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Bahtinov Aid

The Bahtinov Aid is accessible from Tools tab
or with Alt+B shortcut.
 
The Bahtinov Aid uses licensed technology developed by Neils Noordhoek - the very same technology built into the famous Bahtinov Grabber. It allows achieving precise focus using a Bahtinov mask making sub-pixel analysis of the spikes.
 
In order to get accurate calculations you have to enter the pixel size (for DSLRs it is automatically determined), the focal length and aperture of the telescope. Drag the panel over the preview area to frame the star that you want to use. Use the Cross mode to center the star in “Focusing Region”. After that click on Recalc button. You will see two graphical interpretations of the results “Lines” – the usual Bahtinov pattern and “Circles”. To make the focus perfect join the two circles. Also you can see the focus error/distance in pixels. Try to get the lowest possible reading. Usually the atmosphere turbulence is makes the distance to jump all the time. Empirically is found that when there is good focus, without changing the focuser position, the aid starts to give equal values, but with different signs (for example  -0.12 then +0.12). In the bottom of the aid you can see a graph of the values since the last reset (recalc).
 
In the Canon EOS LiveView mode is recommended to use x5 Zoom to get pixel resolution close to 1:1, in Nikon use zoom 100% or bit less to fit the spikes. This will give more accurate calculation of the focus error. Also for DSLR LiveView you can use a stacking algorithm in order to lower the influence of the atmosphere turbulence which makes the measurements more accurate, so after every focus change wait a second or two. You can control the number of the stacked images with the + / - buttons. When you are focusing with images you can increase the exposure to get more stable readings.
 
If the star is very bright, the spikes will be very big and in some cases the Bahtinov Aid will not detect them correctly. You have control by several ways:
 
After focusing is complete it is good to double check the focus using a short exposure.
 
Bahtinov Aid
 
 
Here is a screenshot of Bahtinov Aid when Cross mode is activated. Place the cross in the center of the star and click on Recalc to make spikes detection again.
 
 
1

Focusing Region

1.  Focusing Region
 
It is a "transparent" window that shows the image under the Bahtinov Aid and defines what is the source to be used in the Aid
 
2

Focus error - Circles

2. Focus error - Circles
 
Shows the error as distance between the centers of two circles. Join the circles to get perfect focus.
 
3

Detected Spikes

3. Detected Spikes
 
Shows the position of the detected spikes.
 
4

Recalc

4. Recalc
 
Forces the Bahtinov Aid to make a new spikes detection. Sometimes the aid make false detection. If this happens use the Cross mode, center the star again and click Recalc.
 
5

Cross

5. Cross
 
Enables the Cross mode. Shows a small cross in the Focusing Region in order to achieve better centering of the star.
 
See the screenshot above.
 
6

Focus Error

6. Focus Error
 
Shows the focus error in pixels and with text interpretation.
 
7

Focal length

7. Focal length
 
Defines the telescope focal length. If there is value in the Object Calculator it will be automatically used. This values is needed for accurate sub-pixel calculation of focus error.
 
8

Telescope diameter

8. Telescope diameter
 
Defines the telescope diameter. This values is needed for accurate sub-pixel calculation of focus error.
 
9

Pixel size

9. Pixel size
 
Defines the pixel size of the camera sensor. If there is value in the Object Calculator it will be automatically used. This values is needed for accurate sub-pixel calculation of focus error.
 
For DSLR cameras this parameter is populated automatically using the internal database for all Canon EOS and Nikon models.
 
10

Stacking Pause

10. Stacking Pause
 
Pauses/Resumes the DSLR LiveView stream stacking.
 
11

Stack count

11. Stack count
 
Defines how many frames from the DSLR LiveView stream to stack. Bigger value lowers more influence of the atmosphere turbulence, but needs more time to build an image and to show the effect from the focus tuning.
 
12

Graph

12. Graph
 
Shows graph of the values changes. The Recalc button resets it.