Posts Made By: Mike Hosea

Mark Peterman said:
What do you think is causing this?

I haven't noticed this when wearing my contacts while viewing.

I suspect your glasses have some correction for astigmatism. If so, for the best view it's important to position the eye so that the eyeglass lens is parallel to the top of the eyepiece and the exit pupil is centered where you would normally be looking straight ahead. Otherwise, your eyeglasses may add astigmatism rather than correct it. Although contact lenses may or may not correct astigmatism, depending on their type, they at least will not ADD astigmatism the way glasses do when you are not looking through the glasses in the optimal way.
--
Mike

I don't know how much the warranty is worth, but I've had a couple of instances where Tele Vue made good on it, one was a small scratch at the edge of a Nagler's internal lens which took me awhile to notice (so I can understand how it slipped through QC). The other was an optical correction issue of unknown cause. That one should have been caught in QC, but I guess they employ humans to do QC work, and I hear that humans sometimes make mistakes. Anyway, Tele Vue fixed me up at no charge in both cases. Couldn't have been more satisfied.

My cases were sort of cut-and-dried, and so there was no debate or disagreement about whether they were warranty issues. I have heard, however, of some cases where there was a disagreement over whether a certain line of acceptability had been crossed, e.g. in the appearance and reflectivity of coatings, for example. If you look at

http://voltaire.csun.edu/roland/coating2.html

There are two Barcon lenses there, both of which are "acceptable" by the manufacturer's spec. I can imagine that a mult-element eyepiece having lenses predominately around 0.5% average reflectivity might be less impressive than one with lenses predominately around 0.25%. C'est la vie, apparently. I guess it's normal for a business to define what "as good as we can practically make it and stay in business" is and go with that. The alternative would be to "grade" each item and charge more for some and less for others. That doesn't sound like an attractive business model to me, and in fact it probably doesn't even solve the problem--just buys you disagreements over the grading.
--
Mike

I only used a 3.5mm XW briefly. I owned a 3.5mm Nagler T6 for awhile, and it was fine, but ultimately I found that 3.5mm was too much magnification often enough and a 7mm T6 with a 2x Tele Vue Barlow (a doublet) was good enough that I couldn't justify keeping it. Extrapolating from comparing a 5mm XW to a 5mm T6 for about 3 months, I would say that the XW probably has a tad more contrast in most cases, but I didn't care for seeing reflections of the almost-knife-edge baffles in lunar viewing (which dramatically reduced contrast in one isolated circumstance--viewing the earthshine-lit dark portion of the lunar surface of a quarter moon), and eye positioning with the XW was more finicky than I remembered with the the older XL. I actually found the T6 more comfortable and "stable" from a distance at which you could see about 70 degrees in the 82 degree eyepiece, but some people find it odious not to have the field stop in view as a visual "anchor". If I were buying this focal length, I'd still choose the T6 over trying another XW. I'm sure this would not be agreed upon by everyone, but then again, there don't seem to be a lot of opinions of any kind.
--
Mike

Sure, but that doesn't seem to have anything to do with the discussion you alluded to about planetary performance of T6 Naglers and "glass magic". It's probably just a simple case of a little bit of transmission difference putting some faint stars on either side of the threshold of detection for your eye.
--
Mike

I don't know about quantifying it, but I think I can see a difference. This is one of the two reasons why I don't use short eye relief anymore (the other is fogging), although the eyelash oil problem really depends a lot on the mounting. Usually short eye relief oculars are mounted nearly flush with the surface. This may increase the perception of eye relief, but it worsens eyelash oil problems. It's better to recess the lens slightly.
--
Mike

Seems to me that you have an astronomical item with some market value. The market just needs to be informed that these are NEAF seconds so that it can assign the right value. Selling an item without disclosing its status as a "second" would be fraud. Selling the item with this disclosure should pose no ethical problems. The other thing that happens, however, is NEAF flipping. That sort of thing gets monitored here, I think, because flipping for profit is a commercial use of Astromart. Nothing wrong with that, but then you gotta pay to play.
--
Mike

The 1.25" barrel unscrews from the 2" skirt. Make sure it's snug.
Check the retaining ring of the lens in the 1.25" barrel (on the outside). Make sure it's snug.

