Televue, barlows, Powermates? Which and why?

[Astral Enigma]

Hello,

I'm currently trying to decide about what best to get to amplify my planetary images and have been looking into various barlows. Powermates have a great reputation but are expensive - that won't rule them out though. If I know it's what I'm going to end up feeling i need them I'd rather not go through buying, selling and re-buying what I should have bought in the first place and losing money in the meantime.

So how good are they? I'm thinking initially about a 2.5x magnification. I notice though that Televue also make a 3x barlow - I assume this is different to the Powermate range but I'm not convinced i understand just what, and how important, that difference is.

Alternatively the most likely choice would be the Celestron XCel LX 3x.   I know Skywatcher also make okay (??) ones for a lot less, and the Revelation 2.5x seems highly rated although perhaps as value for money?

Whatever I buy will primarily go on my 925 SCT, but could be used on my Nexstar SE6 SCT too.

I don't think the eventual camera(s) ought to make too much difference to choice of barlow - or should it? At the moment I'm using a Phillips SPC900 webcam but soon I'm intending to upgrade to either the Neximage5, QHY5L-II or ZWO ASI120MC if that matters (no decision yet).

Thanks again everyone!

PS I've also got a Hyperion 10mm eyepiece and the rings etc to connect that to my DSLR so i can do projection imaging - haven't had the opportunity to do it yet though. Can anyone tell me what the equivalent barlow magnification is with that, or how to work it out? It would be a using a 2350mm tube, f/10 if that helps.

[JamesF]

The major difference between the powermates and a barlow is that give or take, a powermate provides the same increase in focal length regardless of the position of the eyepiece or camera sensor relative to the powermate.  With a barlow, the greater the distance between the barlow optics and the eyepiece or camera sensor the greater the effective increase in focal length.  That means, for instance, that be adding an extension between a 2.5x barlow and the camera you can increase the multiplier to, say, 3x or 3.5x.  I have wondered if that isn't exactly what Tele Vue do with their 3x barlow anyhow, as it's not far off twice as long as their 2x barlow.  Or at least, the one I have is.

The powermates work well for imaging, but my suspicion is that their real benefit is for visual use, when a barlow combined with some eyepieces can get a bit messy because you can lose control of exit pupil size etc.

The Revelation 2.5x barlow is just a very good barlow.  It would still be so at twice the price.  If that's what suits your kit then I don't think you'll find much that's better at any price.  I'm not using mine at the moment, but I'd sell others, including my Tele Vue 3x, before I'd consider selling the Revelation.

For imaging with either OTA I'd say the 2.5x would work fine with the QHY5L-II and ASI120MC, and you can squeeze a bit more out of it by adding a 40mm to 50mm extension if you're using the SPC900.  Where I started to find the 2.5x a bit too much was when I had a secondary focuser, flip mirror and filter wheel on my C9.25, with my ASI120MM.  That all got a bit silly so I picked up a second hand Tele Vue 2x barlow.  If I could find a decent quality 1.5x I'd be quite tempted to use one instead.  This is partly due to the way SCTs work, as the focal length (and therefore the focal ratio) changes depending on where primary mirror is when the image is in focus.

James

[JamesF]

PS I've also got a Hyperion 10mm eyepiece and the rings etc to connect that to my DSLR so i can do projection imaging - haven't had the opportunity to do it yet though. Can anyone tell me what the equivalent barlow magnification is with that, or how to work it out? It would be a using a 2350mm tube, f/10 if that helps.

Missed this bit.  The easiest way to work this out is to image something that you know the size of, ideally in terms of the number of arcseconds/arcminutes/degrees it measures across the sky.  For instance, the Moon is about 30 arcminutes across.  If you then count the number of pixels covered by the image and find the size of the pixels you can work out the number of (say) arcseconds per mm on the camera sensor.  From there it's trivial to work out the effective focal length (it's just a simple division, the exact number used depending on which units you're working in).

James

[David_Pickles]

I have not used a barlow but the Tele Vue powermate is extremely good - I have used the 2 x 2" for DSO and Planetary imaging and the results have been really good. Nice sharp stars in the wider shots and lots of details in the planets.