20 replies to this topic

[Nikon-Shooter]

Hello world/universe!

 

I own the following superlative full-frame Nikon SLR lenses,

105mm f1.4  (3")

135mm f1.8  (3")

200mm f2  (4")

300mm f2.8  (4.1")

400mm f2.8  (5.6")

600mm f5.6 (4.1")

800mm f5.6  (5.6")

all of which will outperform expensive telescope objectives at distances of a few hundred metres, and equal them at infinity.

 

In their mounts, they all produce circular images with diameters exceeding 50mm before vignetting is noticeable.

 

I wish these to also serve as telescopes, for occasional terrestrial and frequent general astronomical use.  I live in S.E. England about 50 miles from central London, so night skies aren't that dark..

 

I have devised a method to accurately add (orthogonally) a detachable 1.25" telescope eyepiece to the camera-end of these lenses.

 

There are two ready-to-use existing gadgets which already do this, available second-hand.  One is expensive, Nikon-branded and of medium optical quality.  The other is cheap and of poor optical quality.

 

I want better.

 

What 1.25" eyepieces do you suggest I buy for my own one?  I understand angle-of-view considerations for eyepieces, but don't have an idea how important this will be for this application.

 

Tyoe?

Kellner?  Plossl?  Or else?

 

Focal lengths (3 will do)

3.6mm or 4mm, 9mm, 23mm  were my original idea.

 

SVBony?  (I've read all the threads on this BB relating to SVBony - opinions differ)

Meade?

Celestron? The latter two look like they came out of the same Far East factory as SVBony ones...

 

From https://svbony.aliex...m/store/2801114 you can see (Products > Kind of Eyepiece) that SVBony has many ranges: Kellner, Plossl and 7 different angles from 58 to 80 degrees, with the 62 degree ones the only ones labelled Aspherical, but are also the cheapest...

 

Here's a summary of the SVBonys - the last number is the average price for an objective from that range:

80 deg     11/16 mm      $84 for 2  $42
58 deg     2.5/4/6 mm   $103 for 3  $34

72 deg     9/18 mm       $64 for 2  $32
60 deg     9/12/16/20 mm  $93 for 4  $23

68 deg   6/9/15/20 mm  $89 for 4  $22

66 deg      6/9/15/20 mm  $86 for 4  $21
PLOSSL     4/6.5/10/12.5/15/20/25/30/40 mm  $103 for 9  $11
KELLNER    9/17.5/25 mm  $29 for 3  $10

62 deg ASP 4/10/23 mm    $25 for 3  $8

 

 

Then, we have unknown origin ones, like

https://www.ebay.co....tm/123909677420

which extermally look a bit like the 62 deg Aspherical SVBonys but are at only £10 for 3!

 

The Meade Series 4000 Plossl's are here (UK prices)

http://www.meadeuk.c...-Eyepieces.html

and here's an S/H Meade 9.7 mm on ebay, at only a little less when you factor in postage:

https://www.ebay.co....tm/123754758613

while for 5-6 times less money you can pick up this attractive looking one:

https://www.ebay.co....tm/333335502551

 

 

So, please - WHAT SHOULD I PLUMP FOR, given I will be using them with premium Nikon long primes?

Edited by Nikon-Shooter, 09 October 2019 - 08:20 PM.

[csrlice12]

Why would you want to mate premium Nikon lenses with bottom level eyepieces?  That's like putting regular gas in a Ferrari.

[PatrickVt]

Seems to me that you would prefer Nikon eyepieces.

Edited by PatrickVt, 09 October 2019 - 08:45 PM.

[TOMDEY]

Your two F/5.6 lenses should perform well with premium eyepieces like the Televue Ethos or Naglers.

 

Your F/2.8 lenses will stress the performance of any eyepiece... even the best ones.

 

Your even faster lenses will perform terribly with any eyepiece.

 

Nother thing to check is the location of the exit pupil of your lenses... Telescope eyepieces are designed to perform best with telescope exit pupils that are pretty far from the eyepiece... so that the system exit pupil (which winds up floating behind the eyepiece) is in the nominal/right place for your eye!

 

In general, camera lenses don't perform well when repurposed as visual-use telescopes.    Tom

Edited by TOMDEY, 09 October 2019 - 08:49 PM.

[Eddgie]

In general, camera lenses don't perform well when repurposed as visual-use telescopes.    Tom

TOMDEY beat me to it.   Sounds good on paper, but not so great in practice.

 

Yes, this mirrors my own experience. Someone that spends a lot of money on SLR lenses may not like to hear that, but using a lens at prime focus vs using a lens with an eyepiece are two different things. 

 

For use with most inexpensive eyepieces, the lens will have to be stopped down to f/5.6 or slower, depending on the eyepiece.

