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  #41  
Old 17-09-2021, 04:10 PM
Startrek (Martin)
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Quote:
Originally Posted by gregbradley View Post
The advantage of bias frames is to be able to scale your darks. That means each frame is measured scaled to match. That way a 10 minute dark could correct a 5 minute exposure.

I use same length exposure darks but tend to only use 2 exposure lengths - 300 and 600 seconds.

As far as flats go I subtract a bias from the flat and that is working fine.

In fact I treat my QHY600m the same as my CCDs in all ways and its working fine.

Lower model sensors with amp glow and perhaps oddball issues like uneven flats mean more care is needed with calibration files.

I have even used calibration files from a different read out mode with different gain and offset and it still seemed to work.

But my opinion is for best results use exactly matching calibration files with same temperature, same exposure length, with flats, bias and darks plus
same readout mode (for QHY) and same gain and offset.

So a couple more things to make sure they are the same compared to CCD.

On some of my scopes (AP RHA) I don't get vignetting even with the full frame sensor so flats are not super important. Darks also may not be cleaning much if you use dithered subs. But dithering is also important as there are some random hot pixels with these CMOS sensors it seems and that cleans them up.

I have not seen horizontal banding cause any issues so far with the settings I have settled on except maybe a dim Ha sub and I was only after the small amount of Ha in a galaxy shot. So it was easy to make it go below visibility.

All in all very clean and easy to uses sensors. Only caveat so far is to not blow out the star colours which can occur easily compared to CCD where the star colours retain more easily and tend to be more colourful as a result. Its a result of the high QE of the CMOS sensors and the relatively smaller well depth.

I see these sensors in 2 tiers. Those with amp glow and less than 16 bits and those with no amp glow and 16 bits. These are like superior CCDs.
The amp glow sensors are still very very good but simply not in the same league for ease of use. Amp glow is another thing that can go wrong in an already complicated game.

Greg.
Greg,
Thanks for your input , personal experience and advice
Well received
Iím fortunate to have a camera like the 2600MC which is similar to the QHY 268C
Iíve always had some vignetting issues imaging in Sydney but since I got back from my South Coast dark site after nearly 4 months , the problem is more visually evident and now I realise that I need to use calibration frames to clean up my pre processed images. Itís getting hard and harder to image under Bortle 8 skies and neighbours either side with 2 story homes , flood lit yards , bedroom lights blearing through windows with no blinds and external wall lighting most of the night
Fortunately the 2600MC has the best specs you could ask for in a OSC whilst imaging from a such a challenging (poor) location in Sydney
Low read noise
Almost negligible dark current
FWD 50,000e
No amp glow
QE of +84%
High dynamic range
And so on.....
Iíve never had any banding issues , excessive noise or bloated stars ( even using my L Extreme filter) from this camera. Iíve always dithered every sub from 90sec up to 300 sec, Iíve stacked with and without darks and havenít noticed any discernible difference
Hopefully using Flats ( with Flat Darks ) will minimise vignetting and provide a more even flat field , plus mask those pesky dust donuts
Iíve got some work to do over the coming weeks to incorporate calibration and hopefully get some improvements
Wonít be heading back down south until November/ December , unless we are hit with more lockdowns , the future is always an uncertainty
Thanks again
Appreciate it
Cheers
Martin
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  #42  
Old 19-09-2021, 11:36 AM
Startrek (Martin)
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A nice clear night in Sydney last night so captured M17 as a test target and then full sets of Calibration frames afterwards ( Flats , Darks , Dark Flats , Bias )
Captured 2.5 hours on M17 with the 2600MC set to Lowest Read Noise or Gain 100 , cooled to -10C , with L Extreme filter
Moon was 91% overhead
Capture software APT

Flats were taken at a target 32,768 ADU as advised, and using a stretched white T shirt and a Huion dimmable light panel , dimmed down
Exposure time was 3.6 sec
Captured 40 Flats , 40 Dark Flats, 40 Darks , 50 Bias


Stacked all kinds of variations of Calibration frames ( Deep Sky Stacker )
Lights only to start with
Lights and Flats
Lights , Flats and Dark Flats
Lights , Darks , Flats
Lights Flats and Bias
Lights and Darks

