Recommended x2 Barlow for skywatcher 150p

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Recommended x2 Barlow for skywatcher 150p

Postby Scott.Morrison.139901 » Mon Mar 02, 2015 2:27 pm

Hello All!
I've got a skywatcher 150p telescope with only the two standard eye pieces that come with it. super 10mm and a super 25mm wide angle. I'm looking to get a x2 barlow (uk) and was hoping for some recommendations for a good one to get. I want to get the best for my money and from alittle research i've done im under the impression that I should'nt just be getting and x2 barlow.
Look forward to the responses and thanks in advance
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Re: Recommended x2 Barlow for skywatcher 150p

Postby dave.b » Tue Mar 03, 2015 9:41 pm

A literal answer would be Celestron Ultima or a PowerMate, but I suspect you'll bulk at their cost, especially for the later. You can of course get cheaper ones, but quite frankly they are not worth there cost. The only time I use a Barlow is with my planetary camera.

A better answer is that for visual observing don't bother with a Barlow and invest in better eye pieces. An alternative solution for a beginner is a zoom eye piece, because you can maximise your observing enjoyment without the faff of changing eye pieces and have the bonus of a continuous focal length variation between the zoom limits. A reasonable zoom is the Celestron 8-24mm. The sky watcher one is probably the same with different branding. You can get more expensive ones of course.

A technical answer is that the diameter of the exit pupil limits both the upper and lower limit of magnification because you need to match it to a size that illuminates your eye's macular (high mag) and is no large than your dilated pupil (low mag). Figures of 0.8 to 5.5mm are typically used for the useful working range of exit pupil. Personally I prefer the range 1 to 6mm. Since exit pupil is the objective diameter divided by magnification, that gives for your 'scope a maximum mag of 150/0.8=187.5x (or 150x with a 1mm exit pupil). Magnification is of course the ratio of the 'scopes focal length to that of the EP's. Hence, with a 750mm focal length, a mag of requires an EP focal length of 750mm/187.5=4mm. (Or 5mm with a 1mm exit pupil). So you could use a 2x Barlow with your 10mm EP to get an effective 5mm EP. There would be no point using a 3x or higher Barlow.

A final consideration with magnification is that unless you have very good seeing all you'll end up doing is magnifying the atmospheric turbulence.

Dave B.
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Re: Recommended x2 Barlow for skywatcher 150p

Postby SkyWatcher17 » Tue May 05, 2015 12:18 am

The Celestron AstroMaster kit could be worth looking at. It has 6mm and 15mm Plossl eyepieces that I think are better quality than the standard Sky-Master 10 & 25mm EP's. It has a 2x Barlow and three filters (including a moon filter).

EDIT: The 15mm is a Kellner and the 2xBarlow has a T-ring connection (for a camera) added.
Sky-Watcher Explorer 130M, EQ5 Deluxe, AZ5 Deluxe, SkyMax 102mm, SkyMax 127mm, Orion/SW ST80s (both modified), Vixen Porta II/Vixen SXG Hal-130, Orion 90mm StarMax , Celestron NexStar Evolution 9.25", TS Optics/GSO: SVD series 150mm, f/6.
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