Don Pensack

Designed for use with night vision devices.

On 14/07/2020 at 14:45, Jarvo said:

Thanks for the comments Guys. Yeah all my other caps from televue are black with the company moniker embossed on the bottom. 
 

Looks like even TV are looking at cutting costs. 
 

 

The new caps are VASTLY superior--they stay on.  The older 2-sided caps slowly creep off the eyepieces.

In these days of Covid-19, I wear disposable gloves to get the mail from the box on the street, then put the mail in my garage for 2 days before opening it.

If I had a mail slot to drop the mail through, I'd seal it to keep the mail from coming inside until it hadn't been touched for a couple days at least.

It was created to take the place of a finder in refractors.

It's just plain odd.

34 minutes ago, andrew s said:

Thanks, I often use the telescopeOptics.net site which is liked in the CN discussion. It's very good I find.

What I especially liked about the first paper I posted was the modeling of the complete system, telescope  eyepiece and eye I had not seen that before. It also explained that the eyepiece aberrations were derived by reversing the light as that had not been clear to me, although I had suspected it.

Incidently do you know anything about the Siebert Optics Monocentric IDs?

Regards Andrew 

Yes, they are not truly monocentric.  In a monocentric, all the curves on the 3 elements have a common focal point.

The Sieberts, like the TMBs, are a 3 element eyepiece more closely resembling a Hastings triplet or a Steinheil triplet.

Siebert also makes them in fairly long focal lengths, to be used with his various Barlows to yield short focal lengths.

That's not a bad idea since it greatly expands the eye reliefs (very tight on monocentrics), and reduces induced astigmatism in the eyepiece, making them usable at shorter f/ratios.

Harry doesn't give eye relief or field stop data on them, though some users have mentioned about a 30° field.

2 hours ago, Louis D said:

Never had that issue in Texas, not even on my 127 Mak.  Perhaps because I don't observe in the mornings when the air temperature is closer to the dewpoint.  I've never felt the slightest need for dew strips or shields.

I don't have much of an issue in southern California, either, but when I was last observing in Australia the dew at the site was so heavy it filled a table with a lip on the edge with a pool of water, and all the dobs had to use hair dryers on the secondaries just to keep them usable.  I usually observe all night when I go out, so I run into dew a lot, though it isn't heavy here.  Where I grew up, the morning dew was so heavy it wetted everything as if it had rained.   There are some wet environments out there.

The new caps started with recent Delites, but seems to be spreading into the other eyepieces now.

7 hours ago, markse68 said:

I guess if Don hasn’t come across this design in reality then it must be theoretical. I’ll try contacting the author of that website or maybe Chris Lord. There are a few archaic designs I’ve seen mentioned like the single solid element types that don’t seem to exist except maybe in museums-seems odd though when this one seems to offer real benefits 🤷‍♂️

Could have been some made.  Timing is everything where sales are concerned.

The Takahashi Hi-LE eyepieces (2 focal lengths) were Abbe orthoscopics with 2 element barlows added in the bottoms.  They're now out of production.

An improved design for the Abbe ortho is a nice idea for the modern world, where edge sharpness is more valued.

2 hours ago, Louis D said:

I always go by lowest power (25x to 40x), mid power (65x to 90x), and highest power (150x to 220x) and then do the math for the particular telescope to figure out what eyepieces make sense.  Certainly, there is plenty of room to fill in the blanks at the higher end of the power range when poking at the limits of seeing.

That works fairly well for 3-5" apertures, though the range is a bit low for larger ones.

If you tailor it to magnification per inch of aperture, you adjust for aperture, say:

4-10x/inch--low power

10-20x/inch--medium power

20-30x/inch--high power

>30x/inch--ultra high power

For a 4" refractor, that would be 16-40x, 40-80x, 80-120x, and 120-240x and beyond, so right in line with your magnifications.  You did leave out a very useful range, though.

For my 12.5" reflector, it's 50-125x, 125-250x, 250-375x, and 375-750x, and that pretty much works for me at my observing sites.

19 hours ago, andrew s said:

Can you provide a more accurate reference at least to the key points of science if you feel there are errors in this area.

Personally I am less concerned with the history. 

Regards Andrew 

Well, the article deals mostly with history.  So far as I know, there are no inaccurate representations of design, but I didn't really dig that far into it.

Here is a thread you might find interesting:

https://www.cloudynights.com/topic/657067-comparative-raytracing-from-huygenian-to-ethos-and-beyond/?hl=%2Bray+%2Btrace#entry9278959

Bear in mind some of the information is not accurate.  Plössl designed and was using his eyepiece in the 1830's for example, though

many on-line sources list it as 1860.  It's possible that is a date of patent and that it was designed much earlier.  Often the patent precedes the prototypes, but not always.

