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Old 06-07-2020, 09:24 AM
bluesilver (Peter)
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Are Barlows better to use when viewing planets?

Hi, I am just trying to find out some information in simple terms on how barlows have a visual effect on images through the eyepiece.

Say for example i have f5 Dobsonian.
If i use a 2X barlow it goes to a f10, a 5X barlow goes to a f25
Is this correct so far?

The question is, how dose this focal number effect the visual viewing through the eye piece compared to using the equivalent in a eye piece?
I mean if i were to use a 20mm eyepiece and a 2X balrow, what is the difference from using just a 10mm eyepiece?
Apart form the 10mm eyepiece will still be f5 where the 20mm and 2X barlow will be f10

Just trying to find out what the visual difference is through the eyepiece.
Will the higher the f number like f25 or f10 give more detail that it would at f5, I am also only talking about visual for the planets, not DSO.

I am guessing that also the lower the f number like f5 gives you more light, so f25 will be darker than f5.

I realise this a basic question all up, just having trouble trying to find any real answers.
As my theory was if the higher the f number like f25 gives you a darker image through the eye piece and can then give you more detail on the planets, would you be always better of using barlows to get the f number higher?

If anyone can help with this question in simple terms it would be most appreciated.
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Old 06-07-2020, 10:05 AM
glend (Glen)
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Peter, focal length increases are only as good as the Seeing conditions in your location permit. In theory there is little difference between using a 20mm EP with a 2x Barlow, verses a 10mm EP. However, quality is always going to come into it. There are alot of budget EP and Barlow, and they don't always work well together, in my experience. In your example, I would prefer a good 10mm EP over a barlowed 20mm.
As far as apparent brightness at different focal lengths, this doesn't really make much difference for planets, but yes for dark distributed targets like nebula it certsinly does.
Now back to Seeing, unless the conditions are perfect your going to have a heck of a time trying to force f25 out of your f5 Dob. If you look at a target with say a 15mm EP and see it wobbling around, or looking murky, or stars dancing and spiking, then there is little chance of using high magnification with good detail. There are many folks that carry around very nice 3 to 5mm EP s but rarely get a chance to use them. With experience one can just look at the stars and get a general view of how the Seeing conditions are going to be that night.
Hope that helps.
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Old 06-07-2020, 11:23 AM
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doug mc
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Your eyepieces will handle the longer focal ratio which a barlow offers better. The field of view will be sharper nearer the edge using a barlow. Standard plossl eyepieces become increasingly difficult to peer into when they get shorter than10mm. This means that the20mm with the barlow will be more comfortable to use than the 10mm on its own. Going shorter than10mm using a single eyepiece that has good eye relief means such an eyepiece would most likely have a barlow type lens in it anyway. The only significant problem you are likely to encounter with a barlow is when observing the moon, internal reflections may interfere. I have Meade 2x long barlow that i eliminated such reflections by flocking it. I believe the Celestron excell 2x barlow gets around the internal reflections problem by having a larger internal cavity.
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Old 06-07-2020, 12:43 PM
bluesilver (Peter)
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Thanks for the replies, Yes i thought it was going to be a broad question in general, but overall i think i have got the answers in general to work with.
Appreciate the advice as always.
Thanks again.
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Old 06-07-2020, 05:22 PM
Startrek (Martin)
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I’ve used both Barlow’s and Powermates for observing the moon and planets in various seeing conditions and various Newtonian reflectors 6” up to 12” and without a doubt Powermates reign supreme IMHO ( but the downside is that they can financially out of reach for some folk )
Attached is some Unbiased information from Televue on Barlow’s and Powermates
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