Problem in preprocessing with masterflat !!

[Dominik]

Hello !

First I apologize in advance for my poor english, I'm french, I hope you will understand me ...

Well, for a few weeks I've got a problem when I preprocess my images.
I use the same procedure since a few years and i never had this problem before !

Well, yesterday I took this picture of B33, using my 200/800 Newtonian, a 350D Canon and a UHC Filter (exposure of 5 min) :



After calibration (with masterbias, masterdark and masterflat), I obtain this result :



Dust are not corrected well !!
Last week i took images of Lovejoy and I had the same problem !

Last month, I made this image of M42 (same telescope, 350D, UHC Filter and 5 min exposure) :



So I try to preprocess M42 with the masterbias, masterdark and masterflat I made yesterday for B33.
It works well !!

     

Dusts have been corrected !

So I don't understand what happened :
my calibration images seem to work on M42, but don't work on B33.
And as I said, I've got this problem for a few weeks, I've never had this problem since many years.

So what do you think of that ?

It seems that the dusts are bigger on the images than the dusts on the masterflat...

[sreilly]

The only thing I can think of is maybe the filter is loose and has shifted slightly?

-Steve

[Dimitris Platis]

u should really get flats each time u image....not have them in library because of several issues.

[vicent_peris]

As Dimitris says, it's much better to acquire flats for every imaging session.

It seems to me a focus problem. If the optical elements where you have the dust move along the imaging tray, the size of the shadow cones changes.


Best regards,
Vicent.

[Dominik]

The only thing I can think of is maybe the filter is loose and has shifted slightly?

-Steve

I'm going to look at that !

[Dominik]

u should really get flats each time u image....not have them in library because of several issues.

It's what I do everytime, but I don't know why it doesn't work now ... 

[Dominik]

As Dimitris says, it's much better to acquire flats for every imaging session.

It seems to me a focus problem. If the optical elements where you have the dust move along the imaging tray, the size of the shadow cones changes.


Best regards,
Vicent.

Well, I don't understand why the focus could have change, but you're right, i'm going to look at that too !

[MikeOates]

I also see more dust marks than you have indicated in your images. But what I notice most of all is that the images do not appear to have been flat corrected as there are heavy gradients/vignetting.

Is it possible that you accidentally selected the wrong master flat, or even missed it out completely?

Some thing to bear in mind for the future, there are times when the dust moves during or inbetween exposures. It has happened to me and when it does, a flat will never remove them and I take flats during every imaging session. To remove such marks some will be eliminated during ImageIntegration and any remaining may need the use of the clone tool!

Mike

[lucchett]

Hi,
I once had the same issue.
I acquired several frames in different nights and the problem arose for frames acquired during the worst moon conditions.
In my case I think it was due to reflection hitting the spots differently.
I did the same test as you did, the flats were right and a previous set of frames calibrated well.
andrea