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When barlow lenses are useful/useless, beginner question

PostPosted: Wed Jul 12, 2017 4:44 pm
by Marius
Hello everyone!

I am completely new to astronomy so this question will probably be both easy and a bit dumb. I recently received a Celestron travel scope 70, a fairly weak telescope for a good price in my opinion. I am happy with what it does so far, but I have the possibility of getting a barlow lens (x3) to use with it.
Now my question is wether this lens is even usefull in the first place. I heard that trying to squeeze too much power out of fairly weak telescopes just leads to problems. The travel scope 70 has a 70mm aperature and a focal lenght of 400mm. I have 4 eyepieces. A 4mm one, a 10mm one, and a 20mm one.
I don't know if these specifications are of any help, but here they are anyway.
Thank you for your time!

Re: When barlow lenses are useful/useless, beginner question

PostPosted: Wed Jul 12, 2017 10:04 pm
by Gfamily2
Marius wrote:Hello everyone!

I am completely new to astronomy so this question will probably be both easy and a bit dumb. I recently received a Celestron travel scope 70, a fairly weak telescope for a good price in my opinion. I am happy with what it does so far, but I have the possibility of getting a barlow lens (x3) to use with it.
Now my question is wether this lens is even usefull in the first place. I heard that trying to squeeze too much power out of fairly weak telescopes just leads to problems. The travel scope 70 has a 70mm aperature and a focal lenght of 400mm. I have 4 eyepieces. A 4mm one, a 10mm one, and a 20mm one.
I don't know if these specifications are of any help, but here they are anyway.
Thank you for your time!


Hi Marius - welcome. You're right that you can overstretch a telescope by using too powerful an eyepiece with it, but in your case the Barlow could be of use.
I don't know exactly which eyepieces you have; but as you may well have noticed, for normal priced eyepieces, the eye lens of a lower power eyepiece is bigger, which makes it easier to use. So, my suggestion would be to try out the Barlow with the 20mm eyepiece - as this will give you a magnification nearly as much as that that given by the 4mm eyepiece, but it should be much easier to get your eye in the right place.

Personally, I have only used 2x Barlows, but I know people that use 3x ones.

As for your scope, I bought one as a gift for a family member so I had to try it out before wrapping it (as you do), and I thought it gave quite good results given the size and price. It will benefit from a better tripod if you have one, but it's a nice scope and very portable.

If you're planning on developing astronomy as a hobby, you may find the scope a bit limiting, but the eyepieces and Barlow will be usable in any scope that has takes 1.25" eyepiece, and the 70mm Travelscope will always be a quick and easy to take a scope out into the field.

Re: When barlow lenses are useful/useless, beginner question

PostPosted: Mon Jul 31, 2017 7:31 pm
by The Man with the Corrugated Iron Roof
For your telescope, I would use the 3X Barlow lens with the 10mm eyepiece to achieve a magnification of 120x. With the 4mm eyepiece, it gives magnification of 300x, rather too much. As a comparison, I once used 600x magnification on a slightly smaller telescope and Venus appeared as a comet, it was so bad.

There are some "rules" about maximum magnification and telescopes but there are some exceptions. For a full explanation, you can read the following book:

http://www.philippughastronomer.com/The ... copes.html

I would not try this on your telescope but I have used 2X and 3X Barlow lenses together to image the Moon and Jupiter with my telescope and DSLR.

Re: When barlow lenses are useful/useless, beginner question

PostPosted: Mon Nov 27, 2017 10:21 am
by EIZO
There is no such thing as a "dumb" question

Re: When barlow lenses are useful/useless, beginner question

PostPosted: Thu Nov 30, 2017 2:58 pm
by The Man with the Corrugated Iron Roof
EIZO wrote:There is no such thing as a "dumb" question


Totally agree!! We're all still learning.

Re: When barlow lenses are useful/useless, beginner question

PostPosted: Thu Nov 30, 2017 4:31 pm
by Aratus
People do tend to be wary posting questions on forums, because a lot of them are frequented by impatient experts who have forgotten what it was like to start from the beginning (which they all did!)

There are no such people on this forum. People can ask the most basic questions without fear :) :geek:

Re: When barlow lenses are useful/useless, beginner question

PostPosted: Thu Dec 07, 2017 10:27 am
by EIZO
A1 Aratus

Re: When barlow lenses are useful/useless, beginner question

PostPosted: Sun Jan 14, 2018 8:51 pm
by The Man with the Corrugated Iron Roof
Aratus wrote:People do tend to be wary posting questions on forums, because a lot of them are frequented by impatient experts who have forgotten what it was like to start from the beginning (which they all did!)

There are no such people on this forum. People can ask the most basic questions without fear :) :geek:


... and just when we think we are experts, we do something dumb like forget to remove the cover from a telescope objective or spend ages finding an object we have seen zillions of times already!

Astronomy is a great leveller at times!