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Totally newbie here. I got the Skywatcher Dobsonian 6 inch (150 mm) f/8 Newtonian reflector. I had a brief chance to test it yesterday when the skies here in Melbourne cleared a little bit. I was able to see the Moon that was not that high in the sky.

With the telescope, it came with two 1.25" eyepieces; 10 mm and 25 mm, and a 2" adapter.

I found out the best view with the 25 mm although it wasn't that clear. I still have to check the mirrors alignment and collimate.

However, I found a bit uncomfortable to use it though. It was hard to keep the entire view clear. If I moved slightly I couldn't see the entire view.

After reading some articles, it seem that a 2" eyepiece offer a more comfortable view experience.

I'd like to ask; how can I choose a 2" eyepiece that would be more comfortable for observing the Moon and planets? What should I look for in terms of types or specifications?

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    $\begingroup$ I don't think that you really want the "best" 2" eyepiece, as it costs more than your telescope did (look up the Ethos 21mm for example). For lunar and planetary you want high-power, which will be a 1.25" eyepiece, not 2". Also I don't think that Stack Exchange handles "what is the best product?" questions. Try looking at the forums Cloudy Nights or Stargazer's Lounge. $\endgroup$ – Aaron F May 12 at 9:10
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    $\begingroup$ You might do better to try an amateur astronomy forum for this sort of very specific advice, eg iceinspace.com.au, given where you are located. $\endgroup$ – Dr Chuck May 12 at 18:03
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    $\begingroup$ @AaronF I think the question is okay now. Probably an answer that discussed exit pupil distance and FOV would be suitable. $\endgroup$ – uhoh May 12 at 23:28
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Short version:

The reason that 2" eyepieces will offer more comfortable views is because they are lower powers and therefore have larger exit pupils.


Long version:

In a telescope, the objective is the lens or mirror which collects light and focuses it at a distance equal to the focal length, creating an image at that point.

(You can test this out most easily on the moon: leave the dust cap in your focuser and use your finder to point the telescope at the moon. You should be able to see a clear (after focusing!) image of the moon projected on the dust cap)

When you insert an eyepiece, you're using it to inspect the image created by the objective.

This is why eyepieces with smaller numbers actually magnify the image more: with a 5mm eyepiece you're looking at a smaller central portion (imagine 5mm, though that's not correct, as it also depends on the FoV of the eyepiece) of the image which the objective has collected; and with a 30mm eyepiece you're inspecting a larger part of the image.

And this results in the exit pupil - a function of telescope and eyepiece - which is the size of the image you're looking at. It's the beam of light which enters the pupil of your eye. At lower magnifications (higher eyepiece focal lengths) the exit pupil is larger, and therefore easier to view comfortably.

You can test this out: cut a series of small holes (from 0.5mm up to 7mm) in a piece of paper and try looking through them. You'll find that the larger holes are much easier to see things through.

To calculate the exit pupil, divide the focal length of the eyepiece by the F-ratio of your telescope.
(And, related, to calculate the magnification you'll get from an eyepiece, divide the focal length of your telescope (in mm) by the focal length of the eyepiece (in mm))

For example, your telescope is f/8, so a 30mm eyepiece would give an exit pupil of 3.75mm - nice and sufficiently wide for you to be able to find the view and not lose it.

But if you want to look at a planet then you'll want more magnification, and you'll have to use a shorter focal length eyepiece. Take a 5mm for instance. That would give an exit pupil of only 0.625mm.

Another effect of decreasing exit pupil is decreasing image brightness. You'll find that, apart from the brightest objects, you probably won't want to go below a 1mm exit pupil very often.

Here's an image from the Wikipedia article on the subject which nicely illustrates the concepts:

(Image credit: By Evan Mason - Own work, CC BY-SA 3.0)

You'll see another term in the image: eye relief.

Eye relief is the distance from the surface of the top lens of the eyepiece to the point where the image is formed (the point where you position your eye). This distance increases as the diameter of the lens increases.

If you wear glasses while observing then it's important to buy eyepieces with an eye relief of at least 15mm (actual figure varies, maybe ±5mm, depending on the shape of your face and glasses you wear) so that you can see the whole view.

