1. Announcements
HR Diagram lab will be extended for one week. I’ll talk about it today.
Homework: Chapter 9 # 1, 2 & 3
Next week is a Dark Sky Night. If it isn’t cancelled there will be no class. Meet at the Farm at 6:30pm

2. HR Diagram Lab Exercises 3 & 4
Hipparchus website has changed so can’t use it.
Use Simbad (link in my stuff on APSU astronomy website)
For Exercises 3 & 4 use Coordinates Query Exercise 3 uses degrees instead of hours in RA so be sure to use the correct unit identifier Use Identifiers link on Simbad data page to find visual magnitude (use Ptm Mag for HD catalogue or V Mag for SAO catalogue, SAO is preferred)

3. HR Diagram Exercise 5 For Exercise 5 Use Query by Coordinate
Center of Pleiades is 3h 46m 16s +24d 11m 23s**
Use search radius of 1.0 degree (60 arcmin)
Return Image Data (ID, RA & Dec and Mag)
Plot 100 stars
Don’t worry about parallax data. All stars in the Pleiades are at the same distance so magnitudes will be enough
** convert to decimal degrees

4. CCD Calibrations

5. The FITS file
The FITS Header is a database that is saved with each image of a CCD camera. It contains information on the telescope, time, observers, camera, filters and lots of other stuff.

6. Problem 1: CCD’s suffer from thermal noise
Phonons (thermal vibrations) produce electrons just like photons do. An electron is an electron however it is produced so the heat produced electrons create a constant “noise”

7. Solution 1: cool the chip
Unless the chip is cooled to below -80°C there is still some thermal noise that must be accounted for

8. Solution 2: take dark frames, lots of them, and average them
Take a picture with the shutter closed. The only thing that is imaged is the thermal noise. The image must be at the same temperature and have the same exposure time as your data image. The average of all dark frames will be subtracted from each data image.

9. Problem 2: pre-bias and readout noise
A pre-bias charge is required to “activate” the chip. The readout process can introduce an electronic noise during transfer

10. Solution: Bias images
The bias image is a zero time exposure with the shutter closed. Zero time eliminates thermal noise so the only thing “imaged” is the pre-bias and read-out noise.
The average of all the bias frames is subtracted from each data image unless dark frames are used.

11. Problem 3: irregularities in sensitivity across the chip and dust bunnies.
There are two types of problems here. The first is how the sensitivity changes from pixel to pixel which doesn’t change with time. The second are dust bunnies which do change with time

12. Solution: Flat Frames Take images of a uniform white background
Dome flats are taken of a prepared flat panel inside the dome
A flat box can be made to attach to the front of a small telescope

13. The purist use sky flats or twilight flats
For about an hour after sunset or an hour before sunrise the sky is bright enough to take flats without having a lot of stars in the frame

14. Image Calibration: Master Bias
Be sure the temperature is the same as your data image was taken at
Set the exposure time to zero
Take ~10 images
Average the 10 images
Save the result as Master Bias with a date identifier
Note: bias images are not always needed

15. Image Calibration: Master Dark
Make sure camera is at same temperature as the data image was taken at
Set exposure time to the same as the data image used
Take ~10 images with the shutter closed
Average the 10 images
Save the result as Master Dark with a date identifier
If the camera is below -80°C, darks aren’t needed

16. Master Flat
A set of flats should be taken whenever the camera is taken off the telescope or rotated since dust speck will move.
As with all calibration images, make sure the camera is at the same temperature as the data image was taken at
Find a plane white background to image Large whiteboard evenly illuminated, Flatbox or twilight sky

17. Master Flat 1
Set exposure time to be ~50% saturation. Saturation is found by dividing the full well capacity by the camera gain. Both numbers are found in the camera manual.
Once exposure time is determined, take ~10 dark frames of the same time and create a Master Dark for your flats.
Take ~10 Flat frames.

18. Master Flat 2
Subtract the Master Dark (for your flats) from each flat frame and save each.
If you aren’t doing darks, subtract the Master Bias from each flat frame and save each
Average all the dark subtracted (or bias subtracted) Flat images and save the result as Master Flat with a date identifier.
If you are doing color imaging, each color must have its own independent flat frames

19. Image Calibration: Applying the Masters to the data images
Subtract the Master Dark from each data image*
Subtract the Master Bias from each data image**
Divide each data image - Dark-Bias by the Master Flat
You now have calibrated your images.
*Darks aren’t needed if camera temperature is below -80°C
**Bias isn’t needed if darks are used (the dark frames all have the bias in them)