A few eyepiece tests

[Louis D]

5 hours ago, Mr Spock said:

Being a photographer I can say for certain my DSLR will not image an eyepiece with a lens attached. All you will get is an out of focus image of the eyepiece top and its surroundings.

It will do eyepiece projection of course but I have no means to connect it.

Thank you for the confirmation.

All you have to do is look at the geometry to see that camera lens will be looking well past the edges of the eye lens unless it's a long telephoto lens, and therefore it can't possibly image anything other than the very center of the field of view.

[vlaiv]

11 hours ago, Mr Spock said:

Being a photographer I can say for certain my DSLR will not image an eyepiece with a lens attached. All you will get is an out of focus image of the eyepiece top and its surroundings.

Not sure what you mean by this - simply focus your DSLR at infinity and it will image eyepiece nicely - in the same way phone camera does it.

[vlaiv]

5 hours ago, Louis D said:

Thank you for the confirmation.

All you have to do is look at the geometry to see that camera lens will be looking well past the edges of the eye lens unless it's a long telephoto lens, and therefore it can't possibly image anything other than the very center of the field of view.

Well, I guess that test is in order tomorrow.

I have all the equipment needed and I'll happily test this with DSLR, fast longer FL lens (85mm F/1.4 Samyang), small scope, two or three eyepieces with different magnifications and eye reliefs as well as AFOVs.

[Louis D]

2 minutes ago, vlaiv said:

Well, I guess that test is in order tomorrow.

I have all the equipment needed and I'll happily test this with DSLR, fast longer FL lens (85mm F/1.4 Samyang), small scope, two or three eyepieces with different magnifications and eye reliefs as well as AFOVs.

And pull out your cellphone/smartphone (UK mobile?) camera for comparison shots with each eyepiece.

[vlaiv]

6 minutes ago, Louis D said:

And pull out your cellphone/smartphone (UK mobile?) camera for comparison shots with each eyepiece.

will do.

 

Edited June 4, 2021 by vlaiv

[jetstream]

On 02/06/2021 at 13:18, Mr Spock said:

a Circle-T 12.5mm. Another very sharp eyepiece

I have one and it is super bright and wicked sharp as well. It is a favourite eyepiece.

Edited June 5, 2021 by jetstream

[vlaiv]

Ok, I have to admit - this was one of those surreal experiences

As I promised, I set up to take shots with DSLR and fast lens thru the eyepiece. People say it can't be done - but I was skeptical. If you believe optical science - there is no reason for it not to work - and work good.

I put 30mm F/4 finder guider on photo tripod (that was optics that I had on hand and did not want to set up more serious scope) and took 16mm ES68 eyepiece and put it in eyepiece clamp. I then focused image at distant houses. All looked good for test (there was a bit shimmer in the air from heat that could be seen at even this modest x7.5 magnification but I reconned it won't matter much for the test).

I took my DSLR, attached Samyang 85mm F/1.4 (actually T1.5 - cine version) - it is fully manual lens so I opened up aperture to the max and set focus to infinity.

I then aimed lens at eyepiece and looked thru the viewfinder - and sure enough - there was nice image of what could be seen thru the eyepiece - only magnified and just a tad out of focus. I adjusted focus and when I was happy with the view - I took exposure - and voila:

I looked at the image and asked myself: What in a world just happened?????

So I tried again and again, and then I started to think that I'm loosing my mind or that some extraordinary phenomena is happening. There is mirror involved and maybe some weird polarization thing is happening that is preventing the view that I'm clearly seeing thru the viewfinder - to be captured in image.

In the end I called my wife to verify that I'm just not imagining things. She also clearly saw image of the eyepiece in view finder - but could not get it on image.

I was ready to accept the defeat at this point - and say that you were right and that it simply does not work like I thought it does - and then it dawned on me - Lens is much larger than the eyepiece - what happens to all the light that goes around the eyepiece? Well - it must end up in image.

In above image we see that scope and eyepiece are out of focus - but are there as blur - maybe we could even see a hint of trees in that central part.

