Posts Made By: Mike Hosea

Rob,

You have some choices.

1) Use eyepieces that require less back focus.
2) If you're close (doesn't sound like it, but if so) you can sometimes create a little back focus by moving the mirror cell forward on its collimation bolts (put them all as far forward as they can go and then back off a little for adjustment purposes).
3) If it's a tube scope, you should be able to mount the mirror cell a little closer to the secondary by drilling new holes and moving the cell. If it's a truss scope, you might be able to shorten the truss poles. (I haven't seen how they attach, so seek advice on this.)
4) A different focuser might provide more back focus (as you mention).
5) If a tube scope, the focuser and secondary may be moved closer to the primary (but you almost surely won't want to mess with all that--it's more a theoretical possibility than anything).

Maybe there are other options, but that's all I can think of that seems reasonable at the moment.
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Mike

Looks like we may be having a decent night here in the Dallas area. I just had my homebuilt 7" f/6.7 with Mark Harry primary out and was enjoying some good views of Saturn at 237x with a 5mm Nagler Type 6. It was so sharp, in fact, that I couldn't resist seeing what it would do with more magnification. So I went to 339x (7mm Nagler Type 6 with 2x Tele Vue Barlow). No prob. The image was predictably dimmer but still sharp. Okay, so I swapped the 7mm Nagler Type 6 with the 5mm Nagler Type 6, not expecting a good outcome. This was 475x and almost 68x per inch of aperture. Once again, the image was dimmer but still easily focused and reasonably sharp, still very useful. Not bad, not bad at all.
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Mike

Awhile back we discussed UO orthos, baffling, and field stops. I was somewhat handicapped at the time since I was going on memory, but I recently got an 18mm UO Abbe ortho for a finderscope project I'm working on, and I have some comments about it that relate to our past discussion and a new observation.

First of all, the 18mm UO ortho has a fine field stop, as good as any you'll find in any eyepiece. It has a diameter of 13mm and resides about a cm below the top of the chromed barrel. This is not to say that the eyepiece is well baffled, however, since the field element retaining ring and the spacer ring between the eye lens and field triplet have shiny surfaces despite being black colored. Unfortunately, I don't see an easy user fix. Painting them would give only marginal benefit since even flat black paints are fairly reflective at low angles. It would be better if these rings had a grooved, threaded, or at least textured flat black finish, and the retaining ring doesn't need to be as wide as it is.

I measured the apparent field and came up with 46.8 degrees, which is close enough to regard the quoted 46 degrees on the Kokusai Kohki web site as correct. Measuring the exit pupil also produced results consistent with an 18mm focal length, and if anything it might be a tad longer. I was surprised, however, that the field stop is only 13mm in diameter. To be "orthoscopic" means (among other things) to have low angular magnification distortion, but in this case the angular magnification distortion is positive and significant. The apparent field is about 11% wider than it should be for an 18mm eyepiece with a 13mm field stop and no angular magnification distortion. To get a feel for how far off this is, the 20mm Tele Vue Plossl with 50 degree afov and a field stop of 17.1mm has an apparent field only 2.1% wider than it should have to achieve zero angular magnification distortion.

In other words the 20mm Tele Vue Plossl is a lot closer to being orthoscopic than the 18mm UO Abbe "ortho". Go figure. I would be careful, however, about extrapolating about shorter focal lengths based on the 18mm.
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Mike

Can anybody tell me how about how much focus travel you need to use the 2.8x Klee Barlow with a 1.25" Tele Vue Plossl, Radian, or Panoptic? These eyepieces all have effective field stop locations about a quarter inch below the barrel flange, and if you were to say, focus your scope with one of those eyepieces and then swap in the Klee (inserted to the hilt) with the same eyepiece. What focus adjustment is required?
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Mike

My personal opinion is that the Powermate is a better choice if your goal is to fill magnification gaps in medium focal lengths or gaps in both medium and short focal lengths, especially with eyepieces having relatively few elements such as Plossls, Abbe orthos, RKEs, Kellners, some Erfles, and most Konigs. A Powermate is also more convenient than most Barlow in that it is approximately parfocal even in star diagonals.

When used with most Naglers, Radians, and Pentax XLs, I think there isn't a lot to recommend one choice over the other.

However, for purely short focal length use in a fast telescope (the point is that the effective field stop diameter involved is very small), I prefer a good doublet Barlow (specifically the 1.25" 2x Tele Vue model in my case) since there is less glass, fewer air-to-glass surfaces, and little divergence to speak of anyway.

So what eyepieces do you plan to use it with and in what scopes?
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Mike

What concerns me is that most of the middle eastern Arab populace is now the more hostile towards the US. Apparently we are somehow to blame for all their problems. They oppose the war, which is their right as far as I am concerned, but they need to come to terms with the consequences of that choice, which would seem to be defacto support for the continued oppression of the Iraqi people under a tyrant who styles himself after Stalin and who has demonstrated lethal aggression towards his neighbors, all of whom are muslim and many of whom are Arab as well.
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Mike

I doubt any of these will solve the problem because I think the magnification on all counts is just too low. Assuming this 12.5" Discovery Dob is f/5 and given that you seem to favor lower powers than I would, I suggest an 8-9mm solution at the longest. Actually in my 10" f/5 my most used magnification for Jupiter is 244x, and there is an abundance of contrast at 181x, so much that I don't ever go lower than that on any planetary target. If your 2x Barlow is a high quality model, try using it with your 16mm T5 Nagler. It may vignette a little, but that is of no consequence.
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Mike

Thought you might like to see the mount I made for my ETX90 for grab-n-go use. I didn't like the original mount, a 3-way camera head was too unbalanced.

The Tele Vue Ranger's drawtube does not rotate, just the focusing ring. Low-profile helical focusers on Newts do rotate the eyepiece, but of course the "diagonal" (aka secondary mirror in this case) is not attached to the focuser.
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Mike

I note a particular camera shop now has Pentax XWs up on their web site ($300 for the 1.25" each and $475 for the 2") and only the 3.5mm is listed as not in stock because it is a new product. Anybody pushed hard enough to find out if the others are really in stock or not???
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Mike