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

bias turns my histogram into needles in DSS (data included)

  • Please log in to reply
17 replies to this topic

#1 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 17 July 2019 - 09:18 PM

https://drive.google...O_GWTuOtercISdS

 

There are about 3 hours worth of lights, a master dark (with description) master flat (with description) and about 20 or so bias.  I have no idea why my bias takes all the data away.  I know I am taking them completely in the dark, same sensor temp, and 0.00001 second exposures.

 

I appreciate if anyone wants to stack this and see what they come up with.  

this was my attempt (without the bias).  I am still messing around with it, but I think with 200 second subs, my stars are saturated and most of them lost color (save a few red ones) 

CuHOurzh.jpg

 

I appreciate the help.  


  • lbim and Gipht like this

#2 Kevin Ross

Kevin Ross

    Messenger

  • -----
  • Posts: 435
  • Joined: 28 Mar 2018
  • Loc: Traverse City, MI

Posted 17 July 2019 - 10:15 PM

Hmm, your bias file has a median ADU of 1600. I don't know what is typical for that camera, but that sure seems high to me.

 

I noticed that the specs for that camera say the minimum supported exposure time is .00005s, and your exposure time is .00001s, so shorter than the specified minimum exposure time. Not sure if that has anything to do with it.



#3 Gipht

Gipht

    Surveyor 1

  • *****
  • Posts: 1830
  • Joined: 12 Nov 2016
  • Loc: Prescott Valley, AZ.

Posted 17 July 2019 - 10:20 PM

If you check the exposure time for your flats, and shoot dark-flats, you might get better results.   I have seen quite a number of people recommending dark flats, rather then bias lately.  Hopefully, someone who knows the technology/science better can explain why.

 

Your existing picture is still very good.



#4 TelescopeGreg

TelescopeGreg

    Apollo

  • -----
  • Posts: 1025
  • Joined: 16 Jul 2018
  • Loc: Auburn, California, USA

Posted 17 July 2019 - 11:32 PM

What Gipht said.

 

I've found that Light, Dark, Flat, and Dark-Flat work better than L, D, F, and Bias (though that might be camera-dependent).  And definitely don't include both Dark-flat and Bias at the same time.



#5 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 18 July 2019 - 07:37 AM

If you check the exposure time for your flats, and shoot dark-flats, you might get better results.   I have seen quite a number of people recommending dark flats, rather then bias lately.  Hopefully, someone who knows the technology/science better can explain why.

 

Your existing picture is still very good.

So take a dark flat  and use it as a dark flat in DSS, or take a dark flat and use it as a bias in DSS and skip uploading it as a dark flat?  

 

How do you guys measure the ADU in the bias, and why do you measure it, and why is that important?  Explain it like I am 5.  I like analogies. 

 

I am also my own harshest critic,  but thank you for the complement.  I think my image is overexposed @ 200 second subs 100 gain and 50 offset.  Stars lost all color (save a few super red ones, they are all white).  Still needs more time on the target. Will try 150 seconds on the next go around. and keep bracketing in to find the sweet spot. 


Edited by nateman_doo, 18 July 2019 - 08:18 AM.


#6 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 18 July 2019 - 07:40 AM

Hmm, your bias file has a median ADU of 1600. I don't know what is typical for that camera, but that sure seems high to me.

 

I noticed that the specs for that camera say the minimum supported exposure time is .00005s, and your exposure time is .00001s, so shorter than the specified minimum exposure time. Not sure if that has anything to do with it.

I have tried the 50 microsecond exposure, and it didnt want to take it.  lower was ok, higher was ok.  Strange.  Still the same histogram, and the same results.  Takes almost all the data away.



#7 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 18 July 2019 - 08:19 AM

 And definitely don't include both Dark-flat and Bias at the same time.

Why not?  School me.  Remember... I am a 5 year old. 



