AP Corner - May 2022 - Finding Your Focus – Part 1

By Don Selle

220426km_image.pngIt may seem obvious, but critical focus is one of the key attributes of a good astro-image. Critical focus is the technical term used to mean that the image is focused to the diffraction limit of the optical system (based on the physical properties of the system and how light interacts with it).

Most of us take critical focus for granted in the images we see and take daily. Our DSLR’s with modern lenses, our point and shoot cameras and our smartphone cameras all come equipped with exquisitely accurate autofocus systems. This technology has been available at the consumer level for over 40 years, first in film cameras, later in digital cameras and has been continuously improved ever since.

Not so for astrophotography. Telescopes don’t come equipped with the type of focus sensors that the early film cameras had. Astro-imagers had several different manual focus aids they could use, such as flip mirrors and Focault knife-edge focusers, but these were tedious to use and required considerable experience using them to achieve critical focus.

It was not until the advent of CCD cameras for astrophotography in the early 2000s that telescope autofocus systems started to become available. Still, it took the better part of a decade before the autofocusing a telescope became affordable and routine. 

The images from the camera were available on the computer for the astronomer to review, and imaging software could provide rudimentary focus analysis on screen or to a focus controller. Many (myself included) learned to focus manually using this feedback. Focus motors and controllers soon came after came on the market that interfaced to this software to automate the process.

220426km_phone-camera.pngIf you are just starting your astro-imaging journey, you are in luck, auto-focusing your telescope has become relatively less expensive and is pretty routine stuff. If you are on a budget or just dipping your toe in before you decide to spend money on your dream imaging system, todays focusing aids are really quite inexpensive and accurate. Either way, knowing a little bit about what it takes to achieve good focus on your astroimages will ensure that your time spent imaging is the most productive you can make it.  So let’s start from a the very basic astro-imaging system and work our way up. 

Shooting the Night Sky Directly with Camera Autofocus

Many people star out imaging the night sky directly with a DSLR and lens, or in the last few years with a smartphone or a point and shoot camera. Point and shoot imaging of the night sky requires a lot from your camera. Imaging the night sky typically means you attempting to capture a photograph of a subject that, unlike daylight photography, has low light and very low contrast. It requires a good camera to get decent results this way (TIP – use a tripod as your exposure times will be much longer than daylight photos). Yes the stars offer a high contrast change, but they make up such a small fraction of the image area that the few focus sensors available may not catch them.

If you are photographing the night sky directly with your camera, such as pictures of planetary alignments the moon or the Milky Way, you may be able to use the autofocus system in your camera. The latest smartphones include imaging modes designed for this type of work. If you have trouble reaching focus this way you might try changing the autofocus method your camera uses. 

Modern smartphones and DSLRs allow you to change the focus zones in your image and this tactic can get you focused. Once activated in your camera controls, changing the focus zone is done by tapping your live view screen where you want the camera to reach critical focus. If you tap on the brightest thing in view you may be able to get the whole night scene in focus. This 220426km_telescope.jpgalso can work well with your smartphone attached to a telescope eyepiece adapter and save you frustration getting a well-focused image.

This tactic requires a rather bright object to be in your field of view, which is not always possible. You might try to autofocus on the brightest thing in the sky or a very distant light on the ground, then turn off autofocus while moving to your intended field of view. Once autofocused this way, you must turn the autofocus off and if you are using a DSLR I would suggest you put painter’s tape on the lens focus ring, so you don’t inadvertently change the focus. 

Focusing this way though can be a bit hit or miss. If it’s a miss, then manual focusing is necessary.

Live View Focusing - a Better Alternative

Say for example that you have your DSLR camera and a medium telephoto lens or a small refractor with an eyepiece adapter for your smartphone set up on a camera tracker. Just pushing the shutter button and hoping that autofocus will work will be a very frustrating experience. You will need to use the live view from your camera or device and focus the lens or refractor by hand.

This technique requires full manual focusing, which is built into the vast majority of DSLR camera lenses and high-end point and shoot cameras but may not be on your smartphone. Keep in mind that we are focusing the camera at infinity using the live view from the camera. What some smartphones call live focus (like my Galaxy Note 8) or live view focus is really aimed and blurring the background of your image to highlight the main subject. There may be a camera app out there that gives you full manual focus so take a look at what is out there.

Using live view, find the brightest star in your field of view, and zoom in on it as far as your camera will go. On my Canon DSLRs the live view zoom factor is 10x.

