Post by michaelmorris on Aug 11, 2013 10:33:01 GMT
Hi folks I've just bought a Watec 902H2 Supreme as an upgrade for the Samsung SDC-435 camera I've been using as an all-sky meteor camera. The Samsung was used with a 2.1mm board camera lens + adaptor, but this gives significant vignetting when used with the 1/2" chip in the Watec. I'm struggling to find a suitable C/CS mount lens. I want something that will give >100 degrees field of view and is not £100s to buy. Ideally it would have an auto iris control, but this isn't a deal breaker. Any ideas?
I'm not sure of the fov but the 3.8mm that Alex and Willam use is probably the closest. Even that is only available on the used market now (I think). Wider than that, as you've probably seen, is serious money. Pentax do a range of fish eye lenses but the are manual iris and very expensive.
My first efforts with all sky (CCD) images used a cosmicar 8.5mm lens with a nikon compact camera fisheye adapter. That worked quite well and was cheaper than a dedicated Pentax lens but also fully manual.
Vignetting is an issue as most current lenses are for 1/3in chips not the 1/2in of the watec. It's a problem. If you find something let us know!
Yes, the 3.8mm f0.8 Computar is exceedingly rare ... they used to come reasonably regularly on eBay but not anymore - and on the odd occasion that they do, they go for at least 80 pounds
Alex, operates with a 3.8mm, I use 8mm.
There aren't many alternatives that I'm aware of ... longer focal lengths (6, 8 & 12 mm) do come up with a focal ratio of around f1.2 - obviously a much narrow FOV but they are capable of picking up fainter meteors (due to their longer focal length) ... and of course there are multiple faint meteors for every bright one ...
I hadn't thought of Bill's suggestion ... Bill - what sort of FOV and limiting magnitude were you able to achieve with your combo?
On screen the FOV was 185 degrees on the horizontal axis, and truncated proportionally on the vertical axis.
The image quality wasn't great at the edges, a lot of glass involved!
As for mag this was only 1st mag at video rate. I used it with an old serial starlightxpress camera and a watec 120N at ~10 secs and could get down to mag+5.
I remember giving a lecture once and to demonstrate ultra wide systems I included a shot of myself checking the lens for dew. With the 185degree fov it could basically see behind itself!, I couldn't sneak up un-noticed.
That eBay item is no longer listed, so I couldn't check its focal ratio.
As mentioned, I use a Computar aspherical 3.8mm f/0.8 lens, which gives about the largest FOV that is still useful for determining meteor radiants, velocities, heights and orbits.
My video limiting magnitude is about 3.5, recording meteors to mag 2. A 2.4mm lens would give a larger FOV, but the stellar lim mag would be about 1 or 2, catching mag 0 and brighter meteors, which might not display sufficient background stars for doing useful astrometry, so it would give large angular errors in its estimates of the meteor radiant, etc.
A very wide lens covers most of the visible sky, but it would only record the brighter meteors and the resultant data would be of limited use in doing multi-station measurements.
UFO Capture would probably reject a lot of its data at the Q1 quality level.
It depends on your aims of your project using your camera and lens.