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I have a Celestron Specialty Series Travel Scope 70mm refractor which came with 10mm (40x) and 20mm (20x) eyepieces.

Specifications:

  • Focal ratio: f/5.71
  • Optical Design: Refractor
  • Aperture (mm): 70 mm
  • Focal length: 400 mm
  • Finderscope: 5x24
  • Highest Useful Magnification: 165x
  • Lowest Useful Magnification: 10x
  • Resolution (Rayleigh): 1.99 arc seconds
  • Resolution (Dawes): 1.66 arc seconds
  • Light Gathering Power (Compared to human eye): 100x
  • Optical Coatings: Fully Coated

I’m looking for the best possible magnifications and combinations that would lemme look at planets and deep sky objects more clearly.

I'll soon be moving to a town-city area (Needham, MA) if that is of concern for location.

Can someone help me understand how to choose the eyepieces and accessories that I could buy?

Thanks

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    $\begingroup$ Welcome! I made some small edits to the wording; it's always OK to ask how to choose what to buy, but we have to avoid explicitly asking for "What should I buy" shopping recommendations in Stack Exchange. $\endgroup$
    – uhoh
    Apr 11 at 1:09

3 Answers 3

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To your scope the 165x maximum obtainable magnification mentioned by the manufacturer is bit optimistic. However I would recommend you to get 4mm Plossl eyepiece which will give you 133x magnificafion and a 2x barlow to use with 10mm eyepiece you got with the scope for viewing planets but observing Deep sky objects with this telescope may not be possible. Still you will be able to see majestic rings of Saturn and ribbons of Jupiter. If you want some more information regarding magnification, as thumb rule divide the focal length (400mm) by the focal length of the eyepiece (e.g 10mm) Happy star gazing !

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  • $\begingroup$ I think a 2x Barlow would make the 4mm redundant since they already have a 10mm. This scope will also do fine on large bright DSOs like the Orion nebula and the Andromeda Galaxy. $\endgroup$ Apr 10 at 14:23
  • $\begingroup$ @GregMiller Barlow is not only used to magnify, using it with 10mm and 20mm eyepiece gives you greater eye relief and also increases the magnification of each eyepiece. $\endgroup$ Apr 11 at 7:53
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    $\begingroup$ I would recommend against a 4mm plossl. I have one and the eye relief is so short you basically have to mash your eyeball onto it. Even a 6mm is rather bad in that regard. To push magnification higher I'd suggest either a 3x barlow instead of a 2x; or a slightly more expensive eye piece using a design with better eye relief. (The latter being more or less mandatory if the OP needs to use glasses to observe.) $\endgroup$ Apr 11 at 12:12
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    $\begingroup$ @KavinIshwaran, not disagreeing, just saying the 4mm wouldn't be of much use if you have a 10mm and a Barlow. $\endgroup$ Apr 11 at 14:45
  • $\begingroup$ @GregMiller The point I push with 4mm is that, It gives sharp images with planets where the 10mm with barlow fails to deliver that quality of image and magnification. in my experience :-) $\endgroup$ Apr 11 at 15:00
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Adding a 2x Barlow is almost always recommended regardless of the telescope. This essentially doubles the number of eyepieces you have. They also make looking through a 10mm with a 2x Barlow more comfortable than a 5mm Plossl.

I wouldn't recommend getting anything smaller than what you have now, I would go the other direction and get eyepieces which have a larger field of view like a 32mm or 40mm Plossl. This will make it easier to find objects you can't see in the finder. Then you can switch to a higher power once you've found the object.

If you really think you're going to stick with the hobby and get other scopes, you might want to go ahead and look at getting a set of Plossls. You'll probably get another 10mm, and another 20mm, but you'll likely still save money. They'll often come with a 2x barlow and a carrying case. And the extra eyepieces can actually be useful with multiple scopes set up.

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You have a telescope with a relatively short focal length. Where this telescope excels is with low-magnification wide-field views.

That's not to say it can't be used to look at the planets, but because it's a fast achromat you'll notice colour fringing on brighter objects. This is called chromatic aberration. If you find it distracting then you can buy filters to alleviate the issue.

In your original question you asked for "the best" eyepieces, but you didn't specify a budget.

  • If money is no problem then don't bother with a Barlow lens. Just go straight for eyepieces at the higher end of the market. Look for the names Baader Morpheus, Nikon NAV, Pentax XW, Takahashi TOE, Tele Vue Ethos / Nagler / Delos.
  • If budget is more limited then there are a lot of good eyepieces in the mid-range; names to look for are: APM UFF / XWA, Baader Hyperion, Explore Scientific, Vixen.
  • If budget is very limited then get a Barlow lens and then save up for a good eyepiece. I personally use and recommend the Orion Tri-Mag 3x Barlow. The lens element can be unscrewed from the tube it comes in, and then screwed into the bottom of an eyepiece, to give a ~1.5x increase in magnification.

Don't go overboard buying eyepieces - it's better to have a few higher quality eyepieces than many lower quality eyepieces.

As for which focal lengths to buy. When you buy a telescope which comes with bundled eyepieces, those eyepieces have focal lengths which are chosen because they're best suited to that telescope.
The first eyepiece to buy should have the same focal length as the bundled eyepiece you use the most.
A wide-field (70 degrees AFoV or greater) eyepiece will be a great improvement over the bundled ones.

Another thing to take into account is the "eye relief": the distance from the eyepiece lens closest to your eye and the focal point. If you wear glasses while observing then you'll want to look for "long eye relief" eyepieces - over ~15mm is good.

When it comes to planetary viewing there's a problem: planets are very small objects and therefore require high magnifications; but as you increase the magnification above a certain point the image becomes blurry and loses definition.
The point at which this happens depends on the "seeing". A good website which shows the seeing measured in arcseconds is Meteoblue. The lower the figure the better.
At this point you might notice that your telescope's specifications also have values measured in arcseconds. These are the theoretical maximum resolution your telescope's optics are capable of.
What these values basically mean is that planets in your telescope will be quite small and low resolution. You'll be able to see them, but you should temper your expectations.
As for an eyepiece to use for planets: something around 5-6mm will be the sweet spot for your telescope. It doesn't really matter whether you get this focal length by using a 2x Barlow with your 10mm eyepiece or by buying a dedicated eyepiece. Some nights you might be able to push the magnification higher, but those nights will probably be few and far between.

To give you an idea, here's a photo I took through the eyepiece of my 80mm refractor during the conjunction of Jupiter and Saturn in 2020 (pay no attention to the brightness - your eyes will adjust a lot better than the automatic Android camera settings do!): Jupiter Saturn conjunction I can't remember exactly what focal length eyepiece I was using that evening, but it was as low as I could go without blurring the image.

What you'll notice, however, is that the rings of Saturn are clearly visible.
You'll find that a smaller planet that's in focus is a lot better than a larger planet that's blurry. With time and practice you'll be able to pick out fine details in a small planet image.

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