At the centre of the Virgo cluster lies an alignment of galaxies named the Markarian's chain. Notice the subtle colour differences among galaxies. M84 appears slightly redder than the similar M86. In turn both are less blue than the barred spiral ngc 4407 at the bottom of the field. The field shows hundreds of galaxies.
1600x1122 (1 MB) Full Resolution (3612x2532; 2.1 MB)
The negative image, comes with an annotated overlay: roll over with the mouse to display it. The overlay marks all NGC, IC and PGC objects down to about mag 20. There are about 250 galaxies marked in the overlay.
The limiting magnitude is actually much lower and it can be estimated by comparing the image with the Sloane Digital Survey. In the next negative image I have labelled some faint stars (red) and galaxies (blue) with the magnitude in the visible spectra (g band, 477 nm) obtained from the Sloane survey. The limiting magnitude is well in excess of 21.
An old picture shot with the DSRL through the C8 included only 39 galaxies positively identified and 9 more galaxies that are too dim for clear identification. The limit magnitude of that picture was about 18.9 at the field centre. The limit magnitude of that picture was computed by analyzing the detection probability in each magnitude class as shown in the plot:
Each filled symbol represent a galaxy present in the field. If it can be reliably identified in the image its probability of detection is 1, otherwise is 0. the green symbols indicate the probability averaged in classes 0.5 mag wide. The continuous line is a Boltzmann curve with a midpoint at 18.6 (probability 0.5) which can be considered as an estimate of the limiting magnitude. The measure has been performed on the final image as displayed above and not on the raw stack. Because of noise reduction and non-linear curves modification this is likely to be a lower limit estimate. Therefore, one telescope and a CCD camera later I gained about 2 magnitudes.
Copyright © 2005-9 by Gimmi Ratto. (March 27, 2009)
