[sjastro]

Originally Posted by Rex

Very cool, processes you are using Steven, and a very interesting discussion. Thanks for sharing.

Thanks Rex.

Originally Posted by madbadgalaxyman

Just the g magnitude of an object minus its r magnitude, just as in B-V or V minus R etc.

Color indices using B and V and R and I magnitudes were the traditional ones, developed from photoelectric photometry.
A strongly positive color index usually indicates a red object.

But since the Sloan Digital Sky Survey, the ugriz system has become very popular in the literature;
Here is an example of the transmission curves of the u and g and r and i and z photometric filters ::
http://www1.cadc-ccda.hia-iha.nrc-cn...a/filters.html

The traditional UBVRI magnitudes and filters are briefly compared with the ugriz filterset, in this review paper:
http://www.astrohandbook.com/ch02/be...otosystems.pdf
(This is also a good general reference on various filters used in photometry)

Essentially, a color-magnitude diagram of a cluster or galaxy using B minus V for the colour index is often very similar in appearnce to that using g minus r for the colour index.

To follow the current literature, it is getting ever more necessary to get a good appreciation of u and g and r and i magnitudes ; they are not that different from the standard UBVRI magnitudes.

Thanks Robert.
What I found particularly interesting about the ugriz filter link was the CCD QE performance used on the CFH telescope and how it compares to my ST-10XME.
The CCDs used by the pros absolutely murder the typical amateur CCDs in near ultra violet performance.
I can attest to that as I am currently imaging the SMC in near ultra violet.
For an object that is visible to the naked eye it will take me about 12hrs exposure to get a satisfactory S/N ratio.

Originally Posted by SkyViking

Very impressive Steven. I like how one can usually improve image processing continually and gain new insight into old data. Interesting discussion too for sure. It is true that we as amateurs have an advantage in that we can expose a patch of sky for as long as we want really.
Nice of them to comment on your work like that. I too have found Olivier and Lars to be very approachable and helpful in the past.

Incidentally Lars was my supervisor on an astronomy project back when I studied at Copenhagen Uni in the 90's. We used a Meade LX200 to take images of the double cluster in Perseus from the rooftop of the old Copenhagen Observatory and calculate the age of the clusters. I remember there was no easy image processing back then and all image calibration was done with an excessive number of complicated Unix commands...

A good story Rolf.
In the 1990s I was stumbling around in the dark trying to load my astrofilm onto the reel that went into the developer tank.....

Apart from the feedback from Olivier and Lars what also impressed me was the speed of response. Let's face it, you, me or any other amateur would not exactly rate highly in the order of priority for ESO. It is a credit to each individual to respond, and in a timely manner.

Regards

Steven