How to focus when using Barlow lens.

[Manas]

I have telescope with objective lens 75mm diameter and 700mm focal length. Eyepiece has 10mm. My Barlow has power of 3x magnification. When I trying to use Barlow lens the image got blurry. How to set the Barlow to get sharp focus ?

[Zermelo]

Hello, and welcome to SGL.

Are you saying that when you add the barlow, you can't get it it to focus, however far you move the focuser in or out?
Depending on the equipment, it's possible that you have moved the focus point beyond the distance that the focuser can manage.

Another observation: with that combination you will be getting a magnification of x210, which is probably beyond what you should attempt with a 75mm scope. That may also be contributing to your problem. Barlows with x2 power are more usual.

[AstroMuni]

17 hours ago, Zermelo said:

Another observation: with that combination you will be getting a magnification of x210, which is probably beyond what you should attempt with a 75mm scope. That may also be contributing to your problem. Barlows with x2 power are more usual.

You may have hit the nail on the head I recall that the calculation of max usable magnification is around twice the aperature in mm. That would indicate a maximum magnification of 150x.

[Louis D]

Do you have a 25mm or so eyepiece you could try in the 3x Barlow first to rule out simply pushing your optics too far?

[AstroMuni]

On 30/05/2022 at 17:54, Manas said:

How to set the Barlow to get sharp focus ?

Hi Manas,

If you are trying to view planets and trying to get the largest image out of your scope then try the suggestion by @Louis D Sometimes the blurriness (even at lower maginifications) is also a result of atmospheric disturbances and can vary depending on your viewing location. City viewing is not always the best 😞

[RT65CB-SWL]

Hi @Manas and welcome to SGL. 

A 3x Barlow lens is way to much for your 'scope. It will be a challenge in tracking and keeping the target in the e/p FOV too at the same time as well.

[McQ]

Little scopes need little Barlows, if they work at all. My Xcel 2x won't focus in a 60mm but others have used smaller cheaper barlows in the same scope.