To Barlow, or not to Barlow, that is the question...

[RussellH]

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

So in the meantime, as I’m just about out of money for my new setup, I’m 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 it’s either a cheap Barlow, or nothing.

Any thoughts?

Thanks.
Russell.

[Camelopardalis]

I have one of the Bintel 2x Barlows...perfectly decent enough, I’ve used it for planetary imaging, see here http://www.iceinspace.com.au/forum/showthread.php?t=175567 .

Your biggest challenge will be tracking...it’s 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, they’re practically impossible to use for imaging.

[RussellH]

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?

[Camelopardalis]

By tracking I mean the mount needs to be tracking. Doesn’t 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. Don’t 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 :D Planetary/guide camera sensors are so small that they’re 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 don’t need the wider FOV, it doesn’t matter. Planets are very small relative to the telescope’s FOV.

[RussellH]

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 :)

[bojan]

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).

[Camelopardalis]

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 :thumbsup:

[RussellH]

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?

[bojan]

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

[sil]

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)?

[bojan]

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).

[RussellH]

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.

[bojan]

Try this one (https://www.ebay.com.au/itm/2X-Tele-Converter-Teleconvertor-Extender-Lens-For-T-mount-Lens-T-Mount-Adapter/182580712266?hash=item2a82a8934a&_trkparms=ispr%3D1&enc=AQADAAADAFjVrDbVsZ8oH%2F8PNHtt9 VX4%2Fw7FZcmMuqsX8uaFEduVs8C6qLqMxN nPv6dLEVi%2FbW78ONeuO8D%2FyC0TcUDoK vRJ47hjhr0eg3V2GlnR1NW2MDxLCBnuK6Ri GS5G%2BVg4CW69bKBkoAyYIXATn8HtlN0%2 F4D01K4JK3rkgF80d9p8HX0g61FfyALhD61 gsmXNXKoaZs%2B%2B3l0Jl4lFwDPn8xi3eO ZtnzI80yF4XHlL79Pd35WERqvMQEk0b28gv koeWggbIG9TObBpl9C7siIdx%2BJNRaB2qf r3mbIdmpEqq%2BZF%2FI2RYzdPerZgdtMEv 6vn%2BS%2BJYlTAuAw3l8XRa5uVvsOijOl3 w4w0vOWd0ob2J%2FJjgVwyQUKzFumwA5SEK ErqB4WYUx8fE0arSn3CqPFSzO3rQDfEWlJe y%2FG9Octg%2FdVGM%2BgqdnGDYHsXabP4y ZWXk61M3ekupzRY4hhxOjerrdgdoyxQSZvj 6t4aJ%2FpsFYarOXcGMP7IuYz%2Bd5SxaYP lFf0tktt5YXCPRG7rQP0KWsJ37ioOXZEmYA lO6mTfFzU6nP0snf3EEWzSCmP%2FIQRHYZL E1IbFNUYaWnfz6XbEARNZzn0NAf4BkPUyXB tN2RV4Xv%2Bc%2Bi%2BxmmMMqto1qL2hzqY dK2PGM3gtp9t%2BVVr9qjSzXRgjnnI%2BgK amnge%2BeBEWF%2Fvsc4xBHaomNmCSN16o9 3KrIWS8AWpdSUU2flClGy2EXHgg0UcscdZf C91m9n0YP%2F3%2FTOdQeCA3gpU82ZPv8Un %2BE3o5jFN%2FO0B6OEArFzGFvp3MiI4Sp8 Ang3IERlanuHS28coCjgDCQH8yEP%2ByxOc t9tZY1xK2E%2Fm3ArI%2FbKPD1YfFBWVXMY 4w8I%2FwxD7v3%2B3e8fIJNEguXKwKnIcQp Atc1oa5YWqQIcY7GMzmH2zXLOJ4huJ6r92f cCV5tmQXZdkMOPLnkU%2B05H6m1H0Y557C4 wVMlZ7Ny6LwymTu8TbvDz5f9qx4hem2SUcr PYDifmWHZwXeik1%2BG%2FCy2XPUQ2A%3D% 3D&checksum=182580712266a5e9aebe17934e e18a9fcf69049938fa).. 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 (https://www.ebay.com.au/itm/Benny-2-x-Tele-Converter-for-P-K-Japan/163772778392?hash=item26219e4798:g: ~dAAAOSwz7hc5NFz)) on ebay.
More here. (https://www.ebay.com.au/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR12.TR C2.A0.H0.Xtele+converter.TRS0&_nkw=tele+converter&_sacat=0)

[RussellH]

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...

[RussellH]

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...

[bojan]

Just be aware that cheapest barlow may be plastic... while telexetenders (used, older) will be glass.

[RussellH]

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.

[bojan]

:thumbsup:

[Startrek]

Russell
For planetary imaging with a DSLR and your new 6” 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 6” f6 newt with a 4 x Powermate and 5 x Powermate and my Canon 600D pixel pitch 4.3um

Cheers

[astro744]

Tele Vue may be out of you price range but their advice is free and invaluable. See http://www.televue.com/engine/TV3b_page.asp?id=123&Tab=_con

See also http://www.televue.com/engine/TV3b_page.asp?return=Advice&id=85

[RussellH]

Cool, a nice visual explanation. Will have to read when I have time to study in depth, thanks.

[Orville]

I have a quetion on barlows and hope I am asking in the right place. I want to purchase a 2" x 2 Barlow. It will be used primarily for viewing, and probably in future for astro photography , My budget is around $200 , any recommendation will be appreciated .. thanks.

[xelasnave]

Welcome Simon nice to have you here..I live in Australia also maybe we will run into each other.
Buy a cheap one and after the novelty wears off thank me for the money I saved you. Maybe later when capturing planets go for a 5x but in my view for visual perhaps think eye pieces.
That is my view and hopefully we get someone else to provide some balance.
Alex

[xelasnave]

What scope do you have?
Alex

[Orville]

Hi Alex, I have a 8" celestron nexstar evo on back order. Havent received it yet. Its my first scope and after doing lots of homework looking forward to getting it.. ! in some of my reading i found that using a barlow can improve eye releif on a planetary eye piece, so that got me interested.. thanks for advice and i have turned down the dial on my budget.. i will probably go for a GSO I think they go for around $90.. if the story about improved eye releif is true then it will be worth it as my other half cant see without glasses , cheers Simon

[bojan]

Please note, wearing glasses is relevant only if your other half has astigmatism.
In other cases, you can always adjust focus by means of focuser :-)