Flat fielding an image, Help!

[jimmy10fingers]

Hi everybody,

I am an astrophysics student and am new to this site, this is my first post.

I took some images of the recent supernova SN 2011fe in M101. I have a series of flats for each of the BVR filters. Using the IRIS program I successfully created average flats to reduce the science images. I understand that I must divide the science image by the relevant flat. The average intensity of the science image is 550 and the flat is 6000. Ive tried many methods of reducing the flat, normalizing to the level of the science image and normalizing to 1, but no matter what i do, when i divide by the flat (or indeed any image) all i get is a blank grey image with an average of 1. For example, i say 6000*x = 550, x = 0.091 and use this as a multiplicative coefficent to reduce the flat to the level of the science image.

Clearly Im missing something (probably something silly)

help before this monday would be VERY appreciated!

Thanks in advance

[brantuk]

Hi Jimmy and welcome to SGL

Sorry I can't help cos I'm not too hot at imaging - infact I'm quite naff lol. Someone will be along with a suggestion soon

[Ags]

Hi, welcome to the forum!

I find the subject of flats a bit discouraging - I've never had any luck... I've downloaded IRIS because it supposedly does gradient removal - do you have any links to IRIS tutorials?

I would have expected the application of a flat to a subtractive process - after all, you are just subtracting the sky signal. So your statements about dividing have gone over my head!

[jimmy10fingers]

Hi there,

Iris software

Scroll down the page for the tutorials.

From what i know, while bias's and darks are subtracted, flats are divided. This is because bias's and darks are noise that come from the CCD chip. But flats are a measure of the difference in chips response to light, ie the gain. This means that for the same exposure some of the CCD responds more than others. If the central chips respond more you can imaging a "bulge" in the middle of your image. By dividing you image by a flat you "smooth out" the buldge.

Im open to corrections, as i only found this out 2 days ago.

Any one any ideas on my problem?

[jimmy10fingers]

Problem Resolved!

Turns out that IRIS cant handle the mathematical processes because the intensity level of the pixels is very high, as it is only 16bit, so it just returns zero if large numbers (>323000) are involved. I have an image of RZ-Cas and if I take a slice, the PSF has the chopped off at the maximum level IRIS can handle.

Keep this in mind if anyone is processing images. CCDops can handle the large numbers but you cant batch process images...

[ollypenrice]

I think most of us on here have no great clue as to the maths involved in applying flats, though it's being a process of division is familiar. We just rely n the software to sort it out for us, I'm afraid, by loading in a master flat or even the individual frames and clicking the Get On With It button!

Olly

[Merlin66]

Are you saving the images as signed or un-signed 16bit fits???