So earlier i tried a thin cotton cloth on halpha telescope for acquiring flat field images (if you have no idea about flat field here is a quick link, though web has many more sources on that). Here is a recap image:

With this method i was able to get a good flat.. here is a stack of few images:

One can clearly see the 'dust donuts' appearing in this image.. also notice the image is bit brighter in the lower left portion. Is it a good flat? will it do the job of calibration.. lets test this.

Following is a single raw image.. notice the 'dust donuts' are present in this image.

And here is the calibrated image with the flat field.. you can see all the donuts are gone and the image brightness is also 'balanced'.. this is why it is called 'flat calibration', it really flats all the brightness/darkness errors in the image.