Barlows & Light Reduction

[fauxpas]

OK, my beginner scope came with a plastic piece of crap 3x barlow... Hard to focus thing it is...

So I jump on eBay and get a 2x barlow of much better quality, albeit still very cheap... Much better views through the better quality 2x than the horrible 3x...

But I notice quite a bit of light loss through the barlow... People talk about having a much larger eyepiece selection with the introduction of barlows, but if loss of light is a by-product of using a barlow, I'd rather have a 4mm eyepiece than a 2x barlow under an 8mm...

Do really expensive barlows and eyepieces lose less light or is it a reason to get a bigger aperture scope?

The view last night of Jupiter through my 4" 640mm dob with 6.5mm and 10mm plossls looked good... I'm imagining I'll need a 4mm for a decent view of saturn without needing a barlow...

How much $$$ does one need to spend on a 2x barlow and a couple of decent eyepieces to not be wanting for better optics?

:thanx:

[pluto]

I'm sure someone more knowledgeable than I am can answer this properly but a 2x barlow will halve the aperature of your scope, so f8 becomes f16. Double the magnification, one quarter the light.

[wasyoungonce]

Any Barlow will effectively make the image bigger by increasing the effective focal length.

Your scope will gather a certain amount of light from the object you are aiming at thus "making it bigger" will spread this same amount of light over a larger area (at its focal point) ....thus the object will always look dimmer with a barlow.

Extra optics (glass) will also loose/scatter a % of some incommoding light.

It's a loose loose situation. For visual better to use a decent short FL eyepiece...lets say TV (expensive) or Explore scientific (ES, cheap but very good).

In short, IMHO, dice the barlow buy some ES eyepieces (http://www.vtioptics.com.au/).

[mental4astro]

Hugh, your maths is right though the reasoning is a little off. A Barlow will increase the focal length of a scope by the factor it states, 2X will double it, 3X triple, 5X quintaple it.

Increasing magnification with any scope will result in a dimmer image always. Better quality EPs and barlow's will increase the amount of transmitted light with specialised glass, lens design & quality lens coatings, and give a better overall image. But increasing the aperture of the scope will also result in a brighter image at the same magnification.

There is also a practical limit to how much magnification a given diameter will permit before the image starts to degrade. The rule of thumb being 50X per inch of aperture. So a 4" scope could yield 200X, 8" scope 400X.

BUT! the ultimate dictating factor is the atmosphere! Typical max. being no more than 150X REGARDLESS of your scope! 250X one rare occasions. Anything higher and you should be buying lottery tickets. This though relates to constant steady seeing with nearly no atmospheric thermal distortion.

If you want a larger image, the only way you will achieve this is by 'stacking' multiple images via software that will clean up the image too. A tracking scope isn't mandatory either for this too. There are practical limits, one being just magnifying the image so much for your scope's aperture (and the quality of its components), that the image just becomes too dim, and cheap EPs, such as plossls have an eye lens that is just too small for projection photography. Stacking will also clear up much of the thermal distortion.

Tony, you may be asking too much of your kit for what you want. It's focal length is short, and I wonder about the quality of all its components.

A better quality instrument will be your principle key to better images. If this isn't practical for you right now, try using other photo techniques. You may be asking "Canon" quality images from a Box Brownie.

Edit: Brendan has touched on much the same things as me, :thumbsup:

[barx1963]

I am not a big fan of the old "get a barlow and double your eyepiece collection" advice that is often touted. I have only used good quality barlows (a 2x Televue barlow and a 2.5x TV Powermate) but the view using them is never as good as using a good quality high power eyepiece.
I have used a 13mm Nagler, 13mm Ethos and 10mm Ethos with the barlow and the view in the 6mm Delos or the 7mm Nagler (when I had that) was always better. Specifically, the barlowed 13mm Nag versus the 7mm Nag was only a 7.35% greater mag (231x against 214x) but the image was considerably dimmer and there were noticable more odd reflections with bright objects.
As a result I sold the Powermate and while I still have the barlow, it rarely leaves the case.

Malcolm

[fauxpas]

Thanks for the responses and explanations... It all makes sense now...

I'll grab a couple more eyepieces and throw out those horrible things pretending to be eyepieces, and that 3x barlow...

Then set my sights on a bigger dob...

Do 2" EPs work better than 1.25"?

[mental4astro]

Me too!

The thing with eyepiece barrel size is they will limit how wide the field of view will be.

The best way to explain this is with an example:

Take EPs with 68deg Aparent Field of View (AFOV). The longest focal lenght eyepiece with the 1.25" barrel size that will give this AFOV is 24mm. In the 2" barrel format, this will be 42mm

You can get 40mm EPs in the 1.25" format, but their AFOV will be around 35deg.

I have a 65mm 2" EP, because of the limiting 2" barrel, the AFOV of this lovely EP is around 39degrees.

Now, a 35mm 1.25" EP maximum AFOV is 52degrees. Both the 24mm 68degree EP and this 35mm will show the exact same actual amount of sky, the view through the 24mm EP will appear larger, more magnified and with increased contrast and reduced background skyglow.

[Shiraz]

A good Barlow will certainly not make a poor eyepiece into a good one, but it can make a cheap Plossl into something usable by restricting the angle of the light cone that it has to deal with. A good Barlow should not degrade the image in any noticeable way - it will be darker, but that is simply because it is more highly magnified. Agree with Alexander though, the atmosphere will generally get in the way of high magnification, regardless of what you have.

comment from the Televue site on Barlows:

"I originally designed the Tele Vue Barlow line to complement the high performance of our eyepieces. Only 2 elements of high index glass were required to reduce all aberrations to well below the airy disc in an f/4 system. It has taken a good 15 years, but I think we're finally laying to rest the popular myth of the "degrading Barlow." In a 1997 Sky & Telescope review, Terrence Dickenson wrote "...Technology has erased the old objections. A modern Barlow will not degrade your telescope's optics. Anyone telling you otherwise is using outdated information. Moreover, the highly regarded Nagler eyepieces and their clones have built-in Barlows - ample evidence that the lens is not some detrimental intruder." Thanks Terry, I couldn't agree more. — Al Nagler"

[pluto]

Ah I was just relating it to what I know of modern camera lenses where the aperture is independent of the focal length (mostly), thanks for the explanation.

[Barrykgerdes]

For the price of eyepieces that are likely to give any improvement with a scope of that size you could buy an 8" newtonian OTA and use the existing eyepieces to get a greatly improved view.

Barry

[Vegeta]

I've only ever coupled my 2x barlow with my 6mm eyepiece twice, which yeilded a magnification of 500x in my 12" Dob, but the seeing was PERFECT. any other time, and the image wouldv'e been an optical mess:)

Normally i feel really lucky if i can pull off 250x:D