Afocal

[Legin]

Hello,

I currently own a nice 12inch Dobsonian which is superb for visual work. That is if you can keep it collimated.

I haven't had much luck with prime focus photography. The focus is just a bit short of the prime focus I need.

If I mess about with a Barlow lens and shorten it, I suppose I can get to work be it magnified. I have tried this approach on other scopes and it seems to work quite well. Be it the cost of the Barlow (hmmm I might still go with this idea).

I am tinkering with Afocal photography. If find it near impossible while using my 2in eyepieces. However using my humble 13mm 1.25inch Plossl and a short extension tube it works great.

I don't completely understand why it doesn't work with my 2inch eyepieces. Even my 42mm eyepiece projection eyepiece has trouble focusing. I set the projection to short or as long as possible and it still has trouble focusing. Same deal for my 28mm.

As I said I am somewhat confused why it works so well with my humble Plossl. I thought Afocus was about as simple as it gets. You should be able to simply hold a camera up to the eyepiece. However I seem to have trouble sometimes.

Not sure what I am doing wrong. The projection eyepiece is focused for visual work. Looks absolutely brilliant with such a wide field of view. Then I attach a short tube and the camera. No lens on the camera and I can not focus the Afocal image in the camera.

However I do the same thing with my 13mm Plossl it works beautifully. Be it a bit dim but still working fine.

Any ideas on what is going on. Am I missing something?

[raymo]

Hi Nigel,
I was confused after reading your post, because it is physically
impossible for afocal photography to fail, so I read it again and realised
that what you are doing is not afocal imaging.
Firstly, afocal imaging is exactly what you said , either manually
holding your camera up close to the eyepiece, or using a bracket to hold
it in place. The scope is visually focused and the camera is then auto or
manually focused, and the image taken. The camera MUST have it's own
lens.
Secondly, what you are attempting, is actually eyepiece projection,
and I am pretty sure that your problem is that something is wrong with
one of the dimensions within the optical train between the focusser and the camera's sensor. For example, the eyepiece may be too far from the camera. My eyepieces are around 10-15mm from the lens mounting
face. This part of your problem is actually quite hard to come to grips with as I haven't seen your set up.
Thirdly, many mass produced Dobs have similar focusser
assemblies, and if yours has a 2" with a 1.25" adapter, there is a good chance that you can achieve prime focus with standard DSLRs.
Unscrew and remove the nose piece on the focusser that the 1.25" eyepieces slide into, which should expose a male thread. The female thread on the
camera's T adapter will screw directly onto the male thread without
any tubes in between, so the camera is almost hard up against the focuser. Using a barlow, even when it does work, is rather defeating
the purpose of prime focus imaging, doubling the mag, halving the
field of view, increasing exposure times, and making guiding a bit
more critical, and lastly, degrading the images a little, unless the
barlow is optically first rate.
raymo

[Legin]

Yes eyepiece projection is what I am trying to achieve. Afocal seems easy enough, be it a different thing to projection.

You are correct the projection becomes more and more difficult as you increase the projection length. I am trying to keep it no longer than 3-4cm about 2 of which is inside the camera. This seems to work relatively ok and gives a nice magnification as well.

I thought about few low power eyepieces.

Though I am still puzzled why the 42mm isn't focusing as it is in perfect focus visually. Then even a short 4-5cm (max) projection and it goes all weird and out of focus. The image seems quite bright it just won't focus on the camera sensor. If I look straight through the eyepiece even at the distance of the projection I can see the image perfectly.

It really confuses me, it "should" work. Especially since using my 13mm Plossl in almost the exact same way works perfectly be it a bit dim at times.

I think I have the only telescope I know that has no prime focus thread. I have a few adapters and things but the focus of the scope is a bit short.

I think I might need to shorten the tube or more likely obtain a nice CCD camera that can be used in place of the eyepiece.

If nothing else tinkering around with these ideas are helping me understand my scope a bit more.

