Posts Made By: Mike Hosea

I would say that the Powermate is preferred for the Ranger because of the Ranger's focuser design. Having a parfocal option (one that doesn't require refocusing much when you swap it in) is disproportionately valuable with helical focusers.

No decent Barlow stops down the light to any point in the view. The cause of "vignetting" when Barlows are used with long focal-length Plossls and such is divergence, which is a consequence of the way any Barlow works in the first place. The vignetting occurs (or not) in the eyepiece depending on how well the eyepiece design is able to handle it. Long focal length Plossls are, unfortunately, not very good at handling it. Powermates avoid the problem by using massive divergence after the first lens group and then cancelling it with the second lens group after the divergence has had the desired effect. With conventional Barlows the problem is minimized by using a longer focal length Barlow, but for a given magnification factor this results in a much taller unit that is nowhere near to being parfocal when used in star diagonals. Again, we're back to recommending a Powermate with the Ranger.

The existence of a built-in Barlow in an eyepiece or not is a red herring. You say you want the widest, clearest, 'picture window' view out of the Ranger, but this will be achieved, IMO, with with an ultrawide or at least superwide eyepiece, and the best of these have built-in Barlows (or something like that). But this is a matter of trivia only. The eyepieces are designed to work as indivisible units only.

I think you might very much enjoy a 9mm Nagler T6, which will provide about the true field of a 15mm Plossl, or a 12mm Nagler Type 4, which will provide a true field of view about the same size as a 20mm Plossl. Soon there will be an 11mm Nagler T6 to choose from also. If cost is the issue, then with knowledge of the budget perhaps some more affordable, best-available options could be found.
--
Mike

The 60 degree Tele Vue diagonal is an Everbrite mirror model. The image will be upright but reversed left-to-right.

Generally I think a 90 degree correct-image "Amici" prism is going to give better image quality than a 45 degree prism. Neither is ideal.
--
Mike

Yes, there are problems with it.

1. Correct.

2. No, any details will appear the same size in both eyepieces. However, the one with the wider field will show a larger area of the sky.

3. Wide versus narrow field isn't really the issue. Magnification determines image scale. If you need more magnification, you use a shorter focal length eyepiece or a Barlow.

The exit pupil is not a single column of light with the image packed in any cross section. If that were the case, you'd need to remove the lenses from your eyeball in order to focus on it. Rather, each point in the view emerges from the eyepiece as a separate column of light. Look in a different direction in the apparent field of view, and you are accepting a different column of light and using the lenses in your eyeball to focus it on the most sensitive area of your retina. Other columns focus in other places on the retina. Anyway, the exit pupil diameter is the diameter of each of these columns. With a wide-field eyepiece there simply exist columns of light coming from greater angles.
--
Mike

Scott,

There are different Zeiss Abbe orthos. The 1.25" Carl Zeiss Abbe orthos were limited editions. Here's a set up for sale right now

http://www.astromart.com/viewad.asp?cid=172269

The 0.965" Zeiss orthos have the look of this one here

http://www.astromart.com/viewad.asp?cid=172232
--
Mike

Looks like you don't have enough support in the rear. The problem is often difficult to solve because of the space needed for the OTA to swing through. I see that you've attached the sides to the side of the baseboard of the rocker box. It's better to make the base wider and rest the sides on it. However, you can salvage the status quo by gluing in some, say, 3/4" x 3/4" square dowel or such (plywood scraps can also work) along the inside seams in the rocker box. I use Titebond II wood glue--great stuff. If it won't interfere with your tube, also consider gluing in some wood triangles to stiffen the connection between the sides and the bottom in back.

Once the sides are firmly secured by a glue bond using the square dowel, the only way the sides can wobble is through flexure. To limit this further, I suggest building eyepiece holders. You just take a board and drill 1.25" and or 2" holes at regular intervals and attach it with some framing to the sides. That should stiffen the sides up a bit.