To be honest none of the above produced a even flat field in fact the worst was just using Flats or using Flats and Dark Flats , absolutely terrible
Using Flats and Darks and Flats and Bias produced slightly better results but still grossly uneven on one side of the frame ( I used my processing software Startools to load each stacked file and perform a first stretch to observe the results with Calibration
Then I just loaded my Lights only stacked file into Startools and used the Wipe module set to Narrowband and it cleared up 80 to 90% of the uneven field
In other words the uncalibrated stretched image was not perfect but useable, the calibrated stretched images were horrible , some worse than others

Attached photos of a few
Lights only
Lights and Flats
Lights Flats and Dark Flats
Lights Flats and Bias
Lights only , using Wipe Narrowband preset in Startools

Any ideas ??

Maybe achieve exposure times longer or shorter
Maybe change ADU to lower say 25,000 ADU

Tonight Iím going out again to try imaging on same target M17 with Gain 0 or highest dynamic range plus maybe some different Flats

I was hoping for better results after all the help from everyone

Comments and further advice most welcome

Cheers
Martin
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  #43  
Old 19-09-2021, 06:08 PM
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The_bluester (Paul)
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How does the histogram look on the individual flats? What ADC to target can depend on how the software samples the image. I cant comment for APT as I didn't get advanced far enough to start using flats when I was using it.

If it would not be such a massive transfer I would suggest flicking me your full set of files and I can try running them through Astro Pixel Processor and see what kind of result pops out the other end. If you package up the whole box set of files (Lights and all the individual cal frames) how much data is it? It might be doable via Wetransfer. I think you can transfer a gig with the free version.

Last edited by The_bluester; 19-09-2021 at 06:26 PM.
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  #44  
Old 19-09-2021, 06:37 PM
Startrek (Martin)
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Paul,
Thanks for offering further assistance
Histogram is very basic in APT just saw 3 mountains ( due to colour camera ) across the whole histogram, one on far left , one in the middle and one on the right ) Maybe Iím over exposing ??
Files would be too huge to transfer , total of 12 GB

Iím just going to have to spend time experimenting unless you have other tests I could perform
Iím out tonight on M17 again and will take various flats at different ADU and exposure times later tonight ( plus all other cal frames )
Iím busy for the next 4 or 5 days after that , so at least Iíll capture the data and work from there
Iíve also posted on Deep Sky Stacker user Groupís as well
Maybe Iíll post on APT again , Ivo is also very helpful , just as you folk have helped me to date
Cheers
Martin
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  #45  
Old 19-09-2021, 07:45 PM
Startrek (Martin)
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Originally Posted by Startrek View Post
Paul,
Thanks for offering further assistance
Histogram is very basic in APT just saw 3 mountains ( due to colour camera ) across the whole histogram, one on far left , one in the middle and one on the right ) Maybe Iím over exposing ??
Files would be too huge to transfer , total of 12 GB

Iím just going to have to spend time experimenting unless you have other tests I could perform
Iím out tonight on M17 again and will take various flats at different ADU and exposure times later tonight ( plus all other cal frames )
Iím busy for the next 4 or 5 days after that , so at least Iíll capture the data and work from there
Iíve also posted on Deep Sky Stacker user Groupís as well
Maybe Iíll post on APT again , Ivo is also very helpful , just as you folk have helped me to date
Cheers
Martin
Paul,
Thereís a Tool in APT called Pixel Aid which can be used to access the count across the whole fits image ( Flat ) to ensure your exposure is correct
Iíll be using that Tool to check my Flats
It gives you a Average and Max readings for whole image
Cheers
Martin
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  #46  
Old 19-09-2021, 09:20 PM
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The_bluester (Paul)
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Actually, one thing I did not think to ask, were you using any filter? You mentioned having three distinct peaks in the histogram, one for each colour channel (As you would expect) Were all three peaks away from the sides of the histogram with no stretch applied? If any one channel is clipped on either side of the histogram the flats won't work correctly as the data range will be skewed.

