Eyepieces??

[jninness]

I'm a bit confused about the eyepieces, I had a 4mm a super 10 and a super 25 (plus a barlow but I know what that one does) Can anyone tell me which is the best to use, most powerful and what would be the best to get later?? Does the super 25 = the same as 25mm?? Thanks
Jo

[Argonavis (William)]

The task of the eyepiece (technically called an ocular or ockulair in Europe where some of the leading telescope makers of a previous era resided) is to magnify the image collected by the telescope.
The most significant property of the eyepiece is its focal length - which appears to be 25mm, 10mm and 4mm in your case.
If you know the focal length (in mm) of your telescope it is then a simple matter to calculate the magnification - if you have an 8in (200mm) f10 telescope the focal length is 2000mm.
To get the magnification for each ocular divide its focal length into the focal length of the telescope. So the 25mm will divide x80 times into the focal length of 2000, so has a magnification effect of 80.
Similarily with the 10mm, the magnification is x200, and the 4mm x500.
Do you know the focal lenghth of your telescope?
I was recently asked - why? and I can't say I know, but it clearly has something to do with the ratio of the focal length of the ocular to the focal length of the telescope and the resulting image scale from using the two together.
The higher the magnification the greater the image scale and things will appear bigger.
Higher magnifiactions also magnify the atmospheric "seeing", which is the amount of atmospheric turbulence that you have to look through when using a telescope. With poor to average seeing, too high a magnification will only result in a blurred image, due to the distorting effect of the atmosphere.
It can also be due to the limitation of your telescope. If you have a very small telescope, say 60mm or 114mm, increasing he magnification does not always give you a better image, as the ability of a small telescope to resolve small details is limited. But the main effect on limiting the magnification you can use is seeing. It is unlikely you will ever use the 4mm eyepiece.
Generally you would start observing with a 25mm, and maybe increase the magnification depending on what you are observing and the seeing conditions.

[Argonavis (William)]

As to what is "best to get later"- that is a very open question.

You are best to use what you have got, maybe turn up to a few starparties and look through others scopes (with their permission of course) before embarking on a acquisition spree.

Try to avoid the "gear acquisition syndrome" and enjoy what you have for a while. When you feel you have reached your limitations, then you can look for soemthing better.

Granted, often we don't know what we are missing out on. My first look through a Televue Plossl ocular was a total revelation. I just didn't know that anything could be that good. In my opinion they are amateur astronomy's biggest secret. However, that was many years ago, and I am sure that there are many Plossl design eyepieces on the market that are very good.

[DRCORTEX (Lance)]

And from what I have seen, the biggest cost ( as in EP's ). What does a TV Ep go for ?


Lance

[Argonavis (William)]

Originally Posted by DRCORTEX

And from what I have seen, the biggest cost ( as in EP's ). What does a TV Ep go for ?


Lance

Bintel sell them for $145

[ving (David)]

thats the plossl... naglers are scarey!!

and according to another thread TV are going up

[janoskiss (Steve H)]

Originally Posted by jninness

I'm a bit confused about the eyepieces, I had a 4mm a super 10 and a super 25 (plus a barlow but I know what that one does) Can anyone tell me which is the best to use, most powerful and what would be the best to get later?? Does the super 25 = the same as 25mm?? Thanks
Jo

The 25 and 10mm will be useful for you for low and medium magnifications, respectively. You just got the or are getting the 10" f5 Saxon Dob, right? The focal length (FL) of that telescope is 1250mm. Like Argonavis explained the magnification is (FL of scope)/(FL of eyepiece). So the 25mm FL eyepiece (EP) will magnify 50x, and the 10mm will magnify 125x.

50x is a good magnification to allow the Moon and most star clusters and deep sky objects (DSOs) to fit in the field of view (there are a few spectacular exceptions, try binoculars for these), and 125x is enough to show you some detail on the planets (Saturn and Jupiter being the most notable), and to get close-up views of the lunar surface. It is also a good magnification for taking a close look at larger DSOs, e.g., cores of globular clusters and cores of galaxies, and for looking at smaller DSOs, especially galaxies and planetary nebulae.

Your 4mm EP is unlikely to be useful because I assume it has a tiny lens and it is very uncomfortable to use because of very short eye relief. I find Plossls with FL < 9mm too uncomfortable to use. You would be better off using a Barlow lens on a longer FL EP instead.

For planets you could often use more magnification than your 10mm EP gives you, and many DSOs are better viewed at magnifications somewhere between 50x & 125x you have with the 25 & 10mm. So if you want a bit more versatility, I would recommend you get a 15mm Plossl and a 2x barlow (which effectively halves the FL of an eyepiece, i.e. doubles the magnification). This would give you a very good spread and selection of magnifications.

Edit: Sorry, I see you have a barlow already. Good! To use it stick it in the focuser and the EP in the barlow. It doubles the magnification (if it is a 2x barlow, which is the most common).

[mickoking]

G'day Jo,


As the progress thru this fantastic hobby you will (probably) gravitate towards a particular aspect of Astronomy, this more than anything will define your E/P selection. My interest is mainly deepsky so my E/P collection is biased towards longer focal lengths and I dont own a barlow. 25mm and 10mm are a good start with the 25 being ideal for a lot of deep sky work. cheers

[Miaplacidus (Brian)]

I have to agree with Micko. I started with a 10mm and a 25mm, and as is the way of things, inexorably accumulated more and more expensive eyepieces of pretty much every focal length. Now I'm in the process of gradually whittling them down. More and more I use only two eyepieces. You guessed it, 10mm and 25mm. (Although admittedly they're not the same ones.)

Cheers,

Brian.

[jninness]

Hi there,

Thanks for your help and suggestions. I'm getting a 8" telescope which hasn't arrived yet so I don't know yet which lenses it comes with. I had to return the others when I returned the other telescope in exchange for a bigger one.

[RAJAH235]

Keep us posted. L.