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Old 18-04-2024, 12:23 PM
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gregbradley
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what's your processing routine to calibrate mono images?

I am finding these CMOS sensors a lot harder to calibrate than CCD cameras.

What have you found as a workable approach to calibrating your CMOS images?

Greg.
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Old 18-04-2024, 09:25 PM
Startrek (Martin)
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Greg,
I use both 2600MC and 2600MM cameras in my carbon Newts
Cooling kept at -10C all year
Gain setting for Broadband Low Gain 0 offset 50 Ascom driver
Gain setting for Narrowband Medium Gain 100 HCG mode offset 50 Ascom driver
I shoot a new Dark Library every 8 months for both Gain settings with cooling -10C and a selection of exposures times for both Broadband and Narrowband
I shoot a new Bias Library every 8 months for both Gain settings with cooling -10C and exposure time 0.000032sec

Flat Frames
Flats are captured with Huion A3 and A2 adjustable led light pad
I take Flats for each filter each session ( single night and multiple nights provided camera is not removed )
My TS GPU coma corrector unfortunately introduces vignetting into my image train so Flats are a must
Capture software is APT , I use the CCD Flats Aid tool and Histogram tool
Flats are captured with ADU range on histogram around 11,000 to 32,000 ADU ( ADU target value subject to illumination brightness and exposure times but usually between 20,000 to 28,000 ADU depending on filter type )
Flats exposure times for Broadband are around 0.15 to 0.50 sec max
Flats exposure times for Narrowband are around 1 to 5 sec max

Calibration Process
Originally used DSS for many years
Now I use ASTAP which is excellent ( free open sourced stacking and plate solving software)
Load lights specific filter , analyse all frames
Load Darks from Library same Gain and exposure as lights , analyse all frames
Load Flats specific filter same Gain and temp as lights , analyse all frames
Load Bias from Library same Gain and temp as lights , analyse all frames
Calibrate Flats with Bias ( or Flat Darks ) to create Master Flat
Create Master Dark from Darks
Set Alignment to Star Alignment
Set Stacking Algorithm to Sigma Clipping Average ( improved )
Stack frames
Save stacked file as fits
Repeat for all filters

Never had an issue with Calibration for both the 2600MC and 2600MM cameras
Vignetting eliminated
Clean frame no dust bunnies
Even illuminated field

Hope the above is what youíre referring too ?

Cheers
Martin
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  #3  
Old 18-04-2024, 10:06 PM
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gregbradley
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Quote:
Originally Posted by Startrek View Post
Greg,
I use both 2600MC and 2600MM cameras in my carbon Newts
Cooling kept at -10C all year
Gain setting for Broadband Low Gain 0 offset 50 Ascom driver
Gain setting for Narrowband Medium Gain 100 HCG mode offset 50 Ascom driver
I shoot a new Dark Library every 8 months for both Gain settings with cooling -10C and a selection of exposures times for both Broadband and Narrowband
I shoot a new Bias Library every 8 months for both Gain settings with cooling -10C and exposure time 0.000032sec

Flat Frames
Flats are captured with Huion A3 and A2 adjustable led light pad
I take Flats for each filter each session ( single night and multiple nights provided camera is not removed )
My TS GPU coma corrector unfortunately introduces vignetting into my image train so Flats are a must
Capture software is APT , I use the CCD Flats Aid tool and Histogram tool
Flats are captured with ADU range on histogram around 11,000 to 32,000 ADU ( ADU target value subject to illumination brightness and exposure times but usually between 20,000 to 28,000 ADU depending on filter type )
Flats exposure times for Broadband are around 0.15 to 0.50 sec max
Flats exposure times for Narrowband are around 1 to 5 sec max

Calibration Process
Originally used DSS for many years
Now I use ASTAP which is excellent ( free open sourced stacking and plate solving software)
Load lights specific filter , analyse all frames
Load Darks from Library same Gain and exposure as lights , analyse all frames
Load Flats specific filter same Gain and temp as lights , analyse all frames
Load Bias from Library same Gain and temp as lights , analyse all frames
Calibrate Flats with Bias ( or Flat Darks ) to create Master Flat
Create Master Dark from Darks
Set Alignment to Star Alignment
Set Stacking Algorithm to Sigma Clipping Average ( improved )
Stack frames
Save stacked file as fits
Repeat for all filters

Never had an issue with Calibration for both the 2600MC and 2600MM cameras
Vignetting eliminated
Clean frame no dust bunnies
Even illuminated field

Hope the above is what youíre referring too ?

