Calibration Frames and ZWOASI2600MC

[Startrek]

I’ve never had to use darks , flats and other calibration frames at my dark site ( Bortle 3 ) on the South Coast as vignetting and gradients ( minimal ) are easily handled by my processing software Startools ( with or without NB filter and regardless of exposure lengths ) as this camera has almost zero amp glow and almost negligible dark current at -10C.
However since returning back to Bortle 8 Sydney skies ( with neighbours backyard and side flood lights on all night ) recently after being away for nearly 4 months , the Vignetting , dust donuts and gradients are a real problem when imaging targets like galaxies and globs etc... without a filter ( the 2600MC has a UV/ IR protective window , so in effect a Lum filter ) Even using the L Extreme filter requires additional work in processing to minimise these issues but no where near as bad as without a filter.
So now I’m going to have to take Flats to help mitigate these issues , something I’ve never had to do before. Also I believe the Skyglow in Sydney is getting worse year by year which is so evident.
Question: In regard to my 2600MC should I be taking Flats and Dark Flats to Calibrate in DSS or just Flats only.
In addition,I’ve read in a few forums that Bias frames are not usually required in CMOS cameras like the 2600MC , they are more useful in the older CCD type cameras

Any advice on the above would be very much appreciated

Thanks

[Retrograde]

Hi Martin,

When I first tried flats with the 2600mc (with dark-flats) I found my flats were badly over-correcting.
The solution (which I found on the internet: cloudynights.com/topic/731949-zwo-asi2600mc-pro-flats-cause-issues/page-2 (https://www.cloudynights.com/topic/731949-zwo-asi2600mc-pro-flats-cause-issues/page-2)) was to use flats & bias frames & this seems to work well for me (so far at least).

I don't bother with darks.

[AdamJL]

I've been using Darks, Flats and Flat Darks.

Definitely keen to see what the consensus is here!

[The_bluester]

Flats calibrated with a good master bias is what I have found to work best with my ASI2600MC, and with the MM. Which is very handy as it means you do not need to shoot fixed length flats.

Lights I calibrate with the good master flat produced by that method, and either the master bias, or a master dark of the right gain, offset, exposure time and temp. On very long exposures I have found that without a good master dark (Which contains the bias info in my case) the master flat tends to overcorrect the corners as it amplifies what little dark current there is and brightens the corners.

[kosborn]

I used to take flats, darks and bias with my asi1600mm. With the 2600mc I take flats, darks and flat darks. The subs seem to calibrate well and I now also use that combination for the 1600mm.

[Startrek]

Thanks Pete,
What capture software do you use ?
Sky flats with a white t shirt or LED flat panel with or without t shirt ?
We’re your flats exposure time calculated by the software and a quick check of the histogram ? or you selected around 30,000ADU like most forum posts suggested for this camera and ended up with an exposure time of between 2 and 3 seconds
Cheers
Martin

[Startrek]

Thanks Adam,
How many flats do you take , exposure time and what capture software ?
Cheers
Martin

[Startrek]

Thanks Paul,
Capture software ?
Method of capturing Flats, no of Flats , ADU and exposure time ?
How many Bias frames to create master Bias
Thanks
Martin

[Startrek]

Thanks Kevin,
Sorry to pick everyone’s brains and I know that each optical system is different but trying to gather enough data on the 2600MC so I’m not waisting to much time outside capturing and determining the most suitable calibration methods
Capture software ?
Method obtaining your Flats ( white t shirt or not ? ) no of Flats , exposure times and ADU count ?
Thanks
Martin

[AdamJL]

Hi Martin

40 Flats, 40 Dark flats, 40 Darks
Exposure time is calculated by NINA, depending on the light source. I set NINA to target a histogram mean of 32768 (50%) with a 10% tolerance.
Minimum exposure is usually 2 seconds, maximum is 20 seconds.

For light source, I use either
- a Huon tracing panel, with pieces of white paper between the panel and the scope to dim it a little (even on the dimmest setting, it's usually too bright)
- wait for morning, then slew to zenith, use the sky brightness and pieces of white paper again

I've had the issue of corners overcorrecting though, as mentioned above by Paul. This was a while ago but it seems to be okay these days.

Thanks for starting this topic. I took 100 bias each last night at various gain settings and -10 temperature (temperature probably doesn't make any difference but I did it anyway).