#8 jdupton

jdupton

    Surveyor 1

  • *****
  • Posts: 1875
  • Joined: 21 Nov 2010
  • Loc: Central Texas, USA

Posted 18 July 2019 - 08:35 AM

Nateman,

 

   Yes, your Bias Frames have a higher mean and median ADU value than your Master Dark Frame. That alone indicates something was amiss during the capture of your calibration frames. You probably should check the gain and offset settings for everything in your capture program. I suspect something was set up differently for the capture of Bias Frames, Dark Frames, Flat Frames, and / or Light Frames. A difference in gain or offset are the most common difference. A significant temperature difference between the frames can also result in mismatched calibration frames.

 

   Such a difference in gain, offset, or temperature can cause loss of background levels when you attempt to use frames of mixed capture parameters. It can severely clip the background levels during calibration and cause both a darkening of the overall image as well as the "needle" histogram you are seeing.

 

   To measure the mean or median ADU value of an image, I use the Statistics Process tool in PixInsight. I do not know which image processing programs you have available to you and use. Often, the capture program can give you a readout of the mean and median of an image. I use Sequence Generator Pro for capture and it also shows all the statistics needed to sanity check an image. Look around in the tools you use to see which ones report on the statistics for an image. 

 

   It is always a good idea to sanity check images using statistics. Your Bias Frames should have the lowest ADU mean / median followed by the Flat-Darks, Darks, Lights (target image), and finally Flats. If your "average frames" do not show increasing ADU values in that order then you know something is off in the capture of the data and calibration problems will likely result.

 

 

John

 

PS: Regarding capture of Bias Frames, many capture programs will translate between what exposure time you enter and the capabilities of the camera. For Bias Frames, I always enter 0 for exposure time. Sequence Generator Pro / the camera device driver then uses the shortest exposure possible for the camera. Maxim DLE, and Nebulosity used to work the same when I used them.


Edited by jdupton, 18 July 2019 - 08:39 AM.

  • sharkmelley and bobzeq25 like this

#9 TelescopeGreg

TelescopeGreg

    Apollo

  • -----
  • Posts: 1025
  • Joined: 16 Jul 2018
  • Loc: Auburn, California, USA

Posted 18 July 2019 - 05:24 PM

Why not?  School me.  Remember... I am a 5 year old. 

Yeah, unfortunately, me too.  The "don't use both bias and dark-flats at the same time" is my understanding of the result of a discussion on the DSS Yahoo group recently.  It has to do with the pixel math that DSS uses in its processing, and I think it has to do with double-subtracting the dark levels and some interactions with the "set black point to zero" box.  It pretty well matches my brief experience with using Bias subs late last fall, where the result was a nicely mangled M33.  Taking out the bias frames from the processing set was my solution, and the discussion set the bits in my brain that doing so was in fact the right choice even if I didn't understand why at the time.

 

But beyond that, I'll have to defer to the authors (who hopefully will see this...  David?).  Hopefully I haven't mangled this too.

 

Greg



#10 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 19 July 2019 - 12:29 PM

I just took flat darks and used them as flat darks, no change in the image.  Used them as bias, back to a needle histogram.

 

I am 100% positive I had the same gain from the images, darks, and bias... the only gain I am not sure of is the flats.  I use CCD Flats aid in Astrophotography tool which messes with the gain as I recently found out.  Does that make a difference? 



#11 perdrix

perdrix

    Explorer 1

  • -----
  • Vendors
  • Posts: 72
  • Joined: 28 Sep 2012
  • Loc: Kenilworth GB

Posted 19 July 2019 - 08:21 PM

Perhaps the following wiki entry might cast some light:

https://groups.io/g/...ack-Point-to-0"

 

and also:

 

https://groups.io/g/...-or-Dark-Flats?

 

As for the comment about not using BOTH Bias and Dark Flats the maths says it is OK:

 

(Flat - Bias) - (DF - Bias) = Flat - DF 

 

So unless we've totally messed up in the DSS code, it should be correct.

 

As for the case in point - that bias frame doesn't sound at all "kosher".  



#12 perdrix

perdrix

    Explorer 1

  • -----
  • Vendors
  • Posts: 72
  • Joined: 28 Sep 2012
  • Loc: Kenilworth GB

Posted 20 July 2019 - 06:23 AM

Just had a nasty thought - seeing that the bias frame had an ADU higher than the Master Dark.

 

If you have ever used DSS with option "Set Black Point to 0" selected, and then processed calibration frames of any sort with DSS, then the Master Frames won't be compatible with Master Frames created with that option un-checked (or vice-versa).

