T-shirt flats problem.. Help?

[AlexN]

Heres a stretched master flat frame taken today, 4 sec duration...

I tried many different duration flat frames (0.2s, 0.8s, 1s, 2s and 4s), and I've taken flat-darks for each set. All the different durations looked largely the same, and they do nothing but introduce noise to my final image.. The dont seem to remove dust spots or lessen the vignetting at all.. They just add noise...

Any thoughts?

[RB (Andrew)]

What camera are you using?
You need to make sure you get the histogram between 1/3rd and half way up the x axis and use the same ISO setting as your lights.

This shot does not look evenly illuminated for a flat.
Did you end up getting an EL Panel?

[AlexN]

That was done with the Orion Starshoot Pro..

The Histogram was exposed a bit over 1/2 way across the X axis.

There was a clean, white t-shirt pulled tightly over the dew sheild so that it was trim and flat across the objective, the objective was pointing away from the sun at a clear blue sky.. I did get an EL panel back in the day, however I got one of the small cuttable ones, and at the time I was using a 102mm refractor, which the small cuttable EL Sheet did not fully illuminate, so I sold the panel... I have a little over an hours data of Rho Ophiucuhi that I can't process untill I get some usable flats...

Is it at all possible that the material in the T-shirt is causing the problem? its a fine, microfibre like gym shirt, should I perhaps try something thicker like a cotton shirt?

Thanks for any further info..
Alex.

[mill (Martin)]

White cotton and not stretched too tight or else you will make some spots thinner than others Alex.
Just snug and with no wrinkles is good.

[AlexN]

will try again at dusk.. Cheers.. Failing that, Maybe I'll try using my monitor.. Cant hurt to try I suppose?

[renormalised (Carl)]

Needs washing

[Gama]

Try a white shopping bag from coles, safeway etc.
This works great.

Theo

[AlexN]

Carl - Perhaps...

Theo, Cheers mate, will give that a go at dusk...

One other thing. Considering we're talking about flats, that get converted to a greyscale master-flat, would it matter so much if it wasn't a white shopping bag?? I would assume not, but then, I assumed my T-shirt flats would work too..

[mill (Martin)]

The monitor will work too as long as it is evenly illuminated.

[Exfso (Peter)]

I also do the Tee Shirt method, my speed is usually about 1/200th to 1/400th of a second to get the histogram roughly centred, depending on where I am pointing the scope and cloud cover etc. Here is one I did during the week. Color due to filter still in imaging train