(Also, if you wear glasses while observing, are you sure that you need to? Simple long- or short-sightedness can be corrected by focusing, and mild astigmatism isn't visible at higher powers.)


You ask

how can I choose a 2" eyepiece that would be more comfortable for observing the Moon and planets? What should I look for in terms of types or specifications?

The Moon and planets are objects which you'll generally want to use a high magnification to view. High magnifications are served by 1.25" eyepieces (there's no need to make them as large as 2" - they're looking at a smaller part of the objective image - and making them in 2" barrels costs more to make) and give a smaller exit pupil.


In your telescope, a good starter set of eyepieces, in my opinion, would be:

  • A low power wide field eyepiece, with a focal length of 30-40mm, and a FoV of 68°-82°.
  • A medium power wide field eyepiece, with a focal length of 10-20mm, and a FoV of 76°-100°
  • A high power wide field eyepiece, with a focal length of 5-10mm, and a FoV of 76°-110° (This one's a bit more tricky, as the maximum magnification which will be usable will depend on your local conditions - the "seeing". If seeing is generally good you might be able to go higher than 5mm, but if it's bad then you might not be able to go much higher than 8 or 9mm)

(Notable brand names which come to mind are APM, Baader Morpheus, Explore Scientific, Pentax, Tele Vue)

You don't really need more than three good eyepieces. You could also get away with only one high-power, an 8-24mm zoom, and a Barlow lens. But I think the wide field eyepieces are the better option in your telescope, because with a wider FoV you don't have to nudge the telescope so often to keep the object in view.

(The "good" is quite important: almost all eyepieces look good in the dead centre. Then the view starts to degrade towards the edges. Generally, the more money you pay, the less the view degrades (or starts to degrade further away from the centre). Also, and fortunately for you, the degradation of the view is greater as the F-number gets lower. At f/8, you won't need to spend loads of money to get great views, but you'll have to do your research and find out what other people with the same telescope like to use)


Finally, a couple of tips for comfortable viewing:

  1. Make sure you're sitting down while observing. It makes a huge difference to your ability to keep your head still. If you must stand, then try and have something to hold on to or lean against.
  2. Practice makes perfect: the more you use your telescope and eyepieces, the better you'll get at using them, and you'll find it easier to maintain a steady view.

These related posts Eyepiece diameter for small focal length planetary viewing on a large dobsonian? and What are the advantages and disadvantages of a 2-inch eyepiece versus a 1.25-inch eyepiece? have good answers to similar questions.

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    $\begingroup$ This is a great answer, thanks for taking the time to write such a careful explanation! For the sentence "To calculate the exit pupil, divide the focal length of the eyepiece by the F-ratio of your telescope" is it possible to find a link to a diagram or some other explanation that makes it clear how/why this works? I'm just having trouble visualizing how/why this is so. Also, is eye relief also a factor for why large eyepieces like this offer more comfortable views? $\endgroup$ – uhoh Sep 14 at 23:53
  • $\begingroup$ I'm also trying to understand why an "exit pupil of 3.75mm" is considered "nice and wide" because a night adapted pupil is even wider than that. $\endgroup$ – uhoh Sep 14 at 23:57
  • $\begingroup$ @uhoh thank you :-) I had been wanting to write a reply for a few months. You're right that a diagram would definitely help. I'll have a look for one this evening and will edit the answer to include it. I did think about mentioning eye relief as well, but thought the answer was already quite long, and from OP's description ("If I moved slightly I couldn't see the entire view") it didn't sound to me like a lack of eye relief was the issue. But you're right that it would be worth briefly explaining. I'll address it when I edit later. $\endgroup$ – Aaron F Sep 15 at 8:57
  • $\begingroup$ @uhoh "a night adapted pupil is even wider than that" - yes that's true (although how wide varies from person to person, and unfortunately gets smaller with age). Maybe I should have said "sufficiently wide" instead of "nice and wide"? For me, I find an exit pupil greater than 2mm is big enough and bright enough to view comfortably. If I'm using a nebula filter then I might go above my eye's dilation - up to 8mm - in order to catch as much light as possible, even though some of that light might be "wasted" as it doesn't enter my pupil :-) $\endgroup$ – Aaron F Sep 15 at 9:31

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