Barlow lens ?

[Dazraz]

Hi new to this forum and stargazing as some member may already know. My telecope came with a super 25 wide angle lens and a super 10mm lens. Could anyone tell me what a barlow lens is ? could i do with one ? what do they do ?

And also could i do with any other lenses and can i get a lens that makes me get closer view than what lenses i already have ?

Many thanks for all replies

[Matthew.Blake]

The effectively increase the focal length of your telescope so you can get twice as many different magnifications with your eyepieces. For example if you have a 2xBarlow you could use your 10mm to get the same magnification as a 5mm eyepiece but still have the same eye relief as the 10mm so you don't need to get so close to the eyepiece to have a good view.

[DeanWatson]

Hi Daz

I don't know if you know already (forgive me if you do...) but you can calculate the magnification of an eyepiece by dividing its focal length (10mm, 25mm as you say above) into the focal length of your telescope (say, 750mm), in this case giving 75x and 30x respectively - A barlow would double this.

Bear in mind though that the more optical (lens) elements you have between you and the object you are looking at, the dimmer the image (although on deep sky objects this is sometimes useful as it darkens the background relative to the object.

[Dazraz]

Hi Daz

I don't know if you know already (forgive me if you do...) but you can calculate the magnification of an eyepiece by dividing its focal length (10mm, 25mm as you say above) into the focal length of your telescope (say, 750mm), in this case giving 75x and 30x respectively - A barlow would double this.

Bear in mind though that the more optical (lens) elements you have between you and the object you are looking at, the dimmer the image (although on deep sky objects this is sometimes useful as it darkens the background relative to the object.

what barlow do i look for i have a 8" dob ?

[Merlin66]

You could start with a x2 barlow....Check out FLO's webpage...

[DeanWatson]

Oh, pretty much any barlow will work, they all just come in different qualities! You can get them in 2x, 3x, 4, 5x versions (multiples of your telescopes focal length/eyepiece magnifiaction although generally for anything higher than 2x try to get a better quality one (sadly i'm not an expert on the different brands but you won't go too far wrong if you get something, Meade, Celestron, Skywatcher etc).

You can get them in 1 1/4 inch or 2 inch diameter versions depending on the size of your focussed draw tube (although I expect your 'scope has a removeable adaptor that will take 1 1/4 or 2 inch oculars)

Hope all that helps Daz.

[Dazraz]

Oh, pretty much any barlow will work, they all just come in different qualities! You can get them in 2x, 3x, 4, 5x versions (multiples of your telescopes focal length/eyepiece magnifiaction although generally for anything higher than 2x try to get a better quality one (sadly i'm not an expert on the different brands but you won't go too far wrong if you get something, Meade, Celestron, Skywatcher etc).

You can get them in 1 1/4 inch or 2 inch diameter versions depending on the size of your focussed draw tube (although I expect your 'scope has a removeable adaptor that will take 1 1/4 or 2 inch oculars)

Hope all that helps Daz.

many thanks Dean

[DeanWatson]

You're very welcome Daz. You may find with your new dob that higher mags are generally best for the planets or smaller deep sky objects. Many of the best deep sky targets though (M31 the Andromeda Galaxy, M42 the Orion Nebula, M13 the Hercules Cluster etc) tend to be quite wide in size against the sky (M31 for example is 3x plus the width of the full moon but while one of the brighter deep sky objects is faint overall in comparison to the moon) and so you are better served by just using your 25mm eyepiece on this sort of thing.

[Dazraz]

The effectively increase the focal length of your telescope so you can get twice as many different magnifications with your eyepieces. For example if you have a 2xBarlow you could use your 10mm to get the same magnification as a 5mm eyepiece but still have the same eye relief as the 10mm so you don't need to get so close to the eyepiece to have a good view.

Hi im looking at buying this for my 8" dobsonian - can you tell me if it will fit please ?

http://cgi.ebay.co.uk/Skywatcher-Single-Lens-Barlow-x2-for-1-25-Telescopes_W0QQitemZ390058067530QQcmdZViewItemQQptZUK_Telescopes?hash=item5ad145c64a&_trksid=p3286.c0.m14

[Matthew.Blake]

It will fit. You should, however, look up some of the threads about the vendor - telescope planet - on this forum before committing to buy.