 

For the longer lenses, it would be desirable to fabricate a diagonal:

 

https://www.bobatkin...pe-adapter.html 

 

Now I use SLR lenses for image intensified eyepieces, but my efforts to use them with conventional eyepieces have not proven to be particularly successful.  Yes, you can get it to work, but the results are not as good as one would get with a decent quality astronomical telescope.

Edited by Eddgie, 09 October 2019 - 09:14 PM.

[Eddgie]

I would say to try it though.  I recommend using a 32mm Plossl just as a test.  (Use this at f/5.6).   32mm Plossls are cheap and if it does not work out, then no great monetary investment. 

 

Just because a couple of people on a forum said is would not be as good as a regular telescope does not mean that it will totally fail to produce a result that satisfies you.  I know that it did not satisfy me, but since I did this after using conventional telescopes, I had a real baseline for comparison. Someone that has never used a good telescope might think the result is more than good enough.

[sanbai]

I'm curious about the adapter bayonet-(focuser)-eyepiece you may use. Picture please!

A premium eyepiece costs a small fraction of most of these photo-lenses. However, I also recommend starting with an affordable Plössl for initial test. Alternatively, borrow one from your nearest astronomical society.

Something to take into account is the exit pupile. You don't want more than 7, ideally between 5 to 3 to start with (small ones for daylight use. Formula is
Exit pupile = Focal eyepiece / focal ratio scope.

So for a 5mm exit pupile in a 2.8 lens, you need a 5 x 2.8 = 14 mm eyepiece.

So don't start with a 32 mm, to much of exit pupile.

You can also put a mask on front of the lenses to stop down them (internal aperture control won't work probably, unless these are old Nikon lenses).

Btw, I'm also photography hobbyist. But I don't have such big teles.

Clear skies
Santiago

[noisejammer]

The register on Nikon F-mounts is 46.5 mm. Getting a prism in there will be possible but difficult.

 

That said, I really don't think these lenses will come close to matching a telescope. As a rule, the resolution of a lens just needs to exceed the sensor - so 10 micron spots are more than sufficient at any f-stop. Few lenses actually achieve this in practice - 15 microns is more common and the image is sharpened as part of the in-camera capture process.

 

A telescope will be diffraction limited. As an example, consider an f/7 115mm optic. The diffraction limit is about 1 arcsec so the spot-size is ~ 805/206265 (mm) = 4 microns.

 

On choice of eyepiece - I suggest something that will keep the magnification modest - perhaps 15-20x per inch of aperture and well corrected for fast optics. The Ethos & ES 92deg series work at f/3.3 so they should be OK.

 

edit...

For an 800 mm f/5.6 optic, I get that you might want a 10 mm eyepiece which will produce around 80x magnification. (Bear in mind that camera mfr's don't label their focal lengths accurately.)

 

The 10mm Ethos has a dual 1.25"/2" flange so it can be mounted in a 1.25" barrel... these are large eyepieces so it's going to be precarious.

 

To reach focus at infinity, the eyepiece's effective aperture stop must be located 46.5mm behind the flange. If you use a Baader T2 prism, you will have about 4.5mm spare to construct the rest of your attachment.

[punk35]

I’ll say to get you some nice Pentax eyepieces   After all, a picture is worth a thousand words.

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[Nikon-Shooter]

Thanks to all.  Much for me to digest!  I am doing just that (I'm a physicist; info e.g. on exit pupil size etc. were just what I wanted)

 

It has been suggested to me to buy a Plossl of about 10mm FL, or a Stargazer erecting eyepiece - https://www.rotherva...epiece-125.html

 

I realised that overly large apertures will be an issue; however, I have the latest generation of teleconverters (Nikon 1.4x, 1.7x, 2x) whose effect is like that of a Barlow lens.  They won't work with the 105mm f1.4 or 135mm f1.8 lenses, but they work excellently with the 200mm f2, 300mm f2.8 and 400mm f2.8.  The 600mm f5.6 combination (300mm + 2x) would probably be the combination I used most often.

 

> The Ethos & ES 92deg series work at f/3.3 so they should be OK.

 

An attractive feature - then I looked at their stratospheric prices !!

 

The diffraction limiti is well-known to me; however, you may be out of date with developments in modern photographic lenses and their ability to approach that limit.  The Sony A7RIV sensor (62.5 MP in 24x36) is where tech is currently at, and there is no suggestion that top lenses don't outperform it.  I stand by my view.

[Jon Isaacs]

The diffraction limiti is well-known to me; however, you may be out of date with developments in modern photographic lenses and their ability to approach that limit.  The Sony A7RIV sensor (62.5 MP in 24x36) is where tech is currently at, and there is no suggestion that top lenses don't outperform it.  I stand by my view.

