American Association of Variable Star Observers (AAVSO)
Thu, 05/31/2012 - 16:36

I am trying to figure out what I am doing wrong with the online Basic Observation Planner tool. The query form asks for my equipment's  limiting magnitude. Knowing the telescope is around 14, I enter that value in the appropriate area yet when I get the generated list, I get a listing of stars that is around mag 19+.

So my question is the form taking into account the CCD/photometer is more sensitive than the visual limiting magnitude of the scope? Or is the query wording off? When the list is generated the parameter is specified as:

No fainter than 14 at maximum

Does that mean the selection of stars are at my equipment's limiting magnitude when it is only at maximum?


Thank you in advance,

Mark S. (SMAG)

Limiting magnitude is for maximum

Hi Mark,

Your guess is correct -- the limiting magnitude assumes the star is brighter than your limit at maximum.  There are a number of stars whose range is very large (like R CrB for example).  R CrB is a fine star for binoculars at maximum, but you'd need a telescope to see it at minimum.  If I peg the limits to minimum rather than maximum, I might discourage observers from trying stars that might otherwise be within their capabilities for part of their range.  Further, even if you can't see a star in your telescope, you might still be able to provide a useful "fainter-than" estimate by reporting the faintest comparison star that you can see.  Being able to say R CrB is fainter than 14 is definitely valuable.