These are all perfectly good questions.



I have no idea as to the answers. I just like looking at stuff.

 

Dr Magnus Pyke would have known. Unfortunately he's brown bread now, although he did get to make this video with Thomas Dolby before he popped his clogs.

 

Logic would tell me that the larger the aperture, the more light it will gather and the more you will see. Just maybe not as much as a 20-something, the the rule of aperture will still stand. More light - the greater the light gathered.

 

Yeah, makes sense Dave.

 

Okay, I'll take a stab here.

Not quite, but close. The exit pupil is the term applied to the eyepiece/scope combo - how wide is the shaft of light exiting from the eyepiece. You're talking about the entrance pupil here.

Yup, after 30, the maximum width of the entrance pupils to your eyes starts to gradually shrink from 7mm as the eye muscles controlling the pupil age and atrophy. It's about 1mm per decade, so 40 = 6mm, 50 = 5mm, 60 = 4mm. I think that 4mm is about as small as it gets.

This, however, is a classic example of YMMV - your mileage may vary. The sizes with ages are the averages of the general public. You may be genetically blessed with Arnold Schwartzenegger pupil muscles that defy aging. The only way to really know YOUR entrance pupil size is to have your eye doctor measure it.

You can also measure it yourself by: 1) turning off the red-eye reduction pre-flash on your phone; 2) going into a completely dark room or closet and closing the door behind you; 3) waiting for a good minute; 4) taking a measuring tape or ruler graduated in millimeters, and holding it by your eye for scale; and, finally, 5) taking a picture of your eye with the regular "one-shot" flash.

YES!!! That is exactly it. So, let's be absolutely clear here.

The 7x50 has an exit pupil of 7.1mm - again, that's the size of the light shaft coming out of the eyepiece. Fine if you're in your teens or twenties, but if you're older, if you're in your 50s, like we are, then your entrance pupil is probably about 5mm. That means that a bunch of that light is not making it into your eyeball and down to your retina. The photons are splashing off of your iris (the colored part of your eye) after having travelled millions of light years. Boo! Bad iris!

And this is why 7x50s are relatively useless for older astronomers. May as well save money and get 7x35s instead . . . or, better yet, get 10x50s.

Ah, good question. (Not to imply that the other ones sucked.) So, you can adjust the exit pupil for any given scope. Again, I'll use our 50-year-old 5mm entrance pupils as an example, because I have actually gone through this entire line of thinking.

I am interested in a 6" refractor. There's the standard Synta f/8 that Celestron sells (1200mm f/l), there's the f/6.5 that Explore Scientific sells (988mm), and there's an f/5 floating around out there, too (760mm). (It's from Bresser, and it's a Petzval, but let's ignore that for the moment.)

I'm not as interested in the f/8, because of the increase in moment arm that it has over shorter scopes. (Moment arm is the tendency of long skinny refractors to vibrate in the wind - or even when you touch the focuser - because it's being supported only in the middle. The ends of the scope are just hanging way out there, with all the weight at the ends, willing to vibrate, wanting to vibrate, waiting to vibrate (with apologies to Eliza Dolittle's dad in My Fair Lady). My understanding is that moment arm increases with the square of the extra length, so that shortening the scope by 100mm on each end (1200mm - 988mm = 212mm. 212mm / 2 ~ 100mm) would go a long way to decreasing that moment arm.

(Don't worry, all this refractor talk will lead somewhere.)

In trying to figure out which one to get for myself, I'm looking at a couple of things. I want the widest field of view possible . . . but I want it while keeping the apparent field of view of the eyepiece reasonable . . . which to me is 68 degrees. (I'm one of those weirdos who doesn't like 82-degree EPs - I don't like moving my head around to take in the entire FOV.)

The key thing here is that you have to match the right eyepiece to the scope so that you're matching the resulting exit pupil with your entrance pupil. Let's plug the numbers into a spreadsheet, first with the f/5 Bresser. I'll compare a 34mm ES 68 to a 31mm Baader Aspheric 72 to a 24mm ES 82.

34 ES 68 = 3.04 degree TFOV, but with a 6.8mm exit pupil. No good. No good at all.
31 Baader 72 = 2.94 TFOV at 6.2mm. Nope. Still blows.
24 ES 82 = 2.59 TFOV at 4.8mm. Ding ding ding! We have a winner!

