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Barlow problems...

rutten

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rutten

rutten

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Complete beginner to astronomy. Got bought a Skywatcher Explorer 130p (5.1") reflector for my birthday and I've been capitalising on every scrap of clear sky ever since. I took it out for the full moon a couple of weeks ago, hurt my eyes looking at the moon, consequently bought a moon filter and took it out last night to see a sort of half moon. I also managed to catch Jupiter and the Galilean moons, which was quite the impressive sight. All was well.

The problems started when I affixed my friend's rather budget-looking 2x Barlow lens to the telescope (it goes telescope->Barlow->EP, as I understand it) and it just didn't work at all. All I could see through the EP was the Secondary mirror, the legs that attach it to the inside of the scope and sort of scattered splotches. I tried it with his 3x Barlow, again quite plasticy looking, with the same results.

I have no idea what happened. Could it be that the Barlows were just incompatible or 'too budget' to deal with the scope? Was I doing something wrong? With just telescope + EP it was fine...

Thanks for your help in advance!

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rutten

rutten

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I tried refocusing using the full motion of the focus knobs to no avail. Do you think I should buy a mid-range (30-40 pounds) Barlow lens and see if that makes a difference? Or is it that I have done something wrong? I've read things about collimation, but it's completely fine without the Barlows and only the EP...

Thanks for your quick responses :)

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andrew s

andrew s

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I think you may be pushing the barlow to far into the focuser tube. If you try pulling it out gradually you may get to focus. The magnification you get from a barlow depends on how far it is in past the normal focus point of the telescope (and it own negative focal length). However, if it is in too far it won't work.

Regards Andrew

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rutten

rutten

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Hmmm I never realised I could have the Barlow 'too far or too little in'... I'll definitely try that later and report back. Thanks for the advice! Btw, I was using a 10mm EP that came with the telescope. I never tried it with the 25mm EP that also came with the scope. Could that make a difference? I'll try it out later and see how I get on.

Thanks again!

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brantuk

brantuk

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Which barlow was it? might be a good place to start. Also - try using a 5mm eyepiece to see if the same thing happens. Are you positive it's a 2x and not a 3x nor 4x barlow? The way you describe the problem does suggest a focus issue which can sometimes be solved by setting a 2x barlow further in or out of the focuser tube. Also the scope is f5 which isn't going to perform as well with as it could if using low quality eyepieces. Hth :)

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Stargazer33

Stargazer33

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With a x2 barlow you are doubling the magnification of your eyepiece as I am sure you are aware. What isn't always obvious - and I have been a victim of this - is that if your object isn't dead center in your eyepiece before you put the barlow in, the increase in magnification will mean that your image has shifted outside of your eps view. This may be why you cannot focus on it - it isn't there!

Bryan

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cosmojaydee

cosmojaydee

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An eyepiece of 5 times your focal ratio also gives you an 'exit pupil' of 5mm. This is the longest eyepiece you want to use if you are older, as this exit pupil is approximately equal to an older (45+) person's maximum pupillary dilation. You can't use more light than that. If you are younger, you could go up to 7 times your focal ratio, or an exit pupil of 7mm.

To summarize, for an f/8 scope, we suggest a kit consisting of 6, 10, 16 and 24mm. For an f/10 scope, 7.5, 12.5, 20 and 30mm. For an f/5 scope, 2x Barlow, 8, 18, and 25mm

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rutten

rutten

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First of all, thanks for all the valuable advice. Really helpful.

I've just taken the Barlow 2x lens out for a spin again, this time lining it up with a lampost a few streets over that I've used before to check if the laser finder was aligned properly. I think it's a Tasco Barlow, quite budget/plasticy and it came with my friend's beginner's telescope, which seems of a much lower spec than mine (not sure if that would make a difference). Using only the EP I could see the cobwebs hanging from the lampost in nice detail. Affix the Barlow and it just fills the EP with a kind of splotchy light, the same kind of splotchyness and unfocussed blur that I had last night using it on Jupiter.

I tried pulling the Barlow out of the socket to different levels to 'manually' change the focus, as suggested. No luck. The object (lampost) was lined up perfectly, so no chance of missing the object. I'll try it on a celestial object when there is next clear skies (probably in a few days) but I'm starting to think the Barlows are just non-compatible. Can anyone recommend one that might be?

Again, thanks for all the help!

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brantuk

brantuk

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Ahhhhh Tasco - just about the cheapest most plastic'ish barlow you could have chosen lol. Your scope won't like it at all. One of the best budget barlows is the Tal 2x - but they were in short supply last time I looked - so best to try and get one second hand - they should be around £25-£30 roughly. Great performer for the price and it will work with the 130P straight out of the box (it did with mine). :)

(Check out Astro Buy Sell website for mostly reliable s/h astro kit).

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rutten

rutten

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I think I'm just going to have to scour the market for a good Barlow then. I keep hearing about the TAL Barlow but I've also heard that it's quite rare to see any for sale.

Thanks again for the help guys. I'm sort of glad it's the tools that need fixing and not me :)

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