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Goddard Space Flight
													      Center Sonoma State University
													  Education and Public
													  Outreach
Observatory Activity Taking Astronomical Images for Fun and Science
Overview
Groups and Organization
Technical Parameters
Ready To Observe?

From this activity you will learn...

  • about some interesting types of astronomical objects
  • about how the positions of sky objects are defined
  • how to decide when a sky object is observable
  • basic imaging characteristics of astronomical telescopes
  • basic capabilities of CCD imaging systems
   
Overview

You have been invited to submit an observing proposal for a robotic telescope system with a CCD camera. You should select objects to be imaged by this system. You will select the objects and enter their specifications into a telescope control system. The system will acquire the images you specify. You get to see and receive a copy of the images you have specified. (Note: you should have some familiarity with the material on the Basic Photometry and Astrometry page.

You should select the following types of fields to image:

(1) an image of some field or object in the sky that is of interest to you (fun)

(2) an image of a GTN program object (science)

The objects you select need to be visible at the time and location you are observing, and recordable using the available equipment.

The simplest implementation of this project will involve selecting for your "interesting" object a Messier Object. These are objects that appear in the Messier Catalog.

More demanding possibilities for "interesting" image fields might include an NGC object, a galaxy cluster, a recent supernova, stars known to have planets, comets, minor planets, or... use your imagination.

All GTN program objects have available coordinates, finder charts identifying the objects, and comparison stars with known standard magnitudes. A GTN Object Catalog is available.

In order to select appropriate objects or fields to image you will need to consider the following technical matters...

  • object coordinates (RA and DEC)
  • observable in the sky from the location of the telescope at the specified date and time
  • angular extent of the object or field compatible with the size of the images that can be obtained
  • brightness of the object compatible with the capabilities of the instrumentation
  • desired exposure times within the range of exposure times convenient for the instrumentation
   
Groups and organization

Organize into groups for this activity. Groups should have between 3 and 5 members. Assign or get volunteers for the different research tasks that will be needed. Plan on meeting to discuss your results. Group members should nominate objects to be observed, with the two objects ultimately selected based on group consensus. You will be advised of the date and time that the telescope system will be available.

   

Technical Parameters for the Instrumentation

Coordinates and Telescope Pointing

Accurate coordinates (right ascension and declination, RA and DEC) must be available. Coordinates should be known to the nearest second of time for RA and the nearest second of arc for DEC. Because of the nature of the equatorial coordinate system RA and DEC are actually continuously changing for all sky objects. This is due to the precession of the earth. Thus, RA and DEC must always be specified for a particular instant in time. It is customary to publish and utilize coordinates for the beginnings of key standard years. For example, it is now customary to utilize coordinates designated as 2000.0 , which indicates the coordinates as of the beginning of the year 2000. This is often termed the Epoch or (more correctly) the equator and equinox for the beginning of the indicated year. Modern software systems are prepared to account for precession and will expect users to enter coordinates for a standard equator and equinox (such as 2000.0).

Coordinates must be accurate because astronomical telescopes tend to have very small fields of view and telescope mounts are limited in the precision with which they can point to a designated point on the sky. Small professional or amateur telescopes under computer control should normally be capable of pointing reliably to within about 1 minute of arc of the desired location. Since the field of view of the images obtained (see below) tends to be on the order of about 10 minutes of arc, such telescope pointing is normally acceptable.

GORT uses a pointing model that is formally reliable at the level of about 10 seconds of arc. In reality, GORT's pointing varies somewhat around the sky. In the northern sky the pointing is normally better than 1 minute of arc. In the southern sky it can be a couple minutes worse than that. Thus, if you use coordinates that are accurate at the arc second level for the equator and equinox of 2000.0, when GORT does a pointing you can always be guaranteed that the desired object is in the field (although not necessarily exactly in the center of the field).

   
Image Size

The angular field of view of a GORT image is approximately 12 minutes of arc on a side. This is the expected result for the nominal focal length for a Celestron-14 of 3000 mm and a physical size for the detector in an AP47 CCD camera of 13.3 mm. This angular size can be compared with the angular size of the full moon (or the sun), which is about 30 minutes of arc. GORT does not use a focal reducer to alter the effective focal length. This is considered to be a "small" field of view, but the telescope pointing is sufficiently reliable so that this small field of view is not a problem.

Obviously, objects larger in angular extent than this field of view cannot be imaged in their entirety. Furthermore, objects considerably smaller than this field of view may end up looking like stars (see below). The optimum size for objects to be imaged by GORT is roughly in the range between 1 minute of arc and 8 minutes of arc.