So, I can use ANY 24mm eyepiece, regardless of its AFOV, with the f/5 Bresser, because the exit pupil is only effected by the combo of the two focal lengths. I can use my current 24 ES 68, or bump up to the 24 ES 82. Hell, I can even go for the 25 ES 100 and get an astounding 3.29 TFOV with exactly a 5mm exit pupil.

So, for this scope/eyepiece combo, a 24-25mm EP is the "right" combo to use for our aging eyes with their 5mm entrance pupils. We WILL get down to the lowest possible magnitude that the scope is able to achieve here, because both pupils match. However, I don't like the 82-degree format (not to mention the 100-degree format), so I press on.

Let's run that through again with the ES f/6.5:

34 ES 68 = 2.34 TFOV at 5.23 - not bad, not much light lost.
31 Baader 72 = 4.77 TFOV at 2.26 - that'll work, and I'll deal with the extra 4 degrees AFOV beyond the 68 that I think is the best;
24 ES 82 = 1.99 TFOV at 3.69. That kinda blows, 'cause I want more than 2 degrees here.

So the choice for me is, "Do I want 2.26 degrees TFOV and 72 degrees AFOV, or do I want 2.59 degrees TFOV at 82 degrees?" For me, I would sacrifice some TFOV to get a smaller AFOV.

The basic point I'm making here should be coming out, which is that for a particular focal length telescope, there is a certain focal length eyepiece that you must use in order to keep the two pupils matching each other. And this depends on the age of the user. So, no, scopes aren't gonna put, "The limiting magnitude of this scope is 14.0, but only if you're in your twenties" on their scopes. You can still match the two pupils and get the maximum amount of light that the scope will transmit inside your eye if you choose your eyepieces correctly.

By the way, dogs do perceive the color red, just not as intensely or strongly as we do. Yes, they only have cones for green and for blue, but they can interpolate these and still see red. It's not as if anything colored red all of a sudden becomes invisible to dogs.

Any questions?

 

Cats also see red - into the infrared. This is how they can hunt successfully at night. All warm blooded animals emit heat and that's infrared. We do too, just not as well as a Cat. Ever see how a fireplace in daylight can appear to be out? Look at the same fireplace in the dark - you'll see it still glowing.

 

What about colour blind dogs? lol

Cats can't see infra red any better than we can.

https://www.quora.com/Can-cats-see-IR

https://www.livescience.com/40459-what-do-cats-see.html

I agree about sixty eight degree eyepieces, it's more than enough. My favourite EP's are the TV Panoptics.

 

Perhaps I have dated material. But then why is it said the Cats can see in the dark?? Hmmm....

Just as I don't have a favorite colour, I also don't have a favorite eyepiece. Or a favorite of of just about anything else. Such as cuisine. Think of how dull the world would be if everything blue. Or all we had to eat was stale bread.

I think you get the general idea. But as to TV Panoptic, they are an excellent eyepiece. For a 10mm I think I'm quite partial to an original Speer-Waler which were available in both 72° as well as 82° and look intimidating as they're 1.25" and are 6" and 7" long. These were invented by a Canadian optician the name of Speer, the Waler standing for 'Wide Area Long Eye-Relief.

​

And many other makes and models!

Currently I'm quite taken with my Astromania 9mm & 6mm 58° Planetary Eyepiece, which I think you said are BST.

​

Of course, I've been rather occupied observing a planet. But you get tired of looking at a dust-storm! I think I know how Rommel must have felt. So my mind has made itself busy thinking about cause and effect.

Off to do some research.....

 

If I had to choose a favourite EP it would be the 19mm Panoptic, basically as it is my most used eyepiece. Obviously it isn't suitable for everything, but it wins out over everything else I own in several categories. Firstly its build quality obviously, the drawtube is virtually perfect. It's compact and light. Optics are superb and its eye relief is 13mm which is basically my sweet spot. More than this though ... it just 'feels' right, in an ergonomic way I guess. Nothing I have around the same focal length beats the view through it. In fact, I liked it so much I bought it twice lol.

I had one of the new Speers WALER EP's for a short period of time, unfortunately it had to be returned as there was visible debris. I think I ordered it for the last Mars opposition lol.

Oh yeah, my favourite 15mm EP at the moment is the BST HR. If you see one of these as an 'Astromania' don't hesitate to pull the trigger. I really enjoy using mine for low power views, in fact I actually prefer it to the 14mm ES. The 15 HR has very good contrast and gives a jet black sky at low power (28x on the ED72). It's superb in the ST80 and ST102 as well.

