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Old 12-07-2019, 08:01 AM
RussellH
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To Barlow, or not to Barlow, that is the question...

So now that Iíve decided on an f6 6Ē Newtonian scope (900mm focal length), one of my considerations is how Effective it may be at getting some sort of planetary images. I know itís not made for that, and somewhere down the track (a long way probably) I may get a planetary scope.

So in the meantime, as Iím just about out of money for my new setup, Iím wondering whether a cheap Barlow of some kind will give me any pleasure at least visually, even if not for imaging. Something like a Powermate is out of my price range at this stage and would probably be low on the priority list of things to get for the immediate future. So itís either a cheap Barlow, or nothing.

Any thoughts?

Thanks.
Russell.
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Old 12-07-2019, 08:42 AM
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I have one of the Bintel 2x Barlows...perfectly decent enough, Iíve used it for planetary imaging, see here http://www.iceinspace.com.au/forum/s...d.php?t=175567 .

Your biggest challenge will be tracking...itís a must once you get into the focal length range where you can see interesting details.

Small SCTs and Maks can be effective planetary scopes because of their inherent long focal length, made longer with a simple Barlow, but the same problem applies regardless....without some kind of motor drive, theyíre practically impossible to use for imaging.
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Old 12-07-2019, 08:51 AM
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I hope to have good tracking eventually, although I’m starting with a generic 50mm guide scope and an ASI120MM mini, so room for error with long focal lengths.

You your image was taken at 4500mm? I’d need a 5x Barlow for that. Been about 35 years since I’ve looked through a Barlow lens. Back then it degraded the image horribly. Can’t imagine what a 5x would have looked like.Are they better these days, or is it just a matter of needing a stable enough image to be able to take super long exposures?
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Old 12-07-2019, 09:08 AM
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By tracking I mean the mount needs to be tracking. Doesnít need to be guided. The mount should be able to track well enough to keep a star/planet in the FOV for a short while without intervention. Donít over complicate it with guiding

Yeah so that image was captured at ~5000mm, maybe slightly higher. The beauty of a classic Barlow is that you can vary the spacing from the lens to get a different multiplication factor. The Edge 11 is 2800mm native.

Even cheap Barlows can be pretty decent these days, as maybe is demonstrated in the image Planetary/guide camera sensors are so small that theyíre really only using the centre of the glass. You might notice more aberrations with a low power eyepiece, compared with a Powermate say, but if you donít need the wider FOV, it doesnít matter. Planets are very small relative to the telescopeís FOV.
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Old 12-07-2019, 09:30 AM
RussellH
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OK. Big difference between 900 & 2800 though, so not sure your lovely image is really something I can judge buy.

I’m buying a 2nd hand HEQ5 Pro with the belt mod. Hopefully that tracks good enough. I’m going to work on my Pa alignment with whatever modern method I can find to get it as spot on as possible. Probably levelling the tripod will be my worst alignment
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Old 12-07-2019, 09:38 AM
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Also, 2x Teleconverter is also barlow (if you do not mind M42x1mm thread).
I have Zenit tele extender, it is 4-element system, it was quite cheap on rugift.com ~10-15 years ago
It can be boosted to 5x with just longer extension tube between camera and converter ( I can not say if it is better or worse than much-more-expensive TeleVue Powermate, but I can tell I havent noticed even a hint of CA (contributed by converter) when using it on planets last year).

Last edited by bojan; 12-07-2019 at 09:54 AM.
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Old 12-07-2019, 09:44 AM
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As I said, SCTs/Mak have that inherent focal length advantage for planetary, but no reason you shouldn’t try with a 6” and suitable Barlow. I’ve seen perfectly good images taken with 6” scopes. It all depends on your expectations (hint: a bigger scope helps, but isn’t the be all and end all in the right hands)

Nothing wrong with a HEQ5
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Old 12-07-2019, 10:04 AM
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Hmmm. A teleconverter would be nice to do double duty for standard photography as well, but even a Viltrox one is $160. The Bintel Barlow’s are $60-$80. Comes down to my budget again.

Do you end up with too many lens elements affecting barlowing a teleconverter?
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Old 12-07-2019, 10:36 AM
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Adding barlow to existing teleconverter ("barlowing") may prove not to be a good idea... I never tried this, but I did add extension tubes to boost FL even further.
At some point, even perfect barlow (or Powermate) will stop helping when atmospheric turbulence is limiting the resolution of the system
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Old 12-07-2019, 12:36 PM
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Ok I'll show my dumbness here.

