Chapter 4: At the Scope
A person familiar with CCD imaging will find that Alan Holmes (Appendix B) clearly outlines the process for spectral imaging. Additional information on operating the spectrometer’s ST7 camera can be found in SBIG’s ST7 manual and the manual for the control software, CCDops for Windows. In this chapter, I reemphasize a few of Holmes’s points and add some of my own points and tips.
4.1
The SGS on the Scope
I would like to reemphasize some points made by Holmes. First, internal adjustments in the SGS should be made in a stable working environment; do not open the unit while it is attached to the scope. Second, use an f/6.3 focal reducer with the spectrometer. This will increase the field of view of the tracking chip, making it easier to center objects, and it will also make the spectral images brighter. Anything that can intensify the spectrum and make exposure times shorter is an advantage. Third, attach the SGS so that the entrance slit is aligned with the east/west axis of the telescope. With the slit positioned in this manner, any errors in the scope’s right ascension drive will move the object along the slit rather than off the slit. This will blur the image, but as we will see in the next chapter, this can easily be solved.
Holmes does not mention the importance of balancing the scope. The SGS weighs over 5 pounds, and the bulk of its mass is in the camera, which is the farthest component from the scope. Once connected to the scope, the SGS acts as a strong torque arm. If the scope is not balanced properly, the scope will not track or slew accurately and imaging will become extremely frustrating. Also, make sure the finder scope is properly aligned before attaching the unit, this will save time and energy when centering objects.
4.2 Some More Tips
The SGS has an LED to back light the entrance slit. Instead of turning this on and off, I turned it on at the beginning of my observing run and used a dry-erase marker to mark the location of the slit on the computer screen. As long as you do not rotate the spectrometer during the observing session, the marker line accurately represents the location of the line.
Holmes remarks that the entrance slit does cause some diffraction effects and you will see multiple images in the tracking chip when viewing a bright star. Adjusting the contrast levels of the image will help you determine which is the object of interest. Eventually you develop an intuition for which is the real object. This multiple image affect may seem like a nuisance; however, you can use it to your advantage. When I was taking spectra of Saturn, I could only guess at the orientation of the planet. The planet is so bright that even at the shortest exposure the image would saturate and prevent me from resolving the rings. One night I was adjusting the focus and by chance brought a false image of Saturn into focus. The false image was less bright and I was able to resolve the rings and then rotate the spectrometer to position the entrance slit directly across the planets equator. Figure 4.1 shows this false image.
Figure 4.1
