33 replies to this topic

[Highburymark]

The transmissivity is in a recent, otherwise unrelates thread,
https://www.cloudyni...5#entry14078552
post #94 by Procyon https://www.cloudyni...eps/?p=14073894
and in the following posts,

JG

Thanks for digging this out JG

[Bob4BVM]

Light transmission (or whatever you call it), gets mentioned comparing eyepieces. What it matter most to you on, what more does it show you?

What it matter most to you on?

Dim objects.

 

what more does it show you?

Dimmer objects, dimmer details.

Edited by Bob4BVM, 16 April 2025 - 12:43 PM.

[star acres]

I'm a complete sucker for advertisement facts when I buy eyepieces. More lenses mean less light transmission but more chromatic correction. I dive on more lenses and don't worry about a little light loss. My Zhumell's have 7 lenses and SvBony's have up to 6 lenses. The eye relief is also good.

[Jay_Reynolds_Freeman]

I'm a complete sucker for advertisement facts when I buy eyepieces. More lenses mean less light transmission but more chromatic correction. I dive on more lenses and don't worry about a little light loss. My Zhumell's have 7 lenses and SvBony's have up to 6 lenses. The eye relief is also good.

The intent of more lenses is usually wider fields of view or better functionality at fast focal ratios. The main downside is not less transmission but reduced contrast, in the cases in which some of the light that is not transmitted gets spread diffusely over the field of view. A Ramsden eyepiece, made of just two plano-convex lenses of a simple glass type, can give near-perfect correction for chromatic problems, albeit at the expense of a relatively narrow field of view, no eye relief, and poor overall correction at shorter focal ratios.

 

Clear sky ...

[Mike B]

It certainly matters when I am observing objects near the telescope's limiting magnitude, but what also counts is what happens when the light that is not transmitted gets in the way: Depending in detail on the design and construction of the eyepiece and the quality of its coatings, some of it may end up as general glare spread across the field of view, reducing contrast on details or objects that are low-contrast in the first place. The latter issue is of particular importance when the object I am looking at is bright but has low contrast details -- e.g. planets -- or is a bright star with a faint companion, because in those cases there is a lot of non-transmitted light to end up in the wrong place.

 

Clear sky ...

From my own experience, I think THIS is indeed critical! It’s a reality that tends to hide in the open- we just too easily overlook it. I think I did, as I climbed the aperture ladder to arrive finally at a 15” Dob. I optimized the b’jabbers out of it- cooling fans, even a boundary fan, a carefully crafted baffle at the primary… then I collimated the sucker six-ways-from-Tuesday with the best tools on the market, stuck a ParaCorr in the focuser, then popped an Ethos in & drank in the glorious sights! Glorious they were indeed, and when the upper-atmo seeing would permit it, the Trapezium’s E & F stars not only popped into view handily, they looked like distant car headlights with their neighbor stars!

 

Yet the past half decade I’ve spent apart from my beloved Newtonian, instead cruising the nite skies with 80mm & 110mm fracs, where I haven’t enjoyed the luxury of boundless quantities of photons, so I’ve become a LOT more sensitized to optimizing the few I’ve got to work with!

 

Funny, it wasn’t even a widefield that brought this home to me, but a lowly Plossl- okay, possibly a well-bred sample- a Takahashi TPL, where I can only imagine: the glass used was top drawer stuff, the polish was exemplary, as were the optical coatings. Yet beyond these obvious measures, the interior build was of stellar design, very well blackened & baffled, where NO stray light was permitted to splash about, creating subtle glare, and corroding away at fine contrast!

 

The end result, greeting my awaiting eyeball, was a beautifully black space, upon which stars displayed as fine points, accompanied by a plentitude everywhere  of ultra-faint “threshold” stellar pinpricks in the inky black! Plus I’d never before noticed star colors so vividly portrayed! And stellar diffraction rings were so elegantly displayed around the brighter stars in the field, again, vivid on the inky black backdrop! Still more amazing to my eye was the brilliant Jovian orb in the field did NOT wash away over “overwhelm” this masterful stellar display, with several of these threshold pinpricks shining defiantly in the nearby black!

