Barlow / TE advice

[cajen2]

Hi folks.

I'm after a mid-range Barlow for my 150p f/5 dob. Obviously, I want value for money😄 and a minimum of wide-field aberrations or darkening.

I have in mind three options:

Baader Classic Q 2.25x at £45

StellaLyra ED 2x at £59

ES 'Barlow / Focal Extender' at £85

or obviously any other that you can personally recommend.

I currently have only stock EPs but intend to upgrade these, quite possibly to BST Starguiders.

Any help gratefully received.

[John]

Of those I've owned  the Baader and the ES Focal Extender. I currently use the Baader with a zoom eyepiece to create a high power zoom and it works pretty well. I don't use it with my other eyepieces though - I've enough focal lengths without needing to barlow them.

I compared the ES 2x Focal Extender to the Tele Vue Powermate 2.5x and the ES was very good indeed. If I relied more on a barlow / extender more extensively I would go for the ES Focal Extender over the Baader 2.25x barlow I think. The Focal Extender has much less impact on focus position and eye relief than a barlow and does not vignette the field edges of eyepieces with large field stops like a barlow does.

 

[johninderby]

The Altair extender is identical inside to the ES version I believe. I  have one and find it to be excellent.

https://www.harrisontelescopes.co.uk/acatalog/altair-lightwave-2x-flat-field-tele-extender.html?gclid=EAIaIQobChMItt-ryZGd9AIVxu3tCh09JAZBEAQYAyABEgLPmPD_BwE

[Andrew_B]

If you're using a binoviewer is there any advantage to using a tele extender over a standard Barlow lens or does it not matter?

[cajen2]

Thanks, John x2!

I didn't know if the ED glass in the SL made any difference?

[John]

12 minutes ago, cajen2 said:

Thanks, John x2!

I didn't know if the ED glass in the SL made any difference?

I have used the GSO ED version of that 2 inch SL barlow (I'm pretty sure that GSO make the SL one as well). It was decent but not as good as a Focal Extender / Powermate, in my opinion.

To be fair though I have not actually used the StellaLyra version.

 

[cajen2]

Fair enough, thanks John.

I'll leave it a little while in case I get other recommendations before I part with my hard-earned, but (as usual, sigh) the best option seems to be the most expensive one!

[Don Pensack]

1 hour ago, Andrew_B said:

If you're using a binoviewer is there any advantage to using a tele extender over a standard Barlow lens or does it not matter?

If you use the Barlow under the Binoviewer, the magnification will not be the stated magnification because of the large distance between the Barlow and the eyepiece.

If you use a telecentric Barlow, like the TeleVue PowerMate, the magnification will barely change at all with distance from the lenses.

If you use the Barlow under the eyepiece (you'd need 2!), then a Barlow would be fine.

But most Glass Path Correctors (OCAs) used with binoviewers already impart a magnification.  Typically, it's 1.6-2.5x.

So a 24mm eyepiece will give you the magnification of a 15-9.6mm focal length.

That's one of the reasons people don't typically use Barlows with binoviewers.

[Louis D]

20 hours ago, Don Pensack said:

That's one of the reasons people don't typically use Barlows with binoviewers.

I guess I'm in the minority, but I use a Barlow to reach focus, slow down the light cone, and enable using long eye relief, long focal length eyepieces at the higher powers needed for planetary viewing, which is the majority of my binoviewer usage.

[Don Pensack]

53 minutes ago, Louis D said:

I guess I'm in the minority, but I use a Barlow to reach focus, slow down the light cone, and enable using long eye relief, long focal length eyepieces at the higher powers needed for planetary viewing, which is the majority of my binoviewer usage.

But you are probably not using another GPC/OCA with the binoviewers, in which case the Barlow is serving that function.

It will work, sure, but the Barlow probably isn't anywhere near its rated power when doing so.

 

[Louis D]

1 hour ago, Don Pensack said:

It will work, sure, but the Barlow probably isn't anywhere near its rated power when doing so.

My Meade 140 2x bare nose piece operates at 3x in my Arcturus binoviewer.

Edited November 17, 2021 by Louis D