High magnification EP vs. Barlow lenses

[cijunet]

Hi everyone,
I have a tiny dilemma and I am sure you can solve it quickly. I'm interested in buying an EP with a higher magnification power than my 9 mm that I have.
I did my homework and I know that for my 650 mm FL and 130 mm aperture, the maximum magnification power that will be worth investing in is X 325. That will be achieved pretty much with a 3 mm or 2,5 mm EP. There is also the option of using a barlow lens 2x or 3 x. From what I red, a barlow will add more glass to the EP adding some distorsion to the light. How big this inconvenient would be? What should I choose between this two options?  I do already own a 20 mm and 25 mm EP, so reducing to half of the FL, will give me 10 mm, and as I said, I already have a 9 mm one. Any advice would be highly appreciated.



Thank you,

Adrian 

[Deleted]

That is a toughie. I would go with the barlow or powermate 2x.
An eyepiece in that size is hard to look through,but is great for planets.
I am sure some more in the know will chime in.

[Deleted]

The rule of thumb for the Best optics is no more than 50 times magnification per inch of aperture. But 30 times tends to be more practical for standard optics. So your maximum useful magnification should be no more than 260 X Which still makes it a 2.5mm eyepiece that you're looking for.

The exit pupil (size of beam coming out of the eyepiece) of an eyepiece that small is very small - [aperture / magnification = exit pupil ] resulting in an exit pupil of only 0.5 mm. Any exit pupil under 1.0 mm is restricted by floaters in our eyeballs and is therefore not very useful due to impairment / limitations of our natural vision.

A standard plossl eyepiece has four elements. A classic barlow has two. A Televue Powermate has four but makes up for the loss of light through-put with spectacular optics. Most eyepieces of the 3.5 or 2.5mm size are not four element plossls but something with more elements in order to get proper eye relief (the distance from the field lens to the focus point at your eye)as well as a decent sized field lens (not a tiny peep hole). Thankfully a barlow pushes the exit pupil outwards (I believe)

So, with your eyepiece collection, I would go with a 3X barlow. I have the 2X and 3X Televue barlows and really enjoy them. My smallest eyepiece is currently an 11mm Plossl. Note the 3X barlow is very long -- longer than the 2X, which could poke into your light path if you have a Newtonian reflector.

After the barlow the next eyepiece I'd go after wouldn't be an eyepiece but a UHC filter. This greatly enhances all nebulae. It's the ultimate for the Helix. Some of us won't buy a new eyepiece or filter unless we've field tested it first -- usually by borrowing one from one of us while we're all observing. The pitfall of trying out new eyepieces is you can be suddenly dissatisfied with your old ones.

Hope this helps Adrian.

Darrell.

[cijunet]

Thank you gentlemen for your answers. Darrell, I see what your point is and now it makes sense. Although I kind of decided to go with a Televue 3x barlow (found one on ebay at $121), I will wait until I will
get together with one of you and with your kindness, I will borrow one from you and give it a try. As you mentioned, I am afraid that a 3x barlow will interfere with my mirror. Usually high magnification is used only for observing planets, or
it is common for nebulae and other deep sky objects as well?

[Deleted]

High magnification is also good for planetary nebulae -- that is the brighter and smaller ones such as the Ring M57 in Lyra, Cat's Eye NGC 6543 in Draco, Eskimo NGC 2392 in Gemini, even the Little Gem NGC 6818 above the Teaspoon in Sagittarius. For fainter, more diffuse planetaries and other nebulae, even galaxies, the advantage of more magnification can be lost to sky brightness. However, for star clusters, more magnification can bring out fainter stars, so if sky conditions are favourable higher powers can be useful on Globulars and open clusters.

I've never seen it in writing but I've theorized that the Seeing dictates / limits the amount of magnification for all telescopes the same. So some nights the atmosphere stability yields a 100X night or a 150X night or a 250X night. When the seeing is lousy it might not allow anything over 65X and I personally believe that holds for all telescopes regardless of aperture. This might not be the belief of the majority of experts / observers. It's just that the larger scopes start with higher 'low' powers so they can be hooped easier. More conversation on this would be welcome.

On my scope with a focal length of 2231 mm my very lowest power is my 35mm Panoptic at 64X yielding an exit pupil 6.5 mm -- that my old eyes can barely handle. My 11 mm Plossl gives me 203X and sometimes the seeing won't even allow that -- it just turns the stars into blobs. But on an exceptional night my barlows will give me 406X and 609X with the 11mm and I'm ready for that to search out the central star in the Ring, etc. However my field of view at 609X is less than 5' and objects fly across the field terribly fast.

Magnification isn't the end all and the camera store tiny junk scopes abuse the public's ignorance of that with grand power advertisements and Hubble images. Optical quality and seeing greatly limit the use of magnification.

DSD.

[cijunet]

Thank you Darrell for your conclusive explanation on how magnification, "seeing", exit pupil and other factors are closely interrelated.
I guess the best way of learning is to spend as much time as you can under the stars. Unfortunately here, on the West coast, we are not spoiled much with clear sky. On the other hand, as with any other desirable thing that comes short, once you have it you will make the best out of it. See you at the next coming "starry night".

Adrian

[Adhamh]

Adrian,
This can be helpful to understanding the limitations of a barlow and powermate:
www.televue.com/engine/TV3b_page.asp?id=53&Tab=_back

This calculator is very helpful in understanding what EP's to use for any scope once you input all the pertinent information:
www.televue.com/engine/TV3b_page.asp?return=Advice&id=82

And more reading if you are so inclined:
www.televue.com/engine/TV3b_page.asp?id=154&Tab=#Eyepieces

If you have questions please post them. Also, it looks like we should get some clear sky's for the next two nights.

[welshbc]

This is a most interesting discussion, as this past few evenings I have been mulling over whether or not I should be investing in different eyepieces that may be better to use with my new outfit and taking into consideration my 80 year old eyes.
Later this week I will write out what I have on hand for perhaps your opinions.
Alan

[Deleted]

Thanks for the links Adhamh. A good read.

I've leaned heavily into the TV barlows since I only have most of their plossls. Only my low powered, widest field 35mm Panoptic doesn't need it. As for over extended eye relief causing vignetting when using the lower powers / longer focal length eyepieces, I usually only use my barlows for my medium and higher power plossls. Then the slight increase in eye relief is welcome and I've noticed no vignetting. Otherwise I always use my selection of plossls as is so there's fewer elements in the way. (I'm a bit of a contrast fiend.)

DSD.

[cijunet]

Hi Adhamh,
Thank you for the links. Very informative. By the way, last week I tried out the viewing place you were mentioning on a previous post (by the Fraser river). Works pretty well for a quick escape during the week. If the weather behaves well, I may give it a try again tonight.