Difference in view between TV Barlow and TV Powermate?

[RikM]

I have a had great fun this evening looking at the moon through a TV Pronto with an old TV 2.5x Barlow and a Pentax XW 7mm (for about 170x mag). That has got me thinking about what I'm going to ask Santa for this Christmas.

Is there any optical difference between the 2.5x Barlow and a 2.5X Powermate? (I should add that the TV Barlow is borrowed from Cotswold AS).

I was thinking of getting a Pentax XW 5mm (I already have the 20, 14, 10 and 7mm) or possibly a SH Nagler 3-6mm zoom but If I were to get a 2.5x Powermate, that would give me the equivalent of: 20, 14, 10, 8, 7, 5.6, 4, and 2.8mm for less money than the additional XW.

I will give the Barlow a good work out in my other scopes to see how useful the extra focal lengths might be, but in my head it seems like a good option.

Is there any major change in focus position when using a 2.5x Powermate? I'd hate to fork out for one and then find I can't reach focus.

[John]

Hi Rik,

I've owned the 2.5x Powermate. I was using it with Type 6 Naglers and Tele Vue Plossls and it worked exceedingly well with those. Apart from the additional magnification you would really not know it's there - more so than even a really good quality barlow because, with the above eyepieces at any rate, the Powermate did not affect the focal point and it did not change the eye relief.

The barlow that I'd been using before the Powermate was a Celestron Ultima 2x (2.2x really !) which was a nice optic but the Powermate was even better to my eye.

Optically a Powermate has 4 elements I believe, including one highly convex one close to the field lens of the eyepiece. You would not know that extra glass was there though.

What I've not done though is to use one with the Pentax XW's as mine have come after I parted with the Powermate.

The only downside over going the eyepiece only route is it's one more component to fiddle with in the dark I guess.

[JamesF]

Isn't the line cone diverging after it leaves the barlow, but parallel with the powermate? So you can't increase the length of a powermate with an extension tube to get an increase in effective focal length like you can with a barlow?

James

[Merlin66]

Yes,

It's a telecentric design. So the magnification doesn't change (well just a little!) with increasing back focus.

I've used both the TV barlows and the PM. No comparison...the PM wins everytime!

(The best barlow in my "box" is a TMB x1.8....if you find one grab it!!)

[estwing]

a powermate is one of the best things you can have in a EP case...i got one there fantastic, espesh with a 13mm Ethos!