The final thing is that you may get some flexure in your ground board. You've got a nice wide footprint there, but there must be considerable distance between where the rocker box is supported on the ground board and where the ground board is supported by the ground. We usually like to stack these weight bearing points as much as possible, but if the resulting stance is too narrow, some separation, possibly with reinforcement of some kind, is required. Anyway, watch for flexure there and reinforce the ground board if necessary.
--
Mike

I'm surprised there have been no responses. I tried the Nagler Zoom and found it to be exactly what it is supposed to be, but the eye relief at 10mm was little less than I wanted (12mm of eye relief, as with the type 6 Naglers seems like my lower limit). I wondered a little about the longevity of the mechanics since it did not always seem perfectly smooth from 4mm down to 3mm, particularly when the rate of turn is fairly quick. The problem seems to stem from trying to keep the rate of increase in focal length constant as the grip ring is turned--a lot more extension must happen between 4mm and 3mm than between 6mm and 5mm. I would have preferred making the rate of increase in magnification a constant, which I think would have resulted in simpler and more consistently smooth mechanics.

But Tele Vue's warranty is good, and their service after the warranty also tends to be quite good, so there's really nothing to worry about investment-wise. I basically just prefer a little bit longer eye relief and a wider field of view, so I use 7mm and 5mm Type 6 Naglers with and without a 1.25" Tele Vue 2x Barlow. That makes a more expensive solution but one that ultimately pleases me more.
--
Mike

It's about the same with a Dob, EXCEPT that a well-designed Dob will damp vibrations very quickly, so not as much time is wasted waiting for that. However, because a Dob lacks slow motion controls and axis locks, I find that wider fields of view at high powers are very helpful, particularly after changing eyepieces to ensure that the planet is still in view. My patience starts wearing thin when the true field of view is less than 10 arc minutes, but that's almost 500x with a type 6 Nagler or around 350x with a Radian. The good news is that with larger aperture it isn't principally a matter of going to higher magnification. The image is noticeably more detailed at modest magnifications.

Perhaps it will give you a better subjective idea to note that I have an excellent Osypowski equatorial platform, but I usually don't bother with it unless seeing is good enough to exceed 300x.
--
Mike

The 9mm was only produced as a 7-element type 1 before the type 6 was introduced. Type 2 Naglers have 8 elements.

At one point there was a redesign of the formerly smooth-style casing of the 9mm Nagler to add the rubber grip ring, rubber eyeguard, and even later threads and a safety undercut in the 2" skirt. In the meantime coatings may have been updated, but the optical design always remained a 7-element type 1.
--
Mike

I don't have one, but I can offer some observations from experience with a dozen or so Barlows. It is clearly longer than a 2x TeleVue Barlow and hence would be both less convenient in a star diagonal (particularly a 2" star diagonal) and might protrude into the light path of a Newtonian with a reasonably low-profile focuser. Also, neither the 2x Ultima nor the 2.8x Klee are the equal of the 2x Tele Vue Barlow despite the former two having 3-elements to the Tele Vue's 2, so I would not have high hopes for the Ultrascopic being able to match the performance of the Tele Vue 2x Barlow. It's cheaper, but I snagged my last 2x Tele Vue off Astromart for $75, though lately the going rate seems to be about $80 (shipped).
--
Mike

I always cringe when I hear the "blacker background" being used as an evaluation criterion. You might get a blacker background out of an eyepiece in different ways. One is to reduce throughput by using inferior coatings or elements with lower transmission, or just more elements provided the baffling is good and scatter remains low. Another is to use higher magnification (the 8.8mm Meade should have a 4.4% darker background than a 9mm eyepiece on magnification, assuming at least a little light pollution in the mix). Another way is to have higher contrast. For some reason that isn't at all clear to me, people always seem to assume that a blacker background always means higher contrast and better performance. That MAY be so, but not necessarily.
--
Mike