[Startrek]

Thanks Adam
Calibration is not talked about that often on the forum, only when folk need some advice like me , I’ve managed without it for some time but it’s bitten me on the bum now and I have to learn and use it especially in Sydney
Cheers
Martin

[Startrek]

I’m probably reading to much about Calibration and not imaging and experimenting ( inclement weather for the next 5 days ) but in regard to Flats

I’ve read that in regard to the latest ZWO CMOS cameras -
1/ If your Flats are between 1.5 and 3 seconds you only require to take and add Dark Flats for Calibration
2/ If your Flats are under 1.0 second you only require to take and add Bias frames for Calibration

Comments welcome

[AdamJL]

and what happens if you're over 3 seconds? :)

I'm usually anywhere from 7-14 seconds.

[Startrek]

.......and your flats eliminate vignetting and dust motes ? then most of the stuff I’ve read and seen throws that info out the window
Thanks

[Retrograde]

Hi Martin,
I'm currently just using Sharpcap (but plan to transition to NINA)

I use the white T-shirt & flat panel method & go for the correct exposure/histogram.
Exposure lengths are probably shorter than ideal though.

Pete

[Craig_]

I don't have the 2600, but do have a 533 (so same sensor tech), and I find flats when using a dual-band filter are usually like 8s or so targetting 20,000 ADU. They are much (much) shorter when using a UV/IR cut.

However, even from Sydney I don't ever find I need flats on such a small sensor.

[The_bluester]

A few posts since I last posted so I wont quote individual posts, just lay out my answers.

Firstly, I am using Voyager for capture. I aim for at least 25 flats. With my refractor I set an approx 30K ADU target with a 10% tolerance and have Voyager shoot dawn sky flats for me. It starts with a 15 second default exposure and works it out for itself from there with shrinking exposure times as the sky brightens, and big dithers between flats so any stars captured calibrate out.

I would not worry about any split of method for calibrating the flats with the ASI2600. I have had good results calibrating with just a master bias everywhere from around 30 second exposures when I had an NB filter in the image train, to well under 1 second when I have started shooting flats a bit late and the sky is getting bright.

With my newt I am using a light panel (Controlled by Voyager) and shooting flats at night as I need to chase down a light leak that makes sky flats patchy, it is either sneaking around the main mirror cell or around the focuser drawtube. Voyager starts at a default exposure time and default panel brightness and varies both as required to hit the target ADU.

[Startrek]

Thanks Pete
I’ll be using a white t shirt and my Huion A3 dimmable light panel to take flats
I only image with solid tube newts so I’ll also use a shower cap spray painted black and fitted over the back end to stop light leakage or entry
Cheers
Martin

[Startrek]

Thanks Craig,
Great information
I use the Optolong L Extreme duel band to image emission Nebula and the 2600MC has a UV/ IR protective window so I can expect longer exposures using this arrangement.
Your 533MC has a 14bit ADU ie: max ADU of 32,768 so applying 50% would be around 16,400 ADU. Choosing higher at 20,000 ADU at the same illumination level would take a longer exposure , right ??
Cheers
Martin

[Startrek]

Paul
Thanks for your details
I only use solid tube newts , and now have a Huion A3 dimmable light panel to take my flats
I’ll make sure my newt is sealed at the base ( blackened shower cap )
All the information provided by everyone to date is fantastic
The weekend is suppose to be fine so I’ll be out capturing and testing
Cheers
Martin

[kosborn]

To take flats I use a white t-shirt with the Esprit 100ED and the Newtonian. For the Esprit I use a laptop screen set to mid grey. For the Newtonian I use sky flats just before sunset (the 10" aperture is too big for the laptop screen). I use SGPro which allows you to set desired ADU (I choose 25000) and then uses a flats calibration wizard to work out the best exposure time for that ADU. I take 40 exposures per filter for the ASI1600mm mono or total 40 exposures for the ASI2600mc colour. The exposure time for the darks of course matches the exposure times for the subs and the exposure times for the flats.

[Startrek]

Thanks Kevin,
Brilliant !!
Martin

[Startrek]

Paul,


Forgot to ask you a question about your Bias frames ( yourself and Pete are the only folk who use them who replied )

What exposure time setting did you use to take your Bias frames ?
The 2600MC specs show a minimum exposure of 32 micro seconds or 0.000032 sec
This seems awfully quick ?
I don't know whether my capture software APT will register that setting ( Ive posted a query in the APT forum , waiting on an answer from Ivo )



Await your advice



Thanks
Martin

[The_bluester]

I set an exposure of 0 seconds with the camera capped and let the driver sort it out, but I have heard of people using exposures of up to several seconds. I don't know how APT behaves for very short exposures. Given the very low dark current you would probably get away with a second or so with everything capped to prevent stray light impacting things.

