Thank you, IanP.
I did tests with diferents ISOs. And my conclusion is:
(At least with my old Canon Rebel XT)
1. High ISO generates too many noise. Mainly hotpixels. They cause loss of information when filtered by dark frames.
2. High ISO cause saturation of RGB values of stars. There is tendence to show all them white.
3. I can not manipulate the response of G type pixel. Therefore I have problems with color balance and hue.
4. I haven't, yet, filter against light pollution. Mainly from yellowish street lights. High ISO increase the yellow-red color of sky background, and flat frames can't deal with it.
To faint objects I can work with ISO 800, but only at the more dark side of my sky (East and Zenite).
ISO 1600, only with low exposition time.
What do do ?
Buy filter. Buy new camera. Move to a dark site. Broke all street lamps
My budget has only new camera and filter :
http://www.astronomik.com/en/clip-filter-system.html (but they are only on budget paper, yet !
)
(I must pay 120 % over the price, in US dollar, to importation tax, And with cambio... The price of accessories, to me, cost 4 x the original value)
