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how much do relay lenses reduce image quality?

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#1 MeridianStarGazer

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Posted 21 June 2015 - 02:15 AM

Suppose an f5 cone meets two f5 relay lenses (I assume combined they must be twice as strong as the objective). A correct oriented image is then achieved (see my signature). At 27x, how much of a loss in image quality would result?

 

Thank you.



#2 MeridianStarGazer

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Posted 21 June 2015 - 02:04 PM

I'm guessing the answer is that air spaced doublets won't hurt the image quality much, provided the coatings are not bad. I also suspect I might be wrong about the f# being what makes the relay lens match the system. It might be the focal length and placing them in the proper location. I could always start out by not extending the tube all the way and using just an aluminum mirror for an upside down image, and then upgrade to upright and ES later. I somehow doubt that air-spaced doublets are made that small.



#3 MeridianStarGazer

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Posted 21 June 2015 - 05:08 PM

So I've been thinking about the ray trace. The focal length of the first lens must match the distance from the focal point coming off the mirror. F# does not have to be specific. A larger lens catches more off axis light near the edge, but it has a faster f ratio. 

 

It seems the relay lens should be at least as large as the eyepiece field lens. However, this is not necessary if magnification changes are accepted. The slower the relay lens desired, the more off axis light will be lost. To get a thin circle hitting the lens, the lens must have a short focal length, which means fast f ratio, which means more aberrations. ... 

 

Given the need for fast f# relay lenses, and the spherical and chromatic aberration that results, and the cost and extra surfaces, and the light fall off, I think I might just go back to the upside down, left right reversed view. At 27x, it is not like I'd notice except when panning.



#4 raal

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Posted 22 June 2015 - 05:22 AM

At f/5 they are bad...really. I did some ray tracing in OSLO for that and it's nothing good.

I wasn't analyzing if low mag. would make it acceptable, as I wanted relays for minimizing secondary size, but I guess a relay would pass (barely) at 27x mag.

In any case, if a relay is acceptable enough at 27x, a scope with it would be a one trick pony (if that the right saying).



#5 dan_h

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Posted 22 June 2015 - 08:13 AM

The first telescope I ever cobbled together had a 60mm objective at f8.  At that time I was interested in terrestrial viewing so I used a small Kodak projector lens fitted inside a 1 1/4" sink trap extension as an image erector. It worked fine for that and I recall seeing the gap in Saturn's ring's with it so it couldn't have been too, too, bad.   Of course, if you want real optical quality, you need to run some analysis using a ray trace program such as OSLO.  

 

dan



#6 raal

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Posted 22 June 2015 - 08:57 AM

Yes, I imagine that projector lens could be significantly better that a pair achromat doublets that I ray-traced...plus at f/8..

There's a bunch of machine imaging lens these days to buy, but they are not cheap. Without having prescriptions for them and without having a ray-tracing software for more than 10 surfaces, I really don't want to pull the trigger on any of them just to see how they work...

Any way you turn it, it's some amount of glass (moderate to high) with optical power that we employ not to have any power and add vignetting.

With passable image quality or not, I'm advocating against that it should be the first choice for image flipping, if that's the only thing a relay should do.



#7 Dick Jacobson

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Posted 22 June 2015 - 09:29 AM

A few years ago I built a periscope that employed a pair of relay lenses. The periscope also has mirrors at the top and bottom in order to bring the eyepiece down to feet-on-the-ground level for my 20" f/4.5 Newtonian. The upper lens is a 300 mm f/4.5 telephoto oriented "backwards" so the light enters from the focal plane. The lower lens is a 500 mm f/6.25 achromatic telescope, complete with diagonal and focuser. This produces excellent images. There is some added magnification because of the unequal focal lengths of the relay lenses, and the images are upright and correct.

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#8 raal

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Posted 22 June 2015 - 12:34 PM

Niiice!

So, the Telephoto is set to infinity or the image is better at some other setting?



#9 MeridianStarGazer

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Posted 22 June 2015 - 12:50 PM

This spells doom for the ladder industry, no?

Or will optical purists still climb ladders to avoid two more lenses and two more mirrors?

Correct image and lower. Nice.



#10 dan_h

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Posted 22 June 2015 - 01:36 PM

This spells doom for the ladder industry, no?

Or will optical purists still climb ladders to avoid two more lenses and two more mirrors?

Correct image and lower. Nice.

