Posts Made By: Mike Hosea

http://www.televue.com/engine/page.asp?ID=143

Dan, I think maybe you've misunderstood the discussion in Rutten and van Venrooij, although it has been awhile since I read it myself. It has also been some years since I owned a Type 1 Nagler, but I don't remember much curvature to it. There is significant INTERNAL field curvature to the upper half of the eyepiece, but the field group is designed to match it. The resulting eyepiece as a whole has, I think, unremarkable field curvature. Both the Barlow effect and the internal curvature matching could be said to be responsible for the good correction of the eyepiece, but as Nagler will tell you if you ask, the eyepieces were designed as whole systems.

Besides the additional lens surfaces, the deleterious effects of these negative-positive schemes are debatable, but generally I think the internal baffling of the eyepiece becomes that much more challenging, especially if the field group isn't much oversized.
--
Mike

Doug,

The freezing point drops with a higher concentration of alcohol. I don't know the composition of the Orion lens cleaning solution, but I'd guess that it's not likely to freeze until the temperature drops well below 0 degrees Fahrenheit. However, I seriously wouldn't attempt to use any liquid in extreme conditions. If you want some insurance against fingerprint disasters, I'd keep a plastic syringe pump or bulb syringe for blowing off particles and one or two fresh, new, unopened microfiber lens cleaning cloths (look near the contact lens solutions at Wal Mart, for example, for small ones, about 7"x6"). Then if disaster strikes you can blow off particles and break out a fresh, clean microfiber cloth to save the day. I generally do not recommend lens pens, but here is an application for a fresh lens pen as well.
--
Mike

dan coyle said:
Is this a common problem with threaded helical focusers?

I had this problem both with the original Teleport helical focuser and also with the Gary Wolanski unit that I replaced it with, although maybe not quite as bad as what you describe. Otherwise, I loved the Wolanski's fine focusing. It is not a problem with the Kineoptics HC-2 Helical Crayford ( http://www.kineoptics.com/HC-2.html ) that I use now, but the focus rate on it (0.75" per revolution) is just a bit too fast for my taste. Still, it'll have to do. For one reason or another, conventional focusers aren't a good option for either of my lightweight Newts with small secondary mirrors. I have the HC-1 ( http://www.kineoptics.com/HC-1.html ) on my little Newt and like it a lot (it does 0.5" per revolution).
--
Mike

and 5mm Tak LE back together...

I want to share some more tidbits that I've found out about this eyepiece.

Owing possibly to the reuse of tooling for the casing for the 7.5mm Tak LE, you can actually use this particular 5mm Tak LE (and presumably all the other 5mm Tak LEs out there) in 3 different configurations:

1. The intended 5mm configuration.

The following two configurations are achieved without the spacer ring, which is the ring with the lettering on it. When you remove this ring, it exposes female threads that accept the Barlow lens cell and male threads that accept the barrel.

2. Without the Barlow and spacer ring. I haven't measured the focal length of the resulting 3-element eyepiece, but based on the AFoV and field stop it's probably around 11mm. It shows some astigmatism at the edge in my little 120mm f/8.3 Newtonian, but still seems pretty decent at that f-ratio. Note that I said "without the Barlow" rather than "without the Smyth lens". The reason is that I tested the 2-1 positive assembly for field curvature and found nothing significant.

3. Without the spacer ring but with the Barlow. This shaves about 9.4mm off the separation between the field stop and Barlow lenses. Here again I haven't measured the focal length directly, but I think the resulting eyepiece has focal length in the neighborhood of 6mm. I just tested it briefly on a star (lots of clouds out right now), but as you might expect it seems pretty well-corrected in the 120mm f/8.3 Newtonian.

Just thought you might find that interesting.
--
Mike

Nothing wrong with the question or questioner in this case, especially since there really isn't a good answer and doubly so because this is the forum where a beginner should feel free to ask anything related to the hobby, but the title of the thread reminded me of one of those demotivational posters, and if you haven't seen it already, it's good for a chuckle:

http://www.despair.com/cluelessness.html

Retinal coverage for the view as a whole is entirely a function of AFoV, and if you wish to get a larger retinal area into play for a given object, you increase the magnification, which means DECREASING the exit pupil. However, a larger exit pupil means a higher intensity, which will get more reaction from the retina per unit area. It's this balancing act, getting more total retinal area involved with seeing a given object without sacrificing too much intensity per unit area that makes optimal eyepiece focal length selection for DSOs a non-trivial matter.
--
Mike

Dave Adams said:
I have a new TV zoom and wondering if I should clean any eyelash oil off the glass? Is it better to just leave it alone or will this eat away at the coatings over time?

I really don't know about the long term effects of eyelash oil specifically, although I have seen coatings on some used eyepieces that were discolored permanently by what appeared to be just ordinary deposits that weren't cleaned. I suppose the problem occurs when some weak oxider gets on there and has plenty of time to work. Personally, I think you should clean filmy deposits at your earliest convenience using Al Nagler's instructions on the Tele Vue web site. I don't see the point of having a premium eyepiece that has an eye lens covered with gunk. If what you care most about is preserving your investment, don't use the eyepiece at all. If you want to use the eyepiece, clean it properly as needed. That's what I think.
--
Mike

The formula for Barlow magnification is (in one form)

M = 1 - D/F

where F is the focal length of the Barlow (always a negative number) and D is the distance from the optical center of the Barlow lenses to the projected focal plane (which would be at the field stop of an eyepiece, film of a camera at prime focus, etc.)

Obviously, for a 2x Barlow, F = -D. Yes there would be different focal length 2x Barlows. Pick any F you want and then make the Barlow long enough (or short enough) in physical length that D = -F. I only know the focal length of the 1.25" Tele Vue 2x Barlow because Al Nagler once quoted its focal length on the old Ask Al forum. For that one, F=-74.4mm.
--
Mike