Image Scale, Resolution, and Seeing

The image scale for GORT images is 0.68 arcsec/pixel. Thus, objects (or details) smaller than about 1 arc second may never cover more than 1, or at most 4, pixels. Objects or details smaller than this size can never be recorded as anything other than a single pixel.

However, seeing at the Hume Observatory tends to be about 3 arc seconds (at least during the summer). That is, star images tend to have a full width at half maximum intensity (FWHM) of about 3 arc seconds. This corresponds to about 4-5 pixels. All stars on an image have approximately the same FWHM. This is normally the ultimate limitation as to the smallest objects or detail that can be recorded. Any object smaller than the seeing will simply appear as a star. Two objects closer together than the seeing will appear as a single object. Any detail smaller than the seeing will be washed out.

Seeing can vary dramatically from one night to the next and can also vary during a night. Seeing is always best near the zenith and worsens dramatically as the horizon is approached. This is one reason that observers often try to observe objects when they have the smallest possible zenith distance, or the greatest possible altitude. This will always occur as objects cross the meridian (the north-south line on the sky, passing directly overhead.).

Brightness Limits
(magnitudes and exposure times)

In general, brighter objects require shorter exposure times, and longer exposures will record fainter and fainter objects. However, the faintest objects recorded also depend on the sky brightness. For any given exposure, the faintest objects will always be swamped by twilight, or by the moon, or by thin high clouds, or by haze, or by atmospheric effects such as air glow (this is one major advantage of putting telescopes in space, where there is little air to provide background airglow).

On a clear dark night, a 2 minute exposure can allow GORT to record objects as faint as 18th magnitude. That is, 18th magnitude objects can just be seen in the images. Longer exposure times will certainly record fainter objects until swamped by the sky brightness (but see below).

Bright objects will saturate an image. That is, there is a limit to the number of electrons that can be accumulated by a pixel. Once that limit has been reached the image is saturated at its brightest level and more light cannot produce a brighter appearing image. Saturated image regions tend to produce blooming or streaking as excess electrons have no place to go. While bright objects may produce saturated image regions, fainter objects of interest in other regions of the image can still be satisfactorily recorded.

To keep objects of interest from becoming saturated the light levels must be decreased. This can be accomplished by using filters and by decreasing the exposure times. In general, exposure times of more than a few seconds will saturate for objects brighter than 8th magnitude. In general, a 1 second exposure will produce saturated images for objects brighter than about 8th magnitude. Note, however, that while a one second exposure may not saturate an 8th magnitude object, a one second exposure may not record many objects at all unless there happen to be other bright objects in the field.

In very broad general terms, the practical magnitude limits which can be recorded by GORT are between 8th and 18th magnitude.

Brighter objects can be recorded by decreasing the exposure times. Technically exposure times as short as 0.001 second may be specified. This can occasionally be useful for bright planets, but for such short exposure times only the brightest object in the field is likely to be recorded.

Fainter objects can normally be recorded by increasing the exposure times (but see below). Fainter objects can also be recorded by stacking several shorter exposure images to produce an image which effectively has a longer exposure time.

Exposure Times and Telescope Tracking

Two factors can limit the length of the longest possible exposures. The first factor is the sky brightness (see above). Eventually increasing the exposure time does not record fainter objects because the fainter objects become swamped by the sky brightness.

The second factor is the telescope tracking. Since telescope tracking is never perfect, eventually longer exposures will produce trailed images. Normally, the length of time for the longest possible exposure time is set by the time before stellar images appear to be trailed. This limit is normally set by the periodic error in the telescope drive or by faulty adjustment of the drive rate. Under normal circumstances, GORT is capable of producing 2 minute exposures that do not appear to be trailed. This appears to be the longest exposure time possible for an untrailed image.

   

What you need when you think you are ready to observe...

For each object or field you will need to have the following information. If you will travel to the observatory to obtain observations, you must have this information in hard-copy form. (Of course, hand written information will suffice.) If you will submit your observing request on-line you should save this information to a file.

Object name or field designation
  • Type of object
  • Coordinates (RA to the nearest second of time and DEC to the nearest second of arc)
  • Exposure time in minutes or seconds
  • Filter (B,V,R,I,clear)
  • Magnitude of object
  • Angular size of object
  • Your justification for observing this object

What do you expect to see in your images?


If you have a question about the GTN, please contact one of the "Responsible SSU Personnel" below.

This page was last modified on Monday 02nd February 2009 @ 14:20pm

Science Mission Directorate Universe Division

Responsible SSU Personnel:

Dr. Kevin McLin (mclin at universe dot sonoma dot edu)

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