 

 

What do your images look like?  Are you seeing clean Airy disks with symmetrical Airy disks?  How about a star test.  Telescopes are designed to operate at the theoretical limit in the very center of the field. A top notch refractor will have a Strehl ratio of 98%... Are you resolving Dawes limit or even Rayleigh Criterion doubles?  For a 5.6 inch, that would be well under 1 arc-second.

 

 

Jon

[Nikon-Shooter]

Thanks.

What do your images look like?  Are you seeing clean Airy disks with symmetrical Airy disks?  How about a star test.  Telescopes are designed to operate at the theoretical limit in the very center of the field. A top notch refractor will have a Strehl ratio of 98%... Are you resolving Dawes limit or even Rayleigh Criterion doubles?  For a 5.6 inch, that would be well under 1 arc-second.

 

 

Catch-22.  Currently, I can only begin to assess this is by taking photographs - just looking through the camera eyepiece or Liveview, say, at the night sky, tells me little.  Google suggests that few, if any, people are measuring parameters like Strehl's ratio for camera lenses, so I can't look this up.  If you have any links, I'd be grateful.

 

The best I can look up are sagittal/radial and meriodonal/tangential MTFs for these lenses; chalk and cheese, I know.  The lenses I own produce 98% (actual lab measurements, not theoretical diffractionless calculations) wide-open near the centre @ 10 line-pairs/mm, and ~80% @ the very demanding 50 line-pairs/mm (i.e., 10 μm widths).

 

Considering the extreme violet end of the visible spectrum (0.375 μm) imaged at f2.8, 1.22 x 0.375 x 2.8 = 1.3 μm, and the D850's sensor pixels are far larger than this, at 4.3 μm.  So, any assessment on the basis of photographs isn't too useful.

 

For the 400mm f2.8 mounted on a Nikon D850 (47 MP, pixel pitch 4.3 μm) photographing the night sky, I'm very limited by my not having an equatorial mount (that's really my next question, how to select one).  Photographing circumpolar stars (to minimise motion blur) wide open at 1/4 sec, the high ISO rating necessitated makes drawing conclusions fraught.

 

Once I have the equatorial mount sorted, then I'll also need advice about specialiased software combining multiple images to remove noise, increase contrast.

 

Sorry for being so needy.

[noisejammer]

I've used my 1.4x tc on my scope - it's Canon's most recent model and I'm happy with its photographic performance between f/7 and f/10.

 

> The Ethos & ES 92deg series work at f/3.3 so they should be OK.

 

An attractive feature - then I looked at their stratospheric prices !!

 

The diffraction limiti is well-known to me; however, you may be out of date with developments in modern photographic lenses and their ability to approach that limit.  The Sony A7RIV sensor (62.5 MP in 24x36) is where tech is currently at, and there is no suggestion that top lenses don't outperform it.  I stand by my view.

 

Hmmm... yes, they are expensive. The reason I suggested them is they do work on fast optics. If you limit yourself to f/5.6, you'll have a much wider choice but I'd avoid a Plossl, Erfle, Kelner.

 

I think an Abbe orthoscopic will work .. I don't have an f/5.6-ish scope to hand that I can test one on. Someone can chip in about this.

 

On the resolution of camera lenses - I've been wrong many times. As a physicist who has built many scopes, I see experiment as the sole arbiter. Whatever your lens does, I think your 800 f/5.6 will do quite nicely if you select an eyepiece to give you a 2-2.5 mm exit pupil (say 11-14 mm focal length.)

 

It occurred to me that there's no reason to believe the infinity marking at the nominal 46.5mm register is optimal. I'll guess the register is likely to be a compromise over all the lens' likely uses. Before going too far, I'd start by jury rigging an eyepiece into a tube to lie accurately along the lens' axis then detemin the optimal register by observation. It may turn out you can increase the register slightly and get more space for a diagonal while maintaining acceptable performance.

[Jeff Morgan]

The AB Nightvision Mod 3C eyepiece.

 

I use one on a FD Canon 300mm L series and it rocks. Used it earlier in the week to trace Sharpless 2-245 over 16 degrees through Taurus and Eridanus (and there was still more to go). A direct-vision catch under SQM 20.5 skies of my back yard.

 

 

As you know, aside from providing higher magnifications these lenses are a little on the heavy side to hold by hand. And they function as straigh-thru refractors. For steadiness and ergonomics I made a Dob-style mount and right angle mirror to fix those issues. 

 

[Jeff Morgan]

Forgot to mention - the C-mount adapters are available on Amazon and eBay for around $20.

[OneGear]

...

 

The diffraction limiti is well-known to me; however, you may be out of date with developments in modern photographic lenses and their ability to approach that limit.  The Sony A7RIV sensor (62.5 MP in 24x36) is where tech is currently at, and there is no suggestion that top lenses don't outperform it.  I stand by my view.

