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SBIG ST-4/0490

magnitudes the background level must be

subtracted. This is done by moving the crosshair

to a dark area of the image and specifying that

region as being typical of the background

intensity for the image. This specification of the

background allows the magnitude calculations to

be independent of effects such as dark current or

sky background. The factors that affect the

calculation of stellar magnitude are: Exposure

time, Aperture area, and Calibration factor.

The diffuse magnitude (also called surface

brightness) of diffuse objects is the magnitude per

square arc-second, and is calculated identically to

the magnitude calculation discussed above

except that the accumulated pixel intensities are

divided by the area of the 5 x 5 box in square

arc-seconds. The factors that affect the

calculation of diffuse magnitude are: Exposure

time, Aperture area, Focal length, and Calibration

Factor.

The host software also allows you to measure

the angular separation between objects in an

image. This is done by moving the crosshair to

the first object, establishing that position as a

reference position, and then moving the crosshair

to the second object. The software then displays

the angular separation and orientation (in

degrees, clockwise of vertical) of the crosshair's

position relative to the fixed reference position.

The calculation of the separation between two

objects is only dependent on the dimensions of

the CCD pixels and the focal length of the

telescope used (which must be accurately

entered). This usually requires experimental

determination using known double stars for

precision. The direction corresponding to east-

west in a setup can be determined by taking an

image of a star, letting the image drift for a few

seconds, and taking another image. The images

can then be co-added and the line between the

two images of the same star delineates the East-

West direction.

Other Image Processing Techniques

The host software uses other image processing

techniques besides contrast enhancement,

smoothing, inversion, and photometric analysis.

These other techniques are discussed in this

section.

Flipping the Image

Some inspection will reveal that the screen image

is flipped about a horizontal axis relative to the

CCD. The horizontal and vertical flip commands

enable a picture to be oriented correctly no

matter what combination of telescope and prisms

is used to form the image. Also, the flip

commands are quite useful for making an image's

orientation match that of published images.

These commands actually modify the image data,

and hence the results of using these commands

are retained if the image is saved after these

commands are applied to an image.

Zooming

The host software allows you to zoom in on an

area in the image by moving a zoom-box over the

image until the zoom-box is positioned at the

desired region where the zoom may be

completed. The zoom-box is quarter sized (48 x

41 pixels), and the pixels within it are then

zoomed to a full 192 x 165 sized image and

interpolation used to fill in the "missing pixels".

The zoomed image can then be used with all the

photometric analysis software, etc., and can also

be saved. Depending on the amount of host

memory available when the zoom is performed,

the zoom either writes over the original image

data (you are warned first) or the original data is

retained and the image can be un-zoomed later

(unless the zoomed data is saved on disk in

which case the original un-zoomed data is

discarded).

Zooming can be quite handy for examining

close binary stars, and small detail, but is no

substitute for higher magnification images since

the zooming process doesn't contain any more

information than the original 192 x 165 pixel

image contained.

Histograms

The host software can also calculate and display

an image's histogram, which is a count of the

number of pixels at each of the 256 possible (0

through 255) intensity levels. The histogram can

be useful for determining the settings of the

Background

and

Range

contrast parameters (like

the Auto-Contrast does), for determining the

dark current or sky background level, and for

determining the optimum exposure time.

Содержание ST-4

Страница 1: ...cker Imaging Camera Operating Manual Table of Contents Instrument Overview Page 1 Star Tracking Operation Page 4 Imaging Camera Operation Page 8 Host Computer Software Overview Page 10 Using the ST 4 with an IBM PC or Compatible Page 18 Using the ST 4 with a Macintosh Page 28 Observing Suggestions with the ST 4 Imaging Camera Page 37 Problem Solving with the ST 4 Page 38 Appendix A Page 39 ...

Страница 2: ......

Страница 3: ... pixel position 121 132 the computer will determine its position by noting the increased signal from that pixel If the star is drifting due to guiding errors it will appear at a different position in the next exposure say at pixel position 123 132 The computer then calculates how far the star has drifted from the original exposure and toggles the telescope drive accordingly The microcontroller in ...

Страница 4: ...umber of pixels much greater resolution would be degraded by the limitations of most computer graphics screens Note With the CCD running at lower than ambient temperatures you will wonder why dew and frosting aren t a problem First of all the chamber containing the CCD is small and only a small volume of air surrounds the CCD The small volume minimizes the total amount of water vapor in the air wh...

Страница 5: ...r joystick type controllers the four relays in the ST 4 are used to simulate the joystick being pushed to its limits Two relays are used for each axis or rheostat of the joystick as shown in Figure 3 For the Right Ascension rheostat you would use the X and X relays and for the Declination rheostat you would use the Y and Y rheostat c no nc switch common normally open normally closed A Standard Han...