So I took a T shirt and put it as light shroud around scope and lens - just to see if that will make a difference - and, interestingly enough - it did

Look at that - eyepiece view can clearly be seen in image.

We can easily image edge of the field as well.

In the end - I want to explain what happened, why people thought that it can't be done - and how it can be made to work:

This last image is a good example to understand what is happening.

If we don't put a light shroud around the scope and lens - well, lens has much larger surface and is faster than scope + EP + lens combination. Image that comes around the scope will simply be much brighter and will swamp the signal thru the eyepiece and make it invisible.

If we put a light shroud around EP + lens - we will block all that "outside" light. This essentially happens when people use phone - phone lens is small enough that it gets completely covered with eyepiece - and no external light (one that gets focused) can get thru - only ambient unfocused light can get thru and that just impacts contrast a bit - but does not form an image - like light from far away that directly hits camera lens when lens is larger than EP.

In this last image - light shroud was not placed properly and in one corner light did get thru - and we can see distant tower and mountains (that can also be seen in first image). But rest of the image - that did not get external light - shows nicely eyepiece view.

So what is the moral of all of this - yes, you can use DSLR to image eyepiece FOV. If you use long FL lens - you'll have to do mosaic. If your lens is larger than eyepiece and you have outside light hitting sensor - it won't work - so you better put a light shroud around gap between EP and camera lens.

And yes - imaging field stop works just like I said - you just need to tilt the camera, in this case at about 35 degrees and it will capture field stop nicely.

[vlaiv]

I almost forgot - I sad I'll take one with the smart phone as well:

Compared to DSLR method - this was a breeze, and I would happily recommend it over DSLR method if it were not for that exit pupil / lens aperture mismatch.

Only problem is that at this scale - lens distortions and lens sharpness will compound to those of EP. If we want to avoid that - we need to work on much higher scale and then scale back. That way we will minimize any lens issues - as those are tied to working scale.

[Louis D]

Nice work @vlaiv.  Your next experiment is to confirm if eyepiece aberrations are reduced when using the phone camera versus the DSLR lens.  I theorize there is no difference because the eyepiece sees only the telescope and the camera lens can't undue aberrations once introduced into the image train regardless of combined working f-ratio.

I will admit that the enormous depth of field of wide angle phone cameras will compensate for image field curvature.  In particular, the 30mm WideScan clone eyepiece has tremendous field curvature to my presbyopic eyes, but it looks really nice in a phone camera image.  It's the Agena UWA 80° 30mm below.  Notice how readable the rulers remain so close to the field stop.  When viewing with my eyes, those edge images were a blurry mess until I refocused for the edge.  As such, you cannot judge field curvature of eyepieces using these phone camera images.  Your f/1.4 image would probably reveal how it really should look to the human presbyopic eye.

Notice how the strong astigmatism in the Rini MPL 29mm and chromatic aberration in the Kasai Super WideVue 90° remain even in the phone camera images?  No improvement.  That's pretty much what my eye saw through each eyepiece.

Edited June 5, 2021 by Louis D

[vlaiv]

10 minutes ago, Louis D said:

Nice work @vlaiv.  Your next experiment is to confirm if eyepiece aberrations are reduced when using the phone camera versus the DSLR lens.  I theorize there is no difference because the eyepiece sees only the telescope and the camera lens can't undue aberrations once introduced into the image train regardless of combined working f-ratio.

Thank you.

I have just a setup to test for that - very fast scope that will produce both:

- very astigmatic edge of FOV in simple eyepiece design - Plossl

- Exit pupil of such system will be much larger than phone lens aperture - hence possibility to only observe part of wavefront

I also have artificial star - which will give very good comparison. With text or objects - sometimes it is difficult to judge exact aberration or extent of it - with single PSF it is much easier.

Just need to find some time to do the tests. I've been rather pressed for time recently.

