Right Declination Astrophotography

Exposure Time Compensation - Color Filters

Having established the best SNR time using SkyTools and the amount of exposure and time needed to obtain a level of 100 (second best SNR level accordending to SkyTools), you will need to compensate these exposures time depending what filters you will be using: Either RGB or narrowband HOS
.

In the above example the SNR is at100 quality from 20.25 to 21.50, its a 85 min total exposure using lum filter, so its 10 x 8min (or 480 sec), the time total is important, the number of exposure as well as the highest numbers of frames for later stacking the SNR will fall.

Example

Lum  from 20.25 to 21.50 : 85 mins 
Red from 20.25 to 22:00 : 95mins
Green same :95 mins
Blue same :95 mins

The Skytools takes a lots of parameters to establish the best SNR time and number of exposure , but with differeny filters works with different frequencies and therefore we need to establish what proportion / ratios applys.

In order to make "True color" astro images we need to balance the three color channels. This can be done by determining the weight factor for each channel. These weighting factors can be calculated from the response of the CCD chip for each color channel on a pure white light source. Spectral type G2 class V stars are the ideal objects in the sky since they emit pure white light. The measured brightness of such a star through my color filter set tells me how the ICX424 CCD respond to pure white light as passed by each color filter. The CCD response to white starlight, no filters used, will be set as 100%. Due to uneven spectral sensitivity of the CCD (see above) the response through each color filter will be different and only a fraction of the CCD response without any filter, resulting in the following typical color weighting factors for my CCD/Color filter setup.

For deep-sky imaging these weighting factors can be used to determine the exposure times in red, green and blue. Simply multiply the exposure time used for the red channel by the weighting factors for green and blue to get the needed exposure times for the green and blue channel. Such a set of RGB images have "balanced" pixel values so that a white star produces a white image on the computer screen.


For true color imaging the above mentioned factors have to be taken into account.
Example : We want to make a true color image of an object at 30° altitude.
Example : Which RGB weight i needed for this true color image?
CCD response is R : G : B = 1,00 : 0,74 : 0,77
Extinction correction is R : G : B = 1,00 : 1,08 : 1,15 (see graph above)
The total RGB weight for this image will be R : G : B = 1,00 : 0,80 : 0,83
Therefore a set of exposure times could be R : G : B = 60 : 48 : 50 seconds.

R : G : B = 1,00 : 0,74 : 0,77

True color imaging

80/100 x 60 = 48

83/100 x 60 = 50

So, taking the 95 min as base we can work out the following:

80/100 x 95 =  76mins for the green
83/100 x 95 = 78min for the blue
95 mins for the red.