Some examples of flats please.

[rharrison]

Hello everyone,

Thanks in advance for your help.

It's lovely and clear all night tonight in Essex. I'm getting some nice guided subs or lights whatever you like to call them but the winds making it feel colder than usual ! Anyway I've not used flats yet and I'm finding it difficult to find examples, I plan to take some when the sun starts to light up the sky in the morning.

Could you please post any good examples of a master flat or a good sub from your flats. I was thinking to take around 20 but have pretty much no idea on exposure time apart from apparently I should use the same ISO as my lights which is currently 400.

I'm using a Canon 1200d on a 4.5 F/L Newtonian.

The examples I've found have all been mono ccd's.

Regards Rob

[lensman57]

Hi,

Here is a Master Flat ( 35 Flats ) taken with an Atik 383L+ Mono , Baader 2" 7nm Ha filter and my WO Star 71, it has had a histogram stretch in PI  with STF parameters. I set the exposure for an average ADU of 27000, I think it was about 6 or 7 seconds exposure per sub. I used a n Aurora flat panel. For a DSLR it is slightly different and when I use mine I set the dial to Av and increase the exposure compensation to +1.3 stops and it usually works well for me but everyone's set up is different.

A.G

[Carina Lass]

https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcSofPkKvFaBITFRk1md7DVM7rt9zE94RDpNTwiwD3nh9c-vPbypxx_OpDE

Utah

[steppenwolf]

Here's one from my OSC APS C size sensor. 24 x 1.4 secs ADU target 22,000 using an electroluminescent pane. Not sure how much this will help you though as I can stretch it all over the place to show you different aspects - this incarnation lets you see that it has picked up some dust motes close to the sensor!

[Marky1973]

This is an interesting thread as my flats look nothing like that, making me wonder if I am doing it right!

Is this over-exposed? Is my flat "frame" too bright?

Sorry to hijack the OP's thread.

[ollypenrice]

If your flat doesn't have darker corners and a brighter middle it won't be right. I get this using the flattest flatfield astrograph on the planet, the Tak FSQ106, and even so I get this kind of thing:

The peak of your histogram for flats should be between one third and two thirds of the way from left to right. I prefer to be nearer a third. It depends on the rig, but a third is always good for me.

I can't see any reason whatever for needing to use the same ISO but I don't image with DSLRs so maybe there is a reason?

Flats should be calibrated with their own darks but the good news is that a master bias will do for a 'dark for flats.' There is no statistical difference between a master bias and a master short dark frame.

Olly

[Uranium235]

One from the 80ED:

One from the 130pds:

Yes... my camera needs a bit of a clean

Just need to add that the levels were changed as so show the corners a bit more, at first it looked like Olly's flat (except a bit cleaner...lol)   But Ive always been a bit fanatical about keeping dust out of the imaging train.

[Marky1973]

I think I need to play a game with my flat preparation at the weekend then....

[ollypenrice]

One from the 80ED:

MasterFlat_80ED.jpg

One from the 130pds:

MasterFlat_130pds.jpg

Yes... my camera needs a bit of a clean

Just need to add that the levels were changed as so show the corners a bit more, at first it looked like Olly's flat (except a bit cleaner...lol)   But Ive always been a bit fanatical about keeping dust out of the imaging train.

Hey, it's easy for you! We've got chips the size of football fields. Life's 'ard down here, let me tell you!!!

lly

[Marky1973]

If your flat doesn't have darker corners and a brighter middle it won't be right. I get this using the flattest flatfield astrograph on the planet, the Tak FSQ106, and even so I get this kind of thing:

The peak of your histogram for flats should be between one third and two thirds of the way from left to right. I prefer to be nearer a third. It depends on the rig, but a third is always good for me.

I can't see any reason whatever for needing to use the same ISO but I don't image with DSLRs so maybe there is a reason?

Flats should be calibrated with their own darks but the good news is that a master bias will do for a 'dark for flats.' There is no statistical difference between a master bias and a master short dark frame.

