Does a Barlow effectifly extent my useful maximum magnification?

[Luna-tic]

On ‎9‎/‎11‎/‎2017 at 04:05, THL22 said:

 

Lets say my scope has an focal length op 1800mm and a diameter of 150mm. Maximum useful magnification with this scope should be double the diameter. So 300x

For instance if I use a 10mm eyepiece I'll be a 180x magnification (1800/10)
But with a 5mm eyepiece i'll reach 360x (1800/5) this would overwrite my useful maximal magnification right?

But what if I use a barlow of 2x. Thus doubling my focal length. in this case to 3600. My maximum useful magnification will still be 300x.
Can I now use a higher power eyepiece? Like 5?

 

Rather than thinking of the Barlow doubling the f/l of the tube, think rather, of it halving the size of the EP (15mm EP w/2X Barlow = 7.5mm EP) Then, keeping in mind the max useful magnification doesn't change, pick out the EP's you'd like to have, considering the Barlow will double the magnification each one gives by itself.  So, you wouldn't buy both a 30mm EP and a 15mm EP, since the 30mm + 2X Barlow would give essentially the same view as the 15mm by itself (with a few provisions, partly depending on the quality of the Barlow.)

You know you can also unscrew the lens from the bottom of a 1.25" Barlow's tube, and screw it on the bottom of your EP, and get a 'partial' magnification, usually about 1.5X on a 2X barlow. The distance of the Barlow lens from the EP bottom lens will determine the overall magnification you get from the Barlow. Further away gives more mag. You can even install the Barlow between the tube and diagonal in some cases, depending on how much back focus you have available. In most any case, the view through a Barlow won't be as good as without one. Dimmer, for one thing.