Note: This page desribes a way to search a grid for an extended object. The approach works pretty well for large objects, but is difficult to use with smaller objects such as stars. An improved finding technique is described here.
NGC 5907 is yet another image from the bit bucket from 7/20/01. That night was pretty productive all-in-all from my observing site here in Rockville. The NGT-18 was working fairly well, and the ST-9E was just back from SBIG for the re-connection of a wire I had somehow messed up.
NGC 5907 in Drago was my target this time. It is an edge-on spiral of extended magnitude 11.1. This magnitude is extended across an area of 8 byu 0.8 arc-minutes, and so is pretty dim looking at it from Rockville.
Now you might wonder how it is possible to acquire a target such as this from a site like mine. The camera software (CCDOPS) helps with something called dim mode. In dim mode, the 20 micron pixels are binned 2x2 to give an impressive 40x40 micron area. This area is treated as a single pixel, and the sensitivity of the chip goes way up.
Now, using the focuser as a finder, 6-10 seconds of exposure is all that it takes to produce an image of a target such as this. Not only that, but with the number of pixels now only 25 percent of the normal number (with the binnning), downloading is very fast.
Now the hunt begins.
When I set up the scope, I also set the digital setting circles. I use JMI NGC-max which allows me to use the NGC catalog, or the RA/DEC coordinates. This usage puts me within 30 arc-minutes of the target. My camera produces a 17x17 arc-minute field, so the target is not usually immediately in the field. (see image below)
The Search Sequence:
Imagine a Tic-Tac-Toe game having two vertical lines crossed by two horizontal lines. In the center is a closed in box. This is the box where the digital setting circles position the camera as the first attempt. Usually the object is not found here. (11% chance of finding here).
Using the 8xsidreal-speed control, I first move the scope south 10 seconds and examine the image. This amount of movement pretty much moves the scope by 17 arc-minutes. Then I run the scope west for 10 seconds at the 8X rate and check again. The whole search pattern is as follows:
10 sec south
10 sec west
10 sec north
10 sec north
10 sec east
10 sec east
10 sec south
10 sec south
The object will be found in one of these quadrants.
Using this technique image aquisition using the camera insted of the eyepiece takes:
1) Point the scope: 1 minute
2) view the first image: 20 seconds
3) move the scope: 10 seconds
Since on the average the object is found by the fifth image, the average time to acquire is:
60 seconds + 4*30+10 190 seconds or a bit over 3 minutes.
Compare this to travel to a dark site and then star hopping.
Something to think about.
Clear Skies
Steve Robinson
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