Страница 6: ...CCD by completely blocking the open end of the telescope to remove all light the CCD is very sensitive it will saturate in very low ambient light levels 3 Observe the VALUE reading This reading should fall to a reading of around 10 and stabilize within two minutes of the instrument being powered on Press the INTERRUPT button to halt the FIND AND FOCUS mode or any other mode for that matter 4 Push ...

Страница 7: ...e will also tend to smear the image so it may be helpful to watch several sequential exposures when critically focussing the image A table of typical VALUE readings for different magnitude stars is shown below in Table 1 This assumes a 1 second exposure and a typical response of the CCD Use this as a reference for determining whether the system is properly focussed Star ST 4 VALUE Reading Magnitud...

Страница 8: ...ctions Care should be taken to orient the CCD head so the RA and Declination axes line up with the sides of the CCD head you can tell by looking through the glass window at the CCD or by noting the orientation of the Serial Number tag on the rear of the head which is oriented like the CCD This adjustment is not critical The tracker will work in any orientation It just makes it easier to make sense...

Страница 9: ...is changed Many telescopes have severe backlash and the star image may not move for several seconds after a correction is made The parameter is the number of tenth second increments added to the calculated move when the move direction is reversed from the previous move H2 Hysteresis Adjustment Y axis This parameter is identical to the H1 parameter above except for the Y axis b Boost factor This pa...

Страница 10: ...aken down to 0 01 seconds and the bottom half of the array shifted rapidly up to the upper half The 82 lines of short exposure data can then be readout at the normal rate This method works quite well and uses enough pixels such that 0 5 arcsecond per pixel scale factors can be achieved while viewing an entire planet Unfortunately this technique does not work for the moon since the moon s image typ...

Страница 11: ... Film has the advantages of a much larger format color convenience and independence of the wall plug the ST 4 can be battery operated in conjunction with a laptop computer though After some use you will find that film is best for producing sensational color pictures and the CCD is best for planets small faint objects and general scientific work such as variable star monitoring and position determi...

Страница 12: ...ensity less than 25 to be displayed as black any pixel values between 25 and 75 25 50 to be displayed with a gray scale and any pixels above 75 to be completely white You will want to experiment with the settings of the Background and Range parameters to get a good feel for how they affect the image These parameters do not physically change the image data but only affect the way the image is displ...

Страница 13: ...s displayed on the computer screen pixel number 1 1 refers to the pixel with the x y location corresponding to the upper left hand corner of the screen X increases to the right and Y increases toward the bottom of the screen The position of pixel 1 1 on the CCD can be physically determined by referring to the placement of the serial number tag on the rear of the CCD head as shown below in Figure 4...

Страница 14: ...e uses other image processing techniques besides contrast enhancement smoothing inversion and photometric analysis These other techniques are discussed in this section Flipping the Image Some inspection will reveal that the screen image is flipped about a horizontal axis relative to the CCD The horizontal and vertical flip commands enable a picture to be oriented correctly no matter what combinati...

Страница 15: ...nicate at any baud rate from a lowly 1200 baud to the highest rate of 57 6K baud The actual baud rate used will depend on how you configure the host program and or the environment in which the host and ST 4 exist mainly the cable length between the host and the ST 4 The host software can be configured to talk to the ST 4 at the fixed baud rates of 1200 9600 19 2K and 57 6K or can be configured for...

Страница 16: ...easing the signal level As previously mentioned you can not take infinitely long exposures because the CCD will eventually saturate from the effects of the dark current You can however add several long exposures after subtracting the dark frames from each of the light frames to make an image which while not exactly the same as a longer exposure will have the added benefits of greater signal to noi...

Страница 17: ...T 4 subtract that dark frame from the light images immediately after the image is readout from the CCD and prior to it being sent to the host As always you must manually cover the telescope when you ask the host software to take the dark frame This is handy for searching an area for an object of interest but for your final images the ones you really want to keep you will want to capture and downlo...

Страница 18: ...manually The telescope can be guided on a faint star by viewing the screen and making corrections which is much less fatiguing than guiding through an eyepiece in an awkward position This mode can be used to guide on stars too faint for the ST 4 to lock on since the eye brain combination is better at recognizing a faint stellar image in the presence of grainy readout noise In all the Focus sub mod...

Страница 19: ...lescope can be quickly set close to the optimum focus the next time Replace the CCD head taking care to always seat it securely against the tube Now that you have found an object exit the Focus Mode and vary the Background and Range parameters to modify the display Small values of Range have the effect of increasing the contrast Stars get fatter due to the fact that the stray light near the main i...

