Wonder why a lot of times when I zoom in a little with my camera, I get this shadow in the fov, even though when just view the scene through the ep with the naked eye, I dont see the obstuction (or whatever)..? Seems it disappears when I dont zoom in at all with the camera or if I zoom A LOT, it moves out of the fov...... This is with my 127mm Mak-cass.
simple
its the secondry mirror shadow. i have the same on my scope..... yif you move the camera around the EP you will see the "shadow" move across/around the image
its just a matter of getting the right angle on the camera to miss the spot
And also try zooming (optical zoom) in a little, towards the tele end. This should help with the dark area around the FOV, although it will also mean a closer view.
Well, whatever it is, it's not going to be the secondary mirror shadow - when you're in focus there is no "shadow" cast from the secondary. If there was then you'd see it with a normal eyepiece.
Remember, every point in the field of view is generated by light hitting *all* of the visible portion of the primary. The secondary obstruction is present everywhere, equally. There's no reason why the centre of the field would get any less light than the edge.
It's only when you defocus that you can see the secondary shadow, and even then you can see easily enough that the shadow is present everywhere in the field of view. Look at a defocussed star as you move it around - the shadow is visible just as much when the star is at the edge of field as in the centre.
I'd suggest that the real culprit is some optical mismatch between your camera + zoom and the light cone coming out of the eyepiece. Perhaps the light cone is diverging or converging a bit, and your camera's optics can't handle it.
Id like to hear from others that take photos through the EP (whatever you call it) and see what they get also.
No matter what EP i use...i get the "dark spot" and i have used a cannon fuji and a minolta at the EP i have also used a large range of EP's from cheapo plossl's through to Nags and i still get the same dark spot.... oh it also dosent matter if your scope is collimated or not....still the same.
Can somwone else take a digital camera to their EP's and see if they get the same ???? Please....
Anthony, fwiw it's afocal. I have similar view thru mine. Was due to zooming in too far, I think from memory.
Quote:
I'd suggest that the real culprit is some optical mismatch between your camera + zoom and the light cone coming out of the eyepiece. Perhaps the light cone is diverging or converging a bit, and your camera's optics can't handle it.
regards, Bird
Tried diff settings/E/Pcs etc. 25/18/15 mm = O.K. with no/little zoom. If you zoom too much, that's when you get the shadowing.
HTH. L.
Yeah, sounds to me like the light cone from the eyepiece is either diverging or converging, dunno which one, and that's playing havoc with the optics in the camera - end result is that some of the light that is on-axis is being lost, probably missing the ccd sensor in the camera.
Umm - the light form the EP is parallel - that's what EP's do. This is definitely the central obstruction - you have to remember that the EP is maximised for viewing by eye, not magnified through a camera. Try moving your eye about 4 inches above the EP and you will begin to see the secondary obstruction interfering with your view. The camera simply magnifies this effect. If it wasn't there... how could you collimate???
Ambermile - EP's can easily produce non-parallel rays, we call this "out of focus". When you move the EP toward or away from the correct focal point then you change the output rays from parallel to either diverging or converging.
The human eye is only capable of focusing light that is very close to parallel, but I'm not sure about an afocal arrangement with a camera instead of a human eye. Again, I think the culprit is that the focus is incorrectly set on the scope so that the light rays are not parallel, and the operator has tried to compensate for this by adjusting the focus on the camera.
Maybe someone could try an afocal shot with a refractor. To see if the same effect occurs. fwiw, from memory I don't recall this issue with either of my refractors, but I did with my SCT. Though this is only from memory and its been a while since I did any afocal.
Paul, I was thinking the same thing. It's a bit tricky - it seems that it doesn't always happen even with reflecting scopes from the comments above.
Ambermile - I keep trying to convince myself that you're correct, and that it's the central obstruction. Unfortunately I don't know enough optics to really solve it for sure. There are a couple of things that I keep coming back to:
- earlier posts mention that this shadow moves around, and that you can position the camera so that it's "off the screen". I can't see how that is possible if it's really the central obstruction causing the shadow.
- When you are in focus there is no shadow in the middle of the image from the central obstruction. none. Not even a tiny little one. The light rays forming the image in the centre of the fov are using just as much of the mirror as light rays that form an image at the edge (assuming no vignetting).
good luck
L.
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