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Old 22-04-2022, 06:52 PM
AdamJL
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Mono Flats - Please post yours!

Hi all

Wondering if any of you are able to post (screenshots are fine!) images of what your flats look like for your mono cameras and relevant filters?
Would you also mind stating what camera and filter size you're using (and whether they're mounted/unmounted?)

I'm trying to compare with mine. Mine vignette a bit and I am curious if this is common to filters in general and if so, how much.

Cheers!
Adam
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Old 22-04-2022, 08:37 PM
Startrek (Martin)
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Adam,
I spent months working out the best calibration frames for my 2600MC
I never bothered to look at the flat frame , only the stacked calibrated frame after a stretch to check the field
My flat frames by themselves looked crap
I tried Flats , Darks , Flat Darks
Then Flats and Flat Darks
Then Flats , Darks , Bias
Then Flats and Bias
Flats and Bias worked the best for this camera , but , I was using a ADU target of 28,000 to 32,000 ( 2600MC has at 16bit digital analog converter and has a max ADU of 65,536 so recommended ADU target should be around 32,000 or 1/3 to 1/2 along the Histogram , this was totally wrong !!! )
At 32,000 I was way over exposing the Flats , ended up over half way on the Histogram
I adjusted my ADU target in APT down to 20,000 to 21,000 and the Histogram ended up with an ADU range of 8,500 to 31,500 , which is spot on between black and white points
So with and without filters I use an ADU target of 20,000 to 21,000 and end up with a Flat exposure of 0.2 to 0.5 sec for no filter and 7 sec to 13 sec with the L Extreme filter
I calibrate my Flats with Bias frames ( Bias frames set to 0.000032 sec which is the min exposure for the 2600MC )
Usually 40 to 50 Flats and 50 to 70 Bias frames
Lights , Flats and Bias loaded into DSS and bingo , nice flat field after a stretch in Startools
Took me a month or so of experimenting but found the right combo, settings and method
Sorry cant help you with Mono but I know the ( the Bluester) Paul has the 2600MM
and he uses Flats with Bias frames too

Cheers
Martin
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Old 24-04-2022, 08:44 AM
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Depending on how the program you are using auto stretches, flats can look like they show extreme vignetting just because the range of data is typically very small and it will be stretched so it fills the histogram.

This is what my current flats look like (LRGB) but if you don't stretch them you would be hard pressed to see any vignetting at all. This is on my refractor, the ones off the newt look even worse as there is some vignetting from the secondary mirror so it is even more uneven looking.

Edited to add, this is from my ASI2600MM, 36mm unmounted Astronomik filters (Which are basically a mounted filter, they come in an aluminium ring to make putting them in a filter wheel easier) and my Stellarvue SVX80T at F4.8.
Attached Thumbnails
Click for full-size image (MF-IG_0.0-E_1.28s-ZWO_ASI2600MM_Pro-6248x4176--Red-session_1-St.jpg)
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Click for full-size image (MF-IG_0.0-E_2.56s-ZWO_ASI2600MM_Pro-6248x4176--Green-session_1-St.jpg)
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Click for full-size image (MF-IG_0.0-E_6.08s-ZWO_ASI2600MM_Pro-6248x4176--Blue-session_1-St.jpg)
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Click for full-size image (MF-IG_0.0-E_6.87s-ZWO_ASI2600MM_Pro-6248x4176--Lum-session_1-St.jpg)
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Last edited by The_bluester; 28-04-2022 at 08:17 AM.
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Old 28-04-2022, 07:03 AM
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I am about to include dark, bias, flats and dark flats or whatever works best and this thread is most helpful.

At this stage all of these are going to be a hassle with the RASA set up as you can't just put on a cap and fire away and my preoccupation at the moment is how I am going to take flats and bias..two black sheets thrown over the lot for example is sounding like a good idea. Because of the cable routing needed the camera looks like it will be a permanent fitting and to put on its cap it must be removed from the scope.

In any event making a library although a seemingly huge job is becoming appealing as darks will probably need the black bed sheets or the removal of the camera and flats I guess will need to be the t shirt approach unless I can sourse something to use instead.

May I ask what the views are on a calibration library and it's expected life ...I expect things may change as the camera ages such that it probably would need doing again at some point.

