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Advice for novice please on focusing a Barlow lens


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Good evening all, advice for a novice please

Sat in the garden getting used to everything, no problem with the EP focus and also zooming in from the 20mm to the smaller ones but when I am trying to use the Barlow lens I can't seem to get it to focus, even on the moon.

What am I doing wrong, anyone got any ideas?

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Ensuring that the focuser is moving in and out, place the Barlow into the focuser then pop the 20mm into the Barlow.
Depending on the focal length of your telescope, which determines the magnification, Telescope focal length divided by eyepiece focal length equals power/magnification.  If you own a 20mm and a 10mm eyepiece, and use a 2x Barlow, it allows the 20mm to be used  as if its a 10mm.

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A barlow needs more in focus than an eyepiece by itself. In some telescopes there may not be enough focuser travel to reach focus. A star or planet will be at its smallest when focused and larger either side of focus so you can check whether the telescope is going from one side of focus to the other without producing a reasonable image (if as you wind the focuser in the object gets smaller and then bigger) or if it never reaches focus (the object keeps getting smaller until you cannot wind the focuser in any further). 

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Thanks for the replies, I have a Nat Geo Bresser 60/700 with a 1.5 Barlow and 20mm, 12.5mm and 4mm EP. Are you saying I possibly wont be able to use the Barlow because of the limitations of my scope?

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2 minutes ago, pjg said:

Thanks for the replies, I have a Nat Geo Bresser 60/700 with a 1.5 Barlow and 20mm, 12.5mm and 4mm EP. Are you saying I possibly wont be able to use the Barlow because of the limitations of my scope?

Is that the newtonian reflecting scope, like this ?:

https://www.telescopehouse.com/national-geographic-76-700-az-telescope.html?gclid=CNCe6O-K4NMCFe-87QodmKEI4w

 

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7 minutes ago, pjg said:

Thanks for the replies, I have a Nat Geo Bresser 60/700 with a 1.5 Barlow and 20mm, 12.5mm and 4mm EP. Are you saying I possibly wont be able to use the Barlow because of the limitations of my scope?

I had a 76mm scope and couldn't use a 10mm with it..so trying to use a Barlow would of been impossible...

Your seeing conditions will deteriorate  the image more than the power of the scope..

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pjg.......excuse me if this is real basic stuff, but you can't use a Barlow on its own, it must be used in addition to the eyepiece.

You mention that you had no problems with the other lenses, just the Barlow, hence my basic question?

Put the Barlow into the focuser, stick the 20mm eyepiece into the Barlow, then try again. 

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this is my scope if it helps;

www.telescopehouse.com/national-geographic-60-400-az-refractor-telescope.html?gclid=Cj0KEQjwrsDIBRDX3JCunOrr_YYBEiQAifH1Ftt1Ne3-LNb7oLbJMwmLc51Rpws9i8z5bypMHTsfxCwaAkeT8P8HAQ

 

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As in my previous post you will have to work out by winding the focuser in and out whether the image is just rubbish at the point of best focus or whether it never reaches focus. In either event the answer is probably to not use the barlow.

According to the link you should have:

  1. H20, H12.5 and SR4 eyepieces
  2. 1.5X Erecting lens
  3. 3X Barlow

Note that this is slightly different to the 1.5X barlow that you have talked of. You need to know whether it is a 1.5X barlow or erecting lens as these are different.

Now your telescope has an aperture of 60mm and a focal length of 700mm resulting in a focal ratio of approximately 12.

This means that for planetary viewing the shortest focal length eyepiece you can use is going to be in the 10-12mm range and for splitting double stars perhaps you can push it as far as a 6mm if you are lucky, Comparing these numbers with what you have been supplied with we can immediately see that the 4mm eyepiece is too short to ever be of use. The 3X barlow is only possibly of use with the 20mm eyepiece, but a low quality barlow will degrade the image. Given the H and SR designations of the eyepieces signifying that the eyepieces are as cheap as possible, we can safely assume the same is true of the 3x barlow and that even the 20mm eyepiece/3x barlow combination is not worth trying. Unfortunately, both the 4mm eyepiece and 3X barlow have only been included as a marketing ploy so that "525X magnification" can be written on the box, which is well beyond the useful capabilities of the telescope.

The 1.5X erecting lens is included so that you can use your scope for terrestrial viewing with the image flipped the right way up. However, I believe that the erecting lens must be used without the diagonal mirror as shown in this picture:

file.jpg

If it is an erecting lens and not a 1.5X barlow then this would explain why you cannot reach focus with both the erecting lens and diagonal connected. Terrestrial viewing has similar limits as planetary viewing so I think that only the 20mm eyepiece will give satisfactory results when combined with the erecting lens, although it may be that at 52.5X, even this is too much magnification.

So in summary I suggest that you only use the 20mm (35X mag) and 12.5mm (56X mag) eyepieces for astronomical viewing. I'm sorry that this is significantly lower than the 525X the box claimed you could achieve.

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