Страница 20: ...DOS manual and save the original in a safe place After successfully connecting the ST 4 to your computer power up the ST 4 and computer and run the program Note The PC version of the ST4 program CCD EXE requires several other files found on the floppy disk to run The CGA BGI EGA BGI etc files are graphics drivers for the various types of graphics cards supported by the program and the CCD CFG file...

Страница 21: ...voking the commands to examine their associated command screens You can exit from a command screen and return to the menu bar by pressing the ESC key If you hit the ENTER key within any of the command screens that command will immediately execute Each command screen consists of several parameters which can be changed by moving to the desired line and changing the setting You move the flashing curs...

Страница 22: ... the desired file using the cursor keys and hit ENTER again The file will be loaded the Status box will show an annotation attached to the image and the values of the telescope parameters associated with the image and the Data Buffer information box will be updated to reflect that fact that an image has been loaded After loading an image you would typically use the Display command to display it No...

Страница 23: ...ntrast mode where the computer sets the Background and Range parameters based upon the image s histogram If you do want Auto Contrast don t change the setting of the Background and Range parameters below or the software will automatically deselect Auto Contrast before displaying the image This feature allows you to specify manual contrast mode by merely changing the setting of the Background or Ra...

Страница 24: ...e of two things depending on whether the image had been previously zoomed or not shown in the upper left of the screen If the image had been previously zoomed and was not saved on the disk executing the Zoom command restores the image to its un zoomed state Otherwise executing this command enters Zoom Mode where a portion of the image can be enlarged to the full size of the image and is described ...

Страница 25: ...entroid This command is like the Star command above but higher accuracy positioning of the reference is achieved for stellar objects by determining the center to subpixel accuracy Background This command records the background level used in the magnitude calculations to the average value of the pixels inside the box centered under the current position of the Crosshair Toggle box size Executing thi...

Страница 26: ...cases you can set this item to to type of graphics adapter you have Strange and unusual results can occur if you specify a graphics card type that you don t have Note Some graphics adapters will support multiple modes such as VGA adapters that will also do CGA and EGA emulation These adapters can be used in any of their supported modes Also each supported graphics adapter requires a BGI file in th...

Страница 27: ...b command Camera baseline level Set this item to the desired baseline level as described in the Baseline and Gain Parameters section before Gain expansion Set this item to the desired gain as described in the Baseline and Gain Parameters section before Telescope Setup Parameters Focal length Enter in this item the focal length in inches of the telescope you are using When images are downloaded fro...

Страница 28: ...es enables the Auto Contrast feature when displaying the images downloaded from the ST 4 Background and Range These items are the Background and Range parameters used in displaying the images after downloading when the Auto Contrast feature is not enabled Frame size Set this item to the desired frame mode as discussed in detail in the Focussing and Special Imaging Modes section before Note The CCD...

Страница 29: ...ame image needs to be changed at any point you should switch to Full Frame mode and then back to Quarter Frame mode starting the process over again Track Frame Initially this mode works like the Quarter Frame mode described above except that the rectangle is not quarter frame sized but 33 x 27 pixels in size After the rectangle is positioned hitting the ENTER key causes the Focus Mode to resume ta...

Страница 30: ...ng the serial port you should use the Establish link command to establish a communications link with the camera If you didn t double click on one of the image files use the Open command in the File menu to open one of the image files such as the Dumbbell image file Elements of the User Interface This section describes the basic elements of the user interface for the SBIG ST4 application program We...

Страница 31: ...image file gets opened the image is displayed in the Picture window and the Contrast window is opened You can only have one image file open at a time so you must close the current one if you want to open a different one Close Use this command to close an image file releasing it from memory If the image is new or has been changed due to some image processing functions you are asked if you want to s...

Страница 32: ... appropriate settings for the Background and Range parameters the Auto Contrast feature and updates the image display with those settings Finally there are several check boxes in the Processing section of the Contrast window which are described below Expand 2X Selecting this item makes the image be expanded to twice its normal size Note Since the pixels on the Macintosh are square and the pixels f...

Страница 33: ...ameters associated with the image discussed in the Telescope Factors section of the host software description above as well as some image cropping parameters Each item is discussed individually below Focal length This is usually the focal length of the telescope used to take the image but can be changed by zooming etc Aperture This is the aperture area of the telescope used to take the image Expos...

