Barlows?

[JackW07]

I have recently purchased an 8-inch Dobsonian telescope, and am wondering wether its worth it to buy 1 or 2 Barlow lenses, If so which magnification should I go for?

Thanks much

[RT65CB-SWL]

2X seems to be the most popular size. That said, I have three...

• AstroEngineering 4X
• Klee 2.8X
• TeleVue 1.8X

...all are 1.25"

 

** UPDATE **

now four...

• TeleVue 2X

Edited November 18, 2023 by RT65CB-SWL

[Stu]

Welcome @JackW07

I think my questions before answering would be….

What type of observing do you enjoy doing?

Which eyepieces do you have already?

Stu

[bosun21]

A 2x will suffice imo. I think I used a barlow once when I owned the 8”. With a focal length of 1200mm a 5mm eyepiece will provide 240x which is a decent magnification under UK skies. As Stu said which eyepieces do you have or are bundled with the scope? This will make a bearing on the recommendations you receive as they will try to fill in gaps in the focal lengths of your eyepieces.

[NGC 1502]

3 hours ago, JackW07 said:

I have recently purchased an 8-inch Dobsonian telescope, and am wondering wether its worth it to buy 1 or 2 Barlow lenses, If so which magnification should I go for?

Thanks much

As already asked, what eyepieces do you already have?  Based on your answer then advice about barlows can be made.

If you only have the stock eyepieces that came with the 8” Dob, then most of us on SGL would suggest upgrading those first, rather than buy barlows.  Please don’t make the easy mistake of thinking that very high power is best.  Very high power with an 8” Dob is 250x and above.  Very high power may occasionally be useful but not very often.

 

[michael8554]

Hi Jack

If you only have one 20mm focal length eyepiece, it effectively becomes 10mm with a 2X Barlow.

But if you already have a 10mm eyepiece, you don't need the Barlow.

That's why we're asking what eyepieces you have already.

Michael

[M40]

Hello JackW07, my first proper telescope was an 8" dob, I bought a x2 barlow, then I bought a much better x2 barlow, then realising just how fast everything moves I bought a baader zoom with its associated barlow. To this day I have still not used a barlow but I use the zoom.... a lot. So can I suggest, dont buy a barlow but think about maybe getting an eyepiece that you can take with you on your journey. A zoom will likely give you a range of eyepieces between 8 and 24 and removes the need to keep swapping eyepieces which, imo, works perfectly with a dob. All the best.

[Giles_B]

A Barlow is a useful tool, but not one I think is essential to your kit. I think a barlow was one of the first things I rushed to buy, but I found it didn't get much use. I sold it, then later bought another very cheaply second hand, but found it didn't enhance the views at all. Finally I bought a decent Televue x2, and I'm currently looking for a x3 or x5, but only because I've become interested in photographing the planets and its seems like a barlow is useful for that because the camera otherwise is fixed to the telescopes magnification.

Barlows - like eyepieces - vary in quality and it is worth buying a decent one that does not introduce too much distortion into the image. Even a good one will only be as good as the eyepiece you are barlowing, so worth getting a few good quality eyepieces first, then seeing if the barlow would help extend your range.

[RT65CB-SWL]

Celestron [other brands are also available] do a Barlow lens a 2X, that has removable lens element that you can you can screw into the nosepiece of a 1.25” e/p or star diagonal and have a magnification factor of 1.4x to 1.6X. Also has T-ring on the other end too. [link here]... https://www.celestron.com/collections/barlow-lens

 

Edited November 20, 2023 by RT65CB-SWL

[Louis D]

Being an ultrawide, low power freak, I would recommend getting a GSO coma corrector first.  I demand sharp stars from edge to edge while letting objects drift across the field of view.  The inherent coma in a parabolic mirror prevents this with even the finest eyepieces.  I use the CC during all but high power observing sessions where the small amount of central spherical aberrations gets noticed.

I rarely use a Barlow.  Get a Svbony 3-8mm zoom if you want high power views in a compact pacakage.

[Starfazed]

Chipping in here as a noob....I got a cheap 2x Barlow and a 6mm eyepiece for my 150/750 Skywatcher and chanced upon a still cloudless moonless night in a Bortle 4 area a couple of weeks ago (the first proper one in six months...) I was stunned at how much brighter Jupiter and Saturn were compared to what I'd been expecting to see at 250x magnification, and they completely blew out the image when I tried to capture it on my smartphone (just crudely through the eyepiece). It left me wondering if I need to rock the boat a bit and get a more powerful Barlow. Is 3x (375 magnification) overkill or should I live dangerously and look for a 4x or even 5x?

[Zermelo]

1 hour ago, Starfazed said:

Chipping in here as a noob....I got a cheap 2x Barlow and a 6mm eyepiece for my 150/750 Skywatcher and chanced upon a still cloudless moonless night in a Bortle 4 area a couple of weeks ago (the first proper one in six months...) I was stunned at how much brighter Jupiter and Saturn were compared to what I'd been expecting to see at 250x magnification, and they completely blew out the image when I tried to capture it on my smartphone (just crudely through the eyepiece). It left me wondering if I need to rock the boat a bit and get a more powerful Barlow. Is 3x (375 magnification) overkill or should I live dangerously and look for a 4x or even 5x?

Those higher mag Barlow tend to be used for imaging rather than visual.

One rule of thumb is that the maximum magnification in ideal conditions is around twice the scope aperture in millimeters, so ~x300 for your scope. But, in the UK at least, the usable magnification is often less than this due to seeing conditions. I would save your money for something else.