For a master bias, I used 100 bias frames given that by definition they are very quick to shoot.

[Startrek]

Paul,
Thanks again
Cheers
Martin

[Craig_]

I use the L-extreme too, although my OTA is f7 and your newt's should be faster - unsure if OTA f/ratio impacts flats exposure time but I assume it would, so my 8s is partly because of the relative slowness of the Esprit 120 I suppose.

You're right about the ADU target, perhaps a target of 20,000 is too high for the sensor, although it does not appear to cause any issues. I often shoot flats but have never once actually needed to use them, it's more of a "just in case" :lol: I'll try 16,400 next time and see how it goes. Cheers I use an iPad with a white tshirt over the dew shield by the way, although with the standard 10.5" iPad it doesn't actually cover the entire dew shield (about 2-3mm of uncovered dew shield on the short side) but does not appear to have caused issues either.

[Startrek]

To add more confusion to the topic of Calibration, I received a recommendation / suggestion on the APT forum by a member to use the following method -

Calibrate your Flats with Dark Flats

Don’t calibrate your Darks or Dark Flats , you only integrate them into a Master Dark and Master Dark Flat

You then calibrate your Flats using the Master Dark Flat , instead of using the Master Dark , and then integrate your Flats into a Master Flat

To clarify my understanding of the Calibration terminology -

Calibrate means to stack your Light frames with specific calibration frames ( dark, flat , dark flat or bias or alternatively a Master ) ?

Master means create a stacked file of the same type of calibration frame to be used with your Light frames in final stacking process ?

Integrate means to stack 2 types of calibration frames to form a single stack Master calibration file ?

Appreciate advice and confirmation on the above ( by the way I use Deep Sky Stacker )

Whoosh ........,



Thanks

[AdamJL]

That's the shortest exposure the camera can take. I tried it the other night in NINA. I set the exposure to 0 and it shoots at 0.000032.




^ I totally don't get that. I just chuck all of my lights and whatever calibration frames I use into Deep Sky Stacker and let it do the work.

[Startrek]

Adam,
Thanks
Me neither.....
I’m reading to much on different forums and coming with different methods and opinions all the time , one says this one says that ........
It’s either I do a PhD in Calibration at MIT or just experiment with Calibration frames until I get a good result
Cheers
Martin
Post and Thread ended !

[Craig_]

I think different people and different software approaches it differently. But, the idea is that you may need to calibrate your calibration frames.

Eg, darks may be calibrated with bias. Flats may be calibrated with bias or dark flats. Dark flats may be calibrated with bias. Bias is not calibrated by anything. Lights are then calibrated by the masters of all of the above.

Although I don't use PixInsight much the workflow I follow with that is basically - create master bias, create master darks by calibrating from master bias, create master dark flat by calibrating with bias, create master flat by calibrating with master dark flat, then use bias, flat and dark masters (but not dark flat master) to calibrate lights. At least, if memory serves, this was my process.

In APP I just stick all calibration frames into it and it does the work, but if for example I try and make a master flat without adding bias or dark flats it will have a whinge that it needs to calibrate the flats (with bias or dark flat) to make an accurate master flat.

Aside from processing software, individual cameras will also vary in terms of requirements, eg I have heard bias frames do not work with certain cameras, and for example on the 533, I can generally not bother with darks as long as my data is well dithered, as the combo of dithering and pixel rejection algorithms in the integration means that all hot pixels are removed and thermal noise is very low anyway. I have never been able to perceive much, if any, difference with the 533 between a stack from well dithered data sans darks, and the same data with darks.

[The_bluester]

OK, my take on the terminology (And workflow) and trying to to make too much war and peace.

"Integration" is commonly used to describe the stacking of multiple light frames, but it is actually more like averaging. Integration builds values up and up over time, the actual exposures are an integration. The longer the exposure or the more photons arriving in a given time, the larger the pixel value and the bigger the difference to random or repeating noise, which will occur less often so does not integrate up as strongly. I think "Stacking" is better terminology.

"Calibration" is done before stacking, and what the calibration frames do depends on what type they are.

Dark, flat and bias frames. You stack these to make master darks, master bias and master flat frames (Include dark-flats here, they are nothing different to a normal dark frame, just the exposures match your fixed length flats)

Master dark, master dark-flat and master bias frames calibrate lights or flats by subtraction. They are subtracted pixel by pixel from light or flat frames. Master darks and dark-flats subtract fixed noise (Read noise) hot pixels and dark current (All predictable and repeatable) Master bias frames subtract only the fixed noise.