I understand I will have to settle on unity and gain figures that I must stick to so as to be able to use a library if it is at all viable...
Alex
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Old 28-04-2022, 08:22 AM
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Alex, is that with a 2600? I have found that the darks are quite stable, I used my first set for 12 months and only shot new ones as the old default in Astro Pixel Processor used to produce 16 bit calibration frames which could cause some posterisation. I had deleted the raw dark files so I shot a new set last month to create 32 bit master darks. If the camera is not on the scope yet you could shoot darks and bias frames before fitting it and use them for at least a year. It would only take a couple of days to shoot a good number of darks at multiple gains and exposure times and bias frames at multiple gains will not take long at all. I aim for 50 darks of each gain/exposure to make a nice smooth master dark. Just make sure any masters you create are 32 bit. That issue mostly showed up when calibrating with flats, with some difficult to correct brightening of the corners.
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Old 28-04-2022, 08:53 AM
AdamJL
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Quote:
Originally Posted by The_bluester View Post
Depending on how the program you are using auto stretches, flats can look like they show extreme vignetting just because the range of data is typically very small and it will be stretched so it fills the histogram.

This is what my current flats look like (LRGB) but if you don't stretch them you would be hard pressed to see any vignetting at all. This is on my refractor, the ones off the newt look even worse as there is some vignetting from the secondary mirror so it is even more uneven looking.

Edited to add, this is from my ASI2600MM, 36mm unmounted Astronomik filters (Which are basically a mounted filter, they come in an aluminium ring to make putting them in a filter wheel easier) and my Stellarvue SVX80T at F4.8.
That's exactly what I was after. Thank you! Very interesting to see how your vignettes are basically equal.

These were mine. I have off-centred vignettes (I think it's down to the filter wheel not calibrating exactly to the middle) and light reflections (working on fixing that!)
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Old 28-04-2022, 01:33 PM
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Quote:
Originally Posted by The_bluester View Post
Alex, is that with a 2600? I have found that the darks are quite stable, I used my first set for 12 months and only shot new ones as the old default in Astro Pixel Processor used to produce 16 bit calibration frames which could cause some posterisation. I had deleted the raw dark files so I shot a new set last month to create 32 bit master darks. If the camera is not on the scope yet you could shoot darks and bias frames before fitting it and use them for at least a year. It would only take a couple of days to shoot a good number of darks at multiple gains and exposure times and bias frames at multiple gains will not take long at all. I aim for 50 darks of each gain/exposure to make a nice smooth master dark. Just make sure any masters you create are 32 bit. That issue mostly showed up when calibrating with flats, with some difficult to correct brightening of the corners.
Thanks for addressing my concerns Paul I greatly appreciate your help.

Yes it is a ZWO 2600 mono on the RASA 11 that is my concern. I also have a ZWO 2600 MC the OSC version which won't pose a similar problem as it will probably go on the 80mm permanently.

My assumptions turn on being able to use the RASA with a narrow band set up, which still seems a ,long way off..the only way I could fit the filter draw was to space it some 20 mm away from the camera, for me, an uncomfortable distance from the sensor and in time I want to change this to get the filters as close as possible to the sensor.

I just now saw a flat frame light generator on line for $349 (US $ I think) which motivates me to buy a couple of T shirts. The annoying thing is that I have all the stuff sitting in Sydney to make a flash light box..anyways at least taking something with t-shirts is progress ... the concern is that the tshirt will have to be drapped over the dew shield given the placement of the camera..if that does not work I guess a special ring to hold the shirt just above the camera.

I will start my dark library and probably with the camera on the desk while I work out what to set up for flats..hmmmm maybe take some dark flats ..a variety that I can test...and after shopping some flats to get started.

Thanks again.

Alex
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Old 28-04-2022, 08:34 PM
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The best calibration routine I have found to date, is to calibrate flats (Which can be at a range of exposure times) with a master bias, and lights with the resulting master flat and a matching master dark. Using a master bias in place of a master dark resulted in some brightening of the corners, presumably due to applying the flat frame to what little dark current these cameras have.

The benefit of being able to use a master bias to calibrate the lights instead of a master dark-flat (Or flat-dark, whichever term you go with) is that the flat exposure times can vary as much as you like. I set up my software to take dawn flats each session and exposures vary all the way from 40 to about 0.2 seconds depending on the approaching dawn, and which filter is being shot.
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Old 29-04-2022, 09:56 AM
Startrek (Martin)
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Quote:
Originally Posted by xelasnave View Post
Thanks for addressing my concerns Paul I greatly appreciate your help.

Yes it is a ZWO 2600 mono on the RASA 11 that is my concern. I also have a ZWO 2600 MC the OSC version which won't pose a similar problem as it will probably go on the 80mm permanently.

My assumptions turn on being able to use the RASA with a narrow band set up, which still seems a ,long way off..the only way I could fit the filter draw was to space it some 20 mm away from the camera, for me, an uncomfortable distance from the sensor and in time I want to change this to get the filters as close as possible to the sensor.