Страница 34: ...tness of diffuse objects such as nebulae or galaxies Geometry Separation and Angle These are the angular separation and the angle in degrees clockwise of vertical between the current position of the Crosshair and the background reference marker fixed by clicking the mouse in the Picture window Note The Picture window must be active before clicking in it will place the background reference box mark...

Страница 35: ...d selects a new color table which is an exact copy of the currently selected color table We suggest you use this command whenever you wish to modify an existing color table since any changes made to a color table can not be undone otherwise Also this button will be grayed out if the maximum number of 8 color tables already exists Ramp This button can be used to create a blend of colors between two...

Страница 36: ... downloading images from the ST 4 you need to make sure the physical communications link between the Mac and the ST 4 have been established When the software is first run it will try to find the ST 4 If successful you can probably skip this section unless you desire to change the communications speed etc The Communications command in the Camera menu allows you to select which of the Mac s two seri...

Страница 37: ...s you to select the desired frame mode as discussed in detail in the Focussing and Special Imaging modes section of the host software description above Some of the frame modes require further discussion below Quarter In this mode a full image is first downloaded and the Focus Mode is paused You then position a quarter frame sized square drawn in the Picture window and click the mouse to designate ...

Страница 38: ...ight use the Grab Setup and Grab commands to take download and display a single exposure Use the Grab Setup command to set the Exposure time and Exposure delay items as desired Again the Exposure time can range from 0 01 to 600 seconds Setting the Exposure delay item to a non zero value causes the software to delay by that amount prior to taking the exposure giving you time to get back to the tele...

Страница 39: ...tion of Jupiter can be easily seen in exposures only a half hour apart similar measurements can be made of Mars Use of color filters improves the contrast of planetary surface features Not as well known is that galaxies also change in appearance when color filters are used Old pictures of many galaxies were taken with blue sensitive emulsions and the dust lanes in the arms are very prominent At re...

Страница 40: ...eature is helpful for example when no Declination motor is installed on the telescope mount PROBLEM Star images in photos are not round First check to see in what direction the images are smeared If it is in Right Ascension which is most likely consider the following possible problems 1 The guide telescope focal length is so short that adequate tracking is not possible The guide telescope should h...

Страница 41: ...NTA BARBARA INSTRUMENT GROUP SBIG ST 4 0489 Page 39 using a guide star centered 1 degree away from the field of view of your camera this effect would cause each star to form a streak about 2 6 arcseconds long ...

Страница 42: ......

Страница 43: ...ack 2 Alarm relay1 common connection Orange 6 Alarm relay normally closed Blue 1 Alarm relay normally open White Table A2 Power Connector Cable ST 4 Wire PC Cables Mac Cables Pin Function Color PC XT 25 pin D AT 9 pin D 9 pin D 8 pin DIN 2 Transmit data from ST 4 Red 3 2 9 5 3 Receive data to ST 4 White 2 3 5 3 5 Ground Black 7 5 32 43 Table A3 RS232C Connector Cable For additional information reg...

Страница 44: ...SBIG SANTA BARBARA INSTRUMENT GROUP Page 42 SBIG ST 4 0490 Telephone 805 969 1851 FAX 805 969 0299 ...

Страница 45: ...mages 12 COM 1 or 2 16 Connecting to the camera 11 Contrast 8 Crosshair 9 10 D Dark current 2 7 12 Dark frame 1 4 12 13 Dark frame subtraction host 2 ST 4 13 Declination 2 5 Dew 2 Diffuse magnitude 10 Displaying images 8 Dithering 11 DOWN button 5 Downloading images 12 Dynamic range 13 E Error messages 35 Establish link command 12 Establishing communications link 11 Exposure 1 Exposure time 4 12 1...

Страница 46: ... time 2 Interpolation 11 INTERRUPT button 3 Inversion 9 J L Joystick interface 3 Joystick type controllers 2 LEFT button 5 Light frame 1 12 M Macintosh 25 Macintosh software About dialog 25 Auto button 27 Camera Setup command 32 Color Table command 30 Communications command 31 contrast window 26 27 crosshair window 26 crosshairs 28 Customize command 33 Dithering menu 28 Do It button 27 Focus comma...

Страница 47: ...GHT button 5 RS 232 1 16 RS 422 1 S Schmidt Cassegrain telescope SCT 2 Scribing the eyepiece 5 Separations 10 Serial interface 1 11 16 Serial port 11 16 Signal to noise ratio S N 12 Smoothing 9 Spot frame mode 14 Star tracking operation 3 overview 1 T TAKE DARK FRAME button 3 Telescope drive 1 Telescope factors 9 Thermoelectric cooler 2 7 TRACK button 5 Typical star intensities 15 Typical VALUE re...

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