Honestly there is nothing new in camera lenses, just more money spent making and marketing them.  As has been stated already, telescope objectives are designed for the theoretical limit of resolution, while digital sensors are unable to benefit from such effort.    And in the spirit of being honest, commercial photography - especially photojournalism - is published at far below the potential resolution of the sensor/film.  Astrophotography - utilizing the very same telescopes we use visually - often devolves into an investigation of the ultimate resolution limits of the lens (and inherently,the limits of the mount).

[25585]

 

The AB Nightvision Mod 3C eyepiece.

 

I use one on a FD Canon 300mm L series and it rocks. Used it earlier in the week to trace Sharpless 2-245 over 16 degrees through Taurus and Eridanus (and there was still more to go). A direct-vision catch under SQM 20.5 skies of my back yard.

 

 

 

As you know, aside from providing higher magnifications these lenses are a little on the heavy side to hold by hand. And they function as straigh-thru refractors. For steadiness and ergonomics I made a Dob-style mount and right angle mirror to fix those issues. 

 

What is the aperture of that lens?!

[noisejammer]

What is the aperture of that lens?!

300mm f/2.8 so about 107mm.

 

This sort of optic does work really well with an II (image intensifier). I've used my 400/4, 200/2.8 & 100/2 lenses with my II. In this case, the II only delivers about 60 lp/mm so the lens' resolution is totally adequate.

 

I have tried with my Rokkor 58/1.2 . Wide open, it seems to overpower the II and point sources (even stars) get bright haloes. I don't really know why this happens

[nicoledoula]

You could sell some of those lenses and buy a real telescope...just a thought.

[Nikon-Shooter]

Thank you!

 

Is https://www.ebay.co....tm/293013842523

 what you had in mind as a suitable Abbe Orthoscopic (10mm)?

 

And, will this http://televue.com/e...=212&plain=TRUE

 "Eyepiece Calculator" serve to help me judge?  What should I be looking out for, besides the exit pupil diameter?

 

---

 

As a physicist, I assure you that the improvements in 24x36mm frame camera lenses in the past 5-10 years has been spectacular, though since long FL lenses were already good, the improvement was less here.  And the major improvements have been to performance at or near full aperture and away from the centre, again perhaps less relevant for telescope use.

 

As a test a while back, I compared a Zeiss-Opton Sonnar 85mm/f2 (arguably the sharpest optic of the early 1950s, and CLA'd by an expert) wide-open with a very modern Sigma ART DG HSM 85mm f1.4, also wide open and a stop faster.  To say there was no comparison would be true.

 

The gap narrows but does not disappear when the Sonnar is stopped down.  Still wide open, the Sigma outperforms (resolution and contrast) the Opton even when the latter is stopped down to f5.6.  That's how huge the difference is.  The old Sonnar is a 7/3 design, while the Sigma is a 14/12, with 2 elements LD glass and another aspherical.

 

 

 

If you limit yourself to f/5.6, you'll have a much wider choice but I'd avoid a Plossl, Erfle, Kelner.

 

I think an Abbe orthoscopic will work .. I don't have an f/5.6-ish scope to hand that I can test one on. Someone can chip in about this.

 

On the resolution of camera lenses - I've been wrong many times. As a physicist who has built many scopes, I see experiment as the sole arbiter. Whatever your lens does, I think your 800 f/5.6 will do quite nicely if you select an eyepiece to give you a 2-2.5 mm exit pupil (say 11-14 mm focal length.)

[noisejammer]

It's not clear where you live but I'd honestly recommend joining the local astronomy club then go out on an observing expedition with the group. If you've made a simple adapter and have a gimbal mount for your 800/5.6, there will be some interest from the other club members.

 

If your adapter uses a non-marring collet to secure them, I'm sure the others will let you experiment with their eyepieces. You can then judge which offers acceptable performance.

 

On the $35 Abbe you linked to .. I can't comment on that eyepiece.  There are some excellent value for money eyepieces on the market but inexpensive (in this forum at least) usually means around $100.

 

As I noted, I'm not certain that an Abbe design will operate well when coupled to an f/5.6 optic, but I just realised I have a 60mm f/5.8 refractor (350mm focal length) that I can try. Watch this space.

 

In the meantime, you need to understand that the Abbe design is sharp and relatively simple but it has limited apparent field of view (typically 42 degrees.)

 

Edit - I re-arranged the bits and put an 10mm Abbe eyepiece (in this case a ZAO II) on my my 60ED Borg. It seems this eyepiece works fine at f/5.8. At 35x, the magnification is pretty low so I can't usefully say it's resolving anywhere near the scope's capability (roughly 2") but it is very usable.