Another filter issue?

[rubecula]

The image below is a couple of hours worth of stacked Ha frames of the Medusa Nebula that I was hoping to combine with OIII and SII.  I hope you can see in the attached jpg that there's a sort of wide pale halo around the nebula. When I put this into the green channel it is much more noticable.

I wasn't a particularly cold night and I did have the dew heaters running.  The following night I used the same filter on the Whirlpool nebula and there's no sign of the halo.

Any idea of the cause or how I could get rid of it?

I tried a couple of runs of Gradient Exterminator and that removed the darker arear in the corners but left that darker area around the nebula.

Thanks

[Adam J]

Do you take flats? Am assuming that this is not calibrated due to the bad pixels that are evident in the image. If not then the answer is that you need to shoot Bias, Flats and Darks. 

[rubecula]

Yes, I have 24 flats for for this filter.

[kirkster501]

I also get this when the seeing is rubbish and/or there is a mist and you carry on imaging anyway through the murk.  It is then analogous to taking a flat image because there is so much night glow in the sky.  Or of course, you don't have the due heaters on like you think you do...

A way to get rid is by using the DBE tool in Pixinsight correcting for axial symmetry:

http://www.astro-imaging.com/Tutorial/PixInsight/DBE/en.html

 

[Adam J]

Just now, rubecula said:

Yes, I have 24 flats for for this filter.

And the bias and darks? 

Gradient exterminator will cause the effect you have too if you apply it to the entire image. You need to select the background only or it will result in the area around the bright target being considered as background and over corrected. 

[rubecula]

Adam, yes the image was fully calibrated with flats, bias and darks and the image I put up had not been edited with Gradient Exterminator.  I'll try isolating the nebula and run exterminator on the rest of the image.

Mmmm don't use PixInsight...........more expense?

Thanks

[david_taurus83]

5 minutes ago, rubecula said:

........more expense?

 

It never ends!!!

[Davey-T]

Tips on using Gradient Xterminator here

https://www.youtube.com/watch?v=caEIrhmP8IM

I sometimes get this effect caused by condensation on the camera or focal reducer, you can see it by looking down the scope.

Dave

 

[rubecula]

Wow, what a difference Gradient Exterminator makes when you use  it properly ?

Thanks Dave