 

You may imagine my thrill in turning this TPL upon the Double Cluster! Oh, my! What I just described above, but now on steroids! Less Jupiter, naturally…

 

It was hugely satisfying to my star-hungry eyes to view with the confidence that this optical train was wasting no photons on random scattering!- All the heavenly light was going where it was intended, and needed, to be put!

 

For me light transmission is most important when useing a scope that dosent transmit much light.

I am a refractor nut, there is just something special about the view through a good refractor. But I am with Jon, besides seeing of course, aperture matters. I have a couple good refractors in 5 and 6 inch apertures, I think my 5 inch is ridiculously sharp and very much refractor like views. As much as I prefer the refractor view they just dont compare with even a 10 inch dob in light transmission.

So when using a refractor light transmission is most important to me and when I want quality opitics, less glass, all the light transmission I can get out of an optical train.

[Jon Isaacs]

The intent of more lenses is usually wider fields of view or better functionality at fast focal ratios. The main downside is not less transmission but reduced contrast, in the cases in which some of the light that is not transmitted gets spread diffusely over the field of view. A Ramsden eyepiece, made of just two plano-convex lenses of a simple glass type, can give near-perfect correction for chromatic problems, albeit at the expense of a relatively narrow field of view, no eye relief, and poor overall correction at shorter focal ratios.

 

Clear sky ...

 

As was said in another thread, if a Ramsden is configured to minimize chromatic aberration, then the exit pupil is not outside the eyepiece and the eye lens focuses on a surface of the field lens.  Neither of these is desirable and so, according to Sidgwick, Ramsdens are normally not configured to minimize chromatic aberration.

 

In terms of light transmission, there's relative and absolute.  I find absolute to be more important, that is almost entirely a function of the objective or mirror.  

 

In terms of relative and the eyepiece, coatings are an important part of the equation.  Carefully designed coatings can increase the throughput and reduce the scatter.  

 

Sidgwick in his comments about the Kellner states that it is one of the most haunted eyepiece designs, i.e. ghosting. 

 

Jon 

[Jay_Reynolds_Freeman]

As was said in another thread, if a Ramsden is configured to minimize chromatic aberration, then the exit pupil is not outside the eyepiece and the eye lens focuses on a surface of the field lens.  Neither of these is desirable and so, according to Sidgwick, Ramsdens are normally not configured to minimize chromatic aberration.

 

In terms of light transmission, there's relative and absolute.  I find absolute to be more important, that is almost entirely a function of the objective or mirror.  

 

In terms of relative and the eyepiece, coatings are an important part of the equation.  Carefully designed coatings can increase the throughput and reduce the scatter.

I have had several Ramsdens which were configured for minimum chromatic aberration, including (but not limited to) ones which I made. I have not found it difficult to keep the field lens clean enough so the fact that it is in focus is not a problem.

 

The criterion for a two-simple-lens eyepiece to be well corrected for chromatic difference of magnification is that the sum of the lens focal lengths be twice their separation. That opens up a number of possibilities. Huygens eyepieces generally fit this criterion. Classically, Ramsdens were set up as "positive" eyepieces; that is, the image formed by the objective was in front of the field lens, allowing easy use of a micrometer.

 

I thoroughly agree about coatings, but good ones are not guaranteed. Good polish is also required, and also not guaranteed.

 

I also want the objective to lose as little light as possible, but I don't want the eyepiece to steal any either.

 

 

Clear sky ...

[Jon Isaacs]

 

I also want the objective to lose as little light as possible, but I don't want the eyepiece to steal any either.

 

I want the objective/mirror to start out with as much light as possible for its focal length and that precludes the use of eyepieces like Ramsdens. 

 

Jon

[Jay_Reynolds_Freeman]

I want the objective/mirror to start out with as much light as possible for its focal length and that precludes the use of eyepieces like Ramsdens. 

 

Jon

I think you mean that you prefer fast focal-ratio telescopes, but I am not sure. Is that correct? Or what?

 

Clear sky ...