Cheers
Martin
Thanks Martin,

That's helpful.

Greg.
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Old 19-04-2024, 08:55 AM
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Ryderscope (Rodney)
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Quote:
Originally Posted by gregbradley View Post
I am finding these CMOS sensors a lot harder to calibrate than CCD cameras.

What have you found as a workable approach to calibrating your CMOS images?

Greg.
Hi Greg, I have the ASI2600 mono and ASI6200 mono. The main point that I would offer is to not use bias frames when calibrating CMOS sensors. Bias signal with CCD cameras is linear and repeatable but not so with CMOS. Simply take darks of the same exposure length and temperature as the light frames for dark frame calibration (i.e. do not scale/optimise darks to match lights). For flat frame calibration, use flat darks i.e. darks frames with the same exposure time and temperature as the flat frames. That should do the job nicely.

One other point that I would like to throw out there is that I have seen a few forum posts crop up that posit that CMOS sensors can go for longer periods between times when re taking calibration frames such as darks and flats. I found this a little surprising so tried to track down source material that confirmed this. I was unable to find any technical documentation that supported this view and, to the country, found source information that confirmed that CMOS sensors are the same as CCDs in needing to retake calibration frames regularly. In my case I will redo my calibration frames every three to four months, the same as I have been doing with my QSI683 CCD.

Below are links to sources on the SBIG site that talk to CMOS calibration requirements. The first is a paper by Doug Georg, lead engineer at SBIG, that discusses CMOS calibration technicalities. The second link is to a forum discussion where a discussion ensued regarding the length of time between calibration frames. This was in the context of the SBIG Aluma 4040 CMOS camera. These are worth a read as they talk to the issue of bias stability and managing calibration data.


https://forum.diffractionlimited.com...ay-valid.9550/

https://diffractionlimited.com/calibrating-cmos-images/

Clear skies,
Rodney
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Old 19-04-2024, 10:10 AM
Startrek (Martin)
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Yeh Iíve used both Flat darks and Bias to calibrate my Flats with the 2600ís and yes Iíve read lots of information on so many forums and reviews on how Bias will just introduce more unnecessary noise into your image when using CMOS.
I have tested both alternatives and canít find any visual discernible difference ( my eye sight is not the greatest) but that doesnít mean you shouldnít use Flat Darks

Technically Flat Darks are whatís best to use with CMOS

The thermal dark current noise in the 2600ís is so damn minimal itís ridiculous , Iím sure ZWO are working on the next generation cameras ( 3600 series ???)

My biggest issue with noise in general is imaging under B8 Skies in Sydney ( 6 to 8 months of the year ) Been down at my Dome ( B3 skies ) for a week now with cloudy days and nights , another week or so here and back to the madhouse thatís Sydney.

Martin
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Old 19-04-2024, 07:55 PM
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That's great info Rodney. I have noticed not using bias seems to help.

I'll try that out.

Greg.


Quote:
Originally Posted by Ryderscope View Post
Hi Greg, I have the ASI2600 mono and ASI6200 mono. The main point that I would offer is to not use bias frames when calibrating CMOS sensors. Bias signal with CCD cameras is linear and repeatable but not so with CMOS. Simply take darks of the same exposure length and temperature as the light frames for dark frame calibration (i.e. do not scale/optimise darks to match lights). For flat frame calibration, use flat darks i.e. darks frames with the same exposure time and temperature as the flat frames. That should do the job nicely.

One other point that I would like to throw out there is that I have seen a few forum posts crop up that posit that CMOS sensors can go for longer periods between times when re taking calibration frames such as darks and flats. I found this a little surprising so tried to track down source material that confirmed this. I was unable to find any technical documentation that supported this view and, to the country, found source information that confirmed that CMOS sensors are the same as CCDs in needing to retake calibration frames regularly. In my case I will redo my calibration frames every three to four months, the same as I have been doing with my QSI683 CCD.