König was extremely prolific in his design years and it credited with 38 different designs.  So assigning the name "König" to just one eyepiece design is a bit, uh, selective.

Nonetheless, a lot of good info there.

There isn't a lot of photometry of this star for a reason, but most on-line references are 15.1-15.2 for the central star, with a range of 14.7-15.7.

I strongly suspect the magnitudes brighter than 15 are B magnitudes, given the spectrum of the star, and the 15.1-15.2 estimates are V magnitude.

It is not known to be variable.

That's a pretty good limit for an 8" scope under normal dark sky conditions, so the fact it takes larger scopes to see it is indicative of low contrast.

I've found that by viewing at high altitude (2550m), I get better seeing much of the time, allowing 300x on the 12.5" with steady star images.

Some nights, I can hold the center star with direct vision, other nights only occasionally with averted vision, and other nights not at all.

Seeing is the reason--when there is no atmospheric movement, the star becomes fairly easy to see.

Outside the ring, I often get to well past magnitude 17 with averted vision, so it should be easy, but it's not.  However, it is visible about 80% of the time at that site.

At any rate, I have not really attempted to see this with less than 300x, which is a 6mm eyepiece in the 12.5".

10 hours ago, John said:

According to Don Pensack the Ethos range comprise 2 sequences which have logical magnification steps in most scopes:

21 - 13 - 8

17 - 10 - 6

Then you have the 4.7 and 3.7 Ethos SX

At one point I had them all but now I just have the 21-13-8-6 run with the 17mm ES 92 in between the 21 and 13.

 

That does have to do with focal length.  if you have a scope with a 4000mm focal length, then a 25% progression like the Ethos range might make more sense: 21:17:13:10:8:6 etc.

But for those of us with 2200mm or less, a larger progression makes more sense. a 40% jump is usually common: 30:21:15:11 etc.

If you have a 714mm FLf/7 102mm refractor, even larger jumps make sense.  If you start out with 30x magnification, 60x as the next step makes good sense, but that requires an eyepiece jump of 100%: 24:12:8, etc.

Closer than that, and it becomes hard to see a change in magnification.

So my comment about the 21--13--8 set or 17--10--6 set is a bit proprietary to 1200-2200mm FL dobs.

3 hours ago, Louis D said:

I cool mine straight up and down like a chimney.  I don't know what advantage tilting it would provide.

DEW.  It will form on a mirror pointed at the sky, but not on a mirror pointed at trees or buildings.

It will affect the secondary first, but also the primary.

Now, if you have fans, the scope can be horizontal and still cool just as well without risk of dew.

5 hours ago, Barry-W-Fenner said:

Hi Don,

Thanks for the insightful post regarding the primary mirror, It would seem that you have forgotten more about mirrors than I will ever know 🙂 Are you in the optics profession?

So it would appear that i have collimated to a mechanical state by going over the usual processes. How would one know how to collimate to a optical state?

Reference the 3 Cs I think the scope was out for about an hour on each occasion cooling. Perhaps i am not giving the scope the required amount of time to cool down enough. It might explain why the moon looked great at 6am as I left the scope out from my 3am viewing session.

Thanks for your assistance, Much appreciated.

Baz

 

Optical collimation is aligning the optical axes: focuser axis (using a laser or a sight tube crosshairs), and primary mirror axis (using a Cheshire, a collimation cap, or a barlowed laser).

Mechanical collimation is centering the secondary under the focuser and getting it rotated perfectly.  Neither is essential for optical collimation.  And it is optical collimation that yields the best images.

Fans will help cool the mirror and give those morning images earlier in the night.

As for being in the business, yes, I am--since 2005.  But I've been an amateur astronomer since 1963.

And seeing, which seems to bother the big scope owner more than the small scope owner.  I just about ALWAYS see the central star in M57 and, often, the star superimposed on the ring,

with my 12.5" at around 300x (about a 1mm exit pupil in that scope), using a 6mm Ethos.

And, while it is very true that seeing is a strong determinant of the limit magnification, where stars are concerned, if the seeing is up to it, it is a rare circumstance where the limit stars are seen below the magnification yielding a 1mm exit pupil, and exceedingly rare to see even fainter ones as magnification is raised to 30x/inch or 35x/inch or even higher.  I don't really count things like the faint moons of Uranus, where separating the moons from the planet is key in seeing them.  I've never seen Miranda at magnifications below 400x, but I do occasionally see it above that magnification if seeing allows.  That magnitude is not hard for a 12.5"--I regularly see much fainter stars near M57--but the planet's proximity just makes that one a really difficult target.  You just need separation.

So I agree about the "sweet spot" in magnification, but exactly where it is, I can't say.  For me, it is at least at the 1mm exit pupil, but my next jump in magnification from there is 90x, and it's possible something in between might prove better.

