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Barlow or small lens?

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17 years 5 months ago #41108 by dave_lillis
Replied by dave_lillis on topic Re: Barlow or small lens?

Turlock,

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.


There was definitely a post in the last year or so where someone could not reach focus with their barlow using their newtonian, I don't know who or when.
Using one for visual stuff is fairly subjective, some like them others dont, I dont, but I do use them for planetary webcam imaging.

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. :)
+ 12"LX200, MK67, Meade2045, 4"refractor

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17 years 5 months ago #41134 by Turlock
Replied by Turlock on topic Re: Barlow or small lens?
Cheers for all the replies lads, first off I've a Celestron 6N which has a 750mm focal length (what would that be as f/x?)

I still amn't too sure what to go for but in reality the long term answer seems to be to get both :P

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17 years 5 months ago #41135 by michaeloconnell
Replied by michaeloconnell on topic Re: Barlow or small lens?
Focal length = 750mm
Aperture = 150mm (to use the nearest round number metric equivalent)
Therefore, the focal ratio or f/x of the system is f/5....(750 / 150)

To get 200x magnification: 750 / 200 = 3.75. So, you'd need a 3.75mm focal length eyepiece to get 200x magnification.

Realistically, you will have difficulty getting an eyepiece of such focal length. Eyepieces of very short focal length can be difficult to use due to very short eye relief - you might need your eyeball squished up against the eyepiece lens to see anything! ;-) Personally, what I would do is try out a good quality barlow with an eyepiece which would give roughly 200x.

If you use a 2x barlow: 750 x 2 / 200 = 7.5mm

If you use a 2.5x barlow: 750 x 2.5 / 200 = 9.4mm etc.

Hope this helps.

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