I assume that you meant that the peak of the histogram curve was at ADU 2000?
If so, where are the black point and the white point histogram stretch cursors placed at? You don't want the white point cursor set all the way to the maximum ADU (65535), and you also don't want the black point cursor to be set higher than the peak ADU.
It is best to just turn on the Auto stretch switch (to the right of the histogram curve) for the ASIAIR to do an initial guess of the stretch value. Once it has set the white and black point cursors, tap on Zoom to expand the curve so that you can manually fine tune the white point and black point cursors to bring up the stars in the background. Once you manually stretch, the auto switch will turn itself off, so remember to tap on auto stretch again for a subsequent exposure.
Usually, just turning on auto stretch will give you enough stars.
Keep in mind that stars, unlike emission nebulas, are broadband sources -- an L filter will let through about 350 nm worth of light (roughly speaking, since starlight will follow the Planck blackbody curve). A narrowband filter will have a passband that is narrower than 8 nm. So generally, you would need to expose 40 times longer with a 8nm wide filter to see the same number of stars. And, even longer (80 times) if you have a 4 nm filter.
The same is not true with emission nebulas. In that case, an H-alpha filter will pass almost all of the hydrogen alpha emission through (good filters will pass upwards of 90% to 95%), while knocking off broadband sky background (and narrowband emissions from street lamps that emit at other wavelengths than the filter's passband) by the aforementioned amount.
It is this mechanism that allows you bring out the emission nebula above the sky background (and why the narrowest band filters also cost much more than the garden variety stuff; they knock down much more of the sky background than a cheaper filter). By the same token, stars, being broadband emitters, will also be knocked down by the same amount.
Note that by the same token, narrowband filters will also not make a galaxy stand out in a bright night sky, since you are knocking the stars by the same amount as the sky, and thus not increasing the signal-to-noise ratio. You only use narrowband filters to supplement broadband filters, to bring out the hydrogen, sulphur and oxygen -rich regions in the galaxy. If a galaxy is dimmer than the sky background, you would just need to image from darker skies. It helps a little if the sky is illuminated by city lights that have narrow band sources -- in which case, some of the sky background can be knocked out by using light pollution filters, while the galaxy is knocked down by a smaller amount.
Chen
