I make my living in the Washington Metropolitan Area, and have for my entire career. As a boy I could look up at the night sky and see the Milky Way blazing away. Whenever I am back in rural Missouri, where I grew up, I still can. The only thing I can't do back there is make a decent living. So, like a lot of folk who have the money to buy a good scope, I live in a place where having the scope is of questionable value. If I lived back there, I'd have the skies, but probably not the money to purchase a first-class scope.
Sooooooo.
Well, with a city-based income, maybe it is possible to do at least something worthwhile. I recently purchased a used NGT-18. Some folk would say (and definitely would think) it's a waste, but that's ok by me. Let's try a little mind experiment and see what can be done with scopes of various sizes. For our test object, lets choose a 10th magnitude galaxy with a major axis of 2.1 arc-minutes, and a minor axis of 1.1 arc-minutes. This gives us a little tiny shining patch of fuzzy to run through some visibility algorithms. We'll hold the magnification factor at about 30 diameters, and we'll now determine just how good a sky is needed to detect the object.
Pretty saddening, but if I throw a 406 mm mirror at the above object, and if the skies are at LM 4.3, I should be able to see the same object that I could see having a quality 2 inch scope under mag 5 skies. Or something like that.
If I have the scope in my garage, I'm going to use it more than if I have to drive 2 hours (with my 2" scope) to get to a darker site. Ergo, I say, if you are an astronomer, go for the glass. If you live in the right place, you may actually be able to afford it. To really appreciate what all this means, visit the NGT page.
The calculations of visibility used in the above table used the techniques outlined by Roger N. Clark in his book "Visual Astronomy of the night sky". This algorithm is based on the surface brightness of the object against the limiting magnitude of the sky. Surface brightness of an object is a product of it's visual magnitude and it's surface area, which for galaxies is given as the major and minor axis of the eliptical area of the object.
The NGC catalogs give dimensions of galaxies, but pay attention to the sizes of the objects you find in the catalog. A whole lot of the galaxy dimensions are seriously off. Take for instance the image of M106 below. The elipse shows the commonly used dimension for M106. The RealSky image has been superimposed. Note the size difference. Calculations using the large dimensions will show that the object cannot be seen, and when the object is viewed in the scope it is there. The culprit is the incorrectly reported dimensions of the object. In the case of M106 below, the actual object takes up only 20% of the space allocated to it. This represents a 5X increase in the light density over what would be computed. This is an approximately 2 magnitude difference. I have incorporated the proper sizes in the database for the visibility program, and it now gives good numbers.
