FOCUSING THE CAMERA

Carrying out a good focus is a very important element to have a quality image and as sharp as possible. When the CCD camera is mounted at the primary focus of the telescope, there is no infinite position as can be achieved with the lens of a digital camera (APN) for example. In addition, we cannot make a live focus as with a reflex camera (the focus is on the frosted glass of the camera).

Here are 5 focus techniques and other tips for this task:

1. Focusing assistance software
2. Focusing with the help of a mask
3. The Bahtinov mask
4. Re-focus during the imaging session
5. Automate focus

1. Focusing assistance software

To achieve the focus, one can use the focus function of the software that comes with the CCD camera. We aim at a bright star (of magnitude 4 to 8) and we start a series of successive exposures of 0.1 to 5 seconds depending on the brightness of the star. While looking at the image on the screen, turn the telescope's focus knob slightly to the right or left until a focus image is obtained. To assist in this task, the focus utility from the CCD camera software is used. For example, here is a graphic image for controlling the focus:

By turning the focus knob slightly to the left or the right, we try to reproduce the tallest and narrowest "mountain" possible as in the image above. 

Another example of focus assistance is offered with the Maxim DL software: 

Turning the focus knob shows the following information presented in the image above:

• Max Pixel represents the value of the brightest pixels. The more the value increases, the more the focus improves
• FWHM represents the size of stars (or resolved pixels). The smaller the value, the more the focus improves.
• Half Flux Dia. is similar to FWHM. It is used when the image is very defocused. The stars look like 'donuts'. The more the value decreases, the more the focusing improves.

Everything would be fast if it weren't for the air turbulence. Even if we do not touch the focus button, we will notice that the values ​​vary continuously. These variations are due to air turbulence. So you have to pay attention to keep the best values.

2. Focusing with the help of a mask

Focusing assistance software is very useful for a successful home stay. Without any focus assist tool, it is very difficult to achieve good focus. If you do not have focusing utilities with your camera, for example you are using a webcam or a digital camera at the primary focus of your telescope, here is a method that will allow you to achieve a good focus. It requires a bit of tinkering. This involves making a sheet of black cardboard that matches the diameter of the focal opening of your telescope or telescope. Drill 2 circles or 2 triangles a few centimeters in diameter which are distributed at an equal distance from the center of the cardboard. Here is an example with 2 circles and another with 2 triangles:

We install the cardboard on the front opening of the telescope. It can be held in place with the help of adhesive tapes. Here is the procedure:

• Aim for a bright star with a magnitude greater than 4.
• Set the camera to take successive exposures of 1 to 5 seconds.
• Looking at the screen which reproduces the image of the camera, turn the focusing ring in the direction which brings the 2 circles (or triangles) closer together. You will also notice that the 2 circles decrease in size.
• Continue to turn the focusing ring in the same direction until the 2 circles meet and become one. At this time, the focus is optimal. 

The advantage of the 2 triangles over the circles consists in the appearance of diffraction points around the star as in the following image:

Focusing accuracy is confirmed when the diffraction peaks are sharp and evenly distributed around the star. 

With all of these methods, you will notice that the air turbulence creates variations in focus which cause difficulty in ensuring that the focus is optimal. Personally, I put a lot of effort into focusing the camera. I take about 5 to 10 minutes to do the focus, analyzing the variations due to the turbulence to keep the most optimal values. It is definitely worth the time to invest in a good focus, as the quality of the image will depend a lot on good focus. 

3. The Bahtinov mask

The Bahtinov mask is a revolutionary new technique of focusing. It represents an improvement of the mask technique described in point 2. This method eliminates air turbulence making it possible to achieve very precise focusing in a short time. Here is an image of the Bahtinov mask:

You can install the mask at the end of the dew shield which allows you to quickly check the focus at any time during the shooting. 

You can make the mask yourself. On the other hand, it will take a good dose of patience and dexterity to accomplish this task. Personally, I preferred to opt for the purchase of a ready-to-use mask. Here is a website where you can get the mask:
http://www.kendrickastro.com/kwikfocus.html

For those who want to make the mask, here is a site that explains how to do it. It also offers free patterns for most diameters of telescopes or glasses:
http://www.deepskywatch.com/Articles/make-bahtinov-mask.html

Considering all the advantages of the Bahtinov mask over other focusing methods, I recommend this focusing method.

4. Redo the focus during the imaging session

For aluminum tubes, during the imaging session, if the temperature changes by more than 5 degrees Celsius (for telescopes with focal length f / 6 and less i.e. df / 7..f / 10), it will have to consider redoing the focus. Indeed, optical tubes made of aluminum expand or contract during temperature changes, causing a loss of focus. It should be noted that this theoretical tolerance up to 5^o Celsius temperature change for these tubes will keep a very good to good focus. Also, the primary mirror of Schmidt Cassegrain telescopes can move when the equipment passes (tilts) at the meridian, thus causing a loss of focus. 

For carbon fiber tubes, it is not necessary to refocus, as they are not sensitive to temperature changes.

Here are my recommendations to avoid re-focusing during an imaging session:

• Set up the telescope and all astrophoto equipment outdoors at least one hour before the start of the imaging session. Thus, the equipment will adapt to the ambient temperature.
• Use a weather site that shows temperature variations hourly. Choose an imaging time slot that has a difference of 5^o Celsius and less. You will notice that there are rarely nights when the temperature changes by more than 5 degrees (in Quebec, this happens especially in winter when it is very cold). Also, I recommend evaluating the sensitivity of your instrument to temperature changes. All the details here.
• The primary mirror of Schmidt Cassegrain telescopes that moves causing loss of focus. Prefer a telescope which allows the mirror to be blocked.
• For imaging using the LRGB technique, use parafocal filters (with identical focus).
• For telescopes open at f / 5 and above (f / 4..f / 2), use a carbon fiber tube, as this material is not sensitive to temperature variations.
• If possible, compare the first image with another during the imaging session to ensure that the focus has not changed.

In my experience, on the rare nights où I have to redo the focus during the imaging session, I stop the imaging session when I see that the focus needs to be re-focused. I am using the functions of the Maxim DL software to refocus (see point 1 above) by selecting the brightest star in the current framing. Then I schedule an imaging session for the rest of the photos to be taken according to my initial imaging plan. It all takes me less than 10 minutes.

5. Automate the focus

For all those who are interested, it is possible to automate the focusing of its optical instruments. This consists of using a motorized focuser with encoder (or robofocus) in conjunction with software allowing this task to be completely automated. It should be noted that if this equipment is properly configured, it will allow a very good focus to be maintained throughout the imaging session. Here is an example of the material to be used to carry out this automation as well as an estimate of the monetary investment:

Description of the equipment	Utility	Estimated price Canadian $
Robofocus allowing management via a computer
For my Schmidt Cassegrain telescope	Deep sky	625
For my 80 mm telescope (carbon fiber)	Large field	No need
FocusMax software	Automates focus during imaging session	190
Estimated total investment		815

As can be seen, this automation requires a significant investment. Personally, with my permanent observatory which is easy to access, I prefer to continue to manually focus with the Bahtinov mask, as it only takes me a minute of time to complete this task. Then, on the rare nights when I have to refocus, it takes me less than ten minutes to achieve it.

Richard Beauregard
Sky Astro - CCD

Updated the 2021 / 04 / 11