Running Vcurves
Running Vcurves
Some users find that the First Light Wizard is not able to characterize their system or have a working undertanding of the Vcurve generation process and prefer to save time by running manual Vcurves.
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1.Focus the telescope manually
Adjust the focuser so that the focus position is approximately mid-way in the full range of travel on your focuser.
Example - an Optec TCF-S focuser has a total range of 7,000 steps so the focus position should be adjusted to roughly 3,500 steps by adjusting the draw tube, primary mirror (SCT), etc.
Verify that the focus position is approximately mid-way in the focus travel range (3,500 for the above example).
Select a 4th-5th-6th mag star (fainter for larger apertures) near the zenith and center on CCD.
Using your imaging program, set the exposure time to a 'recommended' value of 0.5 sec and take an image. Measure the star intensity and adjust the exposure or use a brighter/fainter star until the target star intensity is mid-way in the camera's ADU range.
Open the Vcurve window, the current focuser position will be entered in the ‘Center’ box.
2.Vcurve Parameter Settings:
Half Width is the movement in steps away from the Center in both directions
End Points are the Initial and Final focuser positions.
Step Increment sets the number of steps the focuser will move at one time
Steps is the resulting number of moves that will be made when generating the Vcurve.
Note that as you make adjustments in the boxes that the parameters will change once you leave the entry box or press Enter.
Enable Autofocus If you want to perform an autofocus run after the Vcurve is completed.
Repeat setting will rerun the Vcurve using the same settings above
Images/position setting will take multiple images and average the HFD measurements at each focuser position. This feature is useful for reducing noise and will yield a more consistent Vcurve. Downside is it takes more time and if the temperature is dripping rapidly then it may negatively impact the overall accuracy of the Vcurve. A setting of 3 under stable conditions is recommended for many users.
3.Method #1 - Determining Half Width
Press the Jog button and move the focuser In or Out 100 units then press the Find button
Continue to move the focuser until you achieve an HFD of 20+ (30 - 40 is better)
Note the focuser position
Bring the focuser back to the focus position and press the Half Width button on the Vcurve window
Enter the difference between the focus position and the position achieved when you manually jogged the focuser
Example:
Focuser position is 3,500 and 4,000 was the position to achieve 30 HFD.
Enter the difference of 500 into the Half Width box
Adjust the Step Increment value until you see 30 - 40 Steps displayed
Adjust the Step Increment until the Steps has an 'odd' number which helps sharpen the 'V' because each side of the 'V' Plot will have an even number of points.
4.Method #2 - Determining End Points
Press the End Points button on the Vcurve window
Press the Jog button and move the focuser 100 units 'Out' then press the Find button
Continue to move the focuser until you achieve an HFD of 20+ (30 - 40 is better)
Enter the focuser position in the Initial position
Move the focuser In 200 units 'In' then press the Find button
Continue to move the focuser until you find the position approximately equal to the HFD value above
Enter the focuser position in the Final position
Bring the focuser back to the focus position
Example:
Focus position is 3,500
'Out' position = 4,000 to achieve 30 HFD
'In' position = 3,000 to achieve 30 HFD
Adjust the Step Increment value until you see 30 - 40 'Steps' displayed
5.Press the Run button and 'enjoy the show' as FocusMax characterizes your system by creating a Vcurve
6.Watch the 'V' plot while running a V-Curve cycle.
As the points on the 'V' near the bottom watch the FMx Log's HFD value for each point on the 'V
When the HFD reaches its lowest value and starts increasing note the lowest HFD's focuser Position. That position will be the (approximate) point of best focus.
7.Upon completion of the V-Curve cycle change the "Center" field before running another V-Curve cycle. {the 'Initial' and 'Final' fields should automatically adjust} This also helps center the apex of the 'V' on the center line of the V-Curve plot.
8.If the lowest HFD in the Log continually changes or the apex of the 'V' in the V-Curve plot drifts this can indicate:
The telescope has not reached thermal equilibrium with ambient temperature - wait until the telescope cools closer to air temperature.
Temperature changed causing the focus point to drift - wait until temperature and telescope stabilize.
Continual drift of lowest HFD in the V-Curve Log, or centering of the 'V' on the V-Curve plot can also indicate the focuser might be slipping.
Notes:
The extremes of the Vcurve should be on the order of 30 - 40 HFD. The primary reason for the large HFD values is to improve the ability for FocusMax to determine the slope of the line. The larger transition from max to min (focus position) the better. This may be a challenge for some telescopes with short range of travel and/or telescopes with low focal rations which yield a shallow Vcurve.
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