'm headed to get some bits of me cut off this Friday as a result of which I may not be imaging for a couple of weeks.

Peter
Reply With Quote
  #9  
Old 26-08-2020, 09:04 AM
multiweb's Avatar
multiweb (Marc)
ze frogginator

multiweb is offline
 
Join Date: Oct 2007
Location: Sydney
Posts: 22,060
You need to get rid of those hot pixels prior to aligning your subs with a dark subtraction and/or a bad pixel map. Dithering alone won't be enough unless you get a lot of subs as they are pretty bright.
Reply With Quote
  #10  
Old 26-08-2020, 04:36 PM
LewisM's Avatar
LewisM
Novichok test rabbit

LewisM is offline
 
Join Date: Aug 2012
Location: Somewhere in the cosmos...
Posts: 10,388
Peter, dithering in MaxIM is really quite easy - in the Camera Control tab, click on EXPOSE : AUTOSAVE : AUTOSAVE SETUP -> DITHER, and then select your options (either via guider or mount, and either coordinates and movement size).

That's how I do it (with a sequence), but there is likely another way.

Last edited by LewisM; 26-08-2020 at 07:29 PM.
Reply With Quote
  #11  
Old 27-08-2020, 02:21 PM
pmrid's Avatar
pmrid (Peter)
Ageing badly.

pmrid is offline
 
Join Date: May 2008
Location: Cloudy, light-polluted Bribie Is.
Posts: 3,665
Quote:
Originally Posted by LewisM View Post
Peter, dithering in MaxIM is really quite easy - in the Camera Control tab, click on EXPOSE : AUTOSAVE : AUTOSAVE SETUP -> DITHER, and then select your options (either via guider or mount, and either coordinates and movement size).

That's how I do it (with a sequence), but there is likely another way.
Lewis, correct me if I'm wrong, (CMIIW) please. I understand that with guided imaging, dithering should be via guider. If that's right, how does that sit with pulse-guiding vias EQMOD?ASCOM?

Also, I'm getting my money's worth here with question-time - is it necessary to do hot-pixel removal (via maps or whatever) with calibration frames or only lights?

Peter

Peter
Reply With Quote
  #12  
Old 27-08-2020, 02:31 PM
Atmos's Avatar
Atmos (Colin)
Ultimate Noob

Atmos is offline
 
Join Date: Aug 2011
Location: Melbourne
Posts: 6,980
You don't have to use via-guider, you can just get Maxim to send the movement via-mount if that is easier for you. If you tell it to via the guider then it'll just make a dither movement the same way that it would do a guider movement. It's not different to when you run your guider calibration routine.
Reply With Quote
Reply

Bookmarks

Thread Tools
Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT +10. The time is now 10:26 PM.

Powered by vBulletin Version 3.8.7 | Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Advertisement
Bintel
Advertisement
Testar
Advertisement