 

I certainly would buy another of the Astromania's. In fact I tried and tried to! I looked under the bushes and behind the trees, as well as under the rocks! But I just couldn't find a single excuse to do so! Oh Bo! Ho! Ho! I actually have all the eyepieces I'm likely ever going to need...

My Speer-Waler 10mm was from his first run. Got them through UniversityOptics - which went out of business in 2016 - and I did hear the 2nd. run of them had quite a number of problems showing up - including what you've run into. My only regret was not being in time to get the S-W Zoom Eyepiece. They were in a very narrow-range, 3mm to 6mm or something like that. They were only sold for a brief period of time. No word on 'why' this was reached my ears or eyes.

The TeleVue® 24mm 68° FOV is like your 19mm. If I was going to shell out $$$ for another, I likely would have gone for that. And these two are the only TV EP's I suggest to those who also want just one. Aside from a Plössl or two.

Any idea what year the first TeleVue® was sold?

 

I have a 24mm Pan. I mainly use it on the ST102. 19mm is more useful for my bino's. I always used my BST HR's for lunar/planetary but I was pleasantly surprised to find they're superb for wide field.

I'm guessing the first TV Plossls were sold in the late 70's.



I've just found the 28mm LET out, I wasn't sure where I'd put it. I think these were included with some Evostars.



Three element (probably reversed Kellner) with a focal length of 28mm, AFOV56° and a field stop diameter of 31mm. Should give me 15x with the ED72, 3°, 44‘ TFOV for a 4.8mm exit pupil.

 

I bought the 24mm Panoptic for my 12" Meade LX200 (Now replaced with an LX90 12."). But I've found it useful on everything!

I have one of those LET Eyepieces too, Mak. I'm not terribly happy with it. It came free with my SkyWatcher 150mm Maksutov-Cassegrain OTA. Seems to be some sort of modified Kellner.

 

I have the 28mm LET, I keep it to do experiment requiring a 2" barrel more then anything. Perhaps I'll appreciate it with a slow telescope someday.

 

I recall using the LET with my ST80, I thought it was OK. I'll give it a spin on the ED72 when I get the chance. I was considering getting a 2" 2x Barlow with a removable cell for it. That way I could turn it into about a 17.5mm EP and not add greatly to its overall weight. Any 2" Barlow suggestions?

 

I have a 2" GSO Barlow too. Nice piece of glass! But the 2" - 1.25" adapter - located in a strange place- got trapped in it's own brass-compression strip and died. So I tossed it and put on an extra WO 2" - 1.25" I had on hand. Problem solved. Providing you have a GSO. Or do you have a TeleVue®?

That LET is like "A Blast From the Past" to me. A Kellner was an uprade to many of us who started out in the early 1970's or 1960's. What 'came-with' a telescope then were usually Ramsden EP's. For good scopes. Or Huygenian's. ! Nowadays Kellner's are often called 'MA's' for Modified Achromats. Sumthin' like that.

"An thet's my story - an' I'm sticken' to it!" - Vermont Colloquialism.

 

Can you remove the Barlow element from the 2x GSO? The GSO is on my list of possibilities, there's also a Meade 2x Barlow and a Sky-Watcher (Barsta).


This is the favourite at the moment.



I'm more sceptical about the Barsta and the Meade, they're probably as good but I don't see any evidence of a removable cell.

I think 'Modified Achromat' is code for a reversed Kellner as there may be copyright issues with the term 'reversed Kellner' lol.

 

I think so, Mak. I'll check it later and let you know!

Good Day/Night.

 

OK cheers Dave. It's afternoon here lol.

 

My understanding is that Modified Achromat = straight up Kellner, not any kind of reversed Kellner (a la the RKE that Edmund sells). MA is just what Meade calls their Kellners for some reason. Who knows why they went that route. Same thing as why they continue to call their plossls "Super Plossls" when there's nothing super about them.

As for removing the lens element from the Barlow, with the intent of screwing it directly into the filter threads of whatever eyepiece you're using to get 1.5x instead of 2x, yes, you can do that with the 2" Barlow. Agena actually advertises the ability to do this on the 2" GSO:

https://agenaastro.com/gso-2-2x-ed-barlow-lens.html