Olly

Olly, just because I'm an idiot - does this mean that my Flat should actually be lighter than it is? I have been using a white t-shirt doubled up and stretch over a light source, but perhaps with one layer, it would be better? Like I said, I have been meaning to play around with my flats for a while, so I have the impetus to actually do something now!

[sharkmelley]

Use something like a diffusing panel instead of a white T-shirt.  I really don't think a T-shirt gives sufficiently evenly spread light.

Search for "3mm OPAL ACRYLIC 500 x 500 mm Light Diffuser Sign Box Plastic Sheet Panel" on eBay or Amazon - just over a tenner including postage or a fiver for an A4 sheet

e.g. http://www.ebay.co.uk/itm/400143709684

Also, the purpose of flats is to quantify the relative sensitivity of pixels as well as vignetting and dust.  None of these are affected by the ISO setting, so I go for the ISO setting that gives the least noise i.e. ISO 100 and take lots of them.

Mark

[rharrison]

This is an interesting thread as my flats look nothing like that, making me wonder if I am doing it right!

F_6503.jpg

Is this over-exposed? Is my flat "frame" too bright?

Sorry to hijack the OP's thread.

Hello,

Looks very similar to my flats and I too am feeling they're wrong !

I'm setting my 1200d to 'av' 400 iso just before the sun goes down and taking around 20 in raw.

When I use the flats in DSS then really stretch the final image in photoshop it seems much better than not using flats so possibly they're ok...

I'll post the master flat I'm using a bit later, just in the garden trying to get M81 damm full moon !

[rharrison]

Use something like a diffusing panel instead of a white T-shirt.  I really don't think a T-shirt gives sufficiently evenly spread light.

Search for "3mm OPAL ACRYLIC 500 x 500 mm Light Diffuser Sign Box Plastic Sheet Panel" on eBay or Amazon - just over a tenner including postage or a fiver for an A4 sheet

e.g. http://www.ebay.co.uk/itm/400143709684

Also, the purpose of flats is to quantify the relative sensitivity of pixels as well as vignetting and dust.  None of these are affected by the ISO setting, so I go for the ISO setting that gives the least noise i.e. ISO 100 and take lots of them.

Mark

So using the pre dawn sky is not good ?

[rharrison]

If your flat doesn't have darker corners and a brighter middle it won't be right. I get this using the flattest flatfield astrograph on the planet, the Tak FSQ106, and even so I get this kind of thing:

The peak of your histogram for flats should be between one third and two thirds of the way from left to right. I prefer to be nearer a third. It depends on the rig, but a third is always good for me.

I can't see any reason whatever for needing to use the same ISO but I don't image with DSLRs so maybe there is a reason?

Flats should be calibrated with their own darks but the good news is that a master bias will do for a 'dark for flats.' There is no statistical difference between a master bias and a master short dark frame.

Olly

I'm getting darker corners but nothing like as dark as that, is that adjusted to show us what should be going on in the image ? I read a lot of your posts on here, thanks for the help. How many flats and dark flats should I take roughly ? I know it's probably a difficult question as it would depend on the camera and other variables but I was thinking 20 flats and 20 dark flats is that about right ?

Thanks

[sharkmelley]

So using the pre dawn sky is not good ?

A pre-dawn or post-sunset sky is excellent to use on its own (though you may end up picking up a few stars in your flats).  So a twilight sky with diffuser is absolutely perfect!  I generally use between 16 and 32 flats at ISO 100.  That allows you to collect as much light (and to produce just as noise-free a flat) as 128-256 flats taken at ISO 800.  There's really no need for flat darks unless the sky is so dim that each flat needs a long exposure time.

The example flats on this page have all been stretched to emphasise what you are looking for.  If your corners really come out that black (without stretching) then you would be having severe vignetting problems with your optics!  Some scope/camera combinations will have no detectable dimming at the corners until the image is stretched.

