Contents
- Index
Focusing
Since the quality of any automated imaging run depends on achieving and maintaining an accurate focus over the course of the evening, CCDAutoPilot provides many choices for automated focusing. In addition to some active methods, you can also use passive methods such as filter offsets and temperature compensation. You can even use a combination of methods. You should choose the strategy that works best for your equipment and seeing. Both FocusMax and CCDSoft's @focus2 focusing methods are supported.
When an equatorially-mounted telescope is pointed east of the meridian, any focusing activity scheduled to occur within 10 minutes of the meridian crossing will be deferred until the after telescope has crossed the meridian and the mount has flipped.This deferral does not take place with fork-mounted telescope. This focus deferral only occurs when Enable meridian flip is checked
FocusMax
Before proceeding, insure that FocusMax is set up properly for your system. Consult FocusMax documentation for specifics. Once all the software is connected to CCDAutoPiulot, you can use the Focus Now button to verify operation.
FocusMax can be used with either CCDSoft or Maxim. When FocusMax is connected, there are a number of active methods of focusing available. Active methods do not rely on predicting the focus position but actually focus the system at appropriate times. As such, this would be expected to give you the most optimal focus at any point in time. The trade-off here is the time to do the focusing. Depending on the method chosen, active focusing can add from 30 to 240 seconds, depending on the method used, camera download time etc.
Focus Now: This button will focus the system using the options specified on this page.
Focus Exposure: As mentioned above, these fields provide a convenient entry point for the FocusMax starting exposure. With most systems, the shortest exposure is suitable for focusing through the clear filter and the exposure time should be increased for less transparent filters. For example, you might set a 1 sec. exposure for color filters and a 2 - 4 second exposure for narrow band filters such as Halpha or OIII.
Brightest star in FOV: FocusMax will choose the brightest object in the field of view. This is fine if there are no bright extended objects such as galaxies in the field but if there are, FocusMax will attempt to focus on that and will most likely be unsuccessful.
Focus at X, Y: One way to avoid the above problem is to select a specific x,y coordinate for a target star. Take an unbinned image and note the coordinates of the brightest star. Enter those coordinates in the X and Y boxes. FocusMax will use that coordinate with a 100 x 100 pixel box around it to focus.
Sky Star: With this technique and TheSky6, CCDAutoPilot will plate solve the current location, slew the scope to an nearby star, focus there using FocusMax and slew back to the original location. If guiding was in process, it will be stopped for focusing and restarted automatically once the telescope has returned to the target coordinates. This technique guarantees a suitable star for focusing and uses the very powerful data query technology of TheSky6 to select stars of an appropriate magnitude to be used for focusing. SkyStar has some additional options: Center Focus Star: When checked, a precision slew to the focus star will be made, insuring the focus star is in the center of the FOV. Magnitude Range: 4 magnitude ranges are provided for the focus star - 4 to 7, 5 to 8, 6 to 9 and 7 to 10. Select the range that is appropriate for your system. Minimum focus star altitude: Regardless of where the telescope is pointing, focusing will always use a star above this minimum altitude. As a minimum, you must have the Guide Star Catalog (GSC) selected as one of the Stellar Core databases in TheSky.
Focus Program Picks Star: This is similar to Sky Star but it is all done within FocusMax using AcquireStar and requires the full version of PinPoint. Unlike SkyStar, whatever filter you use to focus on will be the same filter used for plate solving. This may be problematic when using low transmission filters such as Halpha and other narrow band filters.
None: No focusing is used. This setting might be used if it is desired to use only filter offsets.
CCDSoft/@Focus2
Before proceeding, insure @Focus2 is properly set up for your system. @focus2 is a feature of CCDSoft and can therefore only be used with CCDSoft. @Focus2 requires a calibration of what length exposure is required for what magnitude star to achieve approximately 20,000 ADU counts. This calibration is required for each filter and may be entered in the appropriate fields as shown in the above screen shot. Consult the CCDSoft documentation for proper setup. Depending on the spectral type (star color) of a chosen focus star, you may get different results, even with the same magnitude star. The effect is minimal with broadband filters such as clear filters and gets progressively more pronounced as the filter bandwidth tightens. For example, a red filter will show this effect more than a clear filter but much less than a Halpha filter. For RGB filters, I recommend calibrating on a G2V filter to "center" the spectral range. For best results with @Focus2, you should focus using the clear filter, determine and use filter offsets for your system. Once calibrated and with all the software connected, you can use the Focus Now button to verify operation.
Focus Now: This button will focus the system using the options specified on this page.
Calibration Exposure: This is the exposure time in seconds required to achieve a peak count of 20,000 ADU for the corresponding stellar magnitude below.
Calibration Magnitude: This is the stellar magnitude required to achieve a peak count of 20,000 ADU for the corresponding exposure, ablve.
Brightest Star in FOV: @Focus2 will choose the brightest object in the field of view. This is fine if there are no bright extended objects such as galaxies in the field but if there are, FocusMax will attempt to focus on that and will most likely be unsuccessful.
Sky Star: With this technique and TheSky6, CCDAutoPilot will plate solve the current location, slew the scope to an nearby star, focus there using @Focus2 and slew back to the original location. If guiding was in process, it will be stopped for focusing and restarted automatically once the telescope has returned to the target coordinates. This technique guarantees a suitable star for focusing and uses the very powerful data query technology of TheSky6 to select stars of an appropriate magnitude to be used for focusing. SkyStar has some additional options: Center Focus Star: When checked, a precision slew to the focus star will be made, insuring the focus star is in the center of the FOV. Magnitude Range: 4 magnitude ranges are provided for the focus star - 4 to 7, 5 to 8, 6 to 9 and 7 to 10. Select the range that is appropriate for your system - 4 to 7 is a recommended starting point. Minimum focus star altitude: Regardless of where the telescope is pointing, focusing will always use a star above this minimum altitude. As a minimum, you must have the Hipparcos/Tycho Catalog selected as one of the Stellar Core databases in TheSky.
