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Using minimum zoom on camcorders (1 Viewer)

TazmanianD

New member
I'm am basically a complete noob to the world of telescopes and telestrial and aerial observations. I recently decide to buy a scope just to mess around with and try a few things. Because I wanted both terrestrial and aerial viewing, I bought a Celestron C5 Spotting Scope that has a 1250mm focal length (which I gather is fairly high for a spotting scope) and exchangable 1.25" eye-pieces.

One of the things I wanted to do was try some videoscoping. I previously messed around with this with a very cheap telescope and a non-video digital camera. I just used the video-out from the camera (the video that shows you the same things that's on the LCD monitor) and it worked great, but the problem is that the camera isn't designed for video so I have no control over the video output and specifically, I can't control the shutter speed or get rid of the display information.

So, I went and bought a relatively cheap camcorder yesterday (JVC GR-D250US) and have discovered that it does not work nearly as great as my digital camera (which works because I can put the lense right up into the eye-piece on the scope). With no zoom, I get pretty significant vignetting if I zoom in a bit, the vignetting increases as the camera lense recesses into the camera and eventually at higher zooms, the vignetting disappears.

Because I've got a relatively powerful scope, this means I can't do any observations at moderately close ranges. With a 40mm eye-piece on the scope (which I don't have yet), I can get a magnification of about 32x but if I have to zoom my camera another 5x to remove the vignetting, that gives me a magnification of 160x which is pretty high.

Did I just get the wrong camcorder or am I going to have this problem with any camcorder because I have a scope with a long focal length? I've read about the various adapters but I'm rather confused. Some of them just mount the camera to the scope, but that won't do me any good because the cam's lense doesn't get close enough to the scope. Are there adapters that can provide some optical solution without magnifying the image? Like I said, I don't really want any more magnification. Perhaps getting an eye-piece with a larger field of view (which is probably more expensive than I want to go)? The cam has a pretty small filter size (27mm) so maybe I should return it and get a camera with a bigger lense.

I would appreciate any thought or comments here.
 
Camcorders vary in this increase of vignetting effect at mid-zoom though my Panasonic GS38 has a hint of the same problem, and I've also observed it on a Sony handycam. If you have control of the aperture - set to manual and up the shutter speed from 1/25 (probably 1/30 in your case) to 1/300 or more and the aperture will drop to about f3/5 or thereabouts, just before the CCD gain gets boosted, you may find vignetting much improved in the mid zoom range. All depends where the aperture is located in the lens.

You will have more problems with such a high magnification scope - camcorders already tend to run more mag than a digicam. 4x zoom is good on a digicam whereas 10x zoom is low-end on a camcorder - if you get a 20x optical zoom camcorder you can almost do without the scope!

Even on a standard birding scope people tend to want to go to lower zooms and eagle eye optics make a digiscoping eyepiece which is a 10-12x which is below what most scopes run at. Presumably to addres the sort of problems you've discovered running at the higher zoom of a camcorder to clear vignetting. You end up with far too much magnification so the whole thing is touchy and shaky, you are losing lightetc etc. My camcorder clears vignetting on the scope at about 5-7x. I've often had better results using a Sony HG 2x teleconverter ahead of my camcorder set at 10x which makes the whole thing quicker on the draw in the field.

I don't know if astro eyepieces go lower in mag but that would be a good move to tone down the overall magnification a bit - the wider field of view may mean you can zoom out more on the cam.

Against that you have to think about the cost of an eyepiece in comparison to a teleconverter instead. Maybe you would be better trading in the camcorder for one with a 20x optical zoom and a 2x converter wich would give you 40x which is the magnification of a midrange birding scope and a lot more light gathering power in a manageable package. You could then keep you scope for stargazing and direct vewing.

The cam has a pretty small filter size (27mm) so maybe I should return it and get a camera with a bigger lense.

that is something you probably really don't want to do. Vignetting seems to get worse for cameras with larger lenses. My GS38 has a 37mm thread but the lens doesn't occupy anywhere near the whole area so it is probably a 28mm diameter.

Other solutions are something like the eagle opticzoom 5x lens which is in theory a great match for a camcorder, giving you about 50x mag on a standard 10x zoom camcorder, which is in the upper range for a birding scope. A great idea if only it didn't also look like you were shooting thru a dirty window and introduce awesome levels of colour fringing...
 
TazmanianD said:
With no zoom, I get pretty significant vignetting if I zoom in a bit, the vignetting increases as the camera lense recesses into the camera and eventually at higher zooms, the vignetting disappears.

Because I've got a relatively powerful scope, this means I can't do any observations at moderately close ranges. With a 40mm eye-piece on the scope (which I don't have yet), I can get a magnification of about 32x but if I have to zoom my camera another 5x to remove the vignetting, that gives me a magnification of 160x which is pretty high.

Hi TazmanianD,

Vignetting in digi/videoscoping occurs if the eye-relief of the eyepiece is not sufficiently long for the camera lens. It has more to do with physical depth (length) of the lens rather than its diameter. The 40mm astro-eyepieces usually have very good (long) eye-relief (ER), which probably improves your situation, but it is hard to say if this is enough. Try to find an eyepiece with very long ER and try it out with your own camera. If it is not enough, you have to consider a new camera - usually cameras/camcorders with small, compact size and narrow zoom range are easiest to set up for digi/videoscoping.

Vignetting in digiscoping terminology is IMO not very accurately defined. I understand vignetting as gradual darkening of the image/field edges. Long focal length of the scope is not a reason for vignetting, but it may make sharp black frames because of narrow field-of-view.

Hope I didn't confuse you too much ;)

Best of luck,

Ilkka
 
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