Mark

[Levster]

I've got a feeling that I'm misusing flats. I try to use mine to also compensate for the light pollution gradient. During the lights exposure I will place a doubled over white plastic bag over the front of the lens (I'm still waiting on my first scope to arrive) and then expose for around the same amount of time as the lights. This produces a flat image that includes the vignette of the lens and the gradient of the light pollution. When I shoot Orion it is quite close to the Southern horizon and light pollution is really bad on the bottom-right of the image and improves towards the top-left. Using my flats has really improved the output that I get from DSS so they must be doing something useful!

[rharrison]

A pre-dawn or post-sunset sky is excellent to use on its own (though you may end up picking up a few stars in your flats).  So a twilight sky with diffuser is absolutely perfect!  I generally use between 16 and 32 flats at ISO 100.  That allows you to collect as much light (and to produce just as noise-free a flat) as 128-256 flats taken at ISO 800.  There's really no need for flat darks unless the sky is so dim that each flat needs a long exposure time.

The example flats on this page have all been stretched to emphasise what you are looking for.  If your corners really come out that black (without stretching) then you would be having severe vignetting problems with your optics!  Some scope/camera combinations will have no detectable dimming at the corners until the image is stretched.

Mark

That's great thanks, makes sense the low ISO on flats! I'll have a look at getting a diffuser, I read somewhere you should keep iso the same as your lights but as Olly said that doesn't make much sense. I think it was in the Mark Thompson book I'm reading.

[ollypenrice]

Ignore the screen stretch on my and other flats presented on threads like this. The poster will just apply a screen stretch which exaggerates the contrasts to make the image more informative. In the raw (linear, unstretched) data the difference between dark corners and bright middle will only be two or three thousand ADU (in a range of 65000) at most. So the corners are not 'really' that dark.

The only way to set the brighness is to look at the position of the histo peak or read the white point value or mouse over the linear flat in a software that will give the pixel values in ADU under the cursor. Whatever you max value (65000 in the case of most CCD cameras) the brightest pixels should be about a third of that. Maybe up to two thirds, depending on whom you listen to.

Flats cannot attenuate LP gradients. If you could somehow photograph the pure LP gradient itself, without stars, then maybe you could use it in this way, but since our objects rise, transit and set at different altitudes I don't see this as being possible. Flats correct errors in your lightpath and nothing else. DBE in Pixinsight is the way to nail LP gradients.

I have a poor success rate with twilight/T shirt flats. I don't know why because some people make them work, but typically I get a flat with a gradient when I try them. One edge of the flat ends up brighter than its opposite edge. I've tried all sorts of things to eradicate this but in the end setlled for panel flats shot in the dark. Certainly in the dark. That's vital.

Thinking about DSLR flats, surely the best plan would be to use the ISO setting which gave the lowest noise? Any residual noise in flats will pass into the final image. 20 should be OK.  After all, flats are not stretched when we apply them so we are not punishing the noise as we do in our lights.

Flats not calibrated by the subtraction of a master bias as dark frame will over correct and create inverse vignetting.

Olly

[Marky1973]

So Olly, excuse me "not getting" this.....does my flat (unstretched) look like it would do the job. I can see two spots of dust that show up about 2/3 of the way across from the left in the middle of the picture - and these showed up quite clearly last night when I stacked some lights without the flats as an "experiment". However, they also showed up in the final image with the flats (only not as obvious). No,w clearly, I need to clean the sensor, but I am presuming that they are still showing up as I am not "quite" getting the exposure right on the flats? As I said, I need to take some time to play around with the flats settings in APT during the day when I am less pressured!

[Marky1973]

Hello,

Looks very similar to my flats and I too am feeling they're wrong !

I'm setting my 1200d to 'av' 400 iso just before the sun goes down and taking around 20 in raw.

When I use the flats in DSS then really stretch the final image in photoshop it seems much better than not using flats so possibly they're ok...

I'll post the master flat I'm using a bit later, just in the garden trying to get M81 damm full moon !