Focus Program Picks Star: Working in concert with TheSky6, CCDAutoPilot will first plate solve the current location. @Focus2 will then slew to a suitable focus star and focus the system. If guiding was in process, it will be stopped for focusing and restarted automatically once the telescope has returned to the target coordinates. Magnitude Range: 4 magnitude ranges are provided for the focus star - 4 to 7, 5 to 8, 6 to 9 and 7 to 10. Select the range that is appropriate for your system. Minimum focus star altitude: Regardless of where the telescope is pointing, focusing will always use a star above this minimum altitude. As a minimum, you must have the Hipparcos/Tycho Catalog selected as one of the Stellar Core databases in TheSky.
None: No focusing is used. This setting might be used if it is desired to use only filter offsets.
Both Focusing Programs
Refocus every xx minutes: Depending on the numeric entry, the focus method will be executed at the start of each series and every xx minutes thereafter. The next exposure that comes along that is xx or more minutes after the last focus will be executed. In other words, the time for an interval focus is determined before an exposure starts. If xx minutes has elapsed since the last focus, a focus run will be performed; if it hasn't elapsed, the checking process will repeat before the next exposure starts. You should set up a brief series of exposures to insure FocusMax can focus satisfactorily with all of the planned filters in place, adjusting the focus exposure to be appropriate to a given filter as described below. Make any adjustments to the FocusMax settings required.
Focus using <filter>: If your filters are sufficiently parfocal, i.e. they all focus at the same point, you may choose to use one specific filter for focusing. That way you can avoid having to deal with different focus exposures for different filters. If your filters are not parfocal, you should determine and use filter offsets.
Post Focus Offset: This setting can be used to minimize the effects of OTA field curvature. The intent is to "split the difference" between perfect focus at the center and focusing at the edge of the field of view. For OTA's without a field flattener and/or a large imaging chip, there can be a significant difference in the focus position between center and edge. First, determine the focus at the center of your OTA and then determine it at some point away from the center. A good starting point is 60% of the way to the corner. You can use the Focus Now button to determine these values. Average a number of focus runs at each location. Calculate the offset and enter it in the Post Focus Offset. For best results, the Center Focus Star option and SkyStar focusing should be used. After achieving focus, the post focus offset will be added or subtracted to the focus results, according to the sign of the entry.
Passive Focusing Methods
These are methods that rely on the predictability of your system's optimal focus position. To the degree your system is not predictable, your focus will be less optimal
Temperature Compensation: If this box is checked and your focuser supports temperature compensation, it will be enabled. Temperature compensation will be disabled automatically during the main exposure. At the conclusion of the main exposure, temperature compensation will be re-enabled and a 5 second delay will be initiated to allow time for temperature compensation. To use temperature compensation, the user must provide a Compensation Slope that appropriately characterizes the system. This value should be counts/temperature needed to maintain focus. When Temp. Compensation is checked, the Focuser Temperature will be shown. If "n/a" is shown, the focuser does not report temperature and temperature compensation can not be used. There are many ways to determine the compensation slope. The basic process is to measure the focus point at different temperatures and calculate the slope. Am imaging session with 10 minute exposures and a focus before every exposure during a period of temperature change is a good way to get the raw data. A least squares fit then gives a good slope. The compensation slope should be in units of focuser count per unit temperature. Be sure the temperature reported, °C, °F or counts used for the measurement is the same as that reported as Focuser Temperature. The focus starting point will be determined at the beginning of a run and the starting temperature noted. At each subsequent focus, the starting point and temperature will be redefined. Between exposures, the focus point will be adjusted, based on the current temperature and the user-supplied Compensation Slope. Note: If focuser temperature compensation is enabled in either your focuser or focus control program, it will be disabled when CCDAutoPilot is connected.
Offset: enter how many counts you wish your focuser to move for a given filter. Normally one filter would be the reference and the others would move an amount, either + or -, relative to that reference. The reference filter should be the filter you select for plate solving - usually a clear or luminance filter. If your filters are parfocal, i.e. they all focus at the same focuser position, you would enter 0 for all the filter offsets. The 
button at the top sets all filter offsets to the same value as that of the first filter entry. If some filters, typically some Halpha filters, are sufficiently non-parfocal, you can speed up the active focusing method considerably by entering the offset. That way, automatic focusing will start closer to the ultimate focus position
Focusing Strategies
There are of course many ways to achieve and maintain focus during the course of the evening. For most users, focusing off-target at the start of each series will give the best results. If a given series is lengthy, then adding a periodic focus will insert a focus activity within the series.
Focusing is generally done with the filter you are exposing. This is especially important if your system is not parfocal. Most modern color filters are parfocal, i.e. they have the same substrate thickness. When used with a reflecting telescope, the system will be parfocal. However, the same filters, when used with a refractor, will generally not result in a parfocal filter. This is because refractors are generally a compromise to achieve close to the same focal point with red and blue light. This is what an APO design strives to achieve.
However, there is another approach if you know the difference in focus for each filter (filter offset), say referenced to a clear filter. You can use the "Focus using filter" option and select the clear filter. Then you can program in the offsets as determined for your system, in the Offset fields. When set up this way, you may be able to use a star in the FOV to focus, which is quicker than using off-target focusing. During a run with this configuration, the clear filter would be used for focusing whenever focusing is called, either at the start of a series or at a specific interval. Once focusing is achieved, the correct filter would be inserted and its specified filter offset would be inserted. To the degree the filter offsets are accurate, so too would be focus.