The application provides an easy way to apply differential extinction corrections to visual brightness estimates of variable stars. This is strongly encouraged especially when performing visual photometry for variable stars at an altitude below 20 degrees, when differences in airmass between the variable star and comparison stars may skew the estimated brightness of the variable. It can use a predetermined extinction coefficient, or it can determine one from visual brightness estimates of known stars (see here)
Besides having to indicate (on a generated star map) what comparison stars were used, the application needs the coordinates of the observer (latitude and longitude) and the time when the observation took place. These are used to convert the equatorial coordinates of all stars involved in alt-az coordinates; later on, the altitude is used to compute the airmass for each star used.
Selecting a particular star to be used is done in the following way:
- click on the desired star selection field on the left side of the application
- the selection field becomes active
- now click on a star label on the map to the right
Step-by-step tutorial for Nova Sgr 2015 No. 2
- Date & time: 2015/04/13 01:30:00
- Latitude: 45.705
- Longitude: 25.653
You can also use the Geolocation link to automatically fill in these fields
- Name of the variable star: Nova Sgr 2015 No. 2
- Limiting magnitude: 7
- FOV [arc minutes]: 1200
- hit the 'Update Chart' button to retrieve the photometry table and star coordinates to generate a star map.
Instead of using the name of a variable star, you can also use the AAVSO chart ID of one of your existing charts. In that case, the limiting magnitude and FOV fields are read-only; the values for these two will be the ones used when plotting the initial AAVSO chart
The chart orientation can be changed at any time, without having to click 'Update Chart', and without having to retrieve the data again.
After the map is updated, a new row is created in the Brightness estimate table
- Brightness estimate
- Bright star: select the star labeled 57, in the NE part of the map, above the 'teapot handle'
- specify 2 as number of steps between the bright star and the variable 'V'
- specify 3 as number of steps between the variable 'V' and the dim star
- Dim star: select the star labeled 63, from the small triangle asterism towards the center of the map, just South of Kaus Borealis
This would correspond to an estimation marked as A 2 V 3 B, where A is the 57 and B is the 63
You can add more estimations to the table, and the final number will be the average; the application also computes a standard deviation, and you can use that to judge if the newly added estimation is actually improving things (standard deviation decreases) or making them worse (standard deviation increases)
At this stage, the brightness estimate is 5.9; the application will advise to apply extinction corrections, since the differences in airmass between the variable and comparison stars are too big
- Extinction coefficient (K)
- click the Or compute it from observations radio button
- click the Argelander comparisons radio button (it should be already checked by default when starting the application)
When using Argelander comparisons, we need to provide at least two comparisons, so the table already has two rows
- for the first comparison:
- select the 47 as bright star (in the Western part of the map, on the Northern part of the spout of the teapot asterism)
- specify 2 as brightness steps to the dim star
- select 46 as dim star, from the same region
- for the second comparison:
- select 57 as bright star (from the North-East side of the map, above the 'teapot handle')
- specify 1 as brightness steps to the dim star
- select the same 59 as dim star (just above the '57')
With the corrections required to compensate differential extinction, the brightness estimate of the variable is now listed as 5.57