Serious Advice Needed

[Stub Mandrel]

I need some advice. I have got some nice detailed subs of the North America Nebula and the Pelican from last night.

There is a gradient with the bottom of the subs looking rather dark.

Gradient Exterminator gets rid of the big gradient, but then stretching pulls out a light band across the bottom and another about 1/3 of the way up. The image below is just chosen to enhance the bands.

I've had this before, but on images where masking allows me to use Gradex again and get rid of the bands, but because the nebulosity here is so extensive, it just isn't working on this image.

If I set gradient exterminator to fine or medium to get rid of the bands, it just destroys all the contrast.

So TWO things... where do these bands come from and ho do I get rid of them.

<Edit ... I think they are reversed on my flats, which is strange - I will take some new flats and see if it helps>

[Davey-T]

Make sure the software isn't flipping your images or flats, I've had this happen before, it's actually a good thing to have some nice dust bunnies to check this isn't happening 

Dave

[alan4908]

1 hour ago, Stub Mandrel said:

So TWO things... where do these bands come from and ho do I get rid of them.

Hi Neil

Although I haven't experienced this problem myself, camera read noise can create vertical or horizontal banding. Noel Carboni provides an action to reduce this effect, so you might want to try this.

see http://www.prodigitalsoftware.com/Astronomy_Tools_For_Full_Version.html

Alan

[Stub Mandrel]

It was a flats issue.

I think its because I mixed flats taken using the sky with flats taken using a plain magnolia wall.

I took another 36 flats using the wall, 3 at at each 30 degrees of rotation and the image I got doesn't even need gradient exterminator at all! (which I put down to using a light pollution filter).

I will go back to my images from a couple of weeks ago as they showed the same problem.

[uhb1966]

tbh in my personal setup I've stopped using flats at all... they were doing more harm than good...

[Stub Mandrel]

I have a few marks left after the astro-modding, I'd rather not take it all apart again with no guarantee of success in getting rid of them. But yes, the flats were my problem.

 

 

[ollypenrice]

15 hours ago, uhb1966 said:

tbh in my personal setup I've stopped using flats at all... they were doing more harm than good...

It isn't easy, necessarily, to take good flats but I consider them absolutely essential. Rather than give up, I think it's best to stick at it to find out the problem. Were you calibrating your flats by subtracting a master bias (in effect the ideal dark for almost any flats? This is vital.)

Olly