I will first confess that I have more elaborate blog postings in the works, but I just haven’t been able to finish them this past month. So, this entry is a substitution.
Part of my excuse has been the “crazy” annular eclipse that’s occurring this Saturday. Here in Northern Illinois it will be partial, of course. I was enlisted to conduct a workshop last week to a group of VIPs and it occupied most of my astronomy time. But, of course the weather in the Great Lakes will have a big low pressure system sitting on us. So, the chances are slim that the clouds will break. :-(
One reason I enjoy my smart scopes is because they allow me to experience great astronomical sights without having to deploy a hundred pounds of equipment or travel to distant locals, etc., etc. Here’s a quick Venn diagram of my observing “universe.” You’ll see that eclipses, like this one at least, are close to what are called a set of measure zero. My smart scope objects zone is large and green…. ;-)
There are so many great things I can see any clear night. So, for me it’s important to be able to pull the most meaning out of more mundane things like galaxies, nebulae and star clusters. That’s why this post is to help us interpret our amazing images.
This blog entry is short but it contains some math. I suspect some people will prefer that. Maybe some won't, but trust me, in the end it will be simple arithmetic that will be useful. This posting is to help you gauge the physical sizes of the objects you see.
MAGNIFICATION OR FIELDS OF VIEW?
With the type of astro imaging going on in our smart scopes, magnification doesn’t really make sense to me. Magnification in a normal telescope is determined by taking the ratio of the telescope focal length and dividing it by the eyepiece focal length. But we don’t use eyepieces except for the eVscope, which really is going to be an exception and its eyepiece is just showing a tiny monitor anyways.
The more important quantity is the field of view (FOV). Below I’ve calculated the vertical fields of view for all the smart telescope systems I know.
It is computed by measuring the angle subtended by the little triangle made by the imaging chip width at the distance of the focal length. If you like the math, here’s the formula:
FOV=2*arctan[(chip dimension/2)/(focal length)]
The last column of the table has the important result for each smart scope. It’s lists our “magic numbers”— the ratio between the distance to an object and the FOV. All we need to do is to divide the distance to object — which we can find in a catalog, an app or by just Googling — by this number. The result is that you will know how high your FOV is in terms of dimensions at the object you're observing.
(Determining actual distance to an object is always a major challenge in astronomy, so we’ll just rely upon the work of others. In some cases it was someone’s PhD thesis. Also note that distances are often a bit uncertain and can be revised.)
So, for example, the nebula M16 shown above is about 7,000 light years away. My Vespera image then, at that distance, spans some 7,000/63=110 light years. The pillars of creation by my reckoning are about 0.028 the height of the frame. (I just use a small millimeter scale and measure off my phone.) They come to some 3 light years long. Those pillars are much longer than the 169 meter high Washington Monument!
Sixty three is my magic number for Vespera. You should memorize the number for your scope and use it to estimate the sizes of what you’re seeing. Also keep in mind that this number is for the full height of your frame. If you zoom in, of course, it won't be accurate.
BTW -- If you’re in the path of the annular eclipse and the weather is good, please take a picture too and share it online in one of your users groups too. Oh, and for my Vespera, which is what I’d use on the Sun, since the sun is 93,000,000 miles away, I estimate the height of my FOV is 93 million/63=1.5 million miles. It easily fits the 0.9 million mile Sun.
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