[Louis D]

2 hours ago, Starfazed said:

I was stunned at how much brighter Jupiter and Saturn were compared to what I'd been expecting to see at 250x magnification, and they completely blew out the image when I tried to capture it on my smartphone (just crudely through the eyepiece).

Try decreasing the exposure with the exposure compensation slider.  Slide it to the left into the negative numbers.  Your camera is trying to expose the dark background sky at 18% gray.  Try the -3 setting for starters.  That should get you in the ballpark of the correct exposure for starters.  Try other settings and see what you get.

[Machinetools1]

How can you tell how good a Barlow is?  Obviously from a price point, but what makes a good one in the manufacturing process. Is it in the Lense design, coatings or how they are put together?

[bosun21]

7 minutes ago, Machinetools1 said:

How can you tell how good a Barlow is?  Obviously from a price point, but what makes a good one in the manufacturing process. Is it in the Lense design, coatings or how they are put together?

All the points you made. How accurate the lenses are cut, the quality of the glass, and high quality multi layered coatings.

[JOC]

In the UK 200P 8" dob. isn't happy at much above 200-250x magnification - that's about a 5mm-4mm (on a really good night).  That's a 10mm x 2 if you want a Barlow.  I have one, but don't use it.  It's a lot of faffing around to constantly put it in and take it out.  Most 200P come with serviceable 10mm and 25mm EP's start with those and make sure that you can see things with them first.  Then if you want closer, sport £40 on some BST EP's say an 8mm and a 5mm and you might find you don't need anything else.  Given hindsight I certainly wouldn't go out and buy a Barlow when you can get a perfectly serviceable EP often for <£50 like the BST's

[Louis D]

2 hours ago, bosun21 said:

All the points you made. How accurate the lenses are cut, the quality of the glass, and high quality multi layered coatings.

I'd put polish near the top of the list.  If the lens polish isn't extremely smooth, you'll get scatter.  This scatter can easily ruin high power, low contrast planetary viewing by placing a veil of haze across the image.

[bosun21]

1 hour ago, Louis D said:

I'd put polish near the top of the list.  If the lens polish isn't extremely smooth, you'll get scatter.  This scatter can easily ruin high power, low contrast planetary viewing by placing a veil of haze across the image.

Yeah forgot to add that.

[Second Time Around]

The amplification factor of a Barlow will vary from eyepiece to eyepiece depending on where the focal plane of the eyepiece is situated.  So the stated amplification factors are only approximate.

The amplification factor also depends on the distance between the Barlow and the eyepiece.  With some 2x Barlows the lens cell can be unscrewed from the tube and screwed into the filter cell of the eyepiece.   This gives an amplification factor of approx 1.5x as posted by RT65CB-SWL.

First Light Optics do their own version (probably identical to branded versions) for only £27.  In fact, they currently have an open box return for £24.  Go to https://www.firstlightoptics.com/barlow-eyepieces/astro-essentials-125-2x-barlow-with-t-thread.html

You can also add an extension tube(s) (£16 each) between such a Barlow to increase the magnification (within reason).  See https://www.firstlightoptics.com/adapters/baader-dt-4-1-31-8mm-nosepiece-extension-18mm-long.html  Each extension tube on the Barlow I linked to above will add about an extra 0.25x amplification. 

 

 

 

Edited November 22, 2023 by Second Time Around

[Spile]

If you have the 10 and 25mm eyepiece that comes with the Skywatcher, I would not recommend any Barlow.  A better way forward would be to save up for decent zoom lens in my opinion.

[Second Time Around]

I agree with Spile's post about buying a zoom eyepiece, but not instead of a Barlow.  A zoom plus a 1.5x/2x Barlow and a low power eyepiece is often all I (and a lot of others) use, although I have a number of other high quality fixed focal length eyepieces.  Your 25m will make a decent low power eyepiece to begin with.

In other words, rather than buying 2 Barlows as per your original post, I'd suggest 1 Barlow plus a zoom.

[Chandra]

Barlow yes, Barlow no... It is difficult to make a recommendation without knowing what eyepieces you have, but a Barlow lens can be a good "trick" to extend the range of focal lengths without spending a lot on eyepieces... It has been speculated that you have 10 and 25mm eyepieces... We also do not know the focal ratio of your Dob, although we presume that it is the traditional f/6 or f/5.9. If this is true, a 2x Barlow will provide you with an adequate range of exit pupils for observing all types of objects. 

An inexpensive but decent Barlow Classic Q-2.25x from Baader will turn those same two eyepieces into a complete range of 6 different focal lengths, since it allows you to disassemble the lens cell to screw it into the barrel of each eyepiece as if it were a filter, also offering a factor of 1.3x. Forget, however, about 3x or 5x Barlow, which would be better suited for imaging applications. 

When it is recommended that the Barlow be quality, it is good advice. Any optics that bends light (objective, eyepiece, Barlow, etc.) better be of quality than not to avoid aberrations. But it would be useless to attach a Barlow Televue to entry-level eyepieces, for example, since you must consider that the optical performance of a system will be determined by the worst (or least good) of its components. So if your eyepieces are what came with your Dob's retail package, you shouldn't spend more than £50 on a Barlow.

[VNA]

Hello, using a Barlow  3X, with a 2 inch barrel, it works very well but not under 20mm
Using a 10" SCT tube.