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Davide Simonetti

Newbie eyepiece question

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Hello, we finally had a clear night in London and I was able to take my Celestron NexStar 127SLT out for the first time. After much fiddling about I managed to home in on Jupiter and 4 of its moons with the 25mm eyepiece and was awestruck. I then switched to the 9mm and enjoyed the view even more. Finally I thought I'd try adding my Barlow x2 lens and saw...nothing. Can anyone tell me what I might have been doing wrong please? Many thanks.

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You have used too much magnification, I'm afraid.

The lowest exit pupil human can see is 0,5mm, this is usually given as a scope's highest magnification, 20x (scope diamenter in millimeter), your scope is f12 (f11.8 to be more exact), 0,5mm exit pupil means you can use 6mm as highest mag. When you use 2x barlow  9mm, you're using 4.5mm for too high mag.

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When you magnify the object you reduce the field of view quite considerably. So any inaccuracy in centering is immediately shown up. Depending on the degree of magnification the object can appear to be off center a short way, or a long way, or completely out of the eyepiece altogether.

To counteract the effect remember to align the finder with the main scope using the highest power (lowest mm eyepiece including barlows) that you intend to use during the session. Then it should be accurate for all magnifications up to that power. Hth :)

(Bear in mind not to exceed the highest stated power for your scope)

Edited by brantuk

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Another factor is that when you add a Barlow you might need to refocus quite a bit to get the image back in focus.

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Thanks for the tips folks. I shall remember them next time I go out. It's a pity I messed up just when I was starting to get the hang of my scope...but seeing Jupiter and it's moons tonight...wow!! I hope tomorrow is clear :)

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Jupiter was my first planet too. I was blown away. I've often tried to magnify things too much and only got a larger blurred image. Advice from the forum often suggests that is better to have a small crisp image than a larger blurry one.

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All good advice here. I really dont think it was a case of too much magnification, but certainly pushing it towards the upper limit. 9mm eyepiece +2x barlow = 4.5mm eyepiece. This gives a very narrow field of view and would need to be refocused. If Jupiter was not perfectly aligned in the scope to begin with then trying to observe with 4.5mm would entail a search in the same area by tweaking the scope left/right/up/down until its in view and then refocus.

Just saying what everyone else has said.

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Jupiter was my first planet too. I was blown away. I've often tried to magnify things too much and only got a larger blurred image. Advice from the forum often suggests that is better to have a small crisp image than a larger blurry one.

Very true. I hate fuzziness (except when it's a galaxy, then I let it off, as it's millions of light years away). :smiley:

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hi david, i have the same scope. i would agree that using a 9mm and a x2 barlow was too much for your scope. the mag limit for that scope is 254. a 9mm with a x2 barlow is 333. i have attached a scope calculator for you to look at. it has all the info re: my ep's. (which you can input your own). the yellow boxes indicate reaching my mag limit, and red is beyond my mag limit. hope this helps.ScopeCalculator1.xls

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no worries davide.

it's a handy wee spreadsheet. helps when your looking at future EP purchases. 

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hi david, i have the same scope. i would agree that using a 9mm and a x2 barlow was too much for your scope. the mag limit for that scope is 254. a 9mm with a x2 barlow is 333. i have attached a scope calculator for you to look at. it has all the info re: my ep's. (which you can input your own). the yellow boxes indicate reaching my mag limit, and red is beyond my mag limit. hope this helps.attachicon.gifScopeCalculator1.xls

Great chart.  just fed my combination of Hyperion 5mm, 28mm Fine Tuning Ring and 2 x Powermate and gives me magnification of X313 agreeing with Stellarium. On the basis that the max for my scope is X160 from the formula or X240 according to Skywatcher I should not see anything through this combination. The attached close ups  were taken  through hand held camera body at this mag, difficult to hold central (adaptor in post) OK not the best close ups you will ever see - especially when reduced to a JPG but I was quite pleased and it has enabled me to explore our neighbour in more detail. I guess seeing conditions must also come into the equation and obviously the moon is a bigger target to find. Have viewed Jupiter through same combination, image was acceptable if not quite as good as lower mags. Always worth trying it costs nothing. 

post-34685-0-00316600-1390853900_thumb.j

I also find that at these mags if you don't position your eye correctly everything can appear black. If I put my eye too close the image disappears.

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