In my 4" apo, it is similarly hard to determine, as the 1mm exit pupil is 102x, and my next magnification is 143x, and that jump may be too large to tell.  I think it is at least that 1mm exit pupil or even smaller, though.

So I may agree with theory, here, but I think it is more of a rule of thumb than a fixed optical principle.

The alignment of the crosshairs on the primary center marker, if a coma corrector is not used, has a very loose tolerance of about 0.03D, where D = mirror diameter in mm.

Essentially, if the crosshairs are anywhere on the center marker, you're probably OK.

It is the primary collimation that has tighter tolerances: 0.005 f/r³,which, fortunately is seen with 2X magnification by the typical Cheshire, so, if the scope is an f/5, for example,

the primary alignment tolerance is 0.625mm, but the read of the tool need only be better than 1.25mm, or about 1/10 the diameter of the primary center marker if the primary center marker is 1/2" across.

That is a pretty easy "read" in the tool IF the center marker is close to the same size as the black center in the Cheshire.

Collimation has two main components: mechanical and optical.  And when you are learning collimation, the one to pay attention to is the optical alignment.

So don't worry whether the primary is centered under the focuser, or whether the 3 clips are equally visible, unless you are concerning yourself with the mechanical collimation of the scope.

It is the optical collimation (tilt of secondary and primary) that truly determine the quality of the images you see.

If you have paid attention to optical collimation and still have poor images, remember the 3 "C"s:

Collimation (which you've just taken care of), Cooling (any mirror larger than 6" will need 1-3 hours before it is cooled to the ambient temperature and will yield sharp high power images).  And Conditions,

which includes the seeing, the darkness, and the transparency of the air (with seeing the most important for sharp star images).  Only if all 3 are good will you see good star images above about 20x/inch of aperture.

Gerry,

To test the limit of an eyepiece, it would first have to be used on an appropriate target, such as stars in NGC206 in M31.

Or whether thousands of stars can be resolve in, say, M14.

Then, the focal length should yield a 1mm exit pupil or smaller, since high powers always yield the faintest stars.

So people who test such things would need to specify scope size and f/ratio as well as exit pupil.

And, for such a test to have value, it would have to be compared to another eyepiece of equal focal length.

And, the observer would have to specify whether the limit observation was truly a limit observation (i.e. visible with averted vision only, and only 10% of the time).

3 hours ago, jetstream said:

I sure wish there was a 5mm Vixen HR.... totally satisfied with these.

Vixen has stated the limit to the design is 3.4mm.

On 10/07/2020 at 18:38, Louis D said:

I had always thought the ETX scopes were originally made in the US.  I didn't realize they were getting them from JOC all along.

I think the ETX scopes may pre-date Meade's association with JOC, so the earliest versions may have been made in the US.

But from ~1997-1998 to 2011, they were from JOC.

They are now from another Chinese mfr.  I think I know who it is, but I am not sure.  The better Meade refractors seem to be from KUO, as they look exactly the same and have identical specs.

On 09/07/2020 at 14:31, John said:

Probably but I don't recall the results being any good !

I later realised that stacking a UHC and an O-III is pointless when I thought it through. What you end up with is an O-III filter with a lower peak pass %.

 

And no narrower bandwidth than the O-III filter by itself.  Stacking two identical O-III filters would also reduce the peak transmission, but also give

a narrower bandwidth.  That could be bad or good depending on the filter.

Sigh. 

OK, go ahead.  I have a thick skin and can take it, no matter how thick the sarcasm.

Actually, oddly enough, there are some dyed-in-the-wool planetary observers who look for the older Japanese-made Meade MA eyepieces from the '80s/'90s

because they say they are very sharp and contrasty.  Maybe so--a lot depends on the polish on the lenses and the intangible factor--the closeness of the execution to the design.

An optical designer may pen a superb eyepiece, but if the factory doesn't make it to spec, it won't perform like spec either.

Those guys would argue the Delos has too many lenses and scatters more light.   What say you? 

1 hour ago, Merlin66 said:

I had a Meade Series 5000 x3 Tele-extender and then acquired an Explorer Scientific x3 Tele-extender. There were identical. Same physical size, finish and with the same optical elements. Definitely came out of the same Chinese factory.

 

Meade got their Chinese goods from Jing Hua Optical from the mid '90s till 2011.

Also the ETX scopes and LXD mounts.

Jing-Hua Optical (JOC) is the owner/maker of Explore Scientific.

So if your Series 5000 TeleXtender was from that era, it is, indeed, the same as the ES Focal Extender.

Since 2011, Meade has gotten products from KunMing United Optics (KUO), but kept the name Series 5000, causing confusion.

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