Barlow lenses?!

[MrsAppleton]

What the devil are they?! After reading and reading posts on here and a astronomy magazine, I still can't work out what they are.

If I google it, the explaination of them is so confusing...... I am blonde and a female so you can see my issue!!! Lol!

Can anyone explain it to me in simple terms?

I have a celestron astromaster 130EQ. My lenses are 10mm, 20mm erecting eyepiece, 15mm Kellner eyepiece, 6mm plossl eyepiece, 2x power lens. I also have a red, blue and moon filter.

Would a Barlow lens benefit me?

Also, is it worth getting a 30mm lens? I have seen these mentioned also.

Hope someone can help me or point me in the right direction!

Thank you

Gemma

[Blazar]

Morning,

By the looks of the equipment you have, you already have one (the x2 power lens). A barlow just extends the effective focal length of the telescope. SO depending upon which eyepiece you put into the x 2 power lens, then that will double it. you can get other powers, but you reach a limit of what each scope s capable of. a usual rule of thumb is x50 per inch of aperture. so my Edge 800 should be able to go to around x400..though on most nights in England we cannot go above x250 due to seeing conditions.

Does that help? Might be a good thing to work out your magnifications with your EP's, and then the barlow will give you other options.

Regards

[Blazar]

So your Astromaster has a focal length (as advertised) of 650mm (distance from mirror to where light is focused)

To work out your magnifications, divide the Focal length (650mm) by the size of the EP (also in mm):

650mm / 20mm = x32 magnification

650mm / 15mm = x43 mag

650mm / 10mm = x65 mag

650mm / 6mm = x108 mag

so what the barlow does if x2 is to take the 650mm and double it. Then do the math again:

1300mm (double the effective focal length) / 20mm = x65 mag

1300mm / 15mm = x86 mag

1300mm / 10mm = x130 mag

1300mm / 6mm = x216

A lot of people just double the initial magnification though ;-) if the barlow was x3 then it would be 650mm x 3 (1950mm) and then divid by the EP mm size.

I think you have a good range of powers there bearing in mind the x50 mag per inch of aperture (your scope has a 5 inch mirror...5 x 50 = x250 mag on the best of nights.), though the EP's that come with the scope may not be the greatest of quality. So my advice would be to use the scope and enjoy it for now, and look into some mid priced eyepieces in a few months of so. Do some reading up on here (or ask questions).

If you are new to stargazing, I would recommend the book Turn left at Orion. It has a lot of basic telescope maths that may help you, as well as showing you where to find objects in the night sky.

[RichM63]

That 2x Power lens will be a 2x Barlow. You might be able to use it with all your exisitng EPs except for the 6mm, where the magnification will be very dependant on seeing conditions.

However, the quality of the 2x Power Lens could be somewhat dubious if it is an all plastic affair. I have one that came with a telescope and its only use is as an empty tube.

A good Barlow can extend your eyepiece ranges without buying extra eyepieces. Personally I see no benefit in buying one in addition to what you already have.

A 30mm eyepiece would give you x22 magnification with that telescope and should be OK for large DSO's (deep sky objects), but again it would be a personal choice thing and not a requirement.

[ghostdance]

Blazar has it I think - your 2× is your Barlow, so your 15mm becomes a 7.5mm when you use the 2× between the eyepiece and the focuser, 10mm becomes 5 etc. There can be a tradeoff in terms of some lightloss, clarity when your go to higher magnifications, but Barlows are handy when viewing planets as they make em bigger - not always better tho! I think they're a good thing to have when you need one....

As to the 30mm, that will give you a low magnification so good for looking at and for some DSOs, star clusters like the Pleiades etc. Have a look in FLO in eyepiece section. For example, the Vixen NPL 30mm is *very* nice at a good price, but there's squillions of choices....

Happy hunting in those Cyprus skies - hope they're generally clearer than ours!

[ronl]

Try this site, http://swindonstargazers.com/beginners/eyepieces.htm it will help you

to understand eyepieces, it's a very good guide, and well worth

bookmarking for future reference.

[Kriss Craik]

Book marked, thanks, really useful.

[Charic]

Blazar has it I think - your 2× is your Barlow, so your 15mm becomes a 7.5mm when you use the 2× between the eyepiece and the focuser, 10mm becomes 5 etc. There can be a tradeoff in terms of some lightloss, clarity when your go to higher magnifications, but Barlows are handy when viewing planets as they make em bigger - not always better tho! I think they're a good thing to have when you need one....

As to the 30mm, that will give you a low magnification so good for looking at and for some DSOs, star clusters like the Pleiades etc. Have a look in FLO in eyepiece section. For example, the Vixen NPL 30mm is *very* nice at a good price, but there's squillions of choices....

Happy hunting in those Cyprus skies - hope they're generally clearer than ours!

In reality a 15mm  still remains a 15mm?  A Barlow's  affect is  on the focal length of the telescope, not the eyepiece.

A Barlow doesn't  magically reduce any eyepiece to anything smaller than it already is? Fit the 15mm and you get 43x Power,  Barlow a 15mm and you get 86x Power, but its still a 15mm EP!

A Barlow  will however  maintain some of the eye-relief, so 2x Barlowing the 15mm to get 86x  could be more beneficial than using a 7.5mm eyepiece alone, if eye-relief is important.

[ghostdance]

You may well be right Charic - but with Barlows I tend to think in terms of eyepiece mm rather than scope focal length - so a 2× Barlow makes an F5 scope an F10, F11 an F22. Ish, at any rate...

I agree about your eye relief point - can be useful if needed.

[Charic]

......me too, ghost, that's how I understood it before,  ie. half the EP focal length!  so my 12 becomes 6? 

Too much  theory,  that's my problem, and what with this weather, what else too do? - I learn to much,  its just remembering / recalling that takes time, just glad we have the web

Even discovered  a little about Don Van Vliet recently ? 

[MrsAppleton]

Thank you all for your help and advise. I've still had to read your messages numerous times to understand it all, but your way of wording things has made it so much simpler!!

I knew this site would be helpful......!

[Daveb3650]

Simply double if a x2 ...or triples if a x3