Filter Question

[pjglad]

I recently purchased SkyTools Visual 4 - great app. I have not used SkyTools in 5+ years - migrated to MAC. I used ST3 for 4+ years. As I recall there was an airmass filter I think. If yes, is there something like that in ST4? I guess I could simulate with an obstructed Horizon. Any feedback would be welcome. Thanks.

[PMSchu]

Hi PJ,

No, ST4 Visual doesn't have the Airmass Filter, but the Imaging version does. It's called 'X' in the Nightly Planner (NP) window. You can use the Column scheme controls (Gear icon) to determine which columns are displayed in the NP window. As in ST3 you can define & save personalized column schemes. 

In ST4v since you can sort on any column, you can sort on the Airmass  ('X') column to pick the appropriate objects, though.

Hope this helps,

Phil S.

[pjglad]

OK, I got it - Quality Filter.

[PMSchu]

Right you are. Select 'Best quality only' & only objects that rise above 2 airmass will be listed in the NP.

Phil S.

[pjglad]

Are you sure about that? I assumed that the quality filter was airmass independent. Just because an object is below 2 airmass doesn't imply that it has poor quality. I also assumed that the quality index negated the necessity for an airmass filter. Perhaps Greg can comment. Thanks anyway.

[PMSchu]

Hi PJ,

Have you found a counter example? With the nearly full moon, many things are listed as best after moonset, so this isn't a good time to evaluate the effect of the filter. I reset the date to Oct 6 for an OL containing 461 objects. With the 'Best quality only' filter selected, none of them were >1.4X with magnitudes as faint as 16.2.

Sure thing.

Phil S.

[theskyhound]

The Help section that describes important differences between SkyTools 3 and SkyTools 4 provides a brief yet informative explanation of the quality filter.

The Quality calculation in SkyTools is the most important part of the software. It is the calculation engine that everything works off from. It's full name is "Relative Observation Quality."

It is not the quality of the object itself. It is the quality of the *view* of that object at a moment in time, for the current observing conditions, as compared to the best possible conditions (at this this same observing location, not Australia or something).

To understand it, we need to first consider the different types of objects that are observed:

1. Stars and stellar objects (including asteroids, quasars etc.)
2. Extended surface brightness objects (nebulae, galaxies, etc.)
3. Detail related objects (Sun, moon, planets and double stars).

If you want to look at a planet or double star, then the astronomical seeing conditions matter a great deal. Its what determines how much detail you can see. Seeing worsens as your object moves toward the horizon, so you want to observe as high overhead as possible. Moonlight, (and even sunlight) is not that important.

For an extended object, contrast is what matters. Contrast depends on how high in the sky you are looking, but not to the same degree, and you can go closer to the horizon without as many bad effects.

In its difficulty calculation, SkyTools considers all of these factors, and puts them all on the same numerical scale. So we can calculate the difficulty value under the best conditions for any object, when it is highest in a dark sky with excellent seeing and weather conditions. Then we can calculate the same value under different conditions, maybe when it is lower in the sky, the moon is up, the seeing is poor, or humidity is high. When we take the ratio of these two numbers, by dividing the actual difficulty value by the best difficulty value, we have an index that tells us the quality of the current observation as compared to the best quality.

So the quality contains EVERYTHING. That's why no other filters are required. It cuts straight to what matters, taking into account the properties of the object itself, your telescope, eyepiece, selected observing conditions, etc.. It a decades beyond what other software can calculate.

When you select an object on the planner, you will see a blue quality line. When it is near the top, the quality is near its best. That is when you want to go look. If you make the seeing poor, it will drop across the board. When the moon comes up, it might drop straight to the bottom, or not be very affected. All of the various factors are already taken into account for you. This is why I suggest setting your quality filter to "At or Near Best," and observing your objects only when it meets this criterion. Leave the other objects for another night.