Imaging fundamentals required

[Fo_Cuss]

Imaging fundamentals required

I'm upgrading a scope; and prior to making irreversable moves, the scope was benchmarked a few days ago.
Done, using an LG4 smart phone, and 'Open Camera' software.

A tube fixed to the phone skin, that corresponds to the eyepiece outside diameter, was used to mount the phone/camera.
... the viewing end of the eyepiece is inserted into the tube, and the camera/eyepiece assembly is inserted into the focus tube.

The images captured were, what I consider to be, fine enough for a first attempt.
... mistakes were made, and anyway, the scope engineering is not yet to the required standard.

It was as much a learning exercise as a benchmarking exercise.
... only that, a couple of lessons were not fully comprehended.

Consequently, I'm here, wondering if someone has crossed these bridges, who might be prepared to share their knowledge

I thought that we might examine the first problem, as any answer might inform other questions

Understanding the camera

Open Camera appears to use contrast focus, as it focusses while the lazer emittor is covered.

It was not clear, when taking the images, which was the correct camera focus setting, between infinity, and closeup.
... until viewing the images on a monitor.
As can be seen ... at the infinity setting, the image more or less remans in focus, only that a broad white band accompanies the wind turbine pillar.

Okay ... so I have learned to set focus to close up, though......?
The stated minimum focus is 100mm ... nowhere near how close the camera lens is to the eyepiece lens.

With this in mind, at my desk, I have just checked distance to focus, with 'macro' selected.
It can then focus to 70mm (it auto fucusses, as if on auto focus).

I'm not sure how it is doing this.
It may be that 100mm is an arbritary choice, that suits all phone cameras.

Setting to 'auto' the camera will reliably focus to 70mm (the same as macro).
This, therefore seems to indicate that the actual reliable focus distance is in the region of 70mm

Note:
Interestingly, one can get slightly closer ... the camera still focusses, only the centre of the image is soft, while the exterior is in focus.

Other than that; let's say that the camera will attempt to focus, based upon best contrast, to 50mm.
At 45mm, multiple taps on the screen might result in a best attempt, but often it just fails.
It is a blur.
It probably can't get enough information to set the focus to the very limit

Objective lens, to eyepiece, to camera lens, to sensor

From the images taken, we have established that the image remains in focus, whether the focus is at infinity or close up.
It is this that forms one of my struggles.

Normally, if I manually change the focus ... anything other than the correct focus will result in a blur.
... but not with the image from the eyepiece.

There is obviously a good reason for this, but for the moment, let's call this 'strange'.
That  strange behaviour, calls into question the camera focus system.

If the image remains in focus, when the focus distance is at the opposite of the ideal ... can the auto focus even begin to function?
If not, then I can forget auto & macro, and simply set the camera to a fixed focus of 100mm.
... a shame, because true focus can go down to 70mm at least.

Of course, the camera lens is almost touching the eyepiece (just millimeters away).

So, where is the image, vis a vis ideal camera focus distance?

Yes, yes, yes ... I get the bit about objects further away producing 'closer to parallel light'.
But the eyepiece lens changes that.
(Ha! You've maybe guessed that the next question relates to focus without an eyepiece)

Does an ideal camera focus distance exist?
Does that change for each eyepiece, or perhaps object distance ... or both? 

These multiple questions lead me to believe that I need to understand the fundamentals.

Can anyone explain them ?  🌝

 

 

[happy-kat]

You are using a camera with a lens to take an image that an eyepiece projects, the eyepiece has already been focused on the image the telescope presents. 

Have a search on afocal imaging

I can't help with your specific camera questions.

Edited December 11, 2019 by happy-kat
Corrected my error

[Cornelius Varley]

1 hour ago, happy-kat said:

You are using a camera with a lens to take an image that an eyepiece projects, the eyepiece has already been focused on the image the telescope presents. 

Have a search on eyepiece projection imaging.

I can't help with your specific questions.