I just now saw a flat frame light generator on line for $349 (US $ I think) which motivates me to buy a couple of T shirts. The annoying thing is that I have all the stuff sitting in Sydney to make a flash light box..anyways at least taking something with t-shirts is progress ... the concern is that the tshirt will have to be drapped over the dew shield given the placement of the camera..if that does not work I guess a special ring to hold the shirt just above the camera.

I will start my dark library and probably with the camera on the desk while I work out what to set up for flats..hmmmm maybe take some dark flats ..a variety that I can test...and after shopping some flats to get started.

Thanks again.

Alex

Alex
Ive been using an A3 Huion led dimmable light panel to take my Flats with my 6 and 8 Newts for over a year
Works fine with a T shirt
Attached is the A2 model which I am buying for my 10 newt

https://www.digidirect.com.au/huion-a2-led-light-pad

Only $219.00 from Digi direct ( Australian company )

This should work for your RASA

The dimmer is touch control via the off on button

I just slew my mount vertically using EQMOD ( obviously not tracking ) and sit the light panel on top , but you could use other methods like up against a wall or suspended or use a music stand etc...,

I usually dim the light down to the lowest and stretch the T shirt over the OTA with a rubber band ( A flattened t shirt so 2 layers of material )

Hope the above helps

Martin
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Old 29-04-2022, 12:37 PM
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Quote:
Originally Posted by The_bluester View Post
The best calibration routine I have found to date, is to calibrate flats (Which can be at a range of exposure times) with a master bias, and lights with the resulting master flat and a matching master dark. Using a master bias in place of a master dark resulted in some brightening of the corners, presumably due to applying the flat frame to what little dark current these cameras have.

The benefit of being able to use a master bias to calibrate the lights instead of a master dark-flat (Or flat-dark, whichever term you go with) is that the flat exposure times can vary as much as you like. I set up my software to take dawn flats each session and exposures vary all the way from 40 to about 0.2 seconds depending on the approaching dawn, and which filter is being shot.
Thanks Paul.

I am getting rather serious and learning everything I can and although you suggest a particular method I will do a variety of tests if I ever get a run of decent clear sky and see how I can fit your system into my selected routine. I think the RASA may take a while to get a sensible work flow...

I have formed an initial impression that flats are most necessary whereas darks, given the cameras these days, is not as big an issue as it once was...dithering and many subs already has shown to me, at least to me, noise is usually not too bad..flats however I know will improve my images greatly.

Thanks again for everything.

Alex
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Old 29-04-2022, 12:51 PM
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Alex
I’ve been using an A3 Huion led dimmable light panel to take my Flats with my 6” and 8” Newts for over a year
Works fine with a T shirt
Attached is the A2 model which I am buying for my 10” newt

https://www.digidirect.com.au/huion-a2-led-light-pad

Only $219.00 from Digi direct ( Australian company )

This should work for your RASA

The dimmer is touch control via the off on button

I just slew my mount vertically using EQMOD ( obviously not tracking ) and sit the light panel on top , but you could use other methods like up against a wall or suspended or use a music stand etc...,

I usually dim the light down to the lowest and stretch the T shirt over the OTA with a rubber band ( A flattened t shirt so 2 layers of material )

Hope the above helps

Martin
Thanks Martin...my carer volunteers at the Salvation Army shop so I have put in my order for something "white" to get things started but I have just now ordered one of those units you recommend...I have ordered additional EAF ( focusers) and guide cameras to have each system not reliant upon stealing things from the rig next to it..extravagant but I know it will be worth the money as if one fails it will be simply a case of switching on the unit next to it...I value clear sky and have determined that when I have it I will not be wasting time fixing or adjusting and sincerely hope my preparedness to compromise won't be called upon when I have things set up as I have planned.

I will probably build a light box around the product you suggested ...mainly so I have it all in one unit ( built in t shirt.. flash) and while at it include a mechanism such that the light box can attach securely and not dependant on orientation of the scope just resting in place.

It certainly is hard work compared to my old system of grab anything with high gain and rely on processing heavily and photoshopping...against the rules of real astro photography.

I am looking forward to your efforts with the new gear.

Thank you very much for all your help.

Alex
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Old 29-04-2022, 08:53 PM
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Quote:
Originally Posted by xelasnave View Post
Thanks Paul.

I am getting rather serious and learning everything I can and although you suggest a particular method I will do a variety of tests if I ever get a run of decent clear sky and see how I can fit your system into my selected routine. I think the RASA may take a while to get a sensible work flow...

I have formed an initial impression that flats are most necessary whereas darks, given the cameras these days, is not as big an issue as it once was...dithering and many subs already has shown to me, at least to me, noise is usually not too bad..flats however I know will improve my images greatly.

Thanks again for everything.

