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SkyTools 4 Imaging 4.0d Update
Hello everyone,

The V4.0d update is now live. A list of fixes can be found here:

This is a fairly major update, although it may not be immediately obvious to the user. The update concentrates on aspects of the software that I felt were not working satisfactorily. The main areas updated are exposure calculations for comets, imaging systems with multiple focal changers, imaging systems with cameras having multiple fixed lenses, and minor planets. In addition there are many other small fixes and enhancements.

When you restart SkyTools after the update, it needs to fix some of your databases. This may take a while, so please be patient. Please do not interrupt it while it is starting.

As one of the areas updated was comets, I thought I might say a few words about how exposure calculations for comets work. Every type of object requires a different approach. There are three main types of exposure/SNR calculations: stellar objects, diffuse objects that are defined by their surface brightness, and emission line objects. For every kind of astronomical object, whether stellar, diffuse, or emission, the most important thing is data. We need to know the magnitude, surface brightness, or emission line strengths, or there is no way to make the calculations. Sometimes the data we need isn't directly available, so we have to be clever to obtain it.

Have you ever wondered how SkyTools determines exposures for dark nebulae? The professional catalogs for dark nebulae don't have a lot of information available. There is a general position, idealized size, and often a crude "opacity" that tells us in a very general way how "dark" the nebula is. If you think about it, how well a dark nebula is defined depends on the faintest stars in your image. A short exposure that only captures a small number of bright stars may not reveal the dark nebula at all. A very deep exposure with many faint stars will define it very well. So what we want to know is the magnitude limit for stars that will reveal the dark nebula. Because that isn't something that has been cataloged, I had to create my own database with measurements of the faint magnitude limit for the stars in the region that will reveal the nebula. I couldn't do this for every dark nebula, so I developed a statistical relationship between the catalog opacity and the faint star limit. This relationship is used to estimate the magnitude of the stars that will reveal it. When you calculate an exposure you are exposing for these stars. 

Of all the types of objects, estimating exposure/SNR for a comet is the most difficult. This is because  every comet is different. Some have bright central cores and others are completely diffuse. Some are tiny and some are very large, and they are always changing. The only information published for comets is a set of parameters that can be used to predict the total integrated magnitude, and this is often hopelessly out of date. So the first thing I have to do is compile more data  directly from reported observations. These observations include total magnitude, diameter of the coma, and the degree of condensation (an estimate of how centralized the brightness of the comet is).

The original algorithm was too quirky and relied on a questionable assumption. What it did was calculate the signal of the comet as if it were completely stellar, and then again as if it were completely diffuse. Then the degree of condensation was used to interpolate between these two results. If the degree of condensation was completely stellar, then the stellar result was used. If completely diffuse, then the diffuse result. If halfway in between, then the result would be the halfway point between the two. To do this we assumed that the relationship was linear, and there was no reason for that to be true. It also meant that when we worked the problem forward, by computing the signal for a given exposure time, and then backward by computing the exposure time required to reach the same signal, the result was not always consistent. 

So what I did was to use an iTelescope to obtain photometry for a wide range of comets. Then I analyzed the data looking for statistical relationships that could be used to estimate the mean surface brightness of the comet core and main coma. Applying the new statistical relationships, I created a new algorithm. I tested it on a new set of comet observations and the result was satisfying. The largest errors came from the reported measurements rather than the algorithm itself, which is always what I am striving for, and the calculations were always consistent (forwards and backwards).

It's easy to take all of this for granted while using the software, but I like to say that the reason nobody has done this before is that it is impossible!
Clear skies,

SkyTools Developer

Messages In This Thread
SkyTools 4 Imaging 4.0d Update - by theskyhound - 2019-03-23, 06:36 PM
RE: SkyTools 4 Imaging 4.0d Update - by PMSchu - 2019-03-24, 12:37 AM
RE: SkyTools 4 Imaging 4.0d Update - by PMSchu - 2019-03-24, 04:49 PM
RE: SkyTools 4 Imaging 4.0d Update - by Rowland - 2019-03-27, 05:52 PM

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