Lab 1: Introduction to the sky and making telescopic observations with the CCD camera. AST 152M Lab Instructor: Greg Doppmann Due: Feb 11, 2000

Transcription

1 Lab 1: Introduction to the sky and making telescopic observations with the CCD camera. AST 152M Lab Instructor: Greg Doppmann Due: Feb 11, 2000 Objective: The goal of this lab is to give students their first hands-on experience with the operation of the 16" telescope and the Apogee CCD camera. For this lab, students will find and identify stars in a field inside the Beehive cluster. Converting coordinates of 3 stars in the field to angular separations, students will measure physical separations in the CCD images you will take to determine the plate scale of the telescope in arcsec/mm. You will compare these experimental results to theoretical values derived from the relevant physical quantities of the telescope and CCD camera. Theoretical: Each telescope has a particular plate scale, which is the conversion factor between the angular size (theta) of an object in the sky to its image size (d) on the plate at the telescope focus. The effective focal length (f) of a two mirror telescope system determines its plate scale by the following approximation which is valid for small angles. d(mm) = f(mm) * theta(radians) The effective focal length is a function of the distances from the primary mirror to the secondary mirror and the secondary mirror to the telescope focus. For our telescope, nominally f = 4700mm. Because celestial objects have small angular sizes in the sky, it is convenient to express plate scale in arc seconds per mm. Solve the equation above for theta(arcsec)/d(mm), and substitute our value of f to get the nominal predicted plate scale. Procedure: 1. Using the telescope and CCD camera along with the small finder scope on the side of the telescope tube, find the star field inside the Beehive Cluster that corresponds to the handout with this lab. The finder scope has cross hairs that can be illuminated. You will see all of the cluster in this larger field view and can move the telescope in small amounts with the hand paddle to align the target stars on the cross hairs. You should be able to identify the target stars even in the small finder scope (these are among the brightest stars in the Beehive!) The field of view in the CCD camera (equal to the 16" telescope field of view), is roughly centered on the crosshairs of the finder scope FOV. Once you get images of stars on the CCD, you will want to optimize the telescope focus. Draw a subframe around a star in the display, and put the camera in a continuous readout mode as you make small adjustments to the telescope focus by move the secondary mirror using a button on the hand paddle.

2 2. Once you are confident that you have found the target stars on the CCD image display, move the telescope in small amounts N, S, E, & W. Make note how the image moves on the CCD display. This will give you the orientation of your image on the CCD camera and for your all images you save to disk and image process later 3. Experiment with exposure times of the stars in your field. Take a series of 5 frames for each of the following exposure times: 0.1 sec, 0.5 sec, 1 sec, 5 sec, 10 sec, 30 sec. Do this for each of 4 filter settings (Blue = (B), Visible = (V), Red = (R), Infrared = (I). Whenever you save data taken at the telescope, you need to keep a record of information which corresponds to each filename you save to disk. Otherwise, all you'll have is raw images without the important details that need to be linked to them. It helps to name your image files using a scheme that makes sense to your group. (i.e. "m44_blue_1sec.fits" makes more sense than "image1.fits") For each saved frame you will want record the following information: Filename: GMT time and date: Color Filter: Object: Integration Time: Sky Conditions: 4. FTP your data down to the computers on the 13 th floor for image analysis. 5. Using IRAF(image reduction and analysis facility) on the 13 th floor computers, open up your saved images and find the centroid positions of the target stars in your field in terms of x and y pixels. 6. Using the known coordinates of the target stars, calculate the angular distance between each of the three target stars in arc seconds. You must show your calculations here or no credit will be given. Recall: 60 arc seconds = 1 arc minute 60 arc minutes = 1 degree 1 hour (in RA) = 15 degrees x cos (dec) Once you have the separations of these stars in arcsec / pixel, then convert to arcsec/mm knowing that each pixel on the CCD is 20 microns (the chip is 512 pixels x 512 pixels = 10.2mm x 10.2 mm)

3 Analysis: Include one of your images in the report labeling the target stars you have identified. Mark where north and east are in your image frame determined from step 2. Your lab write-ups should answer the following questions: Is the plate scale consistent for the three target stars? Does is vary from image to image, with differing exposure times? or color filters? Should it? How does it compare to your theoretical estimate? Discuss what sources of error there could be in your results.

4 16 inch Classical Cassegrain Telescope Hyperboloid Secondary Mirror Paraboloid Primary Mirror f/11.6 Cassegrain focus f = 4700 mm CCD Camera 512 x 512 pixels 1 pixel = 20 microns

5 N M 44 Beehive Cluster E Star #3 90' Star #2 Star #1 HD RA (2000.0) Dec m_v Star #1 08h 39m 50.7s +19deg 32' 27" 6.59 Star #2 08h 39m 57.7s +19deg 32' 31" 7.54 Star #3 08h 39m 56.5s +19deg 33' 10" 7.32