Eye peice

[Adidaz 29]

Very new to this new hobby I got into  I put a 25 eye peice into my focus ser  and I never push the eye piece right down  but when I look through it the focus was bang on so why don't they come a bit longer but that was in the day on a roof top chimney not tried it on a night object has I always cloudy   Clear skies and pork pies

[dweller25]

Different eyepieces have different focus points, so it's always best to push the eyepiece all the way into the focuser and then focus using the knobs.

[Knighty2112]

With most eyepieces (EP's) there should be enough back focus and forward focus to achieve correct focus with the EP fully pushed into the telescope. The main reason you place all the barrel of the EP into the telescope is purely to make sure it is well seated to use so it doesn't fall out when you have the scope at a higher viewing angle. However there is the option that you can get small par focul rings to add to the barrel of any EP. This allows you to keep most of your EP's in the same focul range so that you don't have to adjust the focus too much. Never used these rings myself as to be honest there is no real issue in having to adjust focus with any EP really when you change it, but some people do use them.

BTW: the 25 size refers to the focul length of the EP, and is measured in mm's, so you normal state any EP used as for example as a 25mm EP, or  a 10mm EP etc. With this focul length of each EP, you can work out the magnification it gives in your telescope. For this you will also need the focul length of your scope, which the user manual should tell you, or should be printed perhaps on your telescope and this is measured in mm's again (look for something like fl=xxxx, where xxxx is the focul length of your telescope in mm). As an example with a telescope with a fl (focul length) of 1000mm's, all you need to do is divide your telescopes focul length by the EP's focul length. So if we divide 1000mm (telescope focul length) by 25mm (EP focul length) we get a magnification 40x. This is a low magnification and will give quite a wide view through the EP. If we use a 10mm EP in the same scope we get 1000/10 which gives us 100x magnification, this is a higher magnification, so the view will be smaller but more magnified, so shorter EP focul lengths give you greater magnifications, which in general are used for lunar and planetary views, but can be used on other objects too. One thing to note though is that the view through the EP becomes dimmer the more magnification you use.  

[ronin]

If you are not focusing on the stars/planets but a chinmey then the final image will be "outwards", so the eyepiece would have to not be all the way down, or more correctly will operate if they are not fully in the focuser.

The problem may be that when you come to looking at the actual sky the primary image from the main objective will be "inwards" so unless the eyepiece is fully in the focuser you may not manage to get an infocus image. Pointing at a chimney is not a good idea to judge the operation of the optics on, just too close to you.

Also different eyepieces, their designs, will have their object plane at assorted positions, some will be outside the eyepiece sme will be inside the eyepiece, so the eyepiece will have to be repositioned to suit and that means the focuser needs a fai amount of travel.

The other point is if an eyepiece is fully in the focuser then there is less chance of it falling out, even a £50 eyepiece hitting the ground is not a good experience, a £200 one is even worse.

List your scope if possible as that means if there are problems or points to be made it is easier/possible.

[Adidaz 29]

3 minutes ago, Knighty2112 said:

With most eyepieces (EP's) there should be enough back focus and forward focus to achieve correct focus with the EP fully pushed into the telescope. The main reason you place all the barrel of the EP into the telescope is purely to make sure it is well seated to use so it doesn't fall out when you have the scope at a higher viewing angle. However there is the option that you can get small par focul rings to add to the barrel of any EP. This allows you to keep most of your EP's in the same focul range so that you don't have to adjust the focus too much. Never used these rings myself as to be honest there is no real issue in having to adjust focus with any EP really when you change it, but some people do use them.

BTW: the 25 size refers to the focul length of the EP, and is measured in mm's, so you normal state any EP used as for example as a 25mm EP, or  a 10mm EP etc. With this focul length of each EP, you can work out the magnification it gives in your telescope. For this you will also need the focul length of your scope, which the user manual should tell you, or should be printed perhaps on your telescope and this is measured in mm's again (look for something like fl=xxxx, where xxxx is the focul length of your telescope in mm). As an example with a telescope with a fl (focul length) of 1000mm's, all you need to do is divide your telescopes focul length by the EP's focul length. So if we divide 1000mm (telescope focul length) by 25mm (EP focul length) we get a magnification 40x. This is a low magnification and will give quite a wide view through the EP. If we use a 10mm EP in the same scope we get 1000/10 which gives us 100x magnification, this is a higher magnification, so the view will be smaller but more magnified, so shorter EP focul lengths give you greater magnifications, which in general are used for lunar and planetary views, but can be used on other objects too. One thing to note though is that the view through the EP becomes dimmer the more magnification you use.  

Thank you. Vey much I getting to under stand a little better

[Adidaz 29]

6 minutes ago, dweller25 said:

Different eyepieces have different focus points, so it's always best to push the eyepiece all the way into the focuser and then focus using the knobs.

I do push it all the way in  but if I pull it out a little I become more focuses  than push all the. Way in  clear skies and pork pies

[Adidaz 29]

36 minutes ago, ronin said:

If you are not focusing on the stars/planets but a chinmey then the final image will be "outwards", so the eyepiece would have to not be all the way down, or more correctly will operate if they are not fully in the focuser.

The problem may be that when you come to looking at the actual sky the primary image from the main objective will be "inwards" so unless the eyepiece is fully in the focuser you may not manage to get an infocus image. Pointing at a chimney is not a good idea to judge the operation of the optics on, just too close to you.

Also different eyepieces, their designs, will have their object plane at assorted positions, some will be outside the eyepiece sme will be inside the eyepiece, so the eyepiece will have to be repositioned to suit and that means the focuser needs a fai amount of travel.

The other point is if an eyepiece is fully in the focuser then there is less chance of it falling out, even a £50 eyepiece hitting the ground is not a good experience, a £200 one is even worse.

List your scope if possible as that means if there are problems or points to be made it is easier/possible.

 

[Adidaz 29]

Thanks ronin I understand a little better

[Adidaz 29]

2 minutes ago, Adidaz 29 said:

 

It's a 10 in Orion xti  Dobsonian