Easy enough. But the only controls the NGT-18 provides are 1X and 8X siderial controls in both directions of RA and DEC. Given Distances in Arc-minutes, how can I determine how long to move at 8X to accomplish this centering? Answering this question is the topic of the timing studies for this evening.
Telescope configuration: I plan to mate the NGT-18 with the ST-9 camera from SBIG. The ST-9 has 20 micron pixels, which are a bit large for the NGT-18. (For a discussion about pixel matching click here). To make a better match, I will use the Nagler Paracorr. The Paracorr is supposed to lengthen the focal length a bit to something like F/5.2 or something like that. This puts me a little closer to the 100 inch focal length I want to match the camera.
The problem is, I don't exactly know what the focal length of the configuration is, and I need that number to compute how fast in arc-minutes per second the scope will move at 8X Sidreal. To get this number, I will use the Celestron Micro Guide Eyepiece whose reticle is shown above. The primary segment of the eyepiece to be used tonight is the linea scale consisting of a double line of tic marks having a total length of 60 units.
The procedure for this evening will be:
Note: Use a digital tape recorder to gather timings. Have scope roughly polar aligned (within a few degrees)
SET UP TEST
1) Set the scope to 0 degrees declination
3) Turn on drive
4) Find a reasonably bright star in the Micro Guiding Eyepiece
5) Use the 8X RA controls to move the star back and forth across the field and rotate the eyepiece until the parallel lines are parallel to the motion of the star. The double parallel lines on the eyepiece are aligned on the east-west axis of the scope.
TAKE RA 8X measurements
6) move the star to the east end of the linear scale and turn on the tape recorder.
7) Start the motion of the star West and say Start when the star passes the East-most tick of the scale. Continue the 8X motion and when the star passes the West-most end of the scale say STOP. Then say that this test is the 8X RA motion test and turn off the recorder.
7a) Record this test several times to get a good timing average.
TAKE RA DRIFT measurements
8) Move star to eastermost tick on linear scale
9) Turn on tape recorder and say "Beginning drift measurement"
10) turn off drive motor
11) look in eyepiece and as star passes the first large Tick say "ONE" (Note: you will probably miss the start of the scale because of where the drive switch is).
As the star passes he last large Tick on the scale say "Stop". I caught 5 of the 6 large scales by doing this.
12) Turn on the drive and re-capture the star
13) Turn off the tape recorder.
TAKE DECLINATION 8X measurements
14)Rotate the eyepiece 90 degrees in the field so that the axis is now North-Sourth rather than East-West. Use the 8X Declination drive buttons to make sure the linear scale is parallel to the motion of the star as it moves up and down in the field.
15) Move the star to the Northern end of the scale.
16) Turn on the tape recorder and state that this is the 8X declination timing
17) Start the 8X motion southward and as the star crosses the start of the linear scale say START, and when the star passes the end of the scale, say STOP.
18) take a few of these measurements to get a good average. (Note, one of the things I noticed here is that there is a 2 second delay when switching directions in the North-sourth direction. This indicates that there is a small adjustment needed in the DCL drive mechanism.)
END of Measurements
Calculations
Right Ascention 8X timing was measured off the digital tape recorder as 6 seconds.
Drift Timings: With the drive moter turned off, the star drifted across 5 major divisions in 31 seconds. Extrapolating, the star would have drifted across the six divisions in 37 seconds.
Declination Timings 8X was measured at 9 seconds.
Using the calculations that came with the Micro Guider Eyepiece,
SD=37 cos(dcl)/4= 37/4=9.25 arc-seconds between the fine lines.
Focal length= 82506/(t*cos(dcl))=82506/37=2230mm or 87.79 inches.
Since a single tick on the scale is 9.25 arc-seconds, a large tick is 92.5 arc seconds, and the full 60 tick scale is 555 arc seconds. This scale requires 6 seconds to run at 8X sidreal in the direction of RA. It requires 9 seconds to run the scale in Dec.
Therefore in one second, the scope will move 555/6 arc-seconds or 92.5 arc-seconds per second, which is the same as to 1.54 arc-minutes per second in RA. and will move 555/9 or 61.67 arc seconds,or 1.03 arc minutes per second in Dec.
3/14/01
Tonight, at least for the one night, I declared an end of calibration. I have columnated the scope to a high degree of accuracy, I have now cleaned the mirror, and gotten polar alignment down to almost a science.
So, tonight, I decided to go for broke. I decided to take a real honest-to-god picture.
M1 was my target. Click here.
5/5/01 Fine Guidance Notes can be found here.