Alex
Regards different methods. I have found some data sets quite tolerant of calibration with a master bias and master flat (Which itself is produced from flats calibrated with the same master bias) but on others it led to brightened corners. Typically narrowband or LRGB images where I am trying to get away with the shortest subs I can, so the background is only just above the noise floor, master flat/master dark calibration proved better on some of those.

I have not found a data set of my own where calibrating with a master dark and master flat that that was calibrated with a master bias is worse than using a master bias instead of master dark, so I just do everything that way now. But everyone is free to use whatever process gets them a result they are happy with, many ways to pluck that running chicken!
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Old 05-05-2022, 07:55 AM
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Originally Posted by The_bluester View Post
Regards different methods. I have found some data sets quite tolerant of calibration with a master bias and master flat (Which itself is produced from flats calibrated with the same master bias) but on others it led to brightened corners. Typically narrowband or LRGB images where I am trying to get away with the shortest subs I can, so the background is only just above the noise floor, master flat/master dark calibration proved better on some of those.

I have not found a data set of my own where calibrating with a master dark and master flat that that was calibrated with a master bias is worse than using a master bias instead of master dark, so I just do everything that way now. But everyone is free to use whatever process gets them a result they are happy with, many ways to pluck that running chicken!
My experience as well. Flats can be touchy and very often need the correct dark and bias with it to work well.

Sony ICX694 CCD sensor and the KAI16200 CCD are pretty clean and often just needed a bias and perhaps a flat for the KAI sensor as its larger.

Small sensors may not even need flats as vignetting should be minimal.

Greg.
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Old 05-05-2022, 07:58 AM
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Quote:
Originally Posted by The_bluester View Post
Regards different methods. I have found some data sets quite tolerant of calibration with a master bias and master flat (Which itself is produced from flats calibrated with the same master bias) but on others it led to brightened corners. Typically narrowband or LRGB images where I am trying to get away with the shortest subs I can, so the background is only just above the noise floor, master flat/master dark calibration proved better on some of those.

I have not found a data set of my own where calibrating with a master dark and master flat that that was calibrated with a master bias is worse than using a master bias instead of master dark, so I just do everything that way now. But everyone is free to use whatever process gets them a result they are happy with, many ways to pluck that running chicken!
I generally do not subtract a bias from my flats as its double dipping and I have seen it result in a poor calibration. I can always specify subtract a bias from the flat when calibrating in CCDstack.

With CMOS i read its often recommended to do flat darks rather than a bias.

But the pain there is these CMOS cameras don't have a shutter so it makes taking darks a more tedious process due to having to cap the camera

Greg.
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Old 12-08-2022, 10:06 AM
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I think I've fixed my flats issues. Just had a play around with the filter masks first and that seemed to have changed a few things.

Also changed the exposures for two of my filters (SII and OIII) and I'm getting much nicer flats now. I had to bump them to 11 and 6 seconds respectively.

I only have one issue left and that's an issue on my OIII where one corner is very bright. I think it's the edge blackening of the filter that has rubbed off and light is leaking in, d'oh. Need to do more tests.
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Old 12-08-2022, 11:06 AM
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I think I've fixed my flats issues. Just had a play around with the filter masks first and that seemed to have changed a few things.

Also changed the exposures for two of my filters (SII and OIII) and I'm getting much nicer flats now. I had to bump them to 11 and 6 seconds respectively.

I only have one issue left and that's an issue on my OIII where one corner is very bright. I think it's the edge blackening of the filter that has rubbed off and light is leaking in, d'oh. Need to do more tests.
Adam,
Glad you have nearly sorted out Mono calibration
It took me 3 months to sort out calibration with my 2600MC ( and I will have issues from time to time )
If I ever go Mono early next Im expecting heaps of issues to sort out , one of them will be calibration

Looking forward to seeing more of your images

Cheers
Martin
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Old 12-08-2022, 01:00 PM
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Good to see that you are getting there, good flats do make a big difference.

Quote:
Originally Posted by gregbradley View Post
I generally do not subtract a bias from my flats as its double dipping and I have seen it result in a poor calibration. I can always specify subtract a bias from the flat when calibrating in CCDstack.

With CMOS i read its often recommended to do flat darks rather than a bias.

But the pain there is these CMOS cameras don't have a shutter so it makes taking darks a more tedious process due to having to cap the camera

Greg.
Using Astro Pixel Processor the workflow seems to avoid any double dipping. If you load a master bias frame, a master flat will be created with the master bias subtracted (And if you load bias frames, a master bias is created and then that is subtracted) but if you load a master dark flat, that will be subtracted instead, and the master dark flat will have been created without the master bias being subtracted from it, so it does not double dip.

Same deal with a master dark, they are created from darks only so the bias information is included in that master. If you calibrated lights with a master bias, it will be subtracted but if you load a master dark as well, only that is subtracted form the lights, not both.
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