Barlow, Barlow, TeleXtender

[gareththegeek]

Since they are all a similar price, which should I buy?

Celestron Ultima 2x Barlow

TeleVue 1.25" 2x Barlow

Meade 1.25" 2x TeleXtender

Originally I was convinced that I should get the Celestron but then I heard bad things about shorter barlows (vignetting etc with low power EPs). I am using a God awful cheap Barlow ATM and I have found that I have to in-focus almost to the limit. I am wondering if I will definitely achieve focus with a Barlow? I understand that the TeleExtender will not need as much refocusing which is a huge bonus.

My current scope is a C100ED Refractor and I am using Plossls.

[weega]

celestron ultima 2x has a clear aperture of 27mm, it does not vignette even with low focal lenght eyepiece on fast ratio telescopes!

(I use it on my f/4.3 dobson....)

Celestron is an air-spaced apo triplet, same quality of barlow meade 2x #140 (air spaced apo triplet, long barrel)

I've never tried Televue, it is famous, but I can't read nowhere it is apo.

I've never tried meade telextender 2x. It is a telecentric system, similar to Televue powermate.

ciao

dan

[John]

The Ultima is a nice optic and does have 3 lens elements but it does vignette on some ultra-wide eyepieces though - Nagler 16mm and UWAN 16mm for example.

The TV 2x Barlow is as good as the Ultima and does not vignette but it's longer format is not suitable if you want to use it in the diagonal (as opposed to between the diagonal and the scope).

I used a 2" Meade 2x TeleXtender which does work in a similar way to the Tele Vue Powermates, ie: very well indeed !. I've not tried the 1.25" versions.

The best 1.25" amplifier I've used is the TV Powermate 2.5x and it's a shorty format so goes into the diagonal. Superb

Any of them would be an improvement over what you have now of course

Edit: I've just noticed that you use plossls so the Ultima will be fine - it's only ultra-wides where I've noticed vignetting.

[michael.h.f.wilkinson]

I can vouch for both the 2x and 3x TeleXtenders of Meade (1.25"). Really good piece of kit. Neither shows any vignetting on my 14mm UWA, though the 3x UWA and 14mm combo is only useful in EXTREMELY good conditions on my scope (F/10).

[weega]

I can vouch for both the 2x and 3x TeleXtenders of Meade (1.25"). Really good piece of kit. Neither shows any vignetting on my 14mm UWA, though the 3x UWA and 14mm combo is only useful in EXTREMELY good conditions on my scope (F/10).

You have a f/10 telescope: with a slow ratio telescope it's hard to vignette even with ultra wide angle eyepiece.

Look here:

Seeking barlow "clear aperture requirement(?)" formula - Astronomy Forums | Telescope Forums & Reviews | Astronomy Community (please erase it if it's forbidden linking to other forums)

<<You could trace rays. Put the largest eyepiece you want to use in your hypothetical barlow and see how large the negative lens would have to be avoid cutting off the edge rays. The answer would be roughly the size of the field stop of the eyepiece plus the distance from the negative lens to the eyepiece divided by the product of the telescope's focal ratio times the magnification factor of the barlow.

D = d + s/(f * m) approximately.

D = diameter of negative lens

d = diameter of eyepiece field stop

s = distance from negative lens to field stop

f = focal ratio of telescope

m = magnification factor of barlow>>

ciao

dan

[michael.h.f.wilkinson]

You have a f/10 telescope: with a slow ratio telescope it's hard to vignette even with ultra wide angle eyepiece.

Look here:

Seeking barlow "clear aperture requirement(?)" formula - Astronomy Forums | Telescope Forums & Reviews | Astronomy Community (please erase it if it's forbidden linking to other forums)

<<You could trace rays. Put the largest eyepiece you want to use in your hypothetical barlow and see how large the negative lens would have to be avoid cutting off the edge rays. The answer would be roughly the size of the field stop of the eyepiece plus the distance from the negative lens to the eyepiece divided by the product of the telescope's focal ratio times the magnification factor of the barlow.

D = d + s/(f * m) approximately.

D = diameter of negative lens

d = diameter of eyepiece field stop

s = distance from negative lens to field stop

f = focal ratio of telescope

m = magnification factor of barlow>>

ciao

dan

I also stuck it in a little Konus 4.5" F/4.4, and had no problems, same for a little F/5 refractor. In the latter it did show serious chromatic aberration, but that was purely due to the cheapo achromatic objective (it was really only intended as a BIG finder scope).

[weega]

I also stuck it in a little Konus 4.5" F/4.4, and had no problems, same for a little F/5 refractor. In the latter it did show serious chromatic aberration, but that was purely due to the cheapo achromatic objective (it was really only intended as a BIG finder scope).

good to know!

I didn't want to say that meade tele extender is not good

I'm sorry if it seemed so (I need to improve my English! :-P)

ciao

dan

[michael.h.f.wilkinson]

good to know!

I didn't want to say that meade tele extender is not good

I'm sorry if it seemed so (I need to improve my English! :-P)

ciao

dan

No worries: your were right that an F/10 scope is more forgiving. Anyway, your English is probably a lot better than my Italian

Cheers

Michael

[gareththegeek]

So the TeleVue Barlow will not work with a diagonal unless I put the Barlow into the scope and then put the diagonal into the Barlow? That's good to know because that would be annoying.

How about the TeleXtender, will that work with a diagonal in the 'normal' way? I hope so because I have now ordered one

I know I don't have to worry too much about eyepieces with an f/9 scope but one day I will likely get a larger aperture f/5 reflector and I don't wanna start again with my eyepieces!

Many thanks for all replies.

[michael.h.f.wilkinson]

I do not really see why the TV Barlow cannot be put behind the diagonal. By inserting the diagonal into the barlow, you increase the distance between barlow and EP, which means its magnification will go up.

I have used my TeleXtender in a 2" diagonal, and put the EP in, no problems. a smaller 1.25" diagonal might produce some vignetting, but I do not think so. Because of the barlow, you are using a very small part of the image plane of the scope, so vignetting is less of a problem.

[John]

The TV 2x Barlow is a "long" design so you put it between the diagonal and the scope. The "shorty" types like the Celestron Ultima and the TAL 2x go into the diagonal which is more convenient IMHO, especially in the dark !. The "shorty" types can also therefore be used with 2" diagonals.

The TV Powermate 2.5x is an "in the diagonal" device as it the Meade 2x 2" TeleXtender and (I think) the 2" TV Powermates.

[Rossco72]

The TV Powermates can be used either side of the diagonal. I suspect the Meade design would be similar.

[gareththegeek]

Well the TeleXtender arrived from the US of A today so I will know soon enough!

On a side note what is the deal with the box that thing comes in? It is so superfluously huge, nice though.

[John]

Well the TeleXtender arrived from the US of A today so I will know soon enough!

On a side note what is the deal with the box that thing comes in? It is so superfluously huge, nice though.

Yes - I had the 2" 2x TeleXtender for a while and box was the largest of any eyepiece I've ever seen - larger than the Nagler 31mm and the 28mm UWAN even.

Probably not very environmentally friendly and I wonder how much of the price is packaging ?.

Are you expecting to get hit for import taxes on it ?.

[michael.h.f.wilkinson]

The huge box does protect it very well from knocks inflicted on them during transport.

[gareththegeek]

It seems duty is paid on items over £135 (which this wasn't otherwise I would have just ordered it from the UK). Is that right?

[John]

It seems duty is paid on items over £135 (which this wasn't otherwise I would have just ordered it from the UK). Is that right?

It will just be VAT then plus the Parcelforce handling charge.