The Effect
of Focal Length on Images
What do you think will happen
when we image the same object with telescopes of longer and shorter
focal lengths? Will the images size change? How? What
else do you think will change?
Procedure: Using HOU software, you will compare images sets taken with telescopes having different focal lengths. You will make observations and gather data about image size. Then use the slice tool to measure the distances on both images between the same features. Look up the focal length of each telescope in the image info or in the paragraph that follows about the image sets. Compare the ratio of the focal lengths of the telescopes to the ratio of the measurements between the two stars or the same features on the respective images. Create a data table and/or spread sheet and graph to show your results. The images in each set were each taken with the same CCD. Study how much of the object can be seen in the entire image. Consider the effect focal length has on the field of view.
Image Sets:
1. Select an image set to analyze.
2. The images in each set were taken with telescopes of different focal lengths. The focal lengths are in the image name, and in the chart below. You can also look in Image Info to find the information about the telescopes. Image Sets - The images are named in this manner: activity-object_ccd_focallength_telescope date etc.
Image Location:
folders on payton – Barge’s Class
– HOU Images – Telescope – Focal Length
3. With both images open, study the images to find the same features in both. Then use the slice tool (Data Tools, Slice) to measure between the features. Measuring across the images with slice is a way to compare image sizes.
4. Make a table to record your data. Consider entering your data in a spreadsheet and graphing program. For each set of images, record: Image Names; Telescope Focal Lengths; Slice Measures. Then compute the: Ratio of Slice Measures; Ratio of Focal Lengths
5. Compare the ratio of focal lengths to the ratio of slices.
6. Can you predict the ratio of either the slices or the focal lengths if you know one or the other?
7. Repeat your procedures with the one of the remaining image sets.
Conclusions: Look carefully at your results. Use complete sentences to describe:
a) What happens when we image the same object with telescopes of different focal lengths.
b) Does the image size change and if so, how?
c) Does the field of view change and how?
d) If we controlled for aperture and exposure time, what do you think would be the effect on brightness when we changed focal lengths?
e) What new questions do you have?
How could you create experiments to test these questions?
| Image sets | Information |
| Orion Nebula |
Images:
m42_st7_160cm_gh10in.fts, m42_st7_250cm_km10in.fts
Orion Nebula, M42. This set compares two Meade LX200, 10 inch telescopes, with different focal lengths. The f/6.3 has 160 cm focal length and the f/10 has a 250 cm focal length. These images were taken using an SBIG ST7 CCD, both binned 2x2, yielding 18 micron pixels. (Binning combines pixels. Originally the pixels are 9 microns. Binned across and down to be double in size creates 18 micron pixels.) |
| Orion Nebula |
Images:
m42_ap7p_305cm_fm12in.fts, m42_ap7p_824cm_y24.fts
Orion Nebula, M42. The second set compares a Meade LX200, 12 inch, f/10 telescope with the Yerkes 24 inch, f/13.5 telescope. The respective focal lengths are 305 cm and 823 cm. Both images were taken with the Apogee AP7p CCD with 24 micron pixels. |
| Ring Nebula |
Images:
The two M57 images.
Ring Nebula, M57. This set compares a Meade LX200, 8 inch, f/6.3 telescope with the Yerkes 24 inch f/13.5 telescope, using an Apogee AP7p CCD with 24 micron pixels. Focal lengths are 128 cm and 823 cm respectively. |
| Saturn |
Images:
The two Saturn images
Saturn. These images of Saturn were taken during the same week in November of 1999. One image was taken with a Meade 10 inch f/10 telescope. The other images was taken with the Yerkes 24 inch f/13.5. Both images were taken with the SBIG ST7 CCD, both binned 2z2, yielding 18 micron pixels. Focal Lengths are 250 cm and 824 cm respectively |