You have to choose what part of the spectrum is in focus when using a grating filter (Rainbow Optics Visual, Photo and CCD Star Spectroscope) with a refractor. This web page shows screen captures of focusing on the different parts of a star (Denebola) spectrum.

Denebola is spectal class A3V and it has strong Balmer absorption lines which help in the focusing. Also, Denebola rose above the trees before the primary imaging object, quasar 3C 273 rose above the trees.

MaxIm DL V5.07 controls the QSI 532ws-M1 CCD camera on the Tele Vue NP127is refractor. MaxIm DL graph window provides an easy way to see the spectrum at the telescope as soon as the image is downloaded from the camera to the netbook computer.

In the screen captures Denebola is in the bottom left corner (next to the blue cross hairs) and it’s spectrum is to the right with the MaxIm graph window line placed on the vertical center of the spectrum.

The below sequence of screen captures taken at the telescope show the effects of changing the focus from the short wavelengths to longer wavelengths.

Look at the below screen captures and see the following changes as a result of the focus moving to the longer wavelengths:

• Balmer absorption lines change with the focus.
  • Some Balmer absorption lines appear.
  • Some Balmer absorption lines disappear.
  • The depth of the Balmer absorption lines change.
• The height of the spectrum shifts from left side (short wavelengths) to right side (long wavelengths) of the spectrum as the focus moves to the longer wavelengths.
• The focus of the star is larger as the focus moves to the longer wavelengths.
  • The star CCD blooming streak disappears as the star goes out of focus at the longer wavelengths.
• Water absorption lines start to appear at the long wavelengths as the focus moves to the longer wavelengths.

For imaging the 3C 273 spectrum the focus was set for H-beta to H-alpha spectrum range because the 3C 273 redshift H-beta would be in this range.