Should i use a barlow?

[Mohsen Bahremand]

Hello everyone

I have a Sky-watcher  heritage 150p with 750mm focal length. When i use a 4 mm eyepiece which gives 188x magnification, i can still see the details on moon very clearly without any blurriness. This makes me think that there's still room for using a 2 or 2.5 barlow although it passes the max 300x magnification.

Do you think that it might work?

Can it make any problem with focusing? For instance  should i put them so close to the secondary mirror that it doesn't fit the tube?

[Alan64]

With my solid-tube 150/750, I use 2x and 3x barlows, but with a 12mm eyepiece, and for an effective, simulated 4mm(3x).  Then, the 12mm has the benefit of a larger eye-lens with which to look through.  With a manual mount, I've found that to be nigh to the limit with that telescope(188x).  However, according to the 50x-per-25.4mm "rule", a 150mm aperture can at least approach 300x, so the former is not set in stone.  Personally, I love to ramp up the power with my telescopes.

I've had a 70/900 achromat up to 225x, on Polaris, and the image was clear enough, not bad at all.  

You do want short barlows with a Newtonian, the shorter the better.  My own are fairly short, the one in the middle particularly...

Some, if not many, opine that excessive magnification results in so-called "empty magnification" -- blurry, soft images -- and at times to the point where you might feel that the telescope will explode if you do so.

Per said 50x "rule", my 70/900 achromat should only reach 138x, yet I've had it up to 225x, and meaningfully, satisfactorily.

In short, by all means, shoot for the Moon.   

Edited May 17, 2021 by Alan64

[Mohsen Bahremand]

7 minutes ago, Alan64 said:

With my solid-tube 150/750, I use 2x and 3x barlows, but with a 12mm eyepiece, and for an effective, simulated 4mm(3x).  Then, the 12mm has the benefit of a larger eye-lens with which to look through.  With a manual mount, I've found that to be nigh to the limit with that telescope(188x).  However, according to the 50x-per-25.4mm "rule", a 150mm aperture can at least approach 300x, so the former is not set in stone.  Personally, I love to ramp up the power with my telescopes.

I've had a 70/900 achromat up to 225x, on Polaris, and the image was clear enough, not bad at all.  

You do want short barlows with a Newtonian, the shorter the better.  My own are fairly short, the one in the middle particularly...

Some, if not many, opine that excessive magnification results in so-called "empty magnification" -- blurry, soft images -- and at times to the point where you might feel that the telescope will explode if you do so.

Per said 50x "rule", my 70.900 achromat should only reach 138x, yet I've had it up to 225x, and meaningfully, satisfactorily.

In short, by all means, shoot for the Moon.   

Thanks a ton

Complete and satisfactory answer!

[Alan64]

There is one more thing...

As magnification is increased, the telescope, the eyepieces, the optical-accessories(barlows, et al), all have to work harder, and for pleasing images at the higher and highest powers.  A Newtonian is a continual work-in progress, as it must be collimated regularly to maintain those pleasing images.  It must also be collimated as precisely as possible via its mechanisms, the secondary-assembly in particular.  The alignment of the focusser plays a most important part as well, and as it racks inward and outward, ideally straight and true, with no binding nor slop.  All three components, the primary/secondary assemblies and the focusser, must be aligned with one another, as accurately as possible, and in an L-shaped light-path...

It's the secondary-assembly that gives the most fits.

[Louis D]

15 hours ago, Alan64 said:

You do want short barlows with a Newtonian, the shorter the better.

For refractors, I'd agree due to infocus issues with a diagonal, but this is not an issue with Newtonians.  I've had good luck with long Barlows in my Dob without them protruding into the light path.  In general, they've been sharper than the shorty or mid-length Barlows.  The only shorty I've found to be very good and still be under $70 is a used Celestron Ultima (Parks GS) 2x Barlow.  I've never tried a Klee or Dakin Barlow, but they tend to cost more than $70, which is about all I'm willing to spend on a piece of gear I rarely use.

[Alan64]

12 hours ago, Louis D said:

For refractors, I'd agree due to infocus issues with a diagonal, but this is not an issue with Newtonians.  I've had good luck with long Barlows in my Dob without them protruding into the light path.  In general, they've been sharper than the shorty or mid-length Barlows.  The only shorty I've found to be very good and still be under $70 is a used Celestron Ultima (Parks GS) 2x Barlow.  I've never tried a Klee or Dakin Barlow, but they tend to cost more than $70, which is about all I'm willing to spend on a piece of gear I rarely use.

I was thinking more along the lines of a busy "tower of power" jutting up out of the visual-back, rather.  It's not so much an issue with refractors and catadioptrics.

I've always thought of the Tele Vue barlows being a bit on the long side, like this 3x for example...

https://www.firstlightoptics.com/barlow-eyepieces/tele-vue-barlows.html

My 2.8x Klee, a triplet, and no longer available, was $79($150.37 in 2021; madness) when I purchased it in 1992 or '93.  I was simply wanting a barlow at the time, and I was doing mail-order as the internet had not yet arrived, in my area at least.  I got the barlow from University Optics in Michigan, also defunct.  I knew of Astronomics at that time, too, via their mailings.  It turned out, in later years once the internet was in full-swing, that I read that that very barlow is excellent for shorter, "fast" Newtonians, but again, I didn't know it at the time I had purchased it.  It, albeit allegedly, corrects for coma, to some extent at least, if true.  A 2.2x Klee was offered later, but I didn't pursue that one.  Those were the only barlows ever offered by University, in my experience.

The Antares 2x and 3x barlows, shown flanking the Klee, are very good, if not also excellent.  For example, with my 6" f/5 Newtonian, I was using the 3x one night, and that was the first time an image snapped into focus.  It was quite a pleasant surprise.  From that experience, I now suspect that the primary-mirror of the telescope is at least above average in its execution, possibly.

Or, rather, is it possible that the barlow, with its lenses, simply helped the mirror along in that?  We may never know. 

For a 750mm focal-length, one can make use of a 2x, and particularly a 3x, barlow.  3x-barlows, however, are seemingly as scarce as proverbial hens' teeth, if not scarcer.

This is the same 3x-barlow as my own, but updated with a twist-lock...

https://www.rothervalleyoptics.co.uk/antares-x3-twist-lock-barlow-lens-125.html

I don't like spending a lot on eyepieces and accessories, either.  

Then, there is an alternative to barlows: short-to-very short focal-length eyepieces, but they tend to be pricey in and of themselves. 

In the end, I prefer the cheap thrill of a barlow.

Edited May 19, 2021 by Alan64