A master flat is made up of calibrated flats (With a master dark flat or master bias, depending on camera and preference) divided in to your light frames. Say the centre of the master flat frame has a brightness of 100 and the corners have a brightness of 50, that is 0.5 times the illumination in the middle. The corners would be divided by 0.5 to make them as bright as the middle.

After the calibration frames (Which are applied to each light) you should have lights which don't have read noise (On average) Don't have hot pixels, don't have any dark current and the illumination should be flat from centre to edge. THEN they can be registered and stacked to average out the shot noise and reject outliers!

[Startrek]

Thanks Craig
I think next clear night ( hopefully on the weekend with a big ol moon up ) I’ll capture some lights say an hour on a target like M17 , capture my Flats and dark Flats , take some darks and Bias too
Then experiment on calibration with DSS and compare results with various methods
Vignetting and dust donuts are my main issue
I’ve thoroughly cleaned my camera sensor, protective window , coma corrector and L Extreme filter under a led mag lamp ( doesn’t guarantee your free of dust donuts )
Cheers
Martin

[Startrek]

Paul,
Thanks for the detailed explanation and procedures
Already copied it into my Astro notes
Much appreciated
Martin

[The_bluester]

I should add that some programs behave differently. Some programs will subtract master bias information from dark frames if you load a master bias to make a master dark. Astro Pixel Processor does not do that. The master darks do not have master bias information included even if you have a master bias loaded when you make the master dark. From there the light frame calibration depends on what you load. If you load a master bias but no master dark it will subtract the master bias from the light frames, if you load both a master bias and a master dark, it subtracts the master dark, but not the master bias as the master dark contains the bias information as well..

[Startrek]

Paul
Thanks again for the info and heads up
I’ve posted in the Deep Sky Stacker Groups for some relevant information regarding Calibration and my specific situation
It’s quite an extensive topic this calibration, I had no idea of the complexities with different cameras and imaging systems. It’s definitely “NOT” one shoe fits
all.
Cheers
Martin

[The_bluester]

No, I am thankful that both versions of the ASI2600 calibrate pretty nicely using the same techniques, when I had the 2600MC and the ASI294 they were very different to calibrate.

[Startrek]

I sent a post yesterday to the Deep Sky Stacker User Group outlining my complete equipment set ups ,capture software, issues at the moment and recommendations or requirements for Calibration using DSS
Received quite a few replies one of which was from David C Partridge, a DSS Developer
David simply advised for my scenario -
You don’t need Bias frames
Take your Flats
Take your Dark Flats (you may not need them but take them anyway)
If your Dithering in APT , you probably don’t need Darks ( he’s right as I dither and have not seen any difference with or without the use of Darks )
Load the frames and that’s it

Just need some clear nights now
Cheers

[JA]

That's a very good point Martin:thumbsup:

Best
JA

[The_bluester]

While it would probably get pretty decent results, I don't 100% agree with that approach.

Master darks will get rid of most hot pixels without relying on dithering and outlier rejection. Dither and outlier rejection works pretty well too, but if you have pixels with offsets or with more dark current than others, they will get rejected as outliers where using a good master dark might salvage them. Will the difference be visible? Don't know!

Bias frames are the quickest and easiest of the calibration frame types to shoot. It is easy to generate a good master bias for whatever gain and offset settings you are going to use.

If you calibrate your flats with dark-flats, that is fine and will work well, but your flats need to be a matching exposure time. Cameras that produce good bias frames (Which the 2600 does) allow you to shoot flats of varying exposure times and calibrate them with a master bias to get rid of read noise.

I tried an experiment just now. I created two master flats from a recent nights flat frames, one with the master bias and one without, then I loaded both as light frames, and as darks (So they straight subtracted one from the other) When each one was calibrated by the same one, black screen, when calibrated by the other, the attached grey crosshatch is the result.

Then I unloaded them as darks and loaded them as flats, the most visible one was the flat created without bias calibrated by the flat created with bias. The other way round produces a negative image of this and it does not stand out as much.

As always though, if you shoot flats and are happy with the results without calibrating them with dark-flats or bias, feel free to ignore me and continue! With my old 294, you really HAD to use master darks and HAD to calibrate flats with dark-flats or the results really suffered, the 2600 (Both MM and MC) is not nearly as fussy, there are some images I shot with my refractor and 2600MC that I didn't bother using flats for, I just cropped in a little to where the vignetting was not visible.