I mentioned the filter as when I was using a HA filter or an Oiii filter with my 2600MC I had to skew the requested average ADC for the flats downward to account for in the case of HA, three in four pixels having virtually zero data (Given the signal shows up almost entirely in the red pixels) and for Oiii one in four without much data.
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  #47  
Old 19-09-2021, 09:55 PM
Startrek (Martin)
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Quote:
Originally Posted by The_bluester View Post
Actually, one thing I did not think to ask, were you using any filter? You mentioned having three distinct peaks in the histogram, one for each colour channel (As you would expect) Were all three peaks away from the sides of the histogram with no stretch applied? If any one channel is clipped on either side of the histogram the flats won't work correctly as the data range will be skewed.

I mentioned the filter as when I was using a HA filter or an Oiii filter with my 2600MC I had to skew the requested average ADC for the flats downward to account for in the case of HA, three in four pixels having virtually zero data (Given the signal shows up almost entirely in the red pixels) and for Oiii one in four without much data.
Paul,
Yes if you look back in the thread Iím using an L Exteme OSC narrowband filter
In regard to the histogram last night I think it was clipped ( up against the LHS and RHS )
Tonight Iíve lowered the illumination which has produced a Histogram that is not touching the LHS ( only at the bottom or base of the peak but moving away as it arcs up over and a good distance from the RHS ) However exposure time to achieve this Histogram is 15.25 sec for an ADU of 30,000
Iím going to save this one and experiment more , late night ahead
Cheers
Martin
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  #48  
Old 20-09-2021, 06:14 AM
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The_bluester (Paul)
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I would not worry too much about that as an exposure time. I recently had HA flats where I put too many folds of white cloth over the flat panel (An old habit from the SCT which can apparently get reflections between the corrector plate and the panel, probably not much of an issue with the Newt) and the exposure time was nearly 30 seconds. They worked out fine.

When you look at the histogram though, make sure no stretch is being applied, this is the histogram of a flat from my 2600MC with no stretch and a mild stretch. The separation between them disappears in the APP "Background calibration" step.
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  #49  
Old 20-09-2021, 08:05 AM
Startrek (Martin)
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Using the Flats Aid in APT , you enter your details in the aid box
Target ADU:
ADU Range %:
Start Exposure (sec):
Min exposure (sec):
Max exposure (sec):
Bin 1x1
Create Flats Plan checked
Flats count :

Then hit Run
It takes a series of exposures until it reaches your target ADU during which a live histogram is operating and Read out text is updating below
Once achieved is advises the required exposure time

You donít really touch a thing , other than adjust your light panel brightness if your target range cannot be reached or your histogram is clipped either end

The APT histogram is a bit primitive looking but thats all you have to look at , other than the numbers

I really think I need a more precise controlled light panel as the margin for error to fit RGB into the dynamic range of this camera to avoid clipping is really tight

See photos

Martin
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  #50  
Old 20-09-2021, 08:41 AM
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The_bluester (Paul)
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Ah, there is one thing to change, deselect "Log" in the histogram, and it looks like APT is applying an auto stretch?

Do you want to send me a couple of your flats to have a look at in APP?
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  #51  
Old 20-09-2021, 07:22 PM
Startrek (Martin)
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Quote:
Originally Posted by The_bluester View Post
Ah, there is one thing to change, deselect "Log" in the histogram, and it looks like APT is applying an auto stretch?

Do you want to send me a couple of your flats to have a look at in APP?
Paul,
Enabling the Log button on the Histogram just presents the Histogram with a logarithmic view of the stretched data
It has no function in relation to stretching or auto stretching

Iím working on a couple of sets of Flats and other calibration files I captured last night
Iím busy for the next few days and then some more experimenting hopefully next week
Iíll take up your offer to send some Flat files if Iíve exhausted every Avenue and start pulling what hair I have left on my balding head
Appreciate the offer and help
Cheers
Martin
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  #52  
Old 20-09-2021, 09:25 PM
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I know, just linear histogram is more representative to look at for this.

I mentioned the stretch as the range looks like the black point is up at about 11K ADC.
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  #53  
Old 21-09-2021, 06:31 AM
Startrek (Martin)
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I think Ivo is going to redesign the Histogram to show the peaks across the whole band
You mentioned the black point is up a bit
What should the black point generally be in ADU for a good Flat at a target ADU of 30,000 (16 but )
What should the range generally be in ADU for a good Flat with target range of 30,000 (16 bit )

Thanks
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  #54  
Old 21-09-2021, 07:40 AM
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Really all the black point is indicating there is that there is some sort of stretch going on, so you may not be able to tell by looking at the histogram if the levels are good for a flat.

