is this a good flat example?

[newbie alert]

2 hours ago, ollypenrice said:

The illumination in front of the scope must be even but if the resulting flat is even then you don't need it. In reality it never will be.

A good flat is not a flat which looks flat. A good flat is a flat which shows every irregularity of illumination in your optical system. My widefield rig flats have terribly dark corners, a bright middle and a smattering of dark dust bunnies here and there. For precisely this reason they are brilliant flats because they are telling the stacking software exactly what needs correcting - and it corrects accordingly.

Your system is going to have to be pretty disastrous if an unstretched flat doesn't look flat. Why do daytime photographers not need flats? Because they don't stretch their images. Why do we need them? Because we do. And as we do so we stretch the uneven field illumination inherent to any lens.

Atreta's flats look credible to me. Dark corners, not perfectly symmetrical, and a bright middle. They should work - but if they don't then the way they are taken will need to be refined. For reasons I have never understood I sometimes shoot flats which are not effective. I just shoot them again.

Olly

Pretty decent explanation Olly thanks for that ..but I was talking about the even illumination of the subject which in this case was the ceiling..to my mind it must have shadows..

[Atreta]

5 hours ago, ollypenrice said:

Atreta's flats look credible to me. Dark corners, not perfectly symmetrical, and a bright middle. They should work - but if they don't then the way they are taken will need to be refined. For reasons I have never understood I sometimes shoot flats which are not effective. I just shoot them again.

Thanks for the thorough explanation  Olly. Those flats didn't seem to work. 

I'm imaging again tonight and will try to make flats again and check how it goes. 

[Atreta]

I'm amazed how much easier was to process with the flats tonight.  I dimmed the laptop screen to its lowest setting and used av mode. 

Startools was able to remove the gradients easily and where it used to be dark corners it was quite brighter than the rest, i only need to press the vignetting button and voila. 

This is the resulting image so far: 

Still have to process a bit and crop those coma filled corners and denoise

Thanks a lot to everyone who helped. 

[Demonperformer]

It's progressng nicely.

[Atreta]

Thanks,  the next thing to sort is how to use the coma corrector, i tried everything and it doesn't work for me. 

[ollypenrice]

On 08/07/2018 at 23:51, newbie alert said:

Pretty decent explanation Olly thanks for that ..but I was talking about the even illumination of the subject which in this case was the ceiling..to my mind it must have shadows..

People do use 'observatory flats' in which a white painted panel inside is illuminated as evenly as possible by a light source but, as you say, it will be hard to tell if such a panel or ceiling really is evenly lit. I use a graphics panel.

Olly

[wimvb]

1 hour ago, ollypenrice said:

People do use 'observatory flats' in which a white painted panel inside is illuminated as evenly as possible by a light source but, as you say, it will be hard to tell if such a panel or ceiling really is evenly lit. I use a graphics panel.

Olly

Also the pro's (Uni of Wyoming, I wonder what that dark line in the middle does to flats)

 

[ollypenrice]

1 minute ago, wimvb said:

Also the pro's (Uni of Wyoming, I wonder what that dark line in the middle does to flats)

 

Mystery!

Olly

[Demonperformer]

I seem to remember an article (in AN a couple of years back) by Damien Peach on imaging M92 in which he said he used the observatory wall to take flats while it was getting dark enough to image.