How do I use a barlow lens?

[PeterMammoth]

At the expense of making myself appear stupid, I need to know what I'm doing wrong when using my barlow lens. The EPs are fine by themselves, but when I use a barlow between the EP and scope I can't bring the image into focus as it seems that the EP has moved beyond the focus range. Is there something else I need to do?

[barkis]

Which make of scope do you have, also is the Barlow a 2x one.

It seems like the focuser doesn't have enough inward travel to reach the image plane. Have you tried it with your lowest power eyepiece in the Barlow. The barlow has the effect of increasing the light cone from the mirror/lens, so if you can focus with an eyepiece on it's own, you should manage with both in use. Have tried focusing both ways?

Ron.

[PeterMammoth]

It's a 250PX Dob, and yes it's a 2x barlow. I think I only tried the 10mm EP. I'll try a lower power one next time. How do you mean focusing both ways?

[barkis]

To focus a high power eyepiece, you would turn the focus knob so that the eyepiece in the focuser moves towards the scope tube.

You turn it the opposite way to focus a lower power eyepiece.

Usually the knob if turned in a clockwise direction, will move inwards, anti clock will move it out. You should check that though, it won't apply in all cases.

You have to watch in the eyepiece as you do this, as the focus on any eyepiece is a very precise position, and you may go passed it if you are not carefull, so do it slowly

[barkis]

If you use a 10mm eyepiece in the Barlow lens, the magnification will be 250 times, so focus will be very critical. Very small focus movements will be needed to get it spot on. You will soon get used to the feel of it though.

Ron.

[John]

It's a 250PX Dob, and yes it's a 2x barlow. I think I only tried the 10mm EP. I'll try a lower power one next time. How do you mean focusing both ways?

Is it a 2 inch size barlow by any chance ? - I found that there was a rim inside the drawtube (where you put the eyepiece / barlow lens) of the Skywatcher focusser which stops certain 2" barlow lenses being inserted fully - the result being that you can't reach focus when using them

John

[PeterMammoth]

Is it a 2 inch size barlow by any chance ? - I found that there was a rim inside the drawtube (where you put the eyepiece / barlow lens) of the Skywatcher focusser which stops certain 2" barlow lenses being inserted fully - the result being that you can't reach focus when using them

John

I'm sure that it's a 1.25" - it's the same width as the EPs. I'll just have to give it another try. Perhaps I had the moon filter on the EP when it should have been on the barlow.

[TheThing]

Filter shouldn't make too much of a difference as it screws into the EP. Try the focusing trick above - it is very easy to go past focus.

If your focuser doesn't seem to come out far enough, try gently pulling the EP out of the focuser to see if that helps.

[PeterMammoth]

Problem solved. It was me being stupid as suspected. The cap was on very tightly and completely covers the lens section. This led me to think that you had to unscrew this part of the barlow lens. So I was actually looking through an empty tube. No wonder it failed to focus. You live and learn

[John]

Problem solved. It was me being stupid as suspected. The cap was on very tightly and completely covers the lens section. This led me to think that you had to unscrew this part of the barlow lens. So I was actually looking through an empty tube. No wonder it failed to focus. You live and learn

Don't worry - we've all been there

John

[starboy82]

Problem solved. It was me being stupid as suspected. The cap was on very tightly and completely covers the lens section. This led me to think that you had to unscrew this part of the barlow lens. So I was actually looking through an empty tube. No wonder it failed to focus. You live and learn

lol i did this last night took me a while to work it out