Advanced Observing Program at Kitt Peak Visitor Center

CCD Imaging

The following information is designed to help those individuals who wish to devise their own observing schedule and target list. You are more than welcome to come to the observatory with nothing more than just “wanting to see anything.” We will recommend and image everything from planets to other galaxies! Otherwise, if you have particular objects in mind you may refer to the information below to help create and order your target list.

Basic Information

Configurations using the ST10XME CCD3 camera
(2184 x 1472 pixels, 6.8μ pixels)

20in RC
RC Optical Systems 		16in SCT
Meade Lx200 		76mm Televue4
Refractor
Dimension F/8.4
(normal1) 		F/5.9
(w/reducer2) 		F/10
(normal1) 		F6.3
(w/reducer2) 		F5.0
(with field flattener2)
Focal Length (mm) 4200 2949 4064 2561 380
Field of View
(arcminutes) 		12.16’ x 8.19’ 17.31’ x 11.53’ 12.74’ x 8.59’ 20.38’ x 14.72’ 134.3’ x 90.53’
(2.24° x 1.5°)
Platescale
(arcseconds/pixel) 		0.33 0.47 0.35 0.56 3.69

Notes:

  1. Tip/Tilt AO unit typically used.
  2. Tip/Tilt AO unit unavailable- relay guiding alone.
  3. Our back-up CCD camera is an ST8XE.
  4. Available at both Observatories

Things to Consider

Will my object fit?

From the charts above you can determine the configuration necessary to fit your target on to the chip of the CCD camera. The f-ratio is changed by using a focal reducer (or barlow), so a lens will be attached or removed each time a change in f-ratio is desired. This requires that the telescope be refocused after each switch, which can take time. It is highly recommended to make this switch only ONCE a night.

Often potential guests will put objects on their “wish list” like:

Unfortunately these targets are too large in angular size to fit on the chip in a single exposure. For example, in order to fit the entire Andromeda Galaxy (more than 3 degrees across) would require over 20 pictures with the ST8 at f/6.3! It is now possible to create mosaics, but bear in mind that such a project would take a good portion (if not the entire) night to complete.

The addition of the ST8E to our observatory has advanced our image capabilities dramatically. As of October 1999, the images posted in the “Best of AOP” should reflect this new level of image quality. The larger field-of-view is remarkable!

My plan is to take many pretty pictures. How many images can I expect to bring home?

Our new camera and greater understanding of image processing has re-defined what a “pretty picture” truly is. To go home with the image quality currently on display in our Gallery, expect a few high-quality images rather than many poor quality images. Quality takes time. The more time invested in an image, the better it will be, even to the point of taking all night for a single picture.

Black and white images of targets are generally the sum of many “short” (300 second) exposures. Color images are generally created using a technique called LRGB imagery, where color information is added to a black and white image (the “L”) by taking red, green, and blue filtered images. Generally the L component is a high resolution and “deep” exposure.

Shown below is an example for M51:

Click on the image to see the full resolution image. This image is the sum of 10 five minute exposures.

Below are the low resolution color components:

RED: 20 minutes GREEN: 20 minutes BLUE: 40 minutes

Scroll across to see all three images. Note that it is only when the images are combined that the color information is mapped as red, green, and blue pixels. Thus the total integration time for this exposure was 130 minutes! This image also represents one of the best of its type in the world. Click HERE to see the final color production of this image.

When setup, guide star acquisition, focus and image centering is performed in addition to the total exposure time, a black & white image takes between 20-45 minutes to make, and an LRGB image takes about an hour to 90 minutes to complete. We have access to advanced data reduction software; cleaning up your final image can take up to an hour.

On average, perhaps 2 to 4 high quality LRGB images can be completed on a given night. Keep in mind that most amateur astronomers that take pictures in this way would spend an entire session on a single target, and then process the image later. We hope to do much more, taking many images of several targets, then combining them and cleaning them before sunrise!

Color images are quite an investment in time. Why?

The CCD chip is sensitive to different wavelengths (colors) of light by different amounts. The ratio of exposure times for the chip we currently have (ST8E) is 1 : 1.1 : 1.8 for Red:Green:Blue respectively. Therefore, if the exposure in RED is 300 seconds, then GREEN is 360 and BLUE is 540. All three of these images must be taken in addition to the unfiltered “black and white” exposures. Then, all these images must be combined.

How are dark frames handled?

Dark frames are taken at the telescope on your night. It has been found that a library of master darks would be impossible to create and maintain. On a normal night the time devoted to dark frames is less than 30 minutes. As a last resort, morning twilight can be used to create darks, but this is risky since all image combining would be delayed until that time.

How are flat field images created?

Again, with the addition of the ST8, flat fields are now mandatory calibration images for almost all images. Flats are needed to remove vignetting and chip artifacts (dust, gradients etc). Since the orientation of the CCD camera is constantly changing to accomodate target guide stars, almost every target requires a new flat field image. Flats are created by illuminating the inside of the observatory dome with a standard white flashlight or incandescent light bulb. Milk plastic is placed in front of the telescope to further diffuse the reflected light source. Exposure times range between 3-30 seconds, and applying the flat to your images is done simultaneously with dark subtraction.