Optics of Vivitar TEL50600
I hesitate to call what the Vivitar Telescope has, “optics.” But it does, in fact, have something which can be described as a lens up front. Let’s take a little look at the history of optics to see what has gone wrong. The first telescope was patented in 1608 in the Netherlands, and Galileo Galilei was the first to turn a telescope up to the night sky and publish what he saw in 1609. These telescopes used a single lens objective (front-aperture) and a single lens eyepiece. The basic principle of using two lenses to magnify objects had actually been known prior to this, but all instruments anyone had tried were so bad that the image may have been larger, but it was not more detailed than just looking with your eye. It was full of false-color and all sorts of aberrations, and it was unusable for astronomy. The fix was to make a small lens with a very long focal length and stop it down so the actual aperture was tiny and the distortions from the edge of the lens didn’t damage the image formed by the center. Any single-lens-objective or “singlet” therefore must obey this principle in order to form usable images: it must have an extremely long focal length compared to its aperture.
Lens quality enhanced over time, and eventually, the achromatic telescope was introduced in 1758, allowing for correction of the false-color fringes by using two pieces of glass of different compositions, which brings two wavelengths of light to focus at the same point. This also allowed telescopes to shorten in length from a focal ratio of 25-100 to perhaps 5-15. Ever since the introduction of the Achromat (meaning no-false-color), virtually every affordable refractor telescope has used this principle, from high-quality planetary telescopes to department store refractors to binoculars to certain kinds of astrophotography telescopes. When you buy a cheap refractor telescope, even a very poor one, you can be fairly certain that you’ve got an achromatic doublet at the front.
The Vivitar Telescope has a singlet lens. It is a short focal length lens with an aperture of 40mm and a focal length of 500mm, for a ratio of f/11. In theory, this means it should be loaded with false color, and this is true in practice. In fact, the telescope has more false color fringing than what Galileo had to work with in his 1600s telescopes, because the lens is too wide and the focal length is too short.
We often say of cheap telescopes that even if it’s full of aberrations, it’s at least better than what you’d see with your unaided eye. After all, Galileo’s telescope was full of aberrations, and he revolutionized astronomy. Galileo’s telescope was probably sharper than this.
Here is a simulation of what the view looks like in the Vivitar Telescope (I could not take photos–not a chance I could hold anything steady if I tried to take photos, so I’m editing a photo taken a while ago using a reflector telescope which was stopped down to 50mm aperture.)
Through the Vivitar Telescope, I could see that the Moon had craters. I couldn’t see any detail whatsoever in the craters, I could just tell that it was there. The telescope had not just chromatic aberration but spherical aberration, which meant the telescope wouldn’t come to focus no matter where the focuser position was. It sort of smeared from blurry to slightly less blurry back to blurry again, and there was no hope of finding what the best position was.
The Moon is dim and blurry and I can not overstate how much false color it has–I’m not even sure my simulation does it justice. It’s bad.
I didn’t bother to look at anything else. There is nothing left to try, when you can’t show a passable image of the Moon at 50x, you have unequivocally failed the optical test. It is impossible, given the views of the Moon, that any other object you could look for would be sharp enough to observe, even if you could find it. Stars are pointy blobs. There’s nothing else to say about this telescope’s optics, at least at the front end.
Is it possible that there could be some shining beacon of life and hope? Could there be something redeeming about this telescope? Some accessories that could fix these optical problems? Not a chance.
The telescope comes with two eyepieces, a 10mm and a 5mm Huygens, providing 50x and 100x respectively. It also includes a 3x finderscope. Are these optics any good? Not a chance. 10mm is too zoomed in even for nice telescopes when it’s your only eyepiece, and 5mm is a joke. The 10mm is the only chance you have of finding anything because despite its soda-straw field of view it isn’t the 5mm. The 5mm blows up the already bad 10mm image and makes it much worse. If this were a better telescope, I’d complain about the lack of a low-power eyepiece. But it wouldn’t change anything–even a nice Plossl wouldn’t make a difference. It should come as no surprise that the eyepieces are made out of plastic and are loaded with internal reflections.
The finderscope? Worst I’ve ever used, even worse than the terrible plastic 5×25 finder sold on a lot of cheap telescopes. It can’t be focused, it has a silver inside so it’s loaded with internal reflections, I can’t even see the crosshairs. It’s worse than if it had been a peep-sight, at least then you could see the sky behind it.
The Tripod With the Scope
The first thing you notice when you try to use this telescope isn’t the optics, it’s the tripod. It’s so short that even sitting down it’s undersized. Maybe it’s the right size for a kid. The tripod wants to fall down if you even stare at it wrong. It has extendable legs (and to reach the eyepiece you have to extend the legs), but when the legs are extended it’s so much wobblier. The tripod head is a wobbly and pliable plastic and it flexes when you try to move it. Using this tripod I felt I was too harsh on the TravelScope 70. Combining this extremely finicky tripod with the tiny field of view of the 10mm eyepiece meant that I was only ever able to get perhaps 80% of the Moon in the field of view at one time, at best–getting it centered in the tiny field of view is hopeless. Putting your eye up to the eyepiece is enough to cause the view to shake horribly (perhaps my simulation should have had added motion blur).
Put it on a $1000 Equatorial mount and it wouldn’t fix the telescope.