[gregbradley]

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.

[Startrek]

Greg,
Thanks for your input , personal experience and advice
Well received
I’m fortunate to have a camera like the 2600MC which is similar to the QHY 268C
I’ve 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. It’s 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.....
I’ve never had any banding issues , excessive noise or bloated stars ( even using my L Extreme filter) from this camera. I’ve always dithered every sub from 90sec up to 300 sec, I’ve stacked with and without darks and haven’t 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
I’ve got some work to do over the coming weeks to incorporate calibration and hopefully get some improvements
Won’t 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

[Startrek]

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 I’m 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

[The_bluester]

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.

[Startrek]

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 I’m over exposing ??
Files would be too huge to transfer , total of 12 GB

I’m just going to have to spend time experimenting unless you have other tests I could perform
I’m out tonight on M17 again and will take various flats at different ADU and exposure times later tonight ( plus all other cal frames )
I’m busy for the next 4 or 5 days after that , so at least I’ll capture the data and work from there
I’ve also posted on Deep Sky Stacker user Group’s as well
Maybe I’ll post on APT again , Ivo is also very helpful , just as you folk have helped me to date
Cheers
Martin

[Startrek]

Paul,
There’s 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
I’ll be using that Tool to check my Flats
It gives you a Average and Max readings for whole image
Cheers
Martin

[The_bluester]

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.

[Startrek]

Paul,
Yes if you look back in the thread I’m 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 I’ve 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
I’m going to save this one and experiment more , late night ahead
Cheers
Martin

[The_bluester]

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.

[Startrek]

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 don’t 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

[The_bluester]

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?

[Startrek]

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

I’m working on a couple of sets of Flats and other calibration files I captured last night
I’m busy for the next few days and then some more experimenting hopefully next week
I’ll take up your offer to send some Flat files if I’ve exhausted every Avenue and start pulling what hair I have left on my balding head
Appreciate the offer and help
Cheers
Martin

[The_bluester]

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.

[Startrek]

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

[The_bluester]

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.

[Startrek]

After extensive testing over the past week and much advice and support from IIS members , DSS forum and APT forum I’ve finally nailed good calibration with my set up in Sydney using the 2600MC , L Extreme filter and my trusty 6” 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 didn’t 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 didn’t 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

[The_bluester]

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.

[Retrograde]

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. :thumbsup:

[AdamJL]

Very interesting conclusion, Martin!

Thanks for testing this. I'll start editing with flats and bias as well and see how it goes :)

[The_bluester]

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!

[Startrek]

Paul,

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

I’ve 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
I’m leaving my camera at Gain 0 cooled to -10C at both sites all year round

Cheers
Martin

[The_bluester]

No problem, just I saw a similar problem with my own flats recently and went down a rabbit hole trying to work out why something I had nailed a year ago was suddenly giving me grief.

I use my 2600MC exclusively on 0 gain, and the MM on 0 gain for LRGB and 100 gain only for narrowband, I did shoot NB flats at 100 gain but a rough and ready set of sky flats this afternoon at 0 gain did better than a properly done set at 100 gain! I think it comes down to the loss of resolution where 1e=4ADC or so, the quantisation error was just too much for it to work.

[gb44]

What a great resource this is. Thanks Martin and Paul.

Ive been having the same issue with my QHY410c OSC camera where the flat frames are causing bizzare colour casts in a vignetting sort of pattern.
Looking at the thread has given me clues and I can see a few actions I can take.

Another question though - I'm getting error reports in DSS and ImagesPlus
where, even though I have shot frames at 6070x4046 pixels, these programs report differing frame sizes (6070x4044) and IP cant proceed. I can acquire the frames at the same size so its not a real problem. DSS does proceed however, so it is a work around. Just makes me wonder why???

Cheers
GlennB

[gb44]

Heres a look at the histogram Im getting.

Maybe just need the singlet over the scope?

GlennB

[gb44]

Thought I'd post a couple of pics that show the results with the poor flats.
Horsehead nebula , 11x270s QHY410c, low gain mode, gain53,9degrees, ED100, SharpCap, dithered. Stacked in DSS.
One processed in SIRIL and noise reduced in ImagesPlus.
Second processed in Startools.

Cropped out a lot of vignetting rubbish...

I tried reducing gain to zero and differing exposure lengths. Gain didnt seem to change the flats but there was some improvement lowering the exposure time towards ADU range 8-12k so I'll work with that.

GlennB