New Image: NGC 2997

tanguan2001 · Post by **tanguan2001** » Thu Mar 03, 2005 11:08 pm

All,

Another galaxy pic, just been processed. Amazing how a galaxy is still visible in such a rich starfield. Anyway a nice spiral.

TG

NGC 2997

kohjb · Post by **kohjb** » Fri Mar 04, 2005 12:11 am

Amazing as always !! And very envious. I haven't understood what flat fielding is and what a light box is and does to correct that. Care to explain ? :-D

tanguan2001 · Post by **tanguan2001** » Fri Mar 04, 2005 8:43 am

Flat fielding is a way to make sure that the brightness in different points of the image is representative of the actual brightness of the subject.

Many things can change the brightness at different parts of the image - most severe is optical path vignetting, or dust on the sensor, differences in sensitivity of individual pixels, or different transmisibility through a filter.

So, to correct for all that you take an image of a completely evenly lit field. This can be the twilight sky (but need to make sure no stars show), or in my case a light box that provides even illumination over its whole surface.

This resulting flat field shows up all the imperfections (including in my case a colour fringe, probably due to the LPR filter I use). The vignetting appears, and dust shows up as little donuts.

Each frame of the subject is divided by the flat field. This corrects for the imperfections - the vignetting and dust in the image magically disappears (think about the effect of dividing for a moment - not quite obvious...).

My light box is made from 4 white LEDs inside a plastic box, and an arrangement with 3 stages of diffusion to get even illumination - its not perfect, still about 5-7% uneveness, but it's the best I could manage even after a lot of tweaking. Plan to post pictures of it on my website later.

TG

kohjb · Post by **kohjb** » Fri Mar 04, 2005 11:27 am

Thanks so much for sharing !

Regarding the light box usage, do you then just aim your telescope at this in which case, you'll need to place it sufficiently far away ? Do you measure light evenness with a light meter of some sort ? And is dividing a function that is in Photoshop ?

JB

tanguan2001 · Post by **tanguan2001** » Fri Mar 04, 2005 12:04 pm

I just place the light box over the aperture of the telescope and time the exposure to get to about the 75% level on the histogram (yes it's an out-of-focus image).

The initial steps, including removing offset (another story!), dark frames, flat division, registering, and stacking are all done in Iris.

TG

tanguan2001 · Post by **tanguan2001** » Fri Mar 04, 2005 12:07 pm

Oh, and forgot to say that I measure the lightbox evenness by taking a picture of the lit part, loading it into Iris, and making a graph of light levels across a cross section.