To compare apples and apples, this is the histogram of a flat I shot with my ASI2600MC, two histograms here both presented with a log view rather than linear so they present the same way yours would. One is with no stretch applied and you can see it looks good, all data is well away from both sides of the histogram, and the second is the same flat frame with an auto stretch applied. If that was unstretched it would indicate a bad flat as you would say that the red channel is clipped, but it is really just that the black point has been moved up in the auto stretch.
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  #55  
Old 24-09-2021, 03:38 PM
Startrek (Martin)
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After extensive testing over the past week and much advice and support from IIS members , DSS forum and APT forum Iíve finally nailed good calibration with my set up in Sydney using the 2600MC , L Extreme filter and my trusty 6Ē newt
Out of the combinations and methods of calibration , the only one that gave me a nice even field , removed vignetting and dust donuts after an initial stretch in Startools was Lights , Flats and Bias captured in my Capture software APT
All other calibration methods like Flats and Dark Flats or Flats and Darks didnít work , some overcorrected , some just darkened the field and left vignetting etc....

My settings in APT Flats Aid were as follows -
ADU target 22,000 to 25,000 ( 22,000 ADU was easier to reach using the light panel ) Using the recommended 50% or 32,768 ADU didnít work with my light panel etc...
Exposures reached target from 6 sec to 15 sec
Histogram range was from around 8800 to 41,000
I used a Huion A3 dimmable light sketch panel
One or 2 layers of satin nylon cloth stretched over OTA

Bias Frames were captured using my 2600MC shortest exposure of 0.000032 sec ( I tried 0.1 sec and even 1 sec and they all worked fine )

I captured 40 Flats and 40 Bias

Lights frames were 50 x 180 sec dithered guided subs
2600MC was set in Ascom to highest dynamic range or Gain 0 and cooled to-10C

Stacked in DSS ( you just load in your lights , flats and bias and DSS does the rest )

Stacked file was then loaded into Startools, an initial stretch and quick wipe of gradients and the field was clean as a whistle ( excluding normal noise floor which is minimal anyway ) no anomalies

Next hurdle is Calibration without the filter

See attached photos including the Startools initial stretched image before my processing workflow

Thanks again to all that contributed
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  #56  
Old 24-09-2021, 05:10 PM
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Great to see.

I wondered if reducing your flats target might help when you are using a filter. I did have to do that with the NB filters and my 2600MC, but it is a non event with the 2600MM.

Probably for flats without the filter just go back to the 32K ADC and see how it looks as a starting point.
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  #57  
Old 25-09-2021, 11:44 AM
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Good to see you found a combination that works (and matches what I've been doing).

It's great that you investigated this so fully & I think you've provided us all with some worthwhile info.
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  #58  
Old 27-09-2021, 08:58 AM
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Very interesting conclusion, Martin!

Thanks for testing this. I'll start editing with flats and bias as well and see how it goes
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  #59  
Old 27-09-2021, 06:11 PM
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A question for you Martin, you mention in your last post, shooting your lights at 0 gain, what gain did you use for the flats? I was chasing issues with my flats over correcting my Oiii lights with the mono camera, I did all sorts of reprocessing and it just would not behave. I shot the flats at 100 gain.

I reshot them tonight at 0 gain, perfect result!
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  #60  
Old 27-09-2021, 07:19 PM
Startrek (Martin)
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Originally Posted by The_bluester View Post
A question for you Martin, you mention in your last post, shooting your lights at 0 gain, what gain did you use for the flats? I was chasing issues with my flats over correcting my Oiii lights with the mono camera, I did all sorts of reprocessing and it just would not behave. I shot the flats at 100 gain.

I reshot them tonight at 0 gain, perfect result!
Paul,

Lights , Flats and Bias all at Gain 0 and camera left cooled at -10C ( using L Extreme filter )

Iíve used Gain 100 for both short (60sec ) and long exposures ( 300sec ) with and without filter , Bortle 3 and Bortle 8 and I found Gain 0 to achieve slightly better results on average
Better results meaning slightly less noise floor , more fine detail and contrast across the field
Iím leaving my camera at Gain 0 cooled to -10C at both sites all year round

Cheers
Martin
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