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Automatic Refocusing

The SessionCraft is accessible from Tools tab
or with F9 key.
 
The Focus Control section of SessionCraft monitors for various events (triggers) and executes automatic refocusing. The automatic refocusing depends on FocusCraft and Auto Focus Aid so make sure that you have configured them correctly in order to provide valid focusing metrics (like HFD or InvPower) for all of your filters. This means that you need exposure duration and gain that can work well for all your filters.
 
The Focus Control section defines the events (triggers) on which to execute refocusing, but also provide valuable information as information about last refocusing and information for triggers closing.
 
There are different types of triggers that can be configured in any combination:
 
1. Temperature compensation - this is fixed focuser position altering depending on the temperature change. No refocusing is executed. It is tricky but is also the fastest way to keep the scope focused.
2. Refocus on temperature change
3. Refocus after every N images
4. Refocus every X minutes
5. Refocus on filter change
6. Refocus after meridian flip
7. Refocus after the running exposure is completed
 
In the Focus Control it is all about same action - refocusing so when we have configured multiple triggers there is no matter which trigger exactly will start the process, so there is no priority. For example if imaging plan changes the filter and there is refocusing, the temperature, count and time triggers will restart gathering data.
 
Automatic Refocusing
1

Temp Compensation

1. Temp Compensation
 
Enables the temperature compensation. It helps to compensate the focus point changes cased by the changes in the ambient temperature. Note that temperature compensations will not be applied when Focusing Aid, Auto Focus Aid or Bahtinov Aid are open.
 
In order to use temperature compensation you have to determine how many focuser steps have to move per one ambient temperature degree in order to preserve the best focus. This is called "steps per one degree change". There are different approaches how to measure this. The most common is to focus on various temperatures and record the temperature/position pairs. After that to compute the "steps per one degree change" from the sequential pairs and average the results.
 
APT has powerful algorithm for temperature compensation. It averages defined number of temperature readings and calculates the needed focuser position change no matter if there is full degree change. It also keep care for the changes which are not whole number position.
 
Click on T Comp Settings... button to configure the parameters described below.
 
2

T Comp Settings

2. T Comp Settings
 
Defines the parameters for the temperature compensation algorithm.
 
 

Real-time compensation

Enable this option if you want the temperature compensation to be applied even during image exposures. Disabling it will apply compensations after the running exposure is completed.
 

Temperature Source

Defines which sensor to use for determining the ambient temperature. Supported are focuser's internal sensor, external sensor or combination of both. The last option will average the readings of the internal and external sensors.
 

Steps per Degree

Define how many steps to change per one degree drop/rize in the temperature. When the value is positive APT will apply inward move on temperature drop and outward move on temperature rising. Enter negative value if you want to reverse the behavior and make outward moving on temperature dropping. APT works with high precision corrections and support even compensation with fraction of one position.
 

Reading frequency

Defines the frequency of checking the ambient temperature.
 

Averaging

Defines how many readings to average before calculating the needed temperature compensation. Averaging helps to avoid short temporary changes caused by wind. Frequency 10 combined with Averaging 60 will make temperature compensation on every 600s (10x60 = 600s -> 10 minutes). Note that telescopes can't react fast on the temperature changes, so it not advisable to make corrections in very short intervals.
 
3

Temp Refocusing

3. Temp Refocusing
 
Enables refocusing on temperature change trigger. In short SessionCraft will average at least N temperature readings and if the difference between the calculated value and the temperature during the last refocusing is above the defined limit, new refocusing will be executed.
 
The numbers in the brackets show the current status of the trigger [-0.4 #01*] means that there is accumulated -0.4 degrees difference from the previous refocusing. #01 shows the current number of averages readings. When there is asterisk (*) we already have accumulated at least once the defined number of readings. Having asterisk (*) means that as soon as temperature limit is reached, refocusing will be started. No asterisk (*) means that we need more data to measure accurately the temperature change. If 60 is entered in min averaged readings, no asterisk (*) will be displayed till 60 readings are gathered, after that the number of the counter will start again from #01 but now we have enough data and can start the refocusing when it is needed.
 
Click on T Refocus Settings... button to configure the parameters described below.
 
4

T Refocus Settings

4. T Refocus Settings
 
Defines the parameters for the refocusing on temperature change trigger.
 
 

Temperature Source

Defines which sensor to use for determining the ambient temperature. Supported are focuser's internal sensor, external sensor or combination of both. The last option will average the readings of the internal and external sensors.
 

Reading frequency

Defines the frequency of checking the ambient temperature.
 

Averaging

Defines the minimum number of readings to average before calculating the temperature difference form the previous refocusing. Averaging helps to avoid short temporary changes caused by wind. Note that telescopes can't react fast on the temperature changes, so it not advisable to make corrections in very short intervals.
 
5

Refocusing by count

5. Refocusing by count
 
Enables refocusing on images count trigger. For example is possible define to refocusing after every 15 exposures. The number in the brackets shows how many images have been taken since the last refocusing. LiveView by Images and images taken with Shoot button are ignored - only the images taken by plan are counted.
 
 
6

Refocusing by time

6. Refocusing by time
 
Enables refocusing by time trigger. Defines how many minutes after the previous refocusing have to start new one. The number in the brackets shows how many minutes has elapsed since the last refocusing.
 
7

Refocusing by filter

7. Refocusing by filter
 
Enables refocusing after filter change. If the filter is changed manually from the Gear tab, APT will ask for confirmation if you want to start refocusing. If the change is made during imaging plan execution the plan will be paused, the refocusing will be started automatically and after that the imaging will resume.
 
8

Refocusing after flip

8.  Refocusing after flip
 
When enabled automatic refocusing will be executed as last step of the Automated Meridian Flip (AMF) before resuming the running imaging plan.
 
9

Refocusing ASAP

9.  Refocusing ASAP
 
Enable this option if you want to refocus as soon as possible. If there is running exposure it will refocus as soon as it is completed. If the next exposure preparation is in progress APT will wait the whole exposure to finish and after that will refocus. After that the imaging plan will resume and the check box will be unchecked.
 
10

Focusing Log

10. Focusing Log
 
Shows information about the last executed refocusing:
 
1. Time
2. Which trigger started the refocusing
 
3. HFD / FWHM / InvPower value in the focus point
4. The focuser position of the focus point
 
5. The used filter used for the focusing
6. The temperature in the end of the last refocusing