Hi all, this morning comet Lovejoy has moved further away from the Milky Way to reveal the full extent of its tail and still visible to the naked eye. Many have asked how dark are my skies so I did a few tests this morning while exposing the comet. I can easily see the star within the Coalsack, counted 26 and up to 30 stars within Orion and the hardest was 3 stars within Corvus. I have not checked a star chart to see what magnitude they are as yet.
This image was taken with 50mm Canon F1.4 lens at F2, with 2 minute exposure using Canon 20Da at ISO 800. After taking this image and seeing I was not capturing the whole tail I replaced the 50mm lens with the 24mm F1.4L lens.
Nice work, Lester! The 24 f/1.4 is a ripper lens, isn't it?
I like Samir Kharusi's approach to measuring the sky fog because it's quick and easy - i.e. point the lens in a dark region near zenith and do a few test exposures to work out what exposure time would be needed for a middle-histogram sky fog peak at ISO 800 and f/4. He's also provided a translation scale between the exposure time and mag/sq arc sec for a Canon 1Ds.
Thanks Freddy, Dave and Carl for your comments, as always.
Dave my lens is not the latest II version only the I model, and would like to know how much better the II is. This lens is okay, but I have heard the latest model is an improvement. Also I will look into that sky fog measuring, thanks.
Aah, I only have the mark II version of the lens so all I can say is that it's quite fantastic (even compared to other L primes). My usually dilemma is trying to decide whether to open it up to f/1.4 for maximum light vacuuming (still remarkably sharp), or stop it down to f/2.8 - f/4 for better corners.
Yeah, you have to download the help files separately, which is a bit strange.
Actually, if you look onsite, there's tutes that take you through how to do various things with Iris. I haven't had cause to use the gradient removal routine in any pics, so far, so you may have to ask Scott.
In Iris, load the image you want to get rid of gradients from, then go to Processing, go down to Remove Gradient (polynomial fit), tick balance background colour then hit ok, normally the other default settings work well as they are.
Iris is free though has a steepich learning curve, theres all sorts of hidden functions, even I am still discovering things. The main help pages are here http://astrosurf.com/buil/us/iris/iris.htm
Hope that is of assistance