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Understand your telescope Eyepieces from zero to Hero

April 24, 2019 - DSOs, Gear, Starter Pack
Understand your telescope Eyepieces from zero to Hero

Having a good collection of eyepieces is by no means an easy task to accomplish with a normal budget (a limited one) at  your disposal. You should be aware that good eyepieces can and usually do match or exceed the price of a telescope. But it isn’t an impossible task either. It usually depends on your patience to find good quality glass on astronomy forums (second hand). Another factor may be your specific use cases – if you enjoy planetary observations more, then you may be inclined to use Orthos or small field but very good contrast pieces (Plossls). Instead – if you  enjoy observing faint fuzzies for example – you may want a much wider field of view (FOV) – at least 82 degrees – and an image corrected up to the edges of the field.

Even tough I like to test as many eyepieces as possible, I noticed that my 3 favourites are the ones bellow. Meade 34mm for wide objects like the Andromeda or Sculptor galaxies, The Pleiads, The Double Cluster, M44, Veil Nebula, Markarian’s Chain. The 13mm Nagler is my main objects finding eyepiece and then I switch to the 7 mm Nagler when magnification is required and the seeing allows it.

Order of magnification

From an apprentice to a master’s point of view we usually first use low power eyepieces to observe objects (34-18 mm). This offers a good overview of the area/object of interest. Then, if all the other conditions allow it (atmospheric usually) we increase power achieving a so called “zooming” effect. The object is shown closer and bigger – but not necessarily sharper. At medium magnifications (with 17-10 mm eyepieces used) we get most of the times good results – the image is sharp enough. The trouble comes when increasing the power even more (9-2.5 mm eyepieces). Depending on your telescope and atmospheric conditions you may run into 3 scenarios:

  1. You can exceed the maximum useful magnification of the telescope – 2 times the aperture. For example, when using a 130mm reflector (with a 650mm focal length)  the maximum useful power (magnification) is 260x – in ideal seeing conditions. If you’re trying to use a 3.5mm eyepiece and a 2x Barlow lens you’ll obtain 371x ( (650%3.5)*2 = 371) That translates to an undistinguishable blob of light when trying to observe any given object. It’s just too much for 130mm of aperture.
  2. You have a powerful enough telescope (200mm or more) and eyepiece setup in order to get to more than 300x magnifications. But the seeing is not on your side and even though the sky is clear the objects aren’t sharp enough at that power. This can have multiple causes: smoke, dust, air currents, etc.
  3. Here comes the happy scenario as well: your light bucket is ready and the seeing is good and stable. Magnifications of up to 450x reveal amazing details on planets or deep sky objects keeping the overall enthusiasm high. But there’s a catch: this scenario is very rare! Usually if we consider at least an observing session per month or 12 per year – in my experience only 2 or 3 times a year you’ll get the chance to enjoy observing at powers greater than 250x. And that’s the main reason why beginners shouldn’t allow themselves  to get fooled by commercials marketing telescopes that can magnify up to 500x (or other similar BS).

There are different types of eyepieces

Considering the telescope(focuser) connection:

Nowadays maybe we should go only with 2” eyepieces for best results (except from 6mm 13mm and bellow where 1.25” eyepieces are perfectly fine). For ease of observing a 2” connection or more is recommended.

Keep in mind your current telescope when choosing eyepieces

If you use a 200mm reflector or bellow, having 1.25” eyepieces is ok, they’ll serve you very good even for wide objects – that’ll require eyepieces in the range of 15-30 mm. But if you have access to a dark observing site and a large Dobson telescope at your disposal – at least 300mm aperture – you’ll have a lot more fun with 2” eyepieces. You can observe certain features in galaxies, layers in Planetary Nebulas, resolve stars near the center of globular clusters, etc.

For example I also have a Maksutov 127mm that only allows 1.25″ eyepieces. The Plossls bellow are good friends with it. Occasionally I also bring to the party the 7mm Nagler (also on 1,25″).

  • TeleVue 11 mm Plossl
  • TeleVue 20 mm Plossl
  • TeleVue 25 mm Plossl

The filters issue

When choosing your eyepieces you should consider that filters for 2” eyepieces will be about 2 times more expensive than for 1.25”. In the same time having both 1.25” and 2” eyepieces is problematic because you’ll need two types of filters. That’s why from a filters perspective it’s better to use eyepieces with the same connection factor. Taking my own situation as an example – even though I own an 1.25” UHC filter I intend to change it in the future with a 2” version.

Considering their usage:

Taking into account the optical design:

The Holy Graal of eyepieces

Probably without question the Holy Graal of eyepieces is TeleVue. Their optical performance is amazing and the build quality exemplary. If taken care of – they can last a lifetime. It’s understandable why the price is quite high even for used ones. But that’s a trade off that most amateur astronomers are happy to make. If you can afford new ones I encourage you to try them, if not – I recommend to seize the opportunity of purchasing a used one when available on various forums or amateur astronomers platforms.

Eyepieces weight

Last but not least, eyepieces weight is an important thing to consider. As a general rule heavy eyepieces are a good thing – an indicator of their quality. But that comes with usage problems. Wether you have a well balanced Dob or an ecuatorial mount adding up to a Kilo of weight to your setup might bring in some headaches. You may need counter weights or a sturdier tripod. Examples of very good and heavy eyepieces would be the Wiliam Optics 28 mm UWAN (1 Kg), Explore Scientific 30mm (also 1 Kg), TeleVue 21mm Ethos (1.02 kg), etc.


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