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eccentric

Confusing regarding eyepeices

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After messing around with a feature on sky at night with simulation of eye pieces and views through your chosen telescope I am rather confused!!

With choosing my telescope I am planning to purchase (Sky-liner 250 Flextube) then the object i want to see (ex - Jupiter) then putting it into visual mode instead of camera, I then choose an eye piece... say 25-mm. I can hardly see the planet!!! its just a dot!! but then if i put a small eye piece say 4-mm i can see it perfectly well. So does that mean in buying my telescope I should purchase smaller eyepieces? I assume in doing that that the quality of what I see goes down, but is that really how small i am going to see jupiter? I thought it would be the size of a thumbnail or something!

http://www.skyatnightmagazine.com/interactiveIndex.asp# that is the website and then i clicked field of view calculater

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eccentric

Everything to do with astronomy can be very confusing to start with! We've all been through various states of confusion at different times.

An important point to remember is that magnification is calculated as the focal length of the scope divided by the focal length of the EP. So if your scope has a FL of about 1200mm and you're using an EP of say 30mm, the magnification will be 40x. If you use an EP with a FL of 10mm, the mag will be 120x. ie 'smaller' EPs give a larger magnification

Hope this helps you with the confusion

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you can imagine the eyepiece as a magnifying glass with which you are inspecting the real image created by your objective/primary mirror. as do magnifying glasses, different focal length eyepieces will give different magnifications.

in addition to the primer highlighted by andrew, i think there's a good one over on the televue site.

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Pretty much, but in the case of the moon, studying it at high power is even better than having the whole moon enclosed in a lower power eyepiece.

Low power really becomes useful with larger DSOs. IMHO, Open clusters for example are much more impressive when there's lots of black space around

Andrew

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Eccentric

As Andrew says, it very much depends on what you're looking at. It might be helpful for you to think as small EP's, say up to 10mm, as high power, between 10 mm and 20mm as medium power and above 20mm as low power. This better reflects how the EPs are used in conjunction with different 'size' of scope than does small, large etc.

Good luck with researching your scope. There's a whole load more to appreciate

Steve

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You might want to think about getting a Barlow as well/instead. Effectively, a Barlow goes between the scope and the EP. A 2x Barlow will double the power of your EP - so a 10mm with a Barlow will be the same as a 5mm on it's own. A 25mm will become the same as a 13.5mm.

This way you can double the range of your EPs at a stroke!

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so a 4mm eye peice would be a good idea to purchase as the 250 flex tube only comes with 10 and 25mm eye pieces ?

Well the 10mm will give you 120x magnification so a 2x barlow lens will make that 240x which is about as much power you can use on most nights. You might not use a 4mm eyepiece that often to be honest.

John

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Dont forget the maximum power you'll mostly be able to use will be about x200 and mostly you'll be down to about x150 . A common assumption is that you need super powerful magnification - I assumed the same.

In reality I seldom use anything more powerful than a 15mm.

A higher magnification wil make the objects bigger BUT it will also intensify image problems like heat haze - so the object gets bigger but gets more unsteady.

I'd get to use the scope a bit and get some eyetime in with the supplied accesories before juimping in and buying more EPs.

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Focal Length = 1200.

EyeP Mag

==== ===

4 300

6 200

8 150

10 120

12 100

15 80

25 48

Now choose what magnification you want remembering that bigger is not necessarily better.

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Another try as spaces seem to get compressed.

Focal Length = 1200mm

EyeP......Mag

4mm......300

6mm......200

8mm......150

9mm......133

10mm....120

12mm....100

15mm....80

25mm....48

Chose the required magnification and corresponding eyepiece. Remembering that bigger is not necessarily better.

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I assume I get a 2x barlow with the telescope, so maybe picking up a 3x barlow?

I'd check what you get with the scope when you order. Some packages include just the 2 eyepieces. A 2x barlow would get much more use than a 3x one for the reasons that others hve explained.

John

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Hi eccentric

If you think planets are an area of astronomy that you will spend a lot of time involved in then I would get an eyepiece that gives you between 200-240x magnification.

Barlows are excellent for getting you more magnification but if you want to see the maximum detail then a single eyepeice will show more than an eyepeice & barlow combination.

Rgds

Ian

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I just ran the program - sorry Eccentric but you missed something. The circle represents the field of view through the eyepiece but you do not look at the eyepiece from a foot away (as you do a PC screen) but with it against your eye. If you look at the simulator again and put you eye much closer to the screen so the edge of your field of view can just see the circle - now Jupiter will look bigger. This is how it will be in reality.

I was looking at Saturn last night at only 50x and it was easy to see it and the edge on ring. Saturn is only half the size of Jupiter.

Mike

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That site is either misleading or there's something very wrong with my scope. I see Saturn a lot larger than the site shows with a 5mm plossl. On the other hand, M31 is nowhere near that size in the 32mm. You also have no chance of seeing the colours depicted.

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This is a pretty old thread probably best to start a new one with your question.

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