I have a few different telescopes, including the old Greenkat 60mm spotting scope (includes erecting lens to assist light absorption… and make sure things seen on the Earth are the right way up), a Seiko Saturn scope, with 40mm aperture (no extra lenses to absorb stuff, but small aperture), A Tasco 60mm ‘scope with 800mm focal length (f/13.3 and changeable eyepieces (normally used for solar projection) and the Celestron 130mm SLT (f/5 used for astronomy), with a 650mm focal length. But in terms of eyepieces, magnification and the like, what are the ranges I should be looking at?
For the Greenkat and the Seiko, this doesn’t really apply – firstly they are just toy telescopes and secondly they don’t have removable eyepieces, just variable zoom in the case of the Seiko (15-45x) and a fixed 22x eyepiece on the Greenkat. For the Tasco, I can at least wonder if the eyepieces provided are actually appropriate (including the 3.3x Barlow lens, which increases zoom) and for the Celestron, I can get an idea of the range of use of the eyepieces I have.
First a few things to get out of the way. The aperture is the size of the primary lens or mirror of the telescope (the diameter of the opening for example). Larger apertures allow more light in and so make things appear brighter, they also have greater resolution and so can see smaller details. The focal length is the distance between the primary lens (or equivalent) and the focal point of that lens. The focal ratio (eg f/5) is the aperture divided by the focal length. If the x in f/x is a small number, that means the light is concentrating closer to the primary lens and so appears brighter. Cameras and telescopes with a small x are known as ‘fast’ because it takes a shorter exposure for film to record an image from them. Fast telescopes are better for bringing out faint fuzzy things in the sky, but lose details on things like planets due to the amount of light swamping in. Slow telescopes do the opposite job. The Tasco is slower than the Celestron, but both are good enough for planets, the Celestron performing better with fuzzies too. If a camera is mounted onto the telescope, the f-ratio of the camera is multiplied by that of the telescope to reflect the dimming of the image passing through both systems. Additionally, cameras have all sorts of lenses in them to improve the sharpness etc of an image, each lens represents a point where light is absorbed, further dimming an image. The magnification an eyepiece is capable of is the telescope focal length divided by the eyepiece focal length, so a 25mm eyepiece on the Tasco (800mm) gives a zoom of 32x, whereas if it is stuck on the end of the Celestron (650mm), it provides 26x magnification. Barlow lenses increase magnification and are usually seen as either 2x or 3.3x, which is the amount they increase magnification by. I have a 3.3x Barlow for my Tasco, but I can’t use it on the Celestron as it requires the focal rack to move further than is possible.
So it can be seen that the Tasco can make things bigger than the Celestron can due to its focal length. But the Celestron can make things brighter and more detailed than the Tasco due to its aperture size. This means there is a limit to useful magnification for telescopes – a limit that if you go above, you’ll simply be getting a bigger image rather than a bigger one with more details. It should also be pointed out that if you increase the magnification of an extended object (planet, nebula, comet) rather than a point object (star), it will appear dimmer as the concentration of the light at each point of the image reduces. This means that for some hard to see points, there is an optimum magnification for a given aperture.
The highest useful magnification under perfect skies is given by doubling the aperture diameter in mm. So for the Tasco and the Greenkat (60mm), the maximum useful magnification is 120x. For the Seiko (40mm) the maximum is 80x, for the Celestron (130mm), the maximum is 260x. But conditions are hardly ever ideal and so taking this into account, the usual useful magnification is the same as the diameter in mm for each ‘scope. The lowest useful magnification is just over a tenth of the aperture size in mm (4x for the Seiko, 6x for the Greenkat and Tasco, 13x for the Celestron). For small fuzzies, such as globular clusters and tiny nebula, the trade off between magnification and brightness means the best magnification is around half the diameter of the aperture (20x for the Seiko, 30x for the Greenkat and the Tasco, 65x for the Celestron). For larger nebulae and galaxies, quarter the aperture size (10x for the Seiko, 15x for the Greenkat and Tasco and 32.5x for the Celestron – not that anything other than the Celestron is likely to have the aperture required to spot anything beyond a cluster or five). So what do we conclude from all this?
The Seiko, though very small, manages to keep its magnification within reasonable levels (45x is around the normal useful limit and well below the maximum useful limit). The Greenkat is similar, albeit it is stuck with 22x. I have a 4mm, 9mm, 12.5mm and 25mm eyepiece along with a 3.3x Barlow. The Tasco’s eyepiece limits are:
- Lowest useful magnification = 6, eyepiece size = 133mm, no problem with having too small an eyepiece there, I don’t have anything larger than 25mm.
- Magnification of extended faint sources goes from 15-30x, requiring eyepieces of around 50-25mm (my largest eyepiece will produce the best results in this case, though I wouldn’t use the Tasco for that work)
- Maximum usual useful magnification (eg details on planets) is 60x, equivalent to the 12.5mm eyepiece I have.
- Maximum useful magnification (eg for splitting double stars) is 120x, so the 4mm would be pushing it a bit. Stick the Barlow onto that and, well I guess it is useful for the eye relief. Some people (like me to be honest) like a larger image to look at.
The limits for the Celestron 130 SLT are:
- Lowest useful magnification = 13x, about a 50mm eyepiece. Don’t have one, don’t need one…
- Magnification of extended faint sources = 32.5-65x, or somewhere between the 25mm and 12.5mm eyepieces. I normally scan with the 25mm eyepiece and will continue to do so, using the 12.5mm to zoom in when appropriate.
- Maximum usual useful magnification = 130x, or a 5mm eyepiece, meaning the 4mm should do fine (as this is all approximate).
- Maximum useful magnification = 260x or a 2.5mm eyepiece – the 9mm with the 3.3x Barlow (if it could be used on that scope) or the 4mm with a future 2x Barlow perhaps?