Barlow or small lens?

  • Turlock
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17 years 4 months ago #41100 by Turlock
Barlow or small lens? was created by Turlock
When looking through my Celestron 6" reflector at Saturn it shows up absolutly tiny through my smallest lens (10mm) , what do people reckon would be better to magnify it more? a 4mm lens / 2x Barlow / 3x Barlow?

I'd also like to see the moon a bit closer so keep that in mind...


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17 years 4 months ago #41103 by dave_lillis
Replied by dave_lillis on topic Re: Barlow or small lens?
Hi Turlock,
You need to be careful with barlows and Newtonian scope, the design of the Newtonian makes them Barlow unfriendly in the sence that you might not be able to reach focus using one. Try borrowing one of someone nearby to test this before hand.
A 10mm is quite short, the smallest I use is a 9.7, although I do have a 6.4which gives me a magnification of >450x.
I'd advise to forget the barlow and get more eyepieces, make sure you get ones with good eyerelief, something uncommon with short focal length eyepieces.

How much of the FOV does the moon take up with the 10mm eyepiece ?

Dave L. on facebook , See my images in flickr
Chairman. Shannonside Astronomy Club (Limerick)

Carrying around my 20" obsession is going to kill me,
but what a way to go. :)
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17 years 4 months ago #41104 by voyager
Replied by voyager on topic Re: Barlow or small lens?
I honestly can't advise based on that.

What matters here is magnification and how much a telescope magnifies is not determined by just the eyepiece. A 10mm eyepiece will give a different magnification on different telescopes. I did up a wee piece explaining it on my blog: www.bartbusschots.ie/blog/?p=158

That should help you figure out what magnifications you have at your disposal and the people will be able to give you better advice.


My Home Page - www.bartbusschots.ie

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17 years 4 months ago #41105 by philiplardner
Replied by philiplardner on topic Re: Barlow or small lens?

I'd advise to forget the barlow and get more eyepieces, make sure you get ones with good eyerelief, something uncommon with short focal length eyepieces.

Not sure I'd agree with that! There are two ways to increase magnification: use an eyepiece with a shorter focal length or use a barlow to double or tripple the mag using an existing eyepiece. A major benefit of using a barlow is that you retain the eye-relief of the longer focal length eyepiece. A shorter focal length eyepiece will almost always have a much smaller eye-relief (you have to get your eye *much* closer to the lens) Only the more expensive eyepiece designs have equally long eye-relief.

Also - a barlow will double the number of effective eyepieces in your arsnel. A single new eyepiece will only increase that number by one!

Turlock - if you let us know the focal length (or the f-ratio) of your scope, we/I can suggest a range of suitable eyepieces and barlow combinations to give you the best selection of magnifications.


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  • DaveGrennan
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17 years 4 months ago #41106 by DaveGrennan
Replied by DaveGrennan on topic Re: Barlow or small lens?

Dave L is right about checking that you can reach focus with a barlow. However I've never seen a newt that wouldnt reach focus with one. In theory the less items you have between your target and your eye the better, so that would seem to suggest that a new eyepiece would be the thing to go for. However remember that a barlow effectively doubles your set of eyepiece because each one now has two values. With a 6" reflector you shouldn;t expect to go much beyond 200x magnification forget the 50x per inch rule, realistically thats not gonna happen. Your 10mm eyepiece gives 75x on that scope. (750mm focal lenght / 10mm = 75x). Thats not enough power for planetary viewing really. A 2x barlow would bang that up to 150x which sounds just nice for a 6" scope. A 3x barlow would be too much IMO.

Where do you live? If your around Dublin at all you can borrow my 3x barlow if you like. No harm in trying. I can lend you a 6mm eyepiece to to see which one you prefer.

Regards and Clear Skies,

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17 years 4 months ago #41107 by jhoare
Replied by jhoare on topic Re: Barlow or small lens?
It's not so much that you can't rely on the rule of thumb for aperture (50xOD in inches, 2xOD in mm) as that in most of Ireland we don't get more than a few nights a year when seeing supports magnifications beyond 200-240x regardless of aperture size. There are a few places in the mountains where you might be able to use 300x or higher more often with the right scope, but most of us don't live there.

Always remember that what eyepieces people find useful depends entirely on the focal length of their telescopes. For example, a 152mm Newt with a focal length of 762mm (f/5) needs a 4mm eyepiece to achieve 190x or 3.5mm to reach 218x, which is as far as I'd try to push magnification with any scope where I live. On the other hand a Celestron C8 SCT has an aperture of 203.2mm and a nominal focal length of 2032mm (f/10) and gives 203x with a 10mm eyepiece.

The rule of thumb for aperture isn't hard and fast either, it varies depending on the instrument. On a really good night I can use a 2.5mm eyepiece in my f/6.8 Televue Ranger (OD 70mm, f/l 480mm) to get 192x, but I'm more likely to use that eyepiece during the daytime (scopes are not exclusively for astronomy). If I took the Newt into the mountains for a night I'd bring that eyepiece along too because it might be possible to use 304x, but I wouldn't even take it out of the box if I was set up in my own garden because the seeing limits magnifications to around 200x.

Aperture rules of thumb for limiting magnification of various
telescopes(objective diameter OD stated in millimetres):
Newtonian, SCT or Mak, average collimation: 2xOD
Achromatic refractor: 2xOD
Newtonian, SCT or Mak, precisely collimated: 2.35xOD
ED refractor: 2.35 to 2.6xOD depending on model
Apochromatic refractor: 2.5-2.7xOD depending on model

Good quality small refractors used as spotting scopes in daylight can handle more magnification than is stated above for astronomy. A small ED or Apo refractor should be able to push about 3xOD or so but the higher the magnification the shorter the distance at which it will give a clean, sharp image. Small SCTs and Maks also make useful spotting scopes but may not be quite as sharp as a good refractor. Achromatic refractors generally won't do better than 2xOD without showing false colour when used as spotting scopes.

Rough rule of thumb for limit of magnification by average
seeing conditions (you need to refine this based on your
own experience in your locality but it's a useful starting point):
Sea level: 200x
Increases by 10x per 100m of elevation for higher altitude.

200x may be too much for your telescope, see the rules of thumb for aperture. This rule of thumb is for average seeing conditions on a clear night. There will be some nights when it's better and whether or not you can justify investing in an eyepiece that will give you higher magnifications on those nights is up to you, but it is a good idea to have 3-4 eyepieces to hand that can give you a range of lower magnifications so that you can still get some observing time in on nights with poor conditions not to mention having the capability to get nice wide angle views of large DSOs, asterisms and the Moon.

If you live in the vicinity of a bog, fen or river you should expect the seeing limit to be reduced depending on weather conditions and wind direction.


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