eye piece help

[Neo_uk]

hi,

can any one help a newbie out still looking to get my first scope but trying to get to grips with all the technical blurb.

Am i right in thinking that the smaller the eye piece is in mm the narrower field of view it will give but greater magnification ? then if you want to double the magnification you you would add a x2 barlow lens ???? :scratch:.............(being careful of coarse not to exceed the telescopes maximum magnification)

many thanks chris

[Jamie]

Hi Chris

I like it when folk like you ask questions like this and answer them at the same time.

FYI you have the right idea about eyepieces mate... 8)

[Neo_uk]

Hi Chris

I like it when folk like you ask questions like this and answer them at the same time.

FYI you have the right idea about eyepieces mate... 8)

i am only able to answer the question my self by reading all the great posts on this fantastic forum !!!!!!

just wanted to make sure i had got it right though lol.

many thanks chris

[blinky]

yup, spot on

[philsail1]

Chris, whatever eyepieces you go for, try and buy "quality" ones. You'll get plenty of advice from SGL members on which ones are "best buys" for your particular scope.

I'm sure many of us have bought "naff" eyepieces in the past - and put it down to experience!

Regards,

philsail1

[Si Rollett]

How do you work out the max?

[haitch]

How do you work out the max?

I think the theoretical maxis 50x per inch aperture for most objects (you can sometimes beat this on the moon)

[John]

Theoretical maximum magnification for a scope is one thing. The maximum that is, in practice, actually useful, is usually quite a bit lower - say 2/3 rds of the theoretical maximum.

The theory assumes perfect viewing conditions, perfect optics, perfect collimation etc.

[gooseholla]

It isn't always true that smaller number eyepieces have a narrower field of view!

[haitch]

Theoretical maximum magnification for a scope is one thing. The maximum that is, in practice, actually useful, is usually quite a bit lower - say 2/3 rds of the theoretical maximum.

The theory assumes perfect viewing conditions, perfect optics, perfect collimation etc.

I was about to diverge into seeing conditions in the UK etc but that last glass of Cognac has put paid to lengthy replies for tonight.

[newman]

hi,

Am i right in thinking that the smaller the eye piece is in mm the narrower field of view it will give but greater magnification ?

In general, yes, but it's entirely possible to actually see a wider field with a higher magnification eyepiece. That's because every eyepiece has a poperty called Apparent Field of View (AFOV) that defines, in degrees, how wide field an eyepiece is. How much sky will you see in the eyepiece actually depends on the telesope it's being used in too. You calculate that by dividing the eyepiece AFOV with the magnification it gives in your scope.

So let me give an example. Say we have a telescope of 1000mm focal length.

Now let's assume we're lucky sods and have a 10mm Ethos with it. It's a lovely eyepiece with a 100 degree AFOV - which is an enormous ultra wide field. It gives as a 100x magnification, so:

AFOV / mag = true field

meaning 100/100, the eyepieces shows us exactly 1 degree of true sky while giving us 100x magnification.

So, this is nice and all, but say we also want a lower mag ep that shows us more sky. If we were to just look at eyepiece focal lengths, we might conclude that say, a 14mm Radian is a good buy. But let's check what we'll get with it. A Radian has a 60 deg AFOV - it's not narrow, but certainly no Ethos. It will give us about 71x magnification in our scope. So that means, 60deg AFOV / 71x mag = 0.84 degrees of true sky.

What this means is that it will actually show us less than the Ethos, while making the target appear smaller. We gained nothing and have no reason to use it. This makes this eyepiece somewhat redundant, and not a good combination with the 10mm Ethos.

In short, you need to consider eyepiece AFOVs as well as their magnification, and work out how much true field you'll get, before you plan out a collection. Just taking ep focal lengths into account can lead to redundant eyepieces you won't use. Of course, if you buy all of your eyepieces from the same series (say, all Hyperions, or X-Cel LX's), then they'll all have the same AFOV so you can actually plan by only looking at their focal lengths, but if combining different series you need to take different AFOVs into account.

[Si Rollett]

So if your new like me, stick to the same series for now to play it safe.

[newman]

Well, working out true fields is as simple as dividing a few numbers. It does take a bit of experience to actually get a feel for what said numbers actually translate to in real life - it's probably best to hold off on buying stuff before you get an idea on that.

[ismangil]

hi,

can any one help a newbie out still looking to get my first scope but trying to get to grips with all the technical blurb.

Am i right in thinking that the smaller the eye piece is in mm the narrower field of view it will give but greater magnification ? then if you want to double the magnification you you would add a x2 barlow lens ???? :scratch:.............(being careful of coarse not to exceed the telescopes maximum magnification)

many thanks chris

As mentioned above:

• Smaller mm = greater magnification
  
• BUT not always narrower field of view, depends on FOV of eyepiece design
  

The above is important to know especially if you have a light polluted backyard like me, because there is another eyepiece rule:

• greater magnification = darker background sky = better chance of seeing faint fuzzies (deep sky objects)
  

So the idea is to get maximum magnification at sufficiently wide field of view to frame the object.

Here's how it works:

1. Go to TeleVue's Calculator Tele Vue Optics: Calculator, input your scope focal length and aperture
   
2. Select sort by True Field of View (the default)
   

Now you can see that the same FOV can be achieved by higher magnification eyepieces.

In my case the widest possible I can achieve on my 1.25" 200mm/f6 reflector is 1.3deg, using 24mm 68deg eyepiece at 50x

That's why I got a 24mm Hyperion.

So you need to find the sweet spot for your scope between three factors:

• Optimum magnification
  
• Optimum FOV
  
• Optimum price
  

[Si Rollett]

The reason i was asking was i am finding it hard to know where my scope is pointing and was thinking i May need a wider field of view to help, i can find the moon but that because is so big and bright.

[newman]

Consider a Telrad, it does wonders for starhopping. And it's much cheaper than most wide field eps that are worth anything.

So what is a Telrad, you might ask? It's a zero magnification finder - it basically works by seemingly projecting a target reticule consisting of 3 circles onto the night sky. The first night I got to use it (as a complete beginner, and not being in company of anyone else who knows anything about astronomy, so no help) I found 14 Messiers. And the only reason I didn't find more was, I wasn't in a hurry I tend to use it either with a star chart, or with an iphone planetarium app. You figure out where your target is in relation to known constellations on the sky, then use the Telrad to find it. Of course, a good wide field still helps as it's easier to nail a target in a wider field.

[Si Rollett]

It has a StarPointer finderscope which is the same type of thing I think, havent been able to config it yep cust cant work out if there both looking at the same thing. I'm sure it will be easyer to do once I take it out of town somewhere darker.

[ismangil]

It has a StarPointer finderscope which is the same type of thing I think, havent been able to config it yep cust cant work out if there both looking at the same thing. I'm sure it will be easyer to do once I take it out of town somewhere darker.

Have you tried looking at a far away spot like an aerial or chimney in daylight using the supplied 25mm eyepiece (careful don't look at the sun!)? The further away the better.

When the spot is in the center of the eyepiece, look through the Starpointer, and see if the center of that is pointing at the same thing. If not, adjust the Starpointer.

You can then test it by using the Starpointer to go to a different spot, and then look in the eyepiece to see if it is in the field of view.

[Si Rollett]

Haven't no will once i move but where i live you don't want people to think your a tom. But will try that.