Then, using your camera’s manual focus, either the lens focus ring or a manual focus control, make this star as small in diameter as possible. Move the focus control back and forth across the focus point a couple of times, as this ensures you will home in on the best focus point possible.

This technique is reasonably convenient and consistent; however, it relies on how well your camera live view does in low light conditions.  

But when you get involved in astro-imaging, you need to keep this phrase in mind – “there’s always an upgrade!”

Bhatinov Mask and Live View focusing – the Best Focus Aid Ever

220426km_bhatinov.pngBy now, you will have recognized that using the autofocus system in your camera may not be the best approach to  achieve critical focus. There is however a very good and inexpensive focus aid available which is accurate and relatively quick to use. It’s called a Bhatinov mask, named after its inventor Pavel Bhatinov who released it to the community shortly after he created it in 2005. Due to its popularity and the fact that anyone can produce it, these focusing masks are quite inexpensive! (see https://en.wikipedia.org/wiki/Bahtinov_mask for more information).

Let’s assume you have a refractor with a decent Crayford focuser with a fine focus knob on it (most quality doublet or triplet refractors come with one of these) and a DSLR camera you would like to connect to it. You can connect your camera to the OTA by using a T-ring adapter specific to your camera (it interfaces like a lens) plus a 2 inch nose piece with male T-threads on one end (metric 42mm x 0.75). 

If you own an SCT, you need the same camera adapter, however you will also want to upgrade your focuser. The alternatives are adding a new micro-focuser knob which replaces the stock one, or a full blown SCT Crayford focuser which attaches to the visual back of your OTA. The stock focus knob is then used for rough focus, and the Crayford focuser for fine focusing.

Point your telescope and camera to a bright star while viewing the sky through the live view of your camera. Center the star then use the focuser to make the diameter of the star as small as possible. You may need to pay close attention at first since if you are way out of focus, the diameter of the bright star will be so large that its will appear very dim in comparison to the sky background. Once you have achieved rough focus, put the Bhatinov mask over the objective of the OTA.

The Bhatinov mask has a pattern on it with 3 separate zones, each of which creates its own diffraction spike. The zones are arranged such that the spikes all cross each other at an angle while one of the spikes will appear to move with respect to the other two. When this spike is centered, the camera will be at the critical focus point. 

The beauty of this system is that the movement of the center spike is consistent with the movement of the focuser. So once you know which direction the spike moves when the focuser moves out, adjusting the focus is very straight forward and intuitive. Zooming in on your live view will increase the accuracy of your focus as this helps you get the spike fully centered. 220426km_spikes.jpg

To get a really accurate focus after using live view to get close, you can take and review a series of longer focus exposures while making fine focus adjustments between them. You can also make this process more convenient by reviewing the images on a separate screen, such as a laptop or smart device connected via Wifi or BT to the camera. 

The longer exposures make the spikes much longer and finer making the effect of focus adjustments much easier to see. Having a delay built in between the images allows you to make manual adjustments to the focus and see the results in the very next image.

Many newer DSLRs and certainly all high-end ones have a built in intervalometer. This control allows you set up the camera take multiple images automatically. You can specify the number of frames, the exposure time for each frame and the delay period between each image. Push the shutter button once and off the camera goes. 

If your camera does not have this feature, you can either add in an external intervalometer which interfaces to the camera’s shutter release. Alternatively, if your camera can be remotely controlled by a smart device, there will be a camera specific app you can use to manage all of your camera settings remotely. This app will typically also have a built in intervalometer even if the camera does not.

Another alternative to make focusing more efficient, would be acquire and use an astro-imaging software program which controls your camera. This means you will also need to run a laptop in the field with this software on it. There are several very capable low cost imaging programs, some of which include a graphic aid to help you be sure that your spikes are truly centered. (They will also include the ability to autofocus if your OTA is equipped properly, but that is a subject for next time.) 

The one drawback to using this focus technique is that it requires a fairly bright star. These are not always available in the field you want to image, so you will need to focus first on a bright star, remove the mask then move the OTA to the field you want to image. This means that refocusing periodically as you image requires you to move off your field, refocus on a bright star and the re-center where you want to image. (Why and when you need to refocus come in Part 2).

Many astro-imagers find that this focus technique suits them just fine as they are able to get decent results with a minimum of cost and technical complexity. Using a Bhatinov mask is intuitive, accurate and reasonably time efficient so it may be right for you.

Next time, we will address adding true autofocus capability to your system, discuss some best practices and technical details of focusing.

Just remember though that I warned you. “There’s always an upgrade!”.

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