stellar image which has the smallest full width at half maximum intensity (FWHM). To do this, zoom in on the star to a very high zoom. Then using the cursor, ﬁnd the brightest pixel (again, make SURE it is not saturated!), and subtract the background brightness. Next, moving along a row, ﬁnd the pixel with the brightness (after subtracting background) closest to half of the brightest pixel; and write down its column number (x number); then go to the other side and again ﬁnd the pixel at half intensity; the diﬀerence in column number is the FWHM. Check to see that you get a similar number (to within a pixel) in the opposite direction. Keep in mind that as the focus improves, more and more light will be concentrated in fewer and fewer pixels, which may consequently saturate; keep checking, and adjust exposure time accordingly.
Keep in mind that you must adjust the stretch of the image displayed in order to see the essential brighter part of the stellar image. In other words, the maximum in the display stretch should be set maybe 10% above the value of the brightest pixel; the minimum should be around zero.
You do not need to save the focus images.
Although if you know the optical layout of the telescope you can determine how the optics inverts the image, it is generally unclear how the CCD is oriented and read out.
Relative to north, the observed image may be rotated by an unknown angle; in addition, it is possible for the image to be inverted (ie top to bottom or left to right). The simplest way to determine the image orientation is to (1) set the guide speed to ”5 = B:1” (2) begin an exposure of a bright star centered in the CCD frame. (3) Wait about 30 seconds. (4) Then guide the telescope to the North (by pushing the North guide button for about 10 seconds).
(5) Next, drive the telescope to the east for about 30 seconds and stop the exposure. The resulting image should show an L-shaped pattern, with a knob at one end of the L. The knob is where the star was when you started the exposure; the side of the L starting 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