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Celestron Ultima Barlow


mikeDnight

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I recently bought a Celestron Ultima SV 2X barlow and am very pleased with it. Out of boredom I was just googling reviews about this barlow and stumbled across one by Chuck Hawks. I'm just wondering if someone can explain the claim by Hawks that barlows reduce the light reaching the eye by 75%. This claim seems to me to be quite absurd, as the view through the eyepiece and barlow certainly doesn't look like there's any light loss. Surely to reduce the light by 75% the objective would have to be stopped down by that same amount, or the absorption by the barlow elements would have to be pretty diabolical. I'm not a mathematician but I can follow it pretty well. If anyone would care to shed light (that's nearly funny!) on this claim I'd be grateful! 

Just another point regarding barlows, is that many modern high end eyepieces incorporate a barlow into the design, so are these eyepieces underperforming by 75%? Do i need to sell my 100mm Tak and buy a 25mm frac? :help:

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Actually, the image does become 4x darker in terms of surface brightness simply because the same amount of light is spread out over a 4x larger solid angle (because it is a 2x Barlow). Hence, the surface brightness of a planet or nebula will be reduced by 75%, assuming perfect transmission of the Barlow. This is factually correct, but hardly helpful, as I get the same "loss" if I move from my Pentax XW 10 mm to my Pentax XW 5mm, rather than insert e.g. a 2x PowerMate. Point sources should stay the same in terms of brightness (and might even stand out more, because the background gets darker).

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It's funny Mike, I read that very same report the other day, but unlike you couldn't be bothered to challenge what I know to my eye doesn't appear as an image that's only a quarter as bright as a non barlowed one.

Sometimes maybe a little knowledge IS too much!:grin:

On the Ultima itself, I have never been a huge fan of barlows, but I've owned a couple of Ultimas, and a couple of Baader Hyperion zoom barlows and thought both were excellent.  Speaking only for myself, I've not seen any degradation of the image in either of the above barlows, unless the resulting magnification obtained by using the barlow was clearly beyond the reasonable optical limits of the eyepiece being used in particular conditions - which isn't the fault of the barlow, but rather of the observer and their selection of magnifications and eps to barlow at that time..

Only the "old fashioned" set screw on the Ultima lets it down IMO, and that can be replaced with a plastic or delrin tipped end I dare say. Very nice piece of glass and a great part of the multi branded Pseudo Masuyama family:thumbsup:

Dave

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So Hawks claim that "a barlow reduces the amount of light reaching the eye by 75%," is not only misleading its completely wrong, as the same amount of light does in fact reach the eye, its just spread out rather than concentrated and so the object appears dimmer! What a Charlie Chuck is! 

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5 hours ago, mikeDnight said:

So Hawks claim that "a barlow reduces the amount of light reaching the eye by 75%," is not only misleading its completely wrong, as the same amount of light does in fact reach the eye, its just spread out rather than concentrated and so the object appears dimmer! What a Charlie Chuck is! 

I suppose he should have qualified that statement with "by 75% per unit area" to make it clearer.  The other 75% of the light hits the field stop of the eyepiece, so it never reaches the eye unless the eyepiece with the barlow has twice the field angle of the eyepiece used without a 2x barlow.  For example, a 20mm 100 degree eyepiece in a 2x barlow will show all the light a 20mm 50 degree eyepiece without a barlow shows, just spread out over 4 times the area and thus 1/4th as bright per unit area.  So in that case, yes, the same amount of light does in fact reach the eye once integrated over the entire visible area in both cases.

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On 14/03/2018 at 14:35, michael.h.f.wilkinson said:

Actually, the image does become 4x darker in terms of surface brightness simply because the same amount of light is spread out over a 4x larger solid angle (because it is a 2x Barlow). Hence, the surface brightness of a planet or nebula will be reduced by 75%, assuming perfect transmission of the Barlow. This is factually correct, but hardly helpful, as I get the same "loss" if I move from my Pentax XW 10 mm to my Pentax XW 5mm, rather than insert e.g. a 2x PowerMate. Point sources should stay the same in terms of brightness (and might even stand out more, because the background gets darker).

I guess one reason so many different focal length eps are made. If 5mm works out too dark, go up to a 7mm, so more magnification, but also not dimmed as much as a 5mm.

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2 hours ago, 25585 said:

No More Eyepieces!

Hi Pinocchio, I've just seen your signature claim, which is legally binding incidentally. Now I'm certain you really mean it in your heart, but just to test you out I've attached a pic of some astro sweeties. If your breathing increases or your blood pressure rises, even fractionally, you could be in trouble in the very near future! :happy7:

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