 

So if your dark frames were done with "Set Black Level to 0" and your bias frames were processed without that option - then that might explain it.

 

It all depends on the black level reported in the exif data contained in the tiff file created by from the camera.   Does that CCD camera report "Black Level"?

 

If non-zero then this is definitely something to you need to take care over.

 

Clear skies

David


Edited by perdrix, 20 July 2019 - 06:41 AM.


#13 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 20 July 2019 - 10:21 AM

The CCD flats aid doesnt mess with the gain.  Hummm   

 

undecided.gif

 

XoWd0eG.png

 

Either way, I made all new flats.



#14 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 20 July 2019 - 10:22 AM

Just had a nasty thought - seeing that the bias frame had an ADU higher than the Master Dark.

 

If you have ever used DSS with option "Set Black Point to 0" selected, and then processed calibration frames of any sort with DSS, then the Master Frames won't be compatible with Master Frames created with that option un-checked (or vice-versa).

 

So if your dark frames were done with "Set Black Level to 0" and your bias frames were processed without that option - then that might explain it.

 

It all depends on the black level reported in the exif data contained in the tiff file created by from the camera.   Does that CCD camera report "Black Level"?

 

If non-zero then this is definitely something to you need to take care over.

 

Clear skies

David

not really sure where I check the black point set to zero.  I have the offset at 50, so it should never be zero right? 



#15 perdrix

perdrix

    Explorer 1

  • -----
  • Vendors
  • Posts: 72
  • Joined: 28 Sep 2012
  • Loc: Kenilworth GB

Posted 22 July 2019 - 04:52 AM

The infamous "Set Black Point to 0" check box is on the RAW/FITS DDP Settings dialogue at the bottom.

 

I looked at the "manual" for the camera, and the offset does appear to be similar in concept the the Black Level value used by some DSLRs.   This effectively acts as a stand-in for Bias.   However it may be applied in camera and never appear to the software.

 

If your camera outputs tiff files, can you see what exiftool -BlackPoint -BlackLevel imagefile.tiff reports?



#16 Phil Sherman

Phil Sherman

    Mercury-Atlas

  • *****
  • Posts: 2860
  • Joined: 07 Dec 2010
  • Loc: Cleveland, Ohio

Posted 22 July 2019 - 07:52 AM

If you're using flats, you must also use bias frames somewhere in the processing. Short exposure flats can use bias while longer exposure flats may require flat darks. The breakpoint forcing the split is a flat exposire that includes thermal noise. For a cooled camera, this may be as long as a few seconds, while uncooled ones may be less than one second, especially at high temperatures. 

 

Your problem should be researched by analyzing a set of exposures. Take a series of single exposure darks at exposures of 20, 10, 5, 1, 0.5, 0.2, 0.1, 0.05, 0.02, 0.001 seconds. Then take a bias exposure. Compare the average pixel values for each of the frames. Each of the darks should have a smaller average value than the previous one with very little difference at fhe shorter exposures. If the bias frame has a higher average value than the shortest dark, then you have an issue with the bias exposure settings.

 

Don't forget that you can use a very short dark as a bias frame. I'd be very surprised if a 0.001second dark was significantly different from a bias frame.



#17 perdrix

perdrix

    Explorer 1

  • -----
  • Vendors
  • Posts: 72
  • Joined: 28 Sep 2012
  • Loc: Kenilworth GB

Posted 22 July 2019 - 09:11 AM

Not quite true – you don’t need bias frames to use flats.

 

Mathematically simple (Flat – Bias) – (DarkFlat – Bias) = Flat – DarkFlat

 

Now that’s not saying that bias frames are bad – just saying you don’t need them to calibrate your flats.

 

FWIW
David



#18 nateman_doo

nateman_doo

    Viking 1

  • **---
  • topic starter
  • Posts: 761
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 28 July 2019 - 05:56 PM

Thanks. I just opted to not use bias frames. I use dark flats on occasion. Not really sure I need them, as I like my stacks with what I am doing now. Was curious what the benefits were, but no point pulling my hair out over them, when I take quality flats


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