Observation Tid-Bit #2 - Transparency

Observation Tid-Bit #2 - “It should be transparent to the user!”

- by Aaron Clevenson

Last month we talked about Seeing. This month we will talk about Transparency. Seeing is a measure of the stability of the air. It is the quality of how well you can see objects. Transparency is how clear or transparent the sky is. Together these two measures indicate how things should look when you do your observations.

Transparency is really quite easy to measure. The easiest scale to use is: What is the faintest star you can see naked-eye? If it is magnitude 3.0, then the Transparency is 3. So the scale is easy enough, but how do you measure it? I have seen many different suggestions for how to do this, but the one that I have been most impressed with is easy to use, and almost always available.

The little dipper: Ursa Minor. This constellation has stars that get as bright as magnitude 2.0 and they range really well down to quite faint. And if the Transparency is less than 2.0 you might as well go watch TV. So here is the way to measure Transparency using Ursa Minor:

2.0 – Polaris and Kochab are the only two stars visible.

3.0 – Pherkad is also visible (total of 3).

4.0 – The entire little dipper (minus one star) is visible, but there is also another star visible above the bowl (total of 7).

4.5 – Two more stars are now visible: one further away and above the bowl, and one also into Draco off the end of the bowl (total of 9).

5.0 – The entire little dipper is now visible plus a number of extra stars. Pherkad has one very close and it may be hard to split (total of 13 or 14).

5.5 – Lots of stars are now visible. The stars are in a field that goes from Polaris through Kochab, and about one more bowls length more (total of 24).

6.0 – This is probably the limit for naked-eye visibility (total of 37).

6.5 – And for those who have the vision of Superman… (total of about 66).

The problem with the Ursa Minor is that although it is circumpolar and “up” all year long, it does stretch to about 65.5 degrees declination. This means that around January 1^st it comes within 5 degrees of the northern horizon. The problem with Transparency is that below 20 degrees altitude you begin to have atmospheric extinction. So at that time of year, you will not be able to see the full number of stars.