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Trouble with my Barlow


johnnytomorrow
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Can anyone help? Only been observing since Christmas so I am a total noob.

Just arrived home and the skies were clear as a bell so out I went with my small Dobson.

This is the first time since Christmas I've been able to view the moon through it so that was my first stop - stunning.

Next I managed to locate, what Star Walk reliably informs me was Aldebaran (HR 1457) a beautiful orange star and from there to Jupiter and it's moons, unfortunately too soon to see the red spot.

Then I thought I'd try out the Dobson eyepiece which came with the scope. Now I admit that this is a cheap scope (I was bought it as a present at Christmas) and as I have previously said I am sure to be upgrading sooner rather than later. Generally I am really chuffed with it apart from the Barlow eyepiece that came with it.

All of the other eyepieces work perfectly but as soon as I combine them with the Barlow I cannot get a focus at all.

My question is simply, is it the fact that this is a low quality Barlow or am I missing something?

Any help appreciated.

JT

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Have you gone through the complete range of focus adjustment in both directions ? It can be very easy to miss the correct point if you are not concentrating especially when your new to it.

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Have you gone through the complete range of focus adjustment in both directions ? It can be very easy to miss the correct point if you are not concentrating especially when your new to it.

I have Shaun, I move in the direction where the focus starts to sharpen but I reach the limit of the focus wheel before it achieves it

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Hello and congrats on your first scope.

Generally speaking, anything included with a cheap scope is bound to be cheap, included the barlow. It should still work though.

What is the focal length of the included eyepieces?

On to your problem. The most likely cause is probably what Pig said. I'll suggest a few more options.

If you really can't get it to focus with the barlow then something is certainly wrong. One possibility is that it's simply defect.

Could you describe the "out of focus" view? does it look exactly like when you defocus the view with only an eyepiece in or does it look different?

Bear in mind that when combining a barlow with a short focal length eyepiece it's possible to exceed the max usable magnification. How much magnification you can use is determined mainly by the "seeing", in other words the atmospheric conditions. It would not actually prevent you from reaching focus but it makes the view shimmer and wave, which could perhaps be perceived as out of focus. I'm not implying that you can't tell the difference, just trying to explore possible causes :smiley:

Also, make sure to let the scope cool down properly before use (especially since it's winter). If the scope is warm, hot air rising from the mirror will create thermal turbulence in the tube and this will have a dramatic effect on the view. This effect is amplified with more magnification so it's another possible cause. Even though it's a small scope it should be allowed to cool for an hour, especially in cold weather.

HTH

Steve

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Hello and congrats on your first scope.

Generally speaking, anything included with a cheap scope is bound to be cheap, included the barlow. It should still work though.

What is the focal length of the included eyepieces?

On to your problem. The most likely cause is probably what Pig said. I'll suggest a few more options.

If you really can't get it to focus with the barlow then something is certainly wrong. One possibility is that it's simply defect.

Could you describe the "out of focus" view? does it look exactly like when you defocus the view with only an eyepiece in or does it look different?

Bear in mind that when combining a barlow with a short focal length eyepiece it's possible to exceed the max usable magnification. How much magnification you can use is determined mainly by the "seeing", in other words the atmospheric conditions. It would not actually prevent you from reaching focus but it makes the view shimmer and wave, which could perhaps be perceived as out of focus. I'm not implying that you can't tell the difference, just trying to explore possible causes :smiley:

Also, make sure to let the scope cool down properly before use (especially since it's winter). If the scope is warm, hot air rising from the mirror will create thermal turbulence in the tube and this will have a dramatic effect on the view. This effect is amplified with more magnification so it's another possible cause. Even though it's a small scope it should be allowed to cool for an hour, especially in cold weather.

HTH

Steve

Thanks for your time Steve

The focal lengths of the eyepieces are 20mm and 6mm

The view I get is simply one which has the object centred but large and blurred. When I move the focus wheel it starts to get smaller and starts to become clearer, but long before I can get a viewable image I reach the limit of the focus wheel and cannot go any further.

I wonder if it is that I am exceeding the max usable magnification as you suggest. I admit I did not allow the scope to cool for an hour as I did not know this was necessary and the cloud was coming in and I wanted to use the scope tonight while I could. I will bear this in mind in future though, thanks

JT

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Well it does indeed sound like you can't reach focus with the barlow. When the focuser hits the limit, is it all the way in or all the way out? If it is all the way out you might try pulling the barlow (and eyepiece) out of the focuser very slowly and see if that gets you nearer focus. If that's the case then it sounds like you don't have enough backfocus with the barlow, which would be... odd. If that's the case I'm sure someone who knows optics better than me will have something intelligent to say about this.

Assuming it's a 2x barlow then the 6mm would become a 3mm and that's probably more than your scope can handle without perfect seeing. Barlowing the 20mm should be ok though... depends on the scope of course.

Either way, letting the scope cool down before observing is a very good idea anyway. :smiley:

Steve

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Thanks Steve.

The wheel is all the way in before a clear image is achieved. It happens with both the 20mm and the 6mm.

I think I will put it down to being both cheap and possibly faulty. I will look to purchase another soon and keep it when I upgrade the scope.

Thanks for your help

JT

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Np. And to be honest, you don't really need the barlow. You can do a lot of observing with just the eyepieces. Also, even if you reach focus, the combination of a cheap eyepiece and a cheap barlow tends to not produce very satisfying views anyway. I suspect manufacturers mainly include barlows to be able to claim higher mag in the sales sheet.

Steve

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Just a thought, my scope came with a spacer tube to insert into the draw tube, the idea being you remove it to allow for a camera. You don't have a spacer on yours as well do you.?

Richard - you might have something there.

There is a small piece that unscrews and comes away. Whether this is a spacer for the purpose you describe I am unsure but it allows the barlow to sit inside the tube and its removal may well allow me the extra travel needed to achieve an image.

Unfortunately the cloud looks like it is here to stay for the night so as soon as I can get back out to test this I will update this thread.

Thanks all

JT

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Np. And to be honest, you don't really need the barlow. You can do a lot of observing with just the eyepieces. Also, even if you reach focus, the combination of a cheap eyepiece and a cheap barlow tends to not produce very satisfying views anyway. I suspect manufacturers mainly include barlows to be able to claim higher mag in the sales sheet.

Steve

You are quite correct. The images I have without the Barlow are indeed perfectly fine. I just wondered if I was doing something wrong

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You are quite correct. The images I have without the Barlow are indeed perfectly fine. I just wondered if I was doing something wrong

It sounds like Richard may have cracked it for you. If so, all is well and if not I'd just forget about the barlow if I were you.

Clear skies.

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UPDATE

Richard I owe you one. Just got home and spotted a chance to train the scope on to moon. I removed the spacer as suggested and tried both the 20mm and the 6mm eyepiece.

No problem finding an focus at all!!

I knew it would be something I was doing wrong. All your help was much appreciated guys.

Unfortunately the gap in the clouds has closed so cannot test it on anything else but at least I know it works.

JT

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