Barlow, Apo v Achro

[JB80 (Jarrod)]

I'm looking for a new x2, 1.25" barlow as I have to think just about any will be better than my stock skywatcher one and I have narrowed it down to about 2 or 3 but the difference between them is one the TelVue is an achro and the X-cel or Vixen DX are apo's.

What is the difference if any between the choices?

I purely want this as an upgrade from the SW for solar, lunar and planetary imaging but only having ever used my old one I have no experience of anything else.
Maybe there are other options just as good that I don't know of?

Any ideas and recommendations would be greatly appreciated.

edit: I should add that the price of the TelVue is my upper limit so a powermate is probably out of the question.

[MortonH]

I have the Tele Vue 2x (1.25") one and it's great. I've noticed that many of the so-called "apo" Barlows have three elements, whereas a 'proper' Barlow has only two, which the Tele Vue has. The fewer elements the better I would think. Note that the Powermates have four elements.

I bought mine used so it was quite cheap. Last night on Saturn my best view was with a Pentax XF 8.5mm and the Barlow.

[JB80 (Jarrod)]

Thanks for the reply, that is true both the Vixen and Celestron version have 3 elements and are slightly cheaper than the TV.
I agree that at least at first glance the less elements the better, it seems somewhat logical.

TVs are supposed to be quality so I don't I'd be disappointed with it, just curious if there is a better option for imaging.

[MortonH]

Doh. I missed the part about this being for imaging. Maybe you should post the same question in the imaging forum:

http://www.iceinspace.com.au/forum//...splay.php?f=35

[Camelopardalis (Dunk)]

One of the things I like with the Powermate is that the focal point doesn't change much compared with a normal focal extender.

[dannat (Daniel)]

in regard to barlow achro or APO really mean nothing, its marketing hype - -a achro or 2-el barlow does not introduce false colour, its a diverging lens opposite to an objective lens which is convergent

[JB80 (Jarrod)]

Cheers for the replies all.

Another option I'm looking at is the Bresser SA x2, this seems to get good reviews and is around the same price point.

[MattT]

ES 1.25" Focal Extender is very very good. As good as the TV 2x. Has a big thumb screw and the bottom element screws off too, like the GSO 2x barlow.
Had both TV and ES...kept ES.
Matt

[JB80 (Jarrod)]

I have seen them but I am having trouble locating one in Europe. I need to have a thorough dig through all the retailers when I get a spare hour or so.

[Don Pensack]

All barlows are achromatic.
A 3 element is a design that allows the focal length to be reduced (i.e. the tube shortened) without severe curves on the lenses.
Those severe curves could also lead to other aberrations and higher manufacturing costs.
Hence, 3 element barlows.

If the negative lens is followed by a positive lens, the rays can be brought close to parallel, which:
--reduces vignetting
--doesn't expand the eye relief
--keeps the magnification the same at varying distances from the lenses
This type of barlow lens is called telecentric, and it can have better optical qualities than a conventional Barlow.

Examples from TeleVue, Explore Scientific, and harry Siebert, and, in the past, Meade.

But note that the 2-element and 3-element barlows perform roughly the same, but one is shorter than the other.

[JB80 (Jarrod)]

Thanks Don for that.

I'm relatively certain now I'm leaning for the Bresser SA x2, which is basically a revamp of the Meade 5000 telextenders and the same optically as the ES focal extender.

The Bresser sits right in my price range and is readily available plus I struggle to find a bad word written about them so it looks like that is the one I'll opt for.

[Don Pensack]

Since those are all Jing Hua Optical (JOC) products, I see no reason not to shop on price.

[JB80 (Jarrod)]

Availability is a bigger concern for me than the price. I'm struggling to find a ES one in Europe and the Meades I don't think they make anymore so the Bresser it will be.
I'll keep looking until the last minute though.

[Don Pensack]

The ES focal extenders and the TeleVue PowerMates are available from the US, though I don't believe the Bresser named version is available here.
IF you are looking for a Telecentric barlow.

[5ash (Philip)]

Originally Posted by JB80

Availability is a bigger concern for me than the price. I'm struggling to find a ES one in Europe and the Meades I don't think they make anymore so the Bresser it will be.
I'll keep looking until the last minute though.

Bresser bought "Meade Europe " out some years ago , so their Barlow may be closer to the Meade one than you think, after all most are badged imports from china.
Philip