Moondark for April: STV DSO-ing
A couple of months ago, I wrote about my experiences with the club’s 10” LX-200 computerized telescope and STV video system. A real advantage of this combo is its ease of use, especially since there’s no tedious polar alignment required. Thus it’s more likely to be out of the box and under the stars, with a greater chance of catching a night or two of good seeing.

I've found that this system can be used effectively to image bright deep sky objects (DSO’s) like star clusters, nebulae and galaxies. Setup is a breeze in alt-az mode, and I’ve successfully used both the one- and two-star alignment options. Objects can be put on the chip with only a minimum of hunting and re-sync-ing--or even centered by eye with no difficulty at all if visible in the finder. The track and accumulate mode is your best selection here: short images (tens of seconds long) are automatically registered and displayed. When enough detail is visible or before field rotation trails stars at the edges, simply save the image to the internal memory. Move to the next object, or repeat the process for images that can be registered and combined later once downloaded into a computer.

For lunar and planetary imaging high resolution is critical, long focal lengths are used, and there is plenty of light. In contrast, DSO-ing is basically photon limited, and integrations need to be minutes long (rather than fractions of a second) to accumulate enough light for a pleasing, noise-free image. Best results will come when the optics provide a balance between object size and image scale (so it fills the frame) and f-ratio (faster, smaller numbers require less imaging time). Fortunately the STV comes with a focal reducer that can be mounted in several configurations to tweak the effective focal length, shortening the f/10 to f/6 and even f/3.75. Surprisingly, it can be used in “eFinder” mode as the objective itself with the STV piggybacked on the telescope tube. This delivers a focal length of only 4”! Shorter focal ratios yield brighter images and place less demands on tracking and alignment. Shorter total integrations allow less opportunity for things to go wrong (such as bad tracking) and more objects to be captured on any given night. The key is in matching your optical configuration and your object, but of course, it’s easy enough to change things around on the fly as desired.

At right is a selection of DSO’s captured with the club’s hardware. From top to bottom, these are: M35 and nearby NGC 2158 open clusters, the Eskimo Nebula NGC 2392 planetary, M1 Crab Nebula supernova remnant, Hubble's Variable Nebula NGC 2261, M82 peculiar galaxy, and M31, the Andromeda Galaxy. As you can see, I’ve tried a variety of optical configurations on well-known objects that happened to be up when I was out. These images have undergone no processing other than scaling and close cropping, and surely these could be improved upon with modest effort and darker skies. 

When not pointed at Mars this summer, I’d expect the STV to deliver fine views of bright Messier objects nearby, including nebulae like the Eagle M16 and the Swan M17 as well as globular clusters like M13 and M22.  Watching the object build up and pop out of the screen with successive integrations is a large part of the fun. On the screen you'll be able to see far more nebular detail than through the eyepiece. Best of all, you can show off these wonders of the deep sky to neighbors and friends. And even save the best images as a souvenirs of your tour of the Milky Way. Interested in giving it a try?

Moondark is written by Doug Miller, published on the web , and printed in the Delmarva Star Gazers'Star Gazer News and the Delaware Astronomical Society's FOCUS. Please address comments and suggestions to dcmiller@dmv.com. This document was last revised on 25 March 2001. All text and images copyright © 2001 Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.