Below are links to sources on the SBIG site that talk to CMOS calibration requirements. The first is a paper by Doug Georg, lead engineer at SBIG, that discusses CMOS calibration technicalities. The second link is to a forum discussion where a discussion ensued regarding the length of time between calibration frames. This was in the context of the SBIG Aluma 4040 CMOS camera. These are worth a read as they talk to the issue of bias stability and managing calibration data.


https://forum.diffractionlimited.com...ay-valid.9550/

https://diffractionlimited.com/calibrating-cmos-images/

Clear skies,
Rodney
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  #7  
Old 19-04-2024, 09:13 PM
Startrek (Martin)
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I just analysed, calibrated , stacked and aligned my recent image of Omega Centauri tonight using both options ( Bias and Flat Darks )
Processed the image identically and for the life of me could not see any difference in image quality , background brightness uniformity and residual noise.
I also checked with ZWO and they advised Bias is ok for the 2600MM

I guess I’ll just keep taking Flat Darks and process both options ( Flat Darks and Bias ) for the remainder of the year. If my images don’t show any noticeable improvement using Flat Darks for the next 6 or 8 months , I’ll just go back to Bias as it’s certainly a time saver ( I’m not fully remotely automated ie: set and forget. I still take my flats etc .. in the wee hours then shut the rig down ) Always had trouble with Sky Flats

I also looked on Astrobin , Cloudy Nights , Stargazers etc… and the Flat Darks vs Bias for the 2600MM has mixed views and opinions
The Diffraction Limited report whilst super informative mentions don’t use Bias frames but it doesn’t report on what happens if you do use them or what the ramifications are to your final image compared to using Flat Darks. Maybe their logic is just simply a Mathematical equation , matching temperature and exposure time.

Martin

Last edited by Startrek; 20-04-2024 at 08:01 AM.
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Old 20-04-2024, 10:08 AM
Startrek (Martin)
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Greg,
I just found this excellent video from Adam Block regarding the Calibration process for CMOS cameras

https://www.youtube.com/watch?v=sZmH...nnel=AdamBlock

I guess the bottom line is use Flat Darks to calibrate your Flats if youíre unsure if your camera exhibits any dark current at even short exposures.Flat Darks is the safest measure for calibrating your Flats.
It doesnít mean you canít use Bias as some cameras may exhibit little or no dark current at short Flats exposures of less than 1 sec.
With my 2600MM my Flats are extremely short for Broadband imaging 0.11 to 0.17 sec , so hence I couldnít see any difference between using Bias or Flat Darks to calibrate my Flats
Iíll do some more testing with Narrowband images where my Flats are 1 sec to 4 sec ( I suspect some dark current would be creeping in at 4 sec so Flat Darks are most probably the safest option )
Iíll keep testing over the next 8 months on both bands and see how it all
Pans out

Cheers
Martin
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Old 22-04-2024, 07:52 PM
Startrek (Martin)
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A long time member of Cloudy nights has sent a reply to a post regarding using Bias or Flat darks to calibrate Flats using the 2600MM
This is back in 2021 when the 2600MM had been out in the market for only 12 months.

Interesting reply from Bob in Cloudy Nights

I did some more testing with my 2600MM yesterday on my previous Carina Outer Regions image 2 weeks ago
Used Bias to calibrate Flats scenario 1
Then used Flat Darks to calibrate Flats scenario 2
Both images were stacked and processed the same
I certainly couldnít distinguish between both options in the final images

In my recent testing on a Broadband target and Narrowband target using my 2600MM I certainly agree with Bob from Cloudy Nights regarding using Bias.
Other camera types maybe and probably are totally different !!

Iím still going to use both Bias and Flat darks on the same data set and compare over the next 6 months or so

Martin
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Old 26-04-2024, 04:24 PM
Craig_
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I don’t use calibration frames.

No darks, no bias, no flats. For me they are completely unnecessary. I have no material vignette in my image train, no dust, and dithering takes care of hot pixels and other artifacts when stacking.

I have tried stacking my data with and without calibration frames and see no difference at all. So calibration is something I no longer bother with, and see no detrimental impact on my images.

In fairness, I have a smaller sensor in the 533MM Pro which doubtless makes this easier to get away with.
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