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Thread: Barlow question

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    Default Barlow question



    Hey guys, I've just purchased a barlow, it's just a simple Celestron 2x multicoated barlow, nothing fancy. I bought it for planetary viewing, because my 130mm Heritage's highest magnification eyepiece is a 6mm, that gives me a 108 magnification. The box of my scope says the scope can take a maximum of 260 magnification. My local astroshop guy says I should not get a higher than 5mm magnification eyepiece, because there will be aberration and it's not worth it. Now 5mm in my 650 tube would be 130 magnification.

    So if I use the barlow on my 10mm eyepiece that will give me 130, that should be ok. But the barlow on my 6mm eyepiece would give me 216 magnification, which the box says should still be ok but the astroshop guy advised against.

    My question is: will there be aberration in your opinion? If there will be, what exactly can I expect? And are there any ways to combat that? (I can't try it out now, it has been overcast for weeks here It's why I am asking. )
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    Default Re: Barlow question

    One thing to understand is that what you see in advertisements and on boxes usually quotes a theoretical limit based on 50x per inch (more or less). Your local astro guy is being more realistic, as he should be. The typical rule of thumb is that on average nights you will likely hit your limit with an eyepiece with a focal length matching your focal ratio. In other words if your scope is an f/5, then 5mm. You may be able to exceed that on rare nights of exceptional seeing, but typically those will be few and far between. On most nights, I would say for your scope, the 6mm would likely provide the highest best view you would get, though you may be able to use your 10mm barlowed. Of course you can always try the 6mm barlowed, but in almost all cases I would venture to say you will be disappointed.
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    Default Re: Barlow question

    A barlow is a useful accessory. You will probably not be able to barlow the 6mm effectively, however. You can get some useful range with a barlow by turning your 25mm into a 12.5 and your 10mm into a 5. Your view will be dimmer, however, so that sometimes makes the barlow a poor choice. It depends on the target. On something bright like the moon or Jupiter, you can push your magnification much higher than you can on deep space objects. Do not worry about magnification. You will soon see that lower magnification works much better for 80% of your observing. I very rarely use 6mm lenses in my 10" Dob- only for moon and planetary viewing. Most of the time I am at 12.5mm on the moon. (100X in my scope). In your scope, you will get just over 100X with your 6mm, and my guess is that is about all you can get. For deep space viewing I use a 2" 30mm eyepiece (42X) or an 18mm eyepiece (70X) almost exclusively.
    With the eyepieces you have, you are pretty well set. I would suggest that you upgrade to higher quality eyepieces when you can afford it, but your range of eyepieces is spot on. Have fun! (And cross out that maximum magnification on the box- it is a fantasy.)
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    Default Re: Barlow question

    Thanx for the kind responses. I only bought the barlow for planetary viewing, I would really like to see at least two bands on Jupiter, and so far it has only been a luminous disk with four moons. I was thinking that if a barlow makes things a bit dimmer it could dim out the glow of the luminous disk and I could see the bands maybe> Or is that stupid? Please tell me I did not buy the barlow for nothing. Suggest something that it's good for.
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    Default Re: Barlow question

    Well again, you can use the barlow with the 10mm to get you a little bit of a bump in magnification over the 6mm alone. But you have to understand there is a limit to magnification, and what you see in the eyepiece is not only about that. You have to depend on good seeing for clear and crisp high magnification planetary and lunar views. Some nights are going to be better than others. You won't know how high you can go until you try. I am not sure what other eyepieces you have, but you can barlow those as well to get you magnifications in between what your eyepieces alone provide. Personally I don't use or own a barlow, but I know many do like the versatility they give the observer. I simply prefer to use an eyepiece alone. That is why I have seven of them in my case.

    Another aspect of your scope, have you checked the collimation of it to make certain your mirrors are properly aligned with one another. Poor collimation will also limit how high of a magnification you can successfully use. I can tell you that on an average good night with my 5 inch refractor with magnifications under 100x I can easily see the two bright equatorial bands and often the lighter temperate zone bands, as well as the Great Red Spot when its on the visible side of Jupiter. Even on below average nights I will easily see the two largest bands, but the lighter in color temperate zone belts may not be visible.
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    Default Re: Barlow question

    You weren't the one who outbid me on ebay by chance?
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    Default Re: Barlow question

    Thanx much for the tip to get as good a collimation as I can when I want to use the high magnification eyepieces. These are the small but useful tips I need.
    One more question: If I have a 25 and a 10 mm eyepiece, is it good to have something between them? I mean if I wanna find some DSO I use the 25mm. I wanna see it bigger, I use the 10mm. What do I use a 15mm or 12.5 (barlowing my 25mm) for? What can it add to my viewing experience?
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    Default Re: Barlow question

    You won't be able to use the barlow with your 6mm eyepiece very often. The Earth's atmosphere is seldom steady enough to allow it. However, it is not a bad purchase, because, occasionally, the atmosphere steadies out enough that you can use a bit more magnification.

    The way it works is that the maximum resolution of your scope is achieved at a magnification equal to your aperture. So, 130x, in your case. Going beyond the maximum resolution works if the atmosphere is unusually steady: you don't gain extra resolution, but you are able to take advantage of magnifying the resolution you have. This works up to twice the normal maximum, i.e. 260x in your case. This is where the manufacturers get their advertised "maximum".

    But the key words are "unusually steady". This will happen a handful of times per year. The rest of the time, 130x will be the best you can expect. It's not worth getting a special eyepiece for those occasions, but a barlow will do the job nicely.
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    Default Re: Barlow question

    I was also thinking of getting a 2x barlow. seems like everyone has one. but from what you guys are saying my 650mm is as good as it will get with my 6mm EP. Also have a 10mm so maybe just need a 25mm and I can cover most X needs.
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    Default Re: Barlow question

    Quote Originally Posted by milanpicard View Post
    Thanx much for the tip to get as good a collimation as I can when I want to use the high magnification eyepieces. These are the small but useful tips I need.
    One more question: If I have a 25 and a 10 mm eyepiece, is it good to have something between them? I mean if I wanna find some DSO I use the 25mm. I wanna see it bigger, I use the 10mm. What do I use a 15mm or 12.5 (barlowing my 25mm) for? What can it add to my viewing experience?
    When you switch from the 25mm to a 10mm are you always happy with the view you get of the DSO? Does it ever seem a little too dim, or not quite sharp? If so then having an eyepiece of a little bit longer focal length might be what you need to get a higher magnification view of the DSO than what you get in the 25mm. If it is always to your liking in the 10mm, then you may not feel you have a need for something in between. But what if you are looking for a DSO andyou can't find it in the 25mm? Does that necessarily mean you cannot see it at all? Of course not, but hunting for it with the 10mm might be difficult because of the narrower field of view. That is where having an eyepiece in between, that gives you a little more magnification but still a good field of view will come in useful. I hope that makes sense.
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