OK, so you've put your hands on a decent starter telescope. We're not talking about one of the cheap 50 mm aperture "department store" scopes, but a good quality 4 to 6-inch aperture scope. You've seen this and that in the night skies, and now you want a bit of an upgrade. Here's Gizmag's guide to getting there as easily as possible.
When you feel you are outgrowing your starter model, buying a new telescope isn't the only option. Starter telescopes provide a well-guided path into amateur astronomy, but they are the product of a number of design and manufacturing compromises to meet a marketable price point. The secret is that the negative effects of many of these compromises can be ameliorated by using carefully selected accessories.
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Replacing your eyepieces is probably the easiest and least costly step you can take to improve the view through your starter scope. This is one of the places that starter scopes skimp on to reach a marketable price point. The eyepieces that come with your starter scope are generally simple in optical design, which also leads to them being of limited optical quality. Their poor color correction, curved focal plane, and various forms of distortion significantly impact your observing experience.
There are many highly sophisticated eyepieces on the market. An extreme example is the TeleVue Nagler eyepiece line, whose eyepieces have eight optical elements. These are as close to perfectly neutral as is possible, have a huge field of view and amazing eye relief (the distance required between your eye and the eyepiece for proper use). They also weigh from one to two pounds, and cost between US$600-900 apiece. Clearly overkill to replace eyepieces for a starter scope.
Let's chart a more reasonable course for a 4-inch f/10 telescope. The first thing to do is decide what magnifications you want available for observation. On the low end, if the exit pupil of the telescope is larger than your dark-adapted pupil, you won't get the benefit of the whole aperture of your scope. Unless you have a very dark observing site, you will want your scope's exit pupil to be smaller than about 5 mm, which corresponds to about five power per inch of aperture. For our sample scope, magnifications of 20X or above should work.
Eyepieces are available having apparent field of view (how large the illuminated view seems) between about 40 and 100 degrees. The actual field of view is the apparent field of view divided by the magnification. The field of view at 20X will therefore be somewhere between two and five degrees. But there is another restriction – the image at which you are looking has to fit through a 1 1/4 inch eyepiece tube, and may be more restricted if the secondary mirror of the telescope is too small.
The image being formed by the 4-inch f/10 objective lens has a scale of about 0.7 inches per degree, so the widest image that can be fully illuminated by this telescopes optics is about 1.7 degrees. Buying an eyepiece providing an apparent field of view of 100 degrees would be wasteful, as only about 40 degrees of the field of view will be fully illuminated. The remainder of the field will be vignetted, which means that only part of the light from the objective is included in the image. It may be reasonable to have a little excess to improve the feeling of expansiveness, but apparent widths larger than about 60 degrees are not worth paying for in this case.
A reasonable choice for the low power eyepiece would be a 40 mm focal length Plossl. These are symmetric combinations of two achromatic lenses, which are very well corrected to give a diffraction-limited image near the center of the image, with just a touch of blurriness and color appearing at the edge of the 50 degree apparent field of view. Plossls do have limited eye relief, but in a long focus eyepiece this is not a problem. Prices start at about $50, but used or surplus eyepieces are available in the $30 range. This would provide 25X magnification and a field of view of about two degrees, nearly all of which is fully illuminated. Other forms of relatively inexpensive eyepieces, such as Erfles, can be used equally well for low powers.
Next, I am a firm believer in the use of the Barlow lens to stretch one's eyepiece collection. These are achromatic concave lenses, which effectively double the focal length of your telescope. Using a 2X Barlow with your low power Plossl would give you 50x magnification, and a field of view about a degree across – a very useful combination for looking at star clusters and the Moon. Disks and details on some of the planets can also be seen at this power. A Barlow lens may have arrived along with your starter telescope. After testing it with a replacement eyepiece, you may decide to also replace your Barlow. Good ones are available starting at about $40 new, and as low as $20 used.
Now its time to look at your high power requirements. Large magnification can only allow you to see detail which is already present in the image formed by your telescope. If your 4-inch scope had perfect optics, you would be able to resolve two stars separated by about a second of arc. If your vision is perfect, you can separate two stars about a minute of arc apart. This suggests that you can pull all the detail available from this scope at about 60X magnification, or 15 power per inch of aperture. This is nice in theory, but most people's visual acuity rapidly degrades from stress when working at the limits. A good rule of thumb is to top out at 40X or 50X per inch.
For our 4-inch sample scope, that would suggest a maximum magnification in the 150-200X range. If we just obtained an eyepiece to give that level of magnification, it would have a focal length of about 5-6 mm. Here's where things get tricky. Eye relief generally becomes smaller along with the eyepiece focal length. Your eyelashes are about 5 mm long, so using eyepieces with smaller eye relief can be irritating. For example, the eye relief for a 5 mm Plossl is only about 2-4 mm, so it may make sense to choose a different eyepiece design. There are eyepiece designs which give 15-20 mm of eye relief for a 6 mm focal length eyepiece. They require more lenses and more exotic types of glass, and generally cost upwards of $100.
Instead, I suggest solving the eye relief problem by using your 2X Barlow lens. This allows using a 12 mm eyepiece (having about that length of eye relief) with the Barlow lens to obtain the largest magnifications. A 12 mm Plossl would certainly be a good choice here, although some observers prefer a similarly priced orthoscopic eyepiece for high magnifications.
To sum up, for a 4-inch f/10 telescope we suggest an upgrade kit including a 40 mm Plossl eyepiece, a 12 mm Plossl eyepiece, and a two-power Barlow lens. This combination provides 25X, 50X, 80X, and 160X magnification, which serves nearly every need you might have for such a scope. The price for these replacement eyepieces can be $100 or less, given some use of sales and used/surplus optics. While you may need to vary the selections to fit your starter scope, you can easily obtain a similar set of new eyepieces. When you start using these eyepieces, you will see a substantial improvement in the ease and enjoyment of observing the skies. Perhaps enough that you will start thinking about replacing that shaky telescope mount. But that's a tale for another time.View gallery - 10 images