Autofocus
Autofocus Preferences
The Setup Tab is used to specify the important parameters that will be used during the auto focus routine, such as camera Exposure Time, Near Focus HFD, etc.
No filter wheel
Filter wheel
|
The Near Focus HFD (half flux diameter) setting specifies the HFD position on the Vcurve where FocusMax will pause to take multiple images before moving the focus position (default = 10).
The initial camera binning used to identify the target star.
Note: some cameras or imaging software may not support 2x2 through 4x4 binning and may generate an error message.
The subframe binning used during the autofocus run. If you are using a DSLR camera then it is recommended that binning be set to 2 for both Target and Focus binning.
Two autofocus methods are given:
Standard - utilizes one side of the Vcurve to determine the focus position.
Advanced - will use both sides of Vcurve to predict the focus position. This will yield the most accurate final focus position but will require additional time to complete the autofocus process:
Start either inside or outside of focus depending on the focuser Move Direction
Find the predicted focus position
Move to the other side of focus
Find the predicted focus position
Move the focuser to the calculate the average of the two positions
Two methods are available for calculating the focus position:
Mean - the average reading will be use which can be influenced by outlier readings
MinMaxClip - mIn & max values are ignored and the mean determined
This setting specifies the number of readings to be taken for determining the focus position.
Unlike Convergence method, this is a static setting that does not adjust for variability in seeing conditions.
Filter
Users who focus through a filter may need to adjust the Mn/Max Exposure in order to achieve the desired target flux.
Base Exp
Sets the starting exposure that will be adjusted up or down to meet the target flux setting (default = 0.5).
Min/Max Exp
Sets the Min exposure (default = 0.1 sec) and Max exposure (default = 1.0 sec) that FocusMax will use to bring the target star to the flux target setting.FocusMax will abort the focus run if the calculated exposure is less than Min Exposure or greater than Max Exposure
Target Flux (x1000)
Flux is the total integrated light (intensity) of the star on the chip (default = 300). It is important to verify that the target flux value is not set too high which may yield saturated star on the CCD.
FocusMax will attempt to adjust the exposure time to hit the desired target star flux value. If the star flux is too high, then the exposure will be adjusted down, too low the exposure will be adjusted up. If the exposure falls outside to the defined Min/Max Exposure time then FocusMax will stop the autofocus run.
Set the focuser move direction in the direction that your telescope focuses best without excessive backlash or mirror flop. For a typical SCT telescopes, the best Move Direction is In which moves the primary mirror in a direction against gravity. Settle time is the number of seconds to pause after a focuser move.
Images sets the number of images that will taken and averaged after the focuser moves to the final focus position. This will give a better estimate of the final focus HFD due to seeing variation.
Focus Offset allows the user to offset the focus position in steps in or out to defocus the image. This is useful for photometry where it may not be desirable to have perfectly focused stars or if the user wants to defocus the image a known amount to average the focus across the image plane.
Enabling this feature will find the best focus position by taking repeated sub-frame images and measuring until the average HFD falls within a boundary (or tolerance) that is set by the user. This feature is very useful when the seeing is poor where more samples will be taken then when seeing is steady.
Steps are the number of focuser steps (units) that the average HFD must fall within. You can think of as a tolerance.
Samples is the number of consecutive measurements that must fall within the above step setting before FocusMax determines that sufficient sampling has occurred and considers the predicted position as the position for the final focuser move.
Below is an example of an autofocus run in which Focus Convergence was enabled. The default sampling rate is 5 samples which would have yielded a final focus position of 4178. Note that the average position continued to trend upward and did not begin to converge until sample measurement 27 resulting in a final focus position of 4196 which is difference of 18 steps. Note that FocusMax would have stopped at the default of 5 exposures but continued until the focus position 'converged'.
|
When enabled this will send the focuser back to the previous starting position before the autofocus routine was initiated if the final HFD exceeds the Max HFD setting. This is a useful feature for all night robotic observing sessions where thin clouds could influence the autofocus routine and cause it to leave the system out of focus..
Max HFD is the maximum HFD value that is considered a reasonable Best Focus HFD. If the value is exceeded, then an error message is posted to the Log and the focuser will return to the previous position.
Fail Attempts is the number of iterations that FocusMax will attempt repeated exposures if the autofocus routine fails (default = 3). This is useful if scattered clouds encountering during the autofocus run.
Fail Timer is the wait time in seconds between autofocus attempts (default = 5 seconds).
