EP choice, plan for no Barlow?

[Dazzyd]

Hi guys

I used to be an electronics engineer and know that each time you add a connector into an audio chain (for instance) you lose roughly 0.5dB signal at each connector. So, is the same true when you start adding things like a Barlow? Is it better to have a 5 or 6 mm EP and no x2 or a 10mm and a x2? Or does it purely come down to optics quality?

I'm thinking of replacing the stock 10mm and also getting a 5 or 6mm, both Vixen NPL (or maybe BSTs depending on how much overtime I get on Thursday LOL).

[umadog]

Yes, you loose a little light but this is much less of an issue nowadays because the coatings on glass are very good. Historically light loss was a problem but today it's of little concern: probably on the order of 0.5 or 1% loss. Try stopping down the edge of your mirror to drop light throughput by 10%: bet you won't notice the image being dimmer. In fact, you may even see an increase in performance if you have a bad edge on the primary.

[JamesM]

I agree with the above but like so many things in astronomy is does depend on what you pay for. Barlows design tends to include a negative lens element which has the side effect of increasing the 'exit pupil' as you use longer focal length eyepieces. The exit pupil is the distance from the surface of the last lens element to the point or plane of focus. At the shorter focal length levels this effect is hardly noticeable and some argue beneficial for those like yourself who may wear glasses when they observe, but as you progress through the longer focal lengths (25mm) this exit pupil distance becomes more noticeable and it becomes harder to find the correct eye position when focusing. As a way of solving this problem, Televue Powermates include within their design a positive lens element that effectively cancels out this problem allowing existing exist pupil distances to remain the same as the original eyepiece. As you might imaging, anything with a green writing on it doesn't come cheap and this product is expensive but certainly once bought will not need upgrading and is an item worth keeping. If you budget is more restricted, I would certainly recommend using a x2TAL barlow (FLO £35) as they are pretty good performers given the above parameters.

James

[swamp thing]

Hi Daz

Whether you chose to use a Barlow lens or not is up to you but. I would read this quote by Al Nagler, who knows a thing or two about eyepieces/Barlows etc.

Concept: 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

Bear in mind this is from 1997 so Barlow technology has moved on even more since. They take away nothing from eyepiece performance and just add power.

[damnut]

Hi There,

Both good replies above. The TAL barlow has an excellent reputation for the cost. The televue 2* barlow is around £100. So if you go for the Tal, then if you budget a £100, that would leave you two new BST's in the budget. (if the price had not gone up, they were £36 now £47)

If you get from here... http://www.skysthelimit.org.uk/telescope%20eyepieces.html

Then you can try & buy. The BST's are great eps for the money.

Good Luck

[perrin6]

Extra lens surfaces also mean extra dust as well as more glass between you and the objective.

[LukeSkywatcher]

I now prefer to observe without a barlow.