Light pollution gradients before and after meridian flip. DBE?

Fernando Huet

Hello all.

I live in a medium polluted sky, and usually have no problem when imaging in Narrow Band.

But in LRGB I always have strong light gradients.

My concern is about integrating images taken before and after the meridian flip.

I have opposite 180º gradients due to the flip, and I always get complicated master images.

Indeed, I've noticed that if the number of images at each side is largely different, I got dark corners sometimes etc.

In usual unique images is not a big problem, but this time I'm trying a M31 Mosaic, and at the union zone I got a dark corner from one pane next to a very light corner form the other one!!

I attach some images: Red panes, and the L star alligned mosaic.

I also tried local normalisation, linar fit between subs, etc, but the result is no good.

It's very difficult for my pseudo-beginner level!.

I have not finished all your Fundamental videos, but I have not seen a clue.

My question:

What do you do with that kind of gradients?

Do you integrate all the images in a row, and try to deal with that problem at the DBE moment?

Or do you try a minimum DBE to each individual sub once calibrated and just before the integration?

I never heard about applying DBE to individuals subs, so I prefer asking here!

In that way, the integration process has more consistent data, not a gradient changing 180º in some images...

Thank you.

BlockHead

Hi Fernando,

Your questions have some assumptions built into them...and I would like to sort it all out. However..it would require quite a bit of typing! I am hoping to make video that answers your questions. 

Without any explanation this is going to be obtuse.

Gradients in the sky will rotate with the object when you do a flip. 

Thus registering the images will keep the gradients exactly the same...the meridian flip does not matter. 

Gradients that do not rotate are due to your optical system and this points to how the chip is being illuminated. This is supposed to be calibrated with flats... but if not- that is the area to work on.

Local Normalization might help with dynamic gradients caused by high thin clouds. This does not appear to be the issue with your image. Local Normalization is not a place to start trying to fix things... it is tricky (though very cool).  

Note in your image how the dark part isn't so much the corners..but the galaxy. This means it is likely the gradients are potentially caused by your choice of placement of DBE samples... they are too close to the galaxy... there is darker sky surrounding the galaxy. 

There is more to say as well. 

-the Blockhead

Fernando Huet

Hi Adam.

Thank you for your answer.

If I had thought deeper, I had seen the answer was obvious!

You are right: the light pollution gradient is in the sky, not in the camera. So rotation doesn't matter at all.

The images I posted were calibrated images only, without DBE. I just forced STF to show dark differences.

But your answer has given me the clue: I took two original subs, without no calibration, and made a forced STF to show differences. There are no gradients! (I attach them. As you can see, they are flipped. You see the dark dust spots at the same place)

So the solution I think is related to FLATS!!

I use Sky Flats made at dawn! That's the point. They are made in one pier side, and they must have a gradient!.

I think they produce the gradient during calibration.

Never thought about it. The observatory is a remote one, but I have not installed a flat panel yet.

When I'm at the observatory I make flats with a Geoptik panel that I manually put on the scope.

But when not, I use Skyflats, usually at dawn.

Now I see a wall fixed panel flat it's a must!.

I hope at least that's the solution to my question!.

Another question should be how to deal with those SkyFlats. I don't want to lose so many subs and data!

Perhaps making flats a DBE? (If my assumption is right!).

Thanks again!

Fernando

Rodolphe Goldsztejn

Hi

I've seen an experienced and well-known processor who applies ABE systematically to all his subs before stacking them, as part of the pre-processing steps, with great success.

Nonetheless, using DBE in one go after stacking should also bring good results: the gradient will likely show a symmetry in the case you explain that DBE should be able to tackle.

My 2 cents...

Rodolphe