A beginner in Amateur Astronomy often misunderstands the concept of Barlow lenses and their use.Below is my take on the topic. I have cut down quite a bit of information and the information is meant for the absolute novice. The reason I have chosen a Celestron 114mm reflector as an example. I do not intend to support the company or Malign their quality in any way, and it is therefore a mere example.
Barlows are used to increase the focal length of the telescope, thereby increasing the magnification obtained. Every telescope has its upper limit, and from a city/town the upper limit for a Newtonian reflector telescope is 35x per inch. Consider a telescope has 4.5 inch mirror (114mm). With this you can get a max of 157x magnification. That 20mm erecting eyepiece supplied with many such telescopes is useless, so its best thrown away. This telescope has a built in barlow inside the focuser. This is not a good thing and is found in cheap quality telescopes. Adding another barlow to it only degrades the view very much.
Now secondly, as far as viewing deep sky objects is concerned, these objects are faint. To see them clearly, you need maximum amount of light entering the eye. In other words, the field of view of the telescope has to be maximum. To obtain maximum field of view, you need minimum magnification. That is the focal length of the eyepiece has to be as small as possible, preferably a 30 or 32mm eyepiece will work. If the user uses a barlow and assumes that he/she can see deep sky objects bigger, then the user is wrong. A 2x barlow reduces the field of view by half and a 3x barlow is even worse. With the above telescope I will not recommend a barlow at all. For high magnification, one should purchase a 4mm plossl eyepiece, for low magnification a 32mm plossl eyepiece, or any focal length close to that. Together these will cost about 5000 Rs in India or $40-$50 in USA, but will be useful on any telescope.
I have deliberately answered the question of Barlows with reflectors, but they apply towards Refractors and cassegrains as well without any change.
A good video on Barlows and their use can be seen below.
Barlows are used to increase the focal length of the telescope, thereby increasing the magnification obtained. Every telescope has its upper limit, and from a city/town the upper limit for a Newtonian reflector telescope is 35x per inch. Consider a telescope has 4.5 inch mirror (114mm). With this you can get a max of 157x magnification. That 20mm erecting eyepiece supplied with many such telescopes is useless, so its best thrown away. This telescope has a built in barlow inside the focuser. This is not a good thing and is found in cheap quality telescopes. Adding another barlow to it only degrades the view very much.
Now secondly, as far as viewing deep sky objects is concerned, these objects are faint. To see them clearly, you need maximum amount of light entering the eye. In other words, the field of view of the telescope has to be maximum. To obtain maximum field of view, you need minimum magnification. That is the focal length of the eyepiece has to be as small as possible, preferably a 30 or 32mm eyepiece will work. If the user uses a barlow and assumes that he/she can see deep sky objects bigger, then the user is wrong. A 2x barlow reduces the field of view by half and a 3x barlow is even worse. With the above telescope I will not recommend a barlow at all. For high magnification, one should purchase a 4mm plossl eyepiece, for low magnification a 32mm plossl eyepiece, or any focal length close to that. Together these will cost about 5000 Rs in India or $40-$50 in USA, but will be useful on any telescope.
I have deliberately answered the question of Barlows with reflectors, but they apply towards Refractors and cassegrains as well without any change.
A good video on Barlows and their use can be seen below.