Barlow vs. Teleconverter (Extender)

[SJP]

I had a go at planetary photography for the first time using my newish Celestron C6 NGT and Canon EOS 6D at the weekend using eyepiece projection (a Baader Hyperion 5mm, 150x) and decided that a little more magnification was required.

What are peoples thoughts on barlow lens vs. teleconverters/extenders?  After a very brief perusal of what's on offer I've found the Celestron Luminos 2" barlow, and I've been after a reason to invest in a Canon extender for sometime.

A decent 2" barlow is the third of the price of a decent extender, and is useful for visual observations as well as imaging, and would fill-in the gap between my cheap Plossl 20mm (~40x) and the 5mm Hyperion (150x) when using the Hyperion in 22mm mode with the barlow (~70x)

A decent extender (taking consideration of the quality implication) is useful for day-to-day photography.

Are there any optical benefits of a barlow over a teleconverter/extender when imaging?

[annamay]

Powermate

As with everything optical, the more in the light path, the more affects it will have on the final image. Whilst a barlow/powermate/extender will give you more magnification, you will trade off some light/contrast et al.

[michael.h.f.wilkinson]

Powermates and TeleXtenders (or by their generic name: tele-centric barlows) are very different from the original Barlow. The principal rays traverse the image plane orthogonally all over the image plane, which means that many of the problems caused by the classic Barlow design are absent. In particular in wide-angle EPs a traditional Barlow can cause vignetting because the principal rays in the outer area of the field of view diverge too much. In my experience with tele-centric Barlows, they work very well indeed. They really "get out of the way", and in my LS35 in H-alpha it is difficult to see the difference between a Pentax XW 10mm and a MaxVision 20mm 68 deg with 2x TeleXtender.

My only gripe with the tele-centric Barlows I own is that they are a bit more of a hassle than just changing EPs. They are absolutely brilliant for planetary imaging, where they get very reproducible results, regardless of the length of the optical path between lens and chip.

[RikM]

The strength of the Powermate/TeleXtender is also (possibly) a negative for imaging. With a classical barlow, as Michael says above, the light rays diverge. This means you can use extension tubes between barlow of sensor to vary the amplification (I'll use the word magnification but it isn't really correct). For a maximum usefulness, especially with 68° Hyperion EP's, I would go for a Powermate / TeleXtender over a regular barlow, but it isn't as clear cut as you might imagine.

[michael.h.f.wilkinson]

The strength of the Powermate/TeleXtender is also (possibly) a negative for imaging. With a classical barlow, as Michael says above, the light rays diverge. This means you can use extension tubes between barlow of sensor to vary the amplification (I'll use the word magnification but it isn't really correct). For a maximum usefulness, especially with 68° Hyperion EP's, I would go for a Powermate / TeleXtender over a regular barlow, but it isn't as clear cut as you might imagine.

It is true that a regular Barlow can be used to vary the image scale using extension tubes, but a variable image scale can also throw your settings in planetary imaging, I find. A really top-tier (Zeiss, TMB, or APM) Barlow can give excellent result in both imaging and visually, but the latter with the proviso that in super-wide-angle and ultra-wide-angle EPs vignetting may occur.

[SJP]

So Powermates offer a better experience than a typical barlow, but how do they compare with a teleconverter such as Canon's EF 2x iii?  For imaging, £400 for the EF 2x iii which I can use regularly appeals more than £200 on something which I would use irregularly.

[michael.h.f.wilkinson]

So Powermates offer a better experience than a typical barlow, but how do they compare with a teleconverter such as Canon's EF 2x iii?  For imaging, £400 for the EF 2x iii which I can use regularly appeals more than £200 on something which I would use irregularly.

a telecentric system like a powermate only makes sense in astro-imaging of planets and the moon. These objects are typically best imaged with planetary cameras or webcams, all with very small chip sizes, and typically equipped with 1.25" nose-pieces. These fit into a powermate without difficulty, but the EF 2X will need an adapter of some kind.

