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Determining limiting magnitude |
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Wednesday, 30 April 2008 |
This is how I determine the limiting magnitude (LM) of optical instruments using sketches and how it helps me to create better star maps.
Why limiting magnitude is important
Do you know this? You want to find a faint deep-sky object. You are
about to do some star hopping, you have prepared a map and your telescope is positioned at the starting point of the journey. Then
you look through your finding telescope and you see stars, a lot of stars...
Then you try to compare the picture with the star field on your
map... and now... you are confused. If you only had a map that would
show a similar star field as the one that's in your eye piece.
What you
need is a map that matches the limiting magnitude of your finding aids under the current sky condition. The limiting magnitude (LM) is the faintest star that visually can be seen through an instrument. Of course, the number will vary in respect to sky quality and the human eye's condition.
Determining LM for my equipment
My finding aids are a 8 x 50 finder scope and a 102/500 RFT (Rich Field Refractor). The best way to find out the LM is to sketch a known star field under normal conditions and compair the results with a detailed star map (I used HNSKY with the GSC 1.2 star catalogue for that purpose.) I sketched the Polaris star field. Sky conditions were moderate: fst=5m5 and seeing=II.
I decided to do an experiment with the RFT: I reduced its aperture to 50 millimeters in order to see what the difference would be. I was a bit surprised that there only was a difference of a few tenths of a magnitude, but in Milky Way regions that small difference will dim a lot of stars making the star hopper's life easier...
Producing the right mapsIt's obvious, that I will need at least two maps. I will do intensive field testing in the next couple of months with new maps having LM of 9 and 10 while varying the scaling. After testing is done, I can start to create my new star atlas.
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