Barlows - The Emperor's not wearing any pants!

[JethroB76 (Jeff)]

Originally Posted by astroron

My main gripe with barlows would be that they add a lot of length or bulk to whats hanging out of the focuser particularly the 2"ers..

I use a type 1 13mm Nagler with a two inch GSO Barlow on my 16" scope and have no problem whatsoever
The eyepiece and the Barlow weigh about half a kilo.
Anyone who has looked through the scope with this combination has just said WOW

As I said I have no problem with barlow performance; I've been very happy with my 13mm Ethos in my 2" barlow, but it is still a huge contraption hanging out of the focuser, I feel like I need to check that the thumbscrews are tight every few minutes

[astroron (Ron)]

Go to a 16" and it wont seem so bad

[JethroB76 (Jeff)]

nah, 20" is the next step

Hello Phil,

I find a well-made barlow to be a very useful equipment, with my Newtonians.
My Newts are a 8"f/6 ATM Dob and a 4,5" f/4 Newt.

I do never use a barlow with my 80mm f/18,25 Refractor and my 6"f/12 Yolo.

The f/ratio of the scope is an inportant factor to be considered.

Let us assume you own a fastish f/4 Newt with decent optical quality,
you have him well collimated and you want to reach high powers on a
night with good seeing conditions.

The scope will allow you to use 0,7mm exit pupil, magnification will be
about Diameter of the optics in mm x 1,4 or even x 1,5
You would have to take a 2,8mm eyepiece to achieve this magnification(!)
Or you take a 2,5x Barlow plus a 7mm eyepiece,
or 3x Barlow plus a 8,4mm eyepiece.

The eye reliev of the simple eyepieces like Plössl, Abbe Ortho, RKE, Kellner
is about 2/3 of the eyepieces focal length.
Eyerelief of a 2,8mm Plössl would be extraordinary short.
The shortest Plössl and Abbe Orthos I know have 4mm f.l.
eyerelief is around 3mm and you have to pres your eyeball onto the eye-lens
of th eyepiece to see the whole field of view. A 9mm or 8mm eyepiece
is way more comfortable.

For high mags with ma 112/450mm Newt or 200/1200mm ATM Dobsonian
I regularily take a 3x Televue Barlow or a 2,8x Univesity Optics Klee Barlow
and some 13mm, 12,5m 12mm and 11mm quality Plössl/Ortho/RKE eyepieces.

A single 4mm Ortho shows blown-up stars at the edge of field,
whereas
a 12,5mm Orth or 12mm RKE with one of the Barlows does not show
any edge star degradation.

One downside is that the exit pupil is moved further out if a barlow is used.
That is fine with short focal length Orthos/Plössl/RKE/Kellner, but it can be
disgusting with long focal length eyepieces and shorty barlows.

Greetings , Karsten