[SJP]

I think I'm erring towards the Canon teleconverter for it's quality optics and wide (full-frame) FOV, connected via my T-Mount adapter - at least then I can keep everything at 2" and use the T/C for normal photography as well.

Thanks for the input!

[michael.h.f.wilkinson]

I think I'm erring towards the Canon teleconverter for it's quality optics and wide (full-frame) FOV, connected via my T-Mount adapter - at least then I can keep everything at 2" and use the T/C for normal photography as well.

Thanks for the input!

The full-frame will only be useful for lunar, not planetary. Jupiter (largest of the planetary lot) can be captured on the 640x480 pixels of my little DMK21 (1/4" chip). There simply is not enough detail to capture sensibly on a multi-megapixel image.

[SJP]

The full-frame will only be useful for lunar, not planetary. Jupiter (largest of the planetary lot) can be captured on the 640x480 pixels of my little DMK21 (1/4" chip). There simply is not enough detail to capture sensibly on a multi-megapixel image.

Now that's a thought!  I've avoided using video-mode on my full-frame 6D because it means the pixel-density is reduced (my older 60D has a 640x480 crop-mode which samples just the central 640x480 pixels rather than the 6D spreading 1920x1080 across the entire sensor surface) so I've been thinking that I "need" full-frame coverage without vignetting to use the full 20MP stills-mode, but actually that isn't necessary as I'm only using the central few hundred pixels.

Still, if a pukka teleconverter/extender works as well as even a telecentric system, then the extra outlay for the increased usefulness may still be worthwhile.  It has made me question whether a 1.25" barlow/other is out of the question as I first thought  though.

[JED-E3]

Some more food for thought,

1. If you have excess inward focus travel on your scope when using the dslr and teleconverter you can add extension tubes between the camera body and teleconverter to gain a bit more magnification.

2. If you fit a teleconverter to your camera and then fit that to a t-adapter, the camera (Canon) won't shoot stills, it gives a lens error code. You can shoot video though.

In order to take stills you'll need to tape some of the pins in the teleconverter or fit a 'dumb' extension tube between the camera body and teleconverter.

3. You can buy a new Canon 2x mk3 teleconverter from Digital Rev for £319.

Hope some of that is of use.

James.

[SJP]

Some more food for thought,

1. If you have excess inward focus travel on your scope when using the dslr and teleconverter you can add extension tubes between the camera body and teleconverter to gain a bit more magnification.

2. If you fit a teleconverter to your camera and then fit that to a t-adapter, the camera (Canon) won't shoot stills, it gives a lens error code. You can shoot video though.

In order to take stills you'll need to tape some of the pins in the teleconverter or fit a 'dumb' extension tube between the camera body and teleconverter.

3. You can buy a new Canon 2x mk3 teleconverter from Digital Rev for £319.

Hope some of that is of use.

James.

I've read about camera's "locking-up" when using t/c's elsewhere, however it's not something I've experienced when using a t/c on an old manual mirror-lens with an adaptall/T2 mount.  Is the lock-up a real phenomenon, or one of those internet myths?  If the former, then taping the contacts and/or the T2 mount shouldn't be an issue for me.

One thing which has occurred to me though is whether the protruding shroud on Canon's EF 2x iii will actually fit inside the T2 adapter?

Thanks for the Digital Rev heads-up, buying grey-imports is something I've been known to do in the past, and the saving is significant

[SJP]

Having tried an old teleconverter I see you're completely right, the camera gives an error every time.

[SJP]

A quick update on my progress....

Trying to double-up on equipment for two different hobbies doesn't work, nor does scrimping by ignoring good advice from fellow forum members, consequently I've ended up investing in:

Canon extender 2xiii

Celestron NexImage5 camera

Celestron Ex-Cel 3x Barlow

Tele Vue 5x Powermate

What I should have done is listened to good advice and bitten the bullet and bought a better camera and the Powermate at the outset and saved myself some £'s!

I hope this post will help someone else follow the right path