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Target Object Data

Would you be so kind as to explain the notation you use for the target object data -- particularly for emission nebulae? Example from M27:

SBr(V) 23.1 SBr® 23.2 SBr(total) 22.3
SII[6717]/Ha 0.09 NII[6584]/Ha 0.97

Am guessing SBr(V) and SBr® are¬†photogrametic brightness measures. But I don't understand the ratio calculations or the units.


Greg Latiak

That data was not originally meant to be part of the final product. I put it there for debugging and to give beta testers something to report. As a result, when I decided to keep it around, it didn't get properly described in the help system. I apologize for that. There is going to be a major update in the coming months with new features and I am going to go over the help system and add more information to it at that time.

To answer your question, SBr(V) is V magnitude surface brightness. Sometimes this is a catalog value, sometimes it is a measure that I have obtained to support the software, and other times it is an estimate based on the integrated magnitude and size of the object. This is used in the calculations for galaxies and reflection nebulae.

The ratios are emission line strengths for emission objects. These are used in the calculations for HII regions and planetary nebulae. Some of the main emission lines are H (hydrogen), which is mostly Ha (H-alpha) and H-Beta, Oxygen (usually the brighter OIII line), Silicon (usually the main SII line), and Nitrogen (usually the NII line). The number in the [] brackets is the wavelength of the actual line, in Angstroms, to differentiate between lines for the same element. The calculations work by knowing the absolute strength of the Ha line plus the relative strengths of the other lines. So, for instance, if we know that OII/Ha is 1.5, then knowing the actual strength of the Ha allows us to calculate the OIII as 1.5 times stronger.

The main takeaway from these values is to give us an idea of which lines are strong for the given nebula. If the OIII/Ha is very small, then the OIII line is very weak. If larger than 1, then it dominates the Ha and will be stronger than the Ha. So in your example above, we see that the SII line is only 9% of the Ha, which means an SII filter will need a much longer exposure than Ha. But the NII is about the same as the Ha, and the exposures will be about the same in NII. This information is used to make the per filter predictions. Often the numbers serve mostly as an explanation as to why, for instance, the exposure time is so much longer in the SII than the Ha, as for your object above.
Clear skies,

SkyTools Developer
Thanks, Greg.

That is what I wanted to know. I had suspected as much. Nice to have the information, recognizing what a sketchy thing it might be. Glad you left the info in.



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