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Emission Line Estimates
Hi Greg,

From time to time I see people asking on astro forums if there is a reference that would help them decide which narrow band filters might yield results for a given emission nebula.

The usual answer is "there is no reference" and then perhaps a list of a few websites that have some data.

ST4 users know that in the project dialog, under Exposure Goals and Filters, we have emission line data like this:

"Emission lines (relative to Ha)
486.1 0.28, 500.7 0.75, 656.3 1.00, 671.7 0.20, 434.2 0.40
Emission line data is an estimate only."

This seems to be exactly what people are asking for, but very few people know about.

I would like to recommend ST4 as a reference when this question comes up.  

It would be helpful to give people some indication of the number of targets (or rough percentage?) ST4 has this data for, and how to interpret the "the data is an estimate only" disclaimer.

Any guidance?

Hello Rowland,

For planetary nebulae we can assume typical line strenghts relative to Ha, which will often be in the ballpark. But I am sorry to say that HII regions are really no more than a guess. The relative strengths of OIII and other lines vary too much for the estimate to be very useful. I was able to find data for most planetary nebulae, but there remains a lot of work to be done for HII regions. This is why I am currently working on an observing project to measure the line strengths directly. You should start seeing more actual data next year.
Clear skies,

SkyTools Developer
Thanks Greg, that's helpful to know. It's interesting that with so many narrow band pictures taken by so many astrophotographers, this data has not been assembled in any comprehensive way. Good luck with your project!


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