 

Keep in mind that the Newtonian diagonal already puts the image upright but leaves it reversed left to right. This changes as the tube is rotated to put the eyepiece in different orientations but it remains that the diagonal corrects one axis.  You would have to work out the reflections and corrections to determine the final orientation of the image if mirrors and relay lenses are used.

 

For low power viewing you can make an erector/relay using a pair of identical achromats nearly touching.  Surplus Shed sells an array of suitable lenses to experiment with.

 

dan



#11 MeridianStarGazer

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Posted 23 June 2015 - 07:12 PM

With the Ray trace, was each relay lens f5? Did you face the more curved side of the lens to the parallel rays and the less curved side to the focal point?

 

I can see two lenses and long relays having more chromatic aberration than a single objective. That is with stock lenses, though.



#12 Dick Jacobson

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Posted 23 June 2015 - 07:20 PM

Niiice!

So, the Telephoto is set to infinity or the image is better at some other setting?

Yes, the telephoto is set to infinity. I never experimented with other settings.



#13 Spectral Joe

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Posted 23 June 2015 - 07:30 PM

"Ordinary" relay systems using plain achromats will have Petzval (field) curvature. They will also suffer from various off axis abberations, depending on the lens designs. These can be minimized by having a field lens at the prime focus. The best arrangement is a projection lens designed for the distances involved, preferably with an appropriate field lens. These are designed to maintain flat image surfaces. Enlarger lenses (remember those?) and macro lenses like those used in oscilloscope cameras (remember those, too?) have been the best in my experience.



#14 raal

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Posted 24 June 2015 - 08:02 AM

With the Ray trace, was each relay lens f5? Did you face the more curved side of the lens to the parallel rays and the less curved side to the focal point?

 

I can see two lenses and long relays having more chromatic aberration than a single objective. That is with stock lenses, though.

Yes, that's how they are supposed to be used in relays to keep the SA correction, but chromatic aberration doubles. They were f/5 and faster...

 

Spectral Joe, thanks a million for the tip!

 

A while ago, I stumbled upon a PDF paper somewhere on the web, showing some large military missile tracking scope...

Among other goodies, it used a relay to transfer the image back behind the primary and it was a symmetrical Biotar lens. I wanted to get one, but gave upon relays in the meantime...

Anyhow, in light of what you say about enlarger lens being good for relays, I googled around and I see now that for example Nikon El-Nikkor 50mm f/2.8N uses very similar arrangement to Biotar...

 

Here's a nice test of that one:

http://coinimaging.c...n_el50-28n.html



#15 Dick Jacobson

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Posted 24 June 2015 - 10:05 AM

"Ordinary" relay systems using plain achromats will have Petzval (field) curvature. They will also suffer from various off axis abberations, depending on the lens designs. These can be minimized by having a field lens at the prime focus. The best arrangement is a projection lens designed for the distances involved, preferably with an appropriate field lens. These are designed to maintain flat image surfaces. Enlarger lenses (remember those?) and macro lenses like those used in oscilloscope cameras (remember those, too?) have been the best in my experience.

I believe that a telephoto oriented "backwards" would be as good as a projection lens, since the telephoto is designed to produce a sharp image on a flat film or sensor plane. The telephoto focal plane is located at the prime focus of the telescope mirror. The focal plane of the mirror is curved so the image is not perfect, but to my eyes it looks excellent.



#16 MKV

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Posted 24 June 2015 - 10:45 AM

Geometric optics (ray tracing) do not take into account surface errors of each element. If you trace a Newtonian mirror at infinity with several flat mirrors inserted into the light path, you will still get a perfect (dimensionless) dot on axis. This does not correspond to reality because diffraction due to a finite aperture gives rise to an Airy disc of finite size. It also assumes the mirror surfaces are perfect -- no scratches, hills,valleys, edge issues, ripple, coating damage, etc.

 

Commercial-grade optics are normally finished to 1/2 wave (PV) surface accuracy. Precision optics to 1/4 wave. High precision to 1/8 wave of better. Stacking them in series will inevitably add up these errors, sometimes enhancing and most often degrading wavefront quality. There is also inevitable energy loss due to absorption and reflection. Mirror components require higher surface precision than lenses. Cheaper lenses are made of substrate material with non-uniform density (i.e. non-uniform refractive indices across aperture), which also affects the wavefront quality. 

 

The only way to know the quality of the exit wavefront is to test it interferormetrically in a completed optical assembly.

 

Regards,

Mladen


Edited by MKV, 24 June 2015 - 10:47 AM.



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