If that didn't fix it...

The 2" skirt unscrews from the large black housing. Unscrew it. With the eye lens pointed towards the ground (just to settle the lenses), snug the retaining ring holding the lenses in the black housing. Reassemble.
--
Mike

Most Dob Newts, including mine, cannot dump an eyepiece, so the device has little or even negative safety value there. I just accept it as one of those little inconveniences in life that exist entirely for the benefit of other people.
--
Mike

If the lenses were to cause any vignetting, it would be very difficult to calculate it without details on the prescription, but in this case I am just going by the barrel geometry. I was actually fudging a little since the result depends on the focal length of the scope. Specifically, if you let

b = Distance from the open end of the barrel to the field stop in mm.
c = Distance from the field stop to the top edge of the barrel in mm.
d = Barrel ID.
s = Diameter of the field stop.
L = Focal length of the scope in mm.

Then the fastest f-ratio where the edge of the objective can "see" the nearest edge of the field stop is

f-ratio = L / ( s + L*(d-s)/b )

and the fastest f-ratio that can see the lenses beyond the field stop from the opposite side of the objective is

f-ratio = L / ( L*(d-s)/c - s )

So the fastest f-ratio at which there is no vignetting for a telescope of this focal length is the larger of these two f-ratios.

I can't remember details of the barrel geometry of the 24mm Panoptic, but I did measure with digital calipers and report some specific results once. I think this picture is proportional to the actual geometry and illustrates a the edge-of-field light cone for my 10" f/5. On the left in yellow, IIRC, is the unvignetted cone farthest off-axis. On the right is a light cone at the edge of the field stop that gets clipped a little on both sides from the red down to the blue.
--
Mike

I'm not sure it is totally fixed on the 9mm.

On the 4mm I installed Protostar flocked paper on the inside of the black barrel. This wasn't enough because most of the problem is due to the retaining ring on the field group. It may not help much, but it can't hurt. Note that there is a lip several mm wide at the bottom (it's hard to see). You just want to install the flocked paper above the lip where the barrel has uniform ID. The lip fits down beside the field group cell when the barrel is installed and has no access to light.

With the barrel removed and sitting flat on the table, I unscrewed the retaining ring on the field group and carefully set the barrel (now with loose optics) aside. Then I measured the ID and thickness of the ring, cut a strip of flocked papaer, and installed it. Some precision is needed here because the thickness of the paper may be enough to cause vignetting (just reduced brightness) as it is, and you don't want overlap or a gap. I used the conical end of a pencil to seat the paper properly with force from the inside. Then I reinstalled the retaining ring in the barrel, just snugging the retaining ring (no "tightening") and reinstalled the barrel.

Now I can't be sure if this is enough, either, because I also have a cardboard baffle on the outside from a previous (and only partially successful) attempt to fix the problem with baffles on the inside and outside. I can't remember the ID of the baffle (just a bit less than the diameter of the field lens clear aperture), but I just cut it from something like a cereal box, painted it flat black, and pushed it on the outside.

I call this "aggressive baffling" because I wasn't concerned about introducing vignetting at the edge of field, and I may have gotten a little. I've just had one evening with it for use on Mars, and I did not see any glare. The moon has not been available to me during an observing session since I performed these modifications, but when I hold the eyepiece up to a light bulb at arm's length and examine the exit pupil, it looks good, just a circle of light in a sea of black. Previously, the specular reflection off the retaining ring and other surfaces in the field group were almost as bright as the exit pupil itself.

I may be able to take some pictures tonight.

BTW, it's clear that the field lens retaining ring is most of the problem. If a suitable replacement is offered by Burgess, I suspect it will make a dramatic improvement.
--
Mike