At long last. The NGT-18 was put to the purpose for whitch it had been purchased. 2320 17 November, 00, the picture on the right was taken. It is a 10 second exposure. Polar alignment was achieved using a polar alignment scope. Focus was rapidly achieved, and then the photo to the right. In Washington DC suburban skies, the field located in an anonomous area of sky. Taking the image was a snap.
I didn't use the on-board self-guiding chip here, and a careful look at some of the stars show a bit of tracking error. A highly magnified star from the image is shown below. Note that it is not round (or square).
The image appears to show a drift toward the upper right. It could also be a drift toward the lower left just as easily. Regardless of the direction of the drift, the motion took place over the length of the exposure, which was 10 seconds. The drift appears to have moved over onepixel in the X, and one pixel in the Y direction to achieve the diagonal effect.
An 18 inch scope at F/4.5 focusing on a 20 micron chip will cause a 2 arc-second segment of sky to be recorded on each pixel. The error is therefore X=sqrt(2*2+2*2)=2.8 arc seconds in 10 seconds.
So, how far off are we from the pole? If we are 1 degree off the pole, over a 24 hour period, our scope will trace a circle around the pole which is 1 degree in radius. Thus the error circle has a circumference of 2*pi*1degree, or 6.28 degrees in 24 hours.
A degree has 3600 seconds, so 3600*6.28=22608 arc-seconds.
A day has 24 hours, and an hour has 3600 seconds. This means that in one second, we would expect a movement of about 0.26 arc-seconds if the error was 1 degree of the pole. In 10 seconds, we would expect the error would be 2.6 arc seconds.
Our error is about 2.8 arc-seconds, we we are just a tad over 1 degree in error.
Our on-board tracker should be able to handle a 0.26 arc-second adjustment.
Although the image at right was taken two or three days later than the one shown above, this one is from a known area of sky. The Ring Nebula. The image is the same one shown in the description of "How deep can you go"
The image at right was processed to remove the light gradients using AIP4WIN software.
As good as the image looks, it is inferior to the one shown in "How Deep" so far as astrometry is concerned.
The image on the right is of M82. I focused on the galaxy more as an self guiding test set rather than as an attempt to actually image the galaxy. However in this image, the strucure of this galaxy is apparent. The image was processed by removing the dark frame, then averaging in the flat.
I stil had a good bit of light leakage showing up, so I made my first ever use of MIRA to remove background irregularities. I then applied the digital development algorithms and to my amazement saw the structure show up in the galaxy.
I still seem to be having problems with the self guiding. 2/12/01
