Color vs. Mono Camera Match
SBIG ST-2000XCM color camera vs. the ST-2000XM mono w/filters

Exposure Time Comparison

Is 1 hour of a XCM exposures equivalent to 1 hour each of R,G & B taken with the XM?

  This test is to address the misconception that you can save time by taking all 3 color channels at the same time compared to the mono camera using separate red, green and blue filters with equivalent exposure times and expect to have comparable S/N. In other words is 1 hour of exposure the same as 1 hour of exposures for each of the R,G and B filters? While true you can get all 3 primary colors using the XCM at the same time, but their total S/N is not equivalent to the XM camera using RGB filters. The following test examples will prove this point. Each example is calibrated and an equal DDP stretch applied to all 3 images using Images Plus. The standard deviation measurements were taken measuring the same area of the sky background in each image using Adobe CS.

  Below are 3 examples of NGC 6992. The first two are 3 hours of total exposures taken with the XM and XCM respectively and the third is one hour taken with the XCM. Notice the difference in brightness with an equal DDP stretched. The XCM is slightly dimmer due to the lower QE of the Bayer mask. Also, looking closely you can see the S/N of the two 3 hour examples is fairly close. The 1 hour XCM example is much noisier and also has a higher standard deviation measurement compared to the lower and similar standard deviation measurements of the 3 hour examples.

3 hour exposure of NGC6992 taken with the XM
R=6X10 G=6X10 B=6X10 minutes

3 hour exposure of NGC6992 taken with the XCM
18 x 10 minutes

1 hour exposure of NGC6992 taken with the XCM
6 x 10 minutes

Background Standard Deviation Measurements

  Here the background noise level measurements can be seen between the short and long exposures. Using the Std. Deviation measurement tool in Photoshop and selecting the same sky background area the Std. Deviation values can be measured. The long exposure examples display a lower Std. deviation value compared to the short exposure example.

Three hour XM version. Note the Std. Deviation is close to the XCM version measured in the same background area.
Three hour XCM version Std. Deviation sampled in same background area.
One hour XCM version. Note the high Std. Deviation value compared to the XCM 3 hour example.

Exposure Times Comparison Summary

   These test results prove that the exposure times needed for the XCM to get a S/N close to that of the XM with filters is at least the same for a given object. The noisier image of the XCM with 1 hour of exposure time compared to the 3 hour examples demonstrates the need for longer exposures due to the lower QE of the Bayer sensor and its peaky bandwidth of the blue and green Bayer filters and to a smaller extent, the red filters compared to the broader band pass of the mono using astronomical R,G & B filters.

Top of page

© 2008 Michael A. Siniscalchi