Barlow lens or new eyepiece

[Brauman7]

I have a 10 mm eyepiece and wanted to increase my 130 mm telescope's magnification for under $100. Should I buy a 3.2/4 mm eyepiece or get a 2.5/3 x barlow lens?

[Carbon Brush]

The answer to some extent depends on the scope and eyepiece you already have.
The eyepiece supplied with a package is often mediocre.
Then depending on the focal length of the scope, is higher magnification realistic?
This is obviously limited by the scope, but also how many nights of the year the sky will take a hign magnification.

 

Tell us what scope you have and your expectations then we can provide a better answer.

 

Oh yes, and welcome to the forum.

 

[Brauman7]

12 hours ago, Carbon Brush said:

The answer to some extent depends on the scope and eyepiece you already have.
The eyepiece supplied with a package is often mediocre.
Then depending on the focal length of the scope, is higher magnification realistic?
This is obviously limited by the scope, but also how many nights of the year the sky will take a hign magnification.

 

Tell us what scope you have and your expectations then we can provide a better answer.

 

Oh yes, and welcome to the forum.

 

I have a Celestron 130az, it has an aperture of 130 mm and a focal length of 650 mm. I got two eyepieces with it, a 10 mm and a 25 mm

[Carbon Brush]

Hi.

 

I have not had a hold of this particular scope so am relying on limited published information/specifications.
Celestron do not for example even state it has a parabolic mirror. But surely it has.

Apologies if you know all of the following already, but here goes.

 

As a 'rule of thumb' the maximum magnification is around double the mirror diameter.
You can get 260x magnification, with the right eyepiece.
Depending where you live, it is fair to say a lot of nights, the sky is not steady enough to allow 260x magnification.

 

The next problem is whether you can use high magnification.
The object does not stay in view for very long and an alt-az mount involves chasing using both controls.
An EQ mount would be better. Once aligned you only need turn one knob to follow.
This assumes the mount and tripod are rigid, often not the case on kit packages.
Manufacturers usually include the smallest most flimsy mount/tripod they can get away with.

 

Magnification calculation is scope focal length, divided by eyepiece focal length.
This indicates an eyepiece 2.5mm FL or longer is going to work. Subject to further restrictions.

 

The focal ratio is F5 (650/130mm) which means the cheapest eyepieces are going to give poorer performance near the edges of the field. The centre of view will be sharp, probably 1/2 to 2/3 of the field, then go soft towards the edges.
You will have seen that some eyepieces cost more than your complete scope! Performance costs money.

 

Manufacturers often include two eyepieces in their kits. Around 25mm and 10mm. A frequent comment is that the 10mm is marginally better than useless. Don't forget that regardless of the scope badge, the beginner scopes usually come from the same factories in China.

 

I would be tempted to buy an eyepiece around 5mm FL. BST Starguiders are reported by many as giving good performance for the ££ spent and well within your budget. Going above x100 you are going to get improved detail on Saturn's rings, Jupiter cloud belts, etc. This eyepiece should be usable on a lot of (clear) nights by not trying to go too high on magnification. Also, it is not too difficult to chase the object across the sky.

 

A barlow will not improve on the (problably poor performing) 10mm eyepiece.

 

Hope this is useful to you.

David.

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