[raymo]

One thing strikes me. The 42mm EP you are using is way below the
minimum magnification recommended for a 12" f4.9 scope. it gives
an exit pupil of approx. 8.5mm, which is much larger than your eyes will have; probably 5-7mm depending on your age and how dark your
observing site is. This could conceivably be causing your problem.
Around 32mm would be the longest focal length EP commonly used
with your scope. See if projection works o.k. with an EP around 30 mm.
I have a 40mm Plossl which will not work properly with my 8" f5 scope.
raymo

[Legin]

Yep the 42mm is a bit wide for my scope, 28mm seems to be about the limit.
Hmmm I might go get a nice 20mm Plossl so I can get more of the lunar and solar disk in my shots.

Otherwise the CCD option is definitely looking attractive. Hmm maybe in a few weeks.

[Wavytone]

Nigel,

There are a few issues to be aware of with focal digiscoping, I do this occasionally on my scopes too. You didn't say what your camera is, and I am guessing it is possibly a small automatic compact ?

To get the light from your scope into the camera it must pass through the aperture (iris) in the camera lens. Secondly, consider what happens for light entering the scope off-axis at the edge of the field of view - see the animation under "A Basic Telescope" halfway down this page http://www.jayandwanda.com/digiscope/pupils/pupils3.html

- the iris of the camera lens must be placed at the eye-point of the eyepiece., otherwise the off-axis light will not pass through the iris to reach the camera sensor.

So:

1. Set the camera so the lens iris is wide open, as wide as it will go. On my LX5, I have manual control and can set the lens to f/2. Use a rule or pair of callipers to get some idea of the apparent diameter of the iris, in mm. It also varies with the zoom setting. For example, on the LX5, with the lens set to f/2 and the zoom at its widest, the iris is 2.6mm and zooming in to the maximum focal length the iris increases to about 10mm.

Note: If your camera is fully automatic and doesn't let you set the aperture manually, it will be a huge problem and quite likely almost impossible to control.

2. The exit pupil from the eyepiece should be no larger than the iris diameter. This means the magnification required is m = D/i where D is the aperture of the telescope, and i is the aperture of the iris.

The eyepiece focal length required is then f = F/m where F is the focal length of the telescope.
Alternatively f = i x F/D

On your Dob, 42mm will be way too long for sure and quite possibly even 13mm is too long, depending on your camera.

3. You need to place the camera carefully so the lens iris is at the eyepoint of the eyepiece. This should fill the field of view in the camera. If it is too close to the eyepiece, or too far back, the field of view will vignette badly (i.e. the edges will be cut-off).

4. Focussing. Again, a camera with manual focus control is important. You will be lucky to achieve much with autofocus cameras.

Put the camera focus into manual.
Start by looking through the eyepiece and focus visually.
Attach the camera.
Watching the image in the camera, use the eyepiece focusser to get the image focussed in the camera as best you can. If this is reasonably good but not perfect, you can then let the camera's autofocus do its thing.

5. Exposure control

In normal use camera has just three ways to control the exposure - by setting the ISO (sensor sensitivity), shutter speed (exposure duration) or by varying the lens aperture (which alters the image brightness). However, in digiscoping you must use the maximum lens aperture to avoid vignetting, so only adjust the ISO and shutter speed to achieve the desired exposure. The working focal ratio of the overall combination of telescope + eyepiece + camera is

fr = FR x l/f

where FR is the focal ratio of the telescope, l is the focal length of the camera lens, and f is the focal length of the eyepiece.


6. Image stabilisation.

On some cameras IS will work as usual, typically those with optical in-lens OIS (e.g. Panasonic). With other cameras - usually those with IBIS - it may respond incorrectly and makes matters worse rather than improving the image. Try shooting with it on and off to see which is better.

[Legin]

Hey thanks for the information it helps answer many of my observations and experiences.

The camera I am using is a Nikon D3200 for the lens projection I set the camera to manual and remove the lens (of course).

Then I attach the camera behind the eyepiece. I have found the 13mm Plossl works quite well at a variety of projection lengths. Though I try to keep the projection length as short as possible to save the light drop off. It gets dim real quick at longer projection/s. I have tinkered with a 6mm nagler and it also seems to work quite well.

Hmmm it seems the longer I go with the eyepiece the more problems I have. It seems to be to do with the eye relief distance.

I have decided to get myself a CCD camera in the near future (much easier to use). This afocal has at least taught me a bit about my optics.