My images are definitely better with, but I wonder if I could be getting even better by nailing the flats! I have, in the past, been setting to AV and taking shots at the same settings (otherwise) as the lights - what I struggle with is identifying the histogram readings and getting the peak int he right place - so that is something I am going to have to play around with...as I have only remembered I need to think about that when I am outside taking photos....and then it is a bit too late to experiment!

[ollypenrice]

So Olly, excuse me "not getting" this.....does my flat (unstretched) look like it would do the job. I can see two spots of dust that show up about 2/3 of the way across from the left in the middle of the picture - and these showed up quite clearly last night when I stacked some lights without the flats as an "experiment". However, they also showed up in the final image with the flats (only not as obvious). No,w clearly, I need to clean the sensor, but I am presuming that they are still showing up as I am not "quite" getting the exposure right on the flats? As I said, I need to take some time to play around with the flats settings in APT during the day when I am less pressured!

It's very hard to say just by looking at it. It does seem rather flat   but maybe that's how it is. One thing that flats can't correct are the small black patches caused by contaminants sitting right on the sensor. There is literally no information in the affected pixels so no correction is possible. Clone stamp job!

Olly

[Marky1973]

It's very hard to say just by looking at it. It does seem rather flat   but maybe that's how it is. One thing that flats can't correct are the small black patches caused by contaminants sitting right on the sensor. There is literally no information in the affected pixels so no correction is possible. Clone stamp job!

Olly

Ah, so that will be why the affected areas in the image were completely black after stacking! So its cloning or a sensor cleaning.....could it be dust on the mirror? I'm making this up as I go by the way.

[ollypenrice]

Anything really sharp and solid looking is on the sensor. All bunnies are caused by things close to the chip. Think of the secondary mirror on any reflector. It makes no bunnies!

Olly

[Marky1973]

Think I need to get an hand-pump air blower and give it all a clean! - the pitfalls of changing lenses! Is this as much of an issue with CCD, or are they easier to keep clean.....?

I'll shut up now and leave you all in peace....

[rharrison]

Ignore the screen stretch on my and other flats presented on threads like this. The poster will just apply a screen stretch which exaggerates the contrasts to make the image more informative. In the raw (linear, unstretched) data the difference between dark corners and bright middle will only be two or three thousand ADU (in a range of 65000) at most. So the corners are not 'really' that dark.

The only way to set the brighness is to look at the position of the histo peak or read the white point value or mouse over the linear flat in a software that will give the pixel values in ADU under the cursor. Whatever you max value (65000 in the case of most CCD cameras) the brightest pixels should be about a third of that. Maybe up to two thirds, depending on whom you listen to.

Flats cannot attenuate LP gradients. If you could somehow photograph the pure LP gradient itself, without stars, then maybe you could use it in this way, but since our objects rise, transit and set at different altitudes I don't see this as being possible. Flats correct errors in your lightpath and nothing else. DBE in Pixinsight is the way to nail LP gradients.

I have a poor success rate with twilight/T shirt flats. I don't know why because some people make them work, but typically I get a flat with a gradient when I try them. One edge of the flat ends up brighter than its opposite edge. I've tried all sorts of things to eradicate this but in the end setlled for panel flats shot in the dark. Certainly in the dark. That's vital.

Thinking about DSLR flats, surely the best plan would be to use the ISO setting which gave the lowest noise? Any residual noise in flats will pass into the final image. 20 should be OK.  After all, flats are not stretched when we apply them so we are not punishing the noise as we do in our lights.

Flats not calibrated by the subtraction of a master bias as dark frame will over correct and create inverse vignetting.

Olly

Makes perfect sense thanks !

Is there software to get the ADU from my dslr ? It's fascinating really getting down to the actual maths of the data.

When the sun starts to go down later I will take 20 at the lowest iso and some dark falts. I too got a strange gradient towards the bottom, do you think pointing the scope directly up might help ?

Rob