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barlows


rpmiller78
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i am not really sold on my barlow lens. the view through it was not as crisp or clear as i thought it would be. i have tried it with my 6.3 and 7.5 mm eye pieces. are these not the lenses to use it with?

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What telescope are you using with the eyepieces you stated and barlow? There'll be a limit as to how high you can push the magnification depending on your aperture (magnification limited to 50x per inch) and seeing conditions. You'll only be able to push the limits of your telescope in excellent seeing conditions. This could be your problem but without knowing your equipment it's difficult to say

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Like others said, what barlow and scope were you using?

6.3 and 7.5mm eyepiece are high power eyepieces. After you barlow it, you are getting in effect a 3.2 and 3.8 mm. That's quite demanding and it's likely you scope or the seeing simply cannot support the magnification.

The Skywatcher barlow that came with most budget scope is optically rather poor.

The Revelation barlow that came with their eyepiece kit was a bit better, but the added CA is still very visible.

On the other end of the spectrum, my Meade S5k TeleXtender have negligible impact on image quality.

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i have an 8" dobsonian.

That's the problem.

Assuming you have a 200p (200mm f6), balowing a 7.5mm and 6.3mm eyepieces give you 316x and 375x respectively. Those magnifications are well beyond the seeing limit in the UK, which is normally around 200-250x. I don't know whether the atmosphere in your local area can sustain that level of magnification. If it doesn't you will see the break down in image due to atmospheric distortion. Also, you are getting close to your scope's optical limit at 375x. Miscollimation and any other optical defects will show even if your atmosphere can sustain it.

Edited by E621Keith
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Your 8 inch dobsonian has a 200mm mirror. I would say that 200x magnification is your maximum regular use magnification and the mirrors maximum magnification is 400x. You only be able to get above 200x magnification and have a satisfactory view when the atmospheric conditions allow it. Also, regardless of what you telescope actually handle the atmospheric conditions might prevent you from going any higher than 150x magnification

I'm going to assume you have a 2x barlow and the focal length of your scope is 1200mm (f6). The magnifications with the barlow and eyepieces you stated in your original post would give magnifications of approximately 350 and 400. This is getting close to limit of your scope and I would say that you'd only be able to get that on nights of exceptional seeing.

Also not all barlows are equal in quality, some have 2 lenses, some 3 and some more more for better correction of the light path.

Something when considering image quality at high magnifications is to consider the exit pupil that the eye piece gives (the diameter of the light path as it exits the eyepiece). You can calculate it from the focal length of the eyepiece divided by the focal ratio of your telescope (which I think is f6). A 6mm eyepiece will give a 1mm exit pupil and 200x magnification. Anything less than a 1 to 0.75mm exit pupil (so a 6mm to 4.5mm eyepiece) you won't add anymore detail to your view but you will magnify it.

With the barlow and eyepieces you were using you were getting close to 0.5mm exit pupils which is getting close to what you eye can resolve well and you start seeing floaters on your eye etc.

So if your barlow is a 2x barlow the smallest eyepiece I would use in your scope regularly is a 10mm giving 240x magnification and ~0.85mm exit pupil but as said above the atmosphere can bubble in the view and makes these high mags annoying. Sometimes you just have to wait for it to calm down, I get it in my maksutov at x130 magnification often enough :)

edit: check out warthogs sticky in the beginners sections here http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/ ; the multiples he uses to calculate what eyepieces you need are just the exit pupil in mm

Edited by username
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it is all orion equipment(scope, shorty 2x barlow, 6.5, 7.3, 10, 17, 25, and 40mm sirius plossl eyepieces). i figured i wasnt using the barlow correctly. i will try it with my 10mm eyepiece

Don't be too dissapointed if it's a bit mushy with 10mm and shorty barlow. I've read in places that that barlow is often around 2.4x factor if it's the orion shorty 'plus' barlow. If it's just the shorty then it's like the one I have and 2x.

You mihgt get your best regular magnifications with the 10mm plossl on it's own or the 17mm with the barlow. Depends what your atmosphere is like on the night.

Also, have you made sure that your telescope is in collimation? If it is out then it will degrade your views.

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