ing FROM the knob heads due South (lower declination) (since the telescope moved north); then, heading from the 90 degree bend in the L to the non-knob end points due West (lower right ascension). Save this image as a.ﬁt ﬁle. If you do not obtain this image, get it from the common area (but so note in your report).
3.5. Focus and Plate Scale: Observing a Double Star
As a preliminary observation, you are to take an image of a well known double star.
You will use the separation of the stars to determine the plate scale of the CCD. Choose either Lyr (separation 208 ), or β Cyg (Albireo; separation 34.4 ). Albireo is a beautiful double star, with a striking color contrast between the components. You can use the double star as the focus star discussed in section 3.3. But note that the components of Lyr are themselves doubles with ∼2.5 separations, so don’t mistake this for lack of focus! Save one of the best in-focus exposures to disk (as a.ﬁt ﬁle) for measurement of the stars’ separation (and also to get the exact orientation of your images).
3.6. Observing the Nebulae
Locate the Ring Nebula and center it in the main telescope. Take an image with the Hα ﬁlter ﬁrst, since you will get more counts with this ﬁlter than with [O III]. Try 60 sec to start. Look at the stars in the ﬁeld to see if they are sharp. You may get some drift in even this short exposure. You will need an exposure of at least 5 min (300 sec) to get a reasonable number of counts in the [O III] ﬁlter. (This ﬁlter is very narrow in ˙ wavelength, and its peak transmission is only 30%) You should also take a 5 min (or longer) Hα exposure. Don’t bother with the Hβ ﬁlter; the image will look just like the Hα image, but it will be much fainter.
Now locate your second target. Take at least one long Hα and one long [O III] exposure of it.
Save all your “good” images as.ﬁt ﬁles.
3.7. Taking Your Flat Field Images