Barlow 3x Focus problem

[LukeSkywatcher]

I think for a 130mm scope, the simple fact is that a 3x barlow is just too much magnification.

[Putaendo Patrick]

Patricia,

Which Explorascope have you got? In the range, Celestron do three refractors with apertures of 60, 70 and 80mm, all I think with focal lengths of 700mm. They also do a reflector with 114mm aperture, but I think a Jones-Bird design with 1000mm focal length.

In general the theoretical maximum magnification of a telescope is double the aperture in millimeters, beyond this the loss in image quality makes further magnification pointless. Supposing you are using your 10mm eyepiece alone in one of the refractors, you will get x70 magnification; triple this with the x3 Barlow and you get to x210 which is, as Luke says above, just too much.

These Barlows, usually with poor quality optics, are sold so manufacturers can claim higher magnifications and unfortunately many newcomers to astronomy think that a telescope with higher magnification is better. The reality is this is completely false! In addition to how much magnification a scope may theoretically give, in practice atmospheric conditions limit even the very best telescope. In the UK it is difficult to go much beyond about x200, maybe a little more on the Moon, except in very remote sites where the sky is really dark

Just to give you an idea of misleading publicity, I quote Argos.co.uk on the 114mm reflector Explorascope: "Maximum magnification max theorectical magnification is 750x with a 3 x barlow (supplied) or 250x maximum magnification without the 3 x barlow."  Not even in their most fantastic dreams does anyone here achieve that sort of magnification.

 

[tripod tipper]

Nebula,

 and all participants to this thread, please scrutinize my reply as its been a while since I've had to think about such a topic, I'm trying to re learn the barlow myself. 33 years ago it was a rule of thumb to me that the aperture X 2 = the max practical magnification and the barlow magnified without loosing much of the viewing angle at lower power eyepieces. The higher mag of the eyepiece with the barlow also magnifies any flaws. I've heard barlows are best for deep sky especially if your looking at star clusters and nebula's. The higher mag on planetary objects might not focus properly. I did replace the eyepieces I got with the scope I just bought with a 2X barlow and various powered eyepieces and filters of better quality. If I'm wrong, please correct me.