Cliff Mygatt Astronomical League Master Observer
Astronomical League Lunar II & Constellation Hunter programs national coordinator President, Olympic Astronomical Society Bremerton, WA
Orion XT12i Fully flocked with Zambuto Mirror, Protostar quartz secondary and Moonlite CR focuser
Meade 8" LX10 SCT Fully Flocked with Magellan I DSCs
Orion 120mm and William Optics 80mm Refractors Ethos 21mm,17mm,13mm,10mm,8mm,6mm,4.7mm,3.7mm Nagler 31mm and paracorr type 1
David W. Knisely . . . . . . "If you aren't having fun in this hobby, you aren't doing it right." Hyde Memorial Observatory http://www.hydeobservatory.info Prairie Astronomy Club http://www.prairieastronomyclub.org
Quote:Here is a good chart that should answer your questions. Good Luck and welcome to Cloudy Nights!Aperture Resolving Light Max Power Gathering Magnitude Power 50mm 2.32" 51x 10.3 60mm 1.93" 73x 10.7 80mm 1.45" 131x 11.3 90mm 1.29" 165x 11.5 100mm 1.16" 204x 11.8 125mm 0.93" 319x 12.3 150mm 0.77" 459x 12.7 200mm 0.63" 816x 13.3 235mm 0.49" 1127x 13.6 280mm 0.4" 1600x 14.5 355mm 0.3" 2500x 15
Quote:Looking at the html code for that calculator, I see several errors in percentages and secondary obstruction figures.
Here is one that has been corrected to reflect contemporary scopes, coatings, and diagonal sizes:
Quote:Through a telescope at lowest useable magnification, where the exit pupil equals the eye's fully dilated iris, the limiting magnitude will essentially scale as the ratio of the areas objective : eye pupil.For example, let's suppose the observer's pupil opens to 6mm, any at the time the NELM is 5.5 magnitude. A 100mm scope collects (100 / 6)^2 = 278X more light than the eye. This is equivalent to LOG(278) * 2.5 = 6.1 magnitudes.
Quote:Quote:Looking at the html code for that calculator, I see several errors in percentages and secondary obstruction figures.Here is one that has been corrected to reflect contemporary scopes, coatings, and diagonal sizes:http://www.scopecity.com/limiting-magnitude-calculator.cfm I tried that one and it way overshot the magnitude limit for my 9.25 inch SCT the night I went to 15.6 unless I used a 0.3 extinction coeficient. Most of the time, I have trouble passing 15.0 in the SCT. Indeed, in my 14 inch Dob, I can go to 16th magnitude or a few tenths of a magnitude past that on a really good night, but not a great deal fainter. My old "off the cuff" formula yields a figure of 16.0, so it at least gives me something halfway close to reality, even with the variables mentioned earlier. Again, many of these formulae offer ball-park estimates but little else of significant accuracy. If a person needs to know how faint they are going, it is best to do observations of calibrated star fields. Clear skies to you.
Quote:Looking at the html code for that calculator, I see several errors in percentages and secondary obstruction figures.Here is one that has been corrected to reflect contemporary scopes, coatings, and diagonal sizes:http://www.scopecity.com/limiting-magnitude-calculator.cfm
Quote:I always find it mildly amusing to peruse these well meaning threads purporting to define the magnitude limits for various aperture instruments. The honest fact is that no formula and particularly none of the almost pointlessly complex ones being derived these days have serious and practical application for 99.9% of visual observers. They are based on the results of a select few highly experienced individuals typically working with outstanding instruments under truly excellent skies. Unless that also describes your own situation take any predictions derived from them with not just a few grains of salt, but about 2 pounds there of.There are so many individually unknown/undefined variables involved, not the least of which is today's huge range in sky brightness from light pollution and probably an even greater range in observer skill and experience that utterly precludes any proposed formula from having a true "accuracy" of greater than less than one full magnitude and in some situations even twice that. This comes from someone with nearly 60 years of observing experience and participation in a number of the studies from which the complex modern formulae were derived. So, if you really what to have any true idea of how faint a star your scope will reveal the only honest way to determine that figure is to go out and look for yourself.BrooksObs