Barlow, teleconverter, Powermate are all the same thing right?

I assumed they were more a difference in product name (like Celestron, Meade, etc) or are they functionally different too (like Refractor, Reflector, SCT, etc)?
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Old 12-07-2019, 12:46 PM
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Quote:
Originally Posted by sil View Post
Ok I'll show my dumbness here.

Barlow, teleconverter, Powermate are all the same thing right?

I assumed they were more a difference in product name (like Celestron, Meade, etc) or are they functionally different too (like Refractor, Reflector, SCT, etc)?
Well, they serve the same purpose - they increase the folcal length of the telescope by certain factor.

In some telephoto lenses, you will find a last element (closest to the film.sensor) to be negative lens, single or doublet, sometimes called "field lens" (because it also flattens the field curvature of the front gooups of the optical system).
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Old 12-07-2019, 12:46 PM
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Well I’m no expert, but this is my understanding.

First off, a teleconverter is a lens adapter that mounts on the camera, has camera glass type optics, and multiplies it’s focal length. A Barlow is an add on eyepiece for a telescope with a simple lens system that multiplies its focal length. A powermate (brand name, not a term?) is like a Barlow but it has extra optical elements which help overcome some of the optical flaws that’s Barlow’s introduce. I guess it’s almost like a teleconverter for a telescope.

So they’re all slightly different implementations of ways to multiply your focal length.
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Old 12-07-2019, 01:39 PM
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Quote:
Originally Posted by RussellH View Post
Hmmm. A teleconverter would be nice to do double duty for standard photography as well, but even a Viltrox one is $160. The Bintel Barlowís are $60-$80. Comes down to my budget again.

Do you end up with too many lens elements affecting barlowing a teleconverter?

Try this one.. I've got it with Opteka 500mm f/6.3 lens, it is not worse than standard barlow.


There are also heaps of second hand 2x teleconverters (like this one) on ebay.
More here.
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Old 12-07-2019, 01:44 PM
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Hmm. Not sure what I searched for earlier. Was looking for Canon specific, but still, seeing lots more options now than previously? Also never thought to consider T2 lenses. I’ll review what I’ve found.

*edit* ahh now I see, used...
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Old 12-07-2019, 02:07 PM
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Right well, I’ve probably just wasted $20, but I’ve ordered the cheapest nastiest barlows I could find, a set consisting of 3x and 5x (and yes I know 5x is way over what the telescope can cope with). I figure the $20 is worth it to test whether I even want the type of magnification these provide. Then if I know that, at least then I can buy something decent in the size I want. If nothing else, maybe I can use it for birdwatching....

So that’s settled, moving onto the next item...
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Old 12-07-2019, 02:14 PM
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Just be aware that cheapest barlow may be plastic... while telexetenders (used, older) will be glass.
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Old 12-07-2019, 02:21 PM
RussellH
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The listing says it’s glass, and multicoated even, so you know it has to be good ;-). Let’s just call it an experiment. Maybe I can start a youtube channel on what not to buy for Astronomy?

I almost considered the teleconverter, but then I thought about all the extra weight that would add too. Either way, it’s a to-be-continued story in 3 months time.
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Old 12-07-2019, 03:21 PM
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Old 12-07-2019, 03:24 PM
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Russell
For planetary imaging with a DSLR and your new 6Ē f6 newt you need to do the following -

Set your live view to 5 x zoom , this will give you close to 1:1 pixel resolution

As the Canon 450D has a pixel pitch of 5.19 , the general rule of thumb you is you should be imaging at around f21 for average seeing and up to f30 for good seeing

To achieve f21 you will need a 4 x Barlow or Powermate

To achieve close to f30 you will need a 5 x Barlow or Powermate

Your focal ratio for imaging planets is relies on your Cameras pixel pitch um
The increased focal length ensures the pixels can be sufficiently sampled

These images of Jupiter were captured on my 6Ē f6 newt with a 4 x Powermate and 5 x Powermate and my Canon 600D pixel pitch 4.3um

Cheers
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Old 12-07-2019, 03:37 PM
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Tele Vue may be out of you price range but their advice is free and invaluable. See http://www.televue.com/engine/TV3b_p...d=123&Tab=_con

See also http://www.televue.com/engine/TV3b_p...n=Advice&id=85
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