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IDProjectCategoryView StatusLast Update
0002420SkyChart1-Softwarepublic23-12-06 09:12
ReporterAlan Smith Assigned ToPatrick Chevalley  
Status resolvedResolutionfixed 
Target Version4.4 
Summary0002420: Show effect of airmass on apparent object magnitude
DescriptionThe airmass by zenith angle calculations shown both the Object Details > Visibility for your Observatory, and the bottom status bar of the chart, are very useful to determine object visibility.

Please could you add a new entry to show the adjusted apparent magnitude for that object at its current position on the chart, based on viewing it in that air mass (e.g. observing through 2x airmass will result in the linear object brightness of approximately half its catalogue value).

The new entry would be useful in both Object Details > Visibility for your Observatory, and the bottom status bar on the chart.

Many thanks
TagsNo tags attached.


Patrick Chevalley

21-03-28 14:09

administrator   ~0006973

I look a bit at the theory for the magnitude extinction.
This is not so simple because it depend a lot on the observing wavelength, site quality and the meteorological condition. To get it not too bad you need to do photometric observation for every specific night.
A standard mean value for visual band is 0.2 magnitude per air mass, but this can easily vary from 0.1 to 0.3.

I am a bit reluctant to show a value know to be false that unaware user can take as is.
Maybe it is not so hard to remember this value of 0.2 and apply mentally using the air mass value, simply 0.2 magnitude fainter at 2 air mass.
Anyway at most observing location the sky background brightening below 2 air mass is probably worst than extinction.

Alan Smith

23-12-03 11:02

reporter   ~0008666

Hi Patrick, just noticed this feature request was still open.

It is mainly for when teaching others to use bright stars to hop around the sky. to visualise how dim bright stars become at low altitudes.

For example where we live. if I not take into account extinction there are approximately 15 birght stars which can be used to move across the sky quickly, but if I take into account extinction on thoe which only reach lower maximum altitude it limits it to about 12 (Fomalhaut is a good example which is bright magnitude in the catalogue but dims too much to be useful). It is more about visualising the effect to teach others rather than the actual value being very accurate. We are in dark place so generally no issue with sky background lighting except when the Moon is up.

Its really up to you if you wish to add visualisation effects. I realise this sort of visualisation was not the original intent of CDC so understand if you prefer not to use them. I can achieve it in other software (Stellarium), I would just prefer to be able to use CDC for this type of teaching. I'm happy for it to be closed if you prefer not to provide them. many thanks.

Patrick Chevalley

23-12-04 10:08

administrator   ~0008672

I have added the option and it is now available in today CdC beta version.
You can activate the display with reduced star brightness in Setup / Observatory / Horizon.
The apparent magnitude is show with the airmass in the detail info window.

The absorption at the horizon is about 10 magnitude, this make many stars to totally disappear, but I make the choice to still display them with the minimal dot size so they can be selected to show the information.

The computation take account for the observatory elevation and mean atmospheric humidity.
This use the formula from this paper:

Alan Smith

23-12-04 10:51

reporter   ~0008673

Wow thank you, it sounds amazing, i am looking forward to trying it/. If all goes well I will use it in my next teaching session in January.

Alan Smith

23-12-04 20:33

reporter   ~0008674

Hi Patrick, I have tried it on today's beta version v4644 and it is very very good thank you. The effect on the star display is excellent and I also very much like the 'absorded magnitude' feature in the object detail window. It shows a user in a nice visual way the dimming effect on bright stars and also the trade-offs you have to make when observing through the lower altitudes. I am very much looking forward to using it more.

Patrick Chevalley

23-12-06 09:11

administrator   ~0008679

Last edited: 23-12-06 09:12

I make one more change that will be available next Monday.

It now take account for the relative humidity set just above in the Observatory setup.
For that I use an interpolation between the given values for aerosol scattering and interpret the value in Green paper as following:
"winter" = 20%
"average" = 50%
"summer"= 80%

Now absorption take account for altitude and humidity.

Issue History

Date Modified Username Field Change
21-03-28 09:36 Alan Smith New Issue
21-03-28 11:47 Patrick Chevalley Assigned To => Patrick Chevalley
21-03-28 11:47 Patrick Chevalley Status new => assigned
21-03-28 11:47 Patrick Chevalley Target Version => 4.4
21-03-28 14:09 Patrick Chevalley Note Added: 0006973
23-12-03 11:02 Alan Smith Note Added: 0008666
23-12-04 10:08 Patrick Chevalley Note Added: 0008672
23-12-04 10:51 Alan Smith Note Added: 0008673
23-12-04 20:33 Alan Smith Note Added: 0008674
23-12-06 09:11 Patrick Chevalley Status assigned => resolved
23-12-06 09:11 Patrick Chevalley Resolution open => fixed
23-12-06 09:11 Patrick Chevalley Note Added: 0008679
23-12-06 09:12 Patrick Chevalley Note Edited: 0008679