[Louis D]

I captured some images with the field flattened AT72ED while using the following eyepieces:

• Meade 5000 Plossl 40mm: 61° AFOV, 27mm of usable eye relief, and a 43mm diameter eye lens
• Agena UWA 80° 30mm: 81° AFOV, 18mm of usable eye relief, and a 38mm diameter eye lens
• Meade Silvertop Plossl 9mm: 48° AFOV, 5mm of usable eye relief, and an 8mm diameter eye lens

I chose them because the first two have long eye relief and large eye lenses while the last has short eye relief and a small eye lens, and they present a range of AFOVs from 48° to 81°.

I took the images with my trusty Samsung Galaxy S7 and with a Canon Rebel T3i (600D) mounting Olympus Zuiko 21mm f3.5, Olympus Zuiko 50mm f1.4, Sigma 50mm f1.4 EX DG HSM, Vivitar Series 1 90mm Macro f2.5 (otherwise known as the Bokina lens), and Canon 18-135mm at 18mm/f3.5 lenses.

I chose them because the phone represents the baseline I've taken most of my eyepiece AFOV images with, the 21mm is fairly fast while being quite compact, the Zuiko 50mm is fast and compact but a bit poorly corrected, the Sigma 50mm is fast, bulky, and very sharp, the Vivitar is considered one of the finest macro lenses ever produced (though there is a bit of purple abberation at f2.5), and the Canon zoom because it's my everyday lens on the DSLR.

Below are the images grouped by eyepiece.  The Vivitar produced too much image scale with the 9mm Plossl, so I substituted the zoom at the very end.  I was a bit too tired at that point to go back and use it with the other two eyepieces for reference.

If you select them and view at full resolution, you are getting the exact resolution I was getting for each image.  There was no resampling.

Capturing edge images was a royal pain.

After capturing all these images, I wasn't quite sure what I was supposed to be looking for, so I @vlaiv will look at them tell me if they reveal anything.

I know I much prefer capturing the entire image in one go with the camera's phone.  It's quick, easy, and very repeatable.

Edited June 8, 2021 by Louis D

[vlaiv]

I think that it sort of shows what I was referring to.

With long focal length lens - of 40mm (Meade) - edge performance captured with phone is better than captured with DSLR+lens because DSLR+lens does not stop the aperture down and does not cut away part of the wavefront.

For example:

taken with phone

versus:

taken with Zuiko or with Sigma below:

Even 21mm F/3.5 lens shows some improvement because it is acting as slight aperture mask (but very slight - exit pupil of 6.66mm versus entrance pupil of 6mm):

If this is due to stopping down by phone lens aperture - then we should not see the difference with 9mm eyepiece - because it produces very small exit pupil that will fit in both phone entrance pupil and lens entrance pupil

Here is Phone vs Sigma:

 

Here is a diagram that explains what happens:

There is wavefront that hits the telescope and in ideal circumstances - telescope + eyepiece would replicate that wavefront on exit pupil. However, in real life, both telescope and eyepiece add their own disturbances to that resulting wavefront (it is wavefront because emerging rays in exit pupil are again collimated - parallel as if coming from infinite distance).

Question is - what happens with that wave front if we record it with phone camera lens vs large DSLR lens?

Camera lens is small compared to the size of wavefront (if exit pupil is sufficiently large) - and will "see" only smaller and often flatter portion of the wavefront. On the other hand - DSLR Lens has much larger aperture and will not stop down wavefront - it will see whole of it and bent wavefront will do its thing and create blurrier image (or rather - image as is):

In another words - using phone to record eyepiece image will over estimate quality of edge of the field in long focal length eyepieces but will record short FL eyepieces just fine (as long as you center phone properly on exit pupil - you don't have these centering issues with DSLR Lens as it is much larger and picks up rays easily).

Using lens also eliminates most of lens aberrations. You can shoot edge of the field with central part of lens where lens is sharper (lens will act as if stopped down to aperture equal to exit pupil of eyepiece) and any sharpness of the lens issues will be minimized because you are using lens with longer FL then eyepiece (if you are using such lens) - it will grossly over sample - but you can always resample image down to reasonable size / proper sampling. Any lens blur will then be sub pixel in size once you sample back down and won't affect quality of final image.