Comet Holmes, short refractors and wider video fields

I continue to experiment with wide field possibilities with the video camera. The true fov of view of a camera such as the MallinCam Color Hyper that I use is inherently small because of the size of the CCD chip that records the image. Frequently this field is seen as the equivalent to that delivered by an 8 mm eyepiece - and that figure is accurate if you are measuring the diagonal of the rectangular field delivered by the camera. I have no argument with this approach. Certainly it's a standard way of looking at video screen dimensions. But astronomically I'm still locked in to the visual world of round eyepiece fields and so see the MallinCam as delivering the equivalent of a 5mm eyepiece - a very small field indeed - but I am looking at the smallest dimension of the horizonal field.

In any event, one path to a wider field of view is to use a short focal length refractor and the focal reducers designed for use with this camera. To that end I started with an Orion ShortTube 80 - an F5 80mm refractor. I then combined the MallinCam MFR-3 focal reducer with three extensions that add a total of 25mm between the focal reducer lens and the CCD chip that records the image. Adding distance here increases the field of view. It also makes it impossible to bring to focus with the diagonal in place. I removed the diagonal and easily reached focus, but this means that on the mount I'm using - the one intended for the Celestron 8-inch NexStar - I can not point too high in the sky, or the camera will hit the mount. The limit seems to be about 60 degrees.

But this arrangement does give me a true fov of better than one degree no matter how you measure it - though there's some vignetting on the corners. Shooting last night - November 25 - I could fit Mirfak, plus a lot of other stars and Comet Holmes all in the same field of view. This is a shot of the screen taken with a Canon Digital Rebel and image stabilized lens. I then took it into PhotoShop, reduced the scale, and changed the mode to grey scale. made a more pleasing result, I think. This was a 2-second exposure witht he video camera and the screen shot was at 1/6th of a second.

BTW - Thought it 's reported at magnitude 4.2, with the moon in the sky I could not see Comet Holmes with the naked eye. In binoculars it still was easy to see. In the telescope visually I found it disappointing. Too large and faint - better in binoculars.

I played with Comet Holmes with the camera and other scopes, including the slower Orion 80ED (F7.5) and the slower still, Celestron NexStar 5 SE at F10. But with the NexStar I was able to cut the scope down to F6.3 with a Celestron .63 focal reducer between the scope and diagonal. Then I put on the MFR-3 and 25mm worth of extensions. Here are the results on Comet Holmes with these arrangement.

This is Comet Holmes with the 80ED, MFR-3 and 25mm extensions. Notice there is vignetting on the corners which doesn't bother me much with this subject, but could certainly interfere with others. I think the comet is about 36 minutes across in this image and the fov roughly 50 minutes. Cuting back to the MFR3 and 20 mm of extension loses some fov, but also may eliminate the vignetting. However my tests were inconclusive on this. What looked like vignetting might have been the absense of stars.

The vignetting - and a lot of the fov - is gone when you go to the 80ED with just the MFR-3 and a 10 mm extension. This is roughly equivalent to ,64 reduction, so it takes the scope from F7.5 to about F4.8 and a FOV a bit less than 40 arc minutes.

Using the 5-inch NexStar SE with .63 focal reducer, the MFR3 and the full load (25mm) of extensions resulted in a slightly larger fov. Notice that the extra extensions do not result in vignetting in this case. (I should add that the MFR3 was designed for use with a fast Newtonian reflector like the 15-inch Obsession F4.5 - but it works with other scopes. A newer, MFR4 focal reducer is designed for use with SCT scope slike the NextStar 5.)

Finally, I switched to the Pleiades, a favorite target. Keep in mind for all these shots I was contending with a moon that was just one day past full and about 30 degrees from the Comet and about 23 degrees from the Pleiades. To avoid having it totally wash them out I took this shot at "48" - roughly a bit less than one second. Keeping the background dark this way hides the vignetting and gave a pleasing view of the entire cluster. I look forward to returning to this without the moon nearby.

Caveat: Once more, please keep in mind that the goal with astrovideo is to provide a pleasing and useful live, real-time viewing experience - so take these shots as a report on that experience rather than an end in itself. Photographing the screen is hardly the recommended way to get good astro images these days. Experiencing these images onthe screen is certainly better than seeing them here.