This type of imaging is called afocal, not eyepiece projection. In eyepiece projection photography the image from the eyepiece is projected directly on o the sensor.

[happy-kat]

Whoops yes of course why I forgot that I don't know

[Fo_Cuss]

Thank you Happy-Kat and Cornelius Varley for providing the mission critical terminology:

6 hours ago, happy-kat said:

eyepiece projection

5 hours ago, Cornelius Varley said:

afocal

Knowing the terminology enables easy research  👍

Yes, my use scenario is 'afocal imaging'.
 ... a method that seems to be mainly based upon trial and error (from the documents that I read).
The fundamentals appear to be lost in the chaotic mix of variables.

It certainly doesn't help either, when information is contradictory.
... I thought that I had the explanation, when I saw these diagrams:

They both show parallel light being emitted from the eyepiece.
I thought that this might explain the infinity/closeup camera focus question (if I thought long enough on the subject).

However, I was then researching eye relief, as this was repeatedly mentioned as a factor.
This produced this diagram:

Clearly, the light is shown to converge on the eye pupil.

I'm wondering ... perhaps the triple lines are indicating the RGB split.
Perhaps there is some relationship with the white banding on the turbine pillar.

Text
... was not easy to decipher.
Most docs stated to set the camera to infinity.
One doc stated both ... set to infinity, but idealy set to macro!

• Use an eyepiece with long eye relief.
• Couple the end of the camera lens as close as possible to the eye lens of the telescope eyepiece.
• Set the digital camera at macro mode rather than infinity.
• Use digital camera at full optical zoom.
• If possible, use a camera lens with a focal length longer than the eyepiece focal length.

I don't understand why long eye relief is required, if the camera lens is close against the eyepiece lens.
Does anybody have any thoughts on that?

The other complicating factor, is that none of the docs discussed smart phone cameras.
They all discussed DSLR's which have a large CCD sensor.

My Setup

The Omni Plossl eye relief is stated as:

•   4mm = 6mm ER
• 12mm = 8mm ER

I don't think that this is classed as long, but they are what I have.
My camera lens is close up to the eyepiece lens.
Closeup focus does appear to be better than infinity (more tests required).
The full optical zoom ... does he mean digital zoom, or optical zoom?
... why mention 'digital', in this statement?
Either way, the digital zoom (all that I have) is definitely useful for focussing purposes.
Also, some degree of digital zooming may help.
... I believe that it crops the sensor pixels, before running the image through processing software.

Conclusion

Hahaha ... Forget the fundamentals, and test various setups

Further Thoughts

Tube Connector

For testing, I need to firm up the 'camera to eyepiece' tube connector.
I will probably also add an additional support, to enable the phone to be held upside down, hanging above the eyepiece.
This will enable erect images to be recorded.

lock image to camera position

Open Camera provides a 'lock image to camera position' setting.
Without it, the camera can be inverted, but the image isn't.

This latter point highlights the fact that the camera can rotate or not rotate the image.
Theoretically therefore, there must be potential to display an upside down image, as a vertical image.

I have a feeling that this potential has not been exploited ... probably because it is only relevant to telescope imaging.
Hmmm!
I'll have a check; and if not, I'll contact the Open Camera developer, and suggest that this would be a highly useful mod.
... particularly if the image could be flipped horizontally.

Then, there would be no need for a prism.
The image could be recorded as is, and displayed as is.

Macro or wide-angle camera lens attachment

These are magnetic lenses, that are larger than the camera lens in the phone.
The phone lens aperture = 7mm
Macro = 9mm
Wide = 13mm

4mm EP = 5mm
12mm EP = 11mm

The macro lens works at very closeup, producing a sharp centre surrounded by blur.
The wide seems to allow closer focussing, without gaining the detail of the macro, though it eliminates the blur.

I have them, so I will try them  🌝

 

 

[happy-kat]

Eyepiece projection is a different kind of imaging, as corrected by Cornelius Varley you were using afocal

I'll read your post now.