Focusing stars through your telescope
Achieving a sharp focus is an essential step of any deep-sky imaging session. With so much time and effort put into your polar alignment and autoguiding accuracy, it would be a shame to spoil a photo due to poor focus. A Bahtinov mask is a tool designed to help you focus your telescope.
In general, the smaller the stars are in your image, the better. One of the reasons I love my Explore Scientific ED 102 Refractor so much is its ability to capture pinpoint stars. However, this characteristic is only present in my images when my focus is spot-on. Wide-field Apo owners have come to expect razor-sharp details in their photography, so mastering the art of focusing stars is a must.
Different types of telescopes have better ways of achieving a sharp focus than others. In this post, I will be describing tips that work well on a wide-field imaging refractor such as the ED 102 I use.
You can count on the advice I am reporting to hold true to all small apo refractor telescopes including the popular models from Orion, SkyWatcher and Astro-Tech.
Focus Basics: Using Live View
If you haven’t heard of focusing using live view, it means you are quite new to DSLR astrophotography, so I’ll briefly explain how it works. Most DSLR cameras (such as the Canon Rebel XS and beyond) have a feature known as “live view”, where a real-time picture is displayed on-screen.
Because it is so dark at night, your live-view screen may appear as pure black when looking through your telescope. This could be because:
- You do not have your DSLR’s ISO Sensitivity high enough
- You are so far out of focus that stars are not visible
- You are not on long exposure “bulb mode”
- You are not pointed at a bright enough star!
4 tips to ensure that a star appears on your live view screen:
Aim your telescope at a bright star
First, make your life easier by pointing your telescope at bright star such as Sirius, Betelgeuse, Vega, Deneb etc. These stars are all bright enough to appear on your live-view screen.
Use a high ISO sensitivity
Next, make sure that your camera is set to it’s highest ISO when focusing via live view. On the Canon T3i that I use, that happens to be ISO 6400.
Set camera to Manual Mode – Bulb Shutter speed
Make sure your DSLR camera is set to Bulb mode – the longest possible exposure – past 30”. Slower shutter speeds will dim the star, and we want it as bright as possible for focusing.
Adjust the focus knob on your telescope
And finally, focus your telescopes draw tube until you see a bright star appear on-screen. You may be way out, so make sure that you check end-to-end. (If you still can’t reach focus, you may need to purchase an extender tube)
A dual-speed Crayford style focuser really helps when adjusting focus at this level of accuracy. If you are looking at purchasing a refractor for astrophotography
What about using a camera lens?
The same settings apply if you are using a camera lens in place of a telescope. The only extra step you will need to take is to make sure that the lens is set to its fastest aperture.
A lens set to an aperture of F/4 or faster will allow plenty of star light to reach the sensor.
Better focus with BackyardEOS
Using Astrophotography Software for Even Better Results
BackyardEOS was built to help astro-imagers improve their acquisition process in the field. There is an excellent function within BackyardEOS that has provided me with a higher level of focus accuracy than ever before: FWHM.
Find BackyardEOS and all of the other software I use for astrophotography on the Resources page.
How to use FWHM (Full Width Half Maximum)
FWHM is a precision focusing aid feature included with BackyardEOS. It is found within the “frame and focus” tab at the top left of the screen. This function associates a value on the star you have selected in a target window. Using the live view mode within BackyardEOS will show you a real-time image of a bright star in your field of view.
Official Description from the creator of BackyardEOS:
“Full Width Half Maximum is the width of a star’s image at half its peak. Focus is achieved when you get the lowest value for the same star over time, indicating a tighter star. BackyardEOS implements FWHM by calculating the standard deviation (the square root of the variance) of all pixel values in a very small selected area.”
Remember to use the same camera settings as discussed earlier while performing this task. Manual Mode, Bulb Shutter Speed, High ISO, and Pre-Focus.
First, make sure the Live view button is pressed in the lower right-hand side of the BackyardEOS interface. If your camera settings are correct, and your focus is close, you should see a number of stars on screen.
I prefer to choose a medium-sized star within the frame as my target star. Double click the target window around your chosen star. This will show a zoomed-in preview window at the top right of the interface.
You will now notice a number below this zoomed star image, and this is the value we will monitor to achieve a high accuracy of focus. The Zoom box center can zoom in on the star if desired, but I prefer to use the default 3X zoom.
Adjust the fine focus knob on your telescope and watch as the number associated with that star changes. The goal is to get the star as small as possible, with the lowest number. There is no set number to reach, as stars vary in size. On a medium sized star, I usually reach a number as low as 3.6, give or take.
Once you have reached the lowest number possible for that star, go ahead and lock your focus into position using the lock screw on your telescope.
Do not use a Bahtinov mask if you are using the FWHM function of BackyardEOS. The photo above is misleading because you will only be adjusting focus and referring to the FWHM number when the mask is off. The Zoom box is used for an up close look at the star diffraction pattern only. A real reading of the star size can only be obtained when the mask is off. When it comes to focusing with a Bahtinov mask, or the FWHM function, it’s one or the other.
Video: Using a Bahtinov mask to focus my telescope
How to use a Bahtinov Mask
I recently used a Bahtinov mask for the first time with my Explore Scientific ED 102, to see if I could get an ever sharper focus on my astrophotography images. The model I used was made by Kendrick Astro Instruments and was built for telescopes 90mm – 105mm.
How a Bahtinov Mask works
While aiming a bright star, you simply place the Bahtinov focusing mask on your telescopes objective lens, and secure it into place using the provided rubber tabs. The openings in the mask create a set of diffraction spikes on the star that will assist in the accuracy of our focus.
The same process as previously described using Live view on your DSLR, or within BackyardEOS is applied when focusing with the mask installed. The difference is, you will no longer use the FWHM value to adjust focus. This time, you will need to pay attention to the star pattern created.
Test Exposures vs. Live View
For my small refractor the live view image was not bright enough to get a useful star diffraction pattern. Instead, I used short preview exposures (the Snap Image button) and made adjustments back and forth.
After about 3 slight focus adjustments, I was able to produce the ideal star diffraction pattern to indicate that my focus was as sharp as possible.
The nice thing about the Bahtinov focus mask is that the star diffraction pattern provides you with a useful visual aid. Slight changes in focus are evident in the star pattern right away.
Using FWHM with/without the mask
If you watched my video, you’ll know that I am not convinced that a Bahtinov mask is needed to reach perfect focus on a small refractor. Of course, this is assuming you own the BackyardEOS software and use the FWHM method.
I believe that larger, faster telescopes such as a Newtonian Reflector would benefit more from a Bahtinov mask. These telescopes collect more light and thus provide a brighter image when using live view. This would allow you to make focusing adjustments to the star diffraction pattern in real time.
In my experience, the FWHM procedure (without a Bahtinov mask) produced results as sharp as with the Bahtinov mask. I may skip the Bahtinov focusing mask in future imaging sessions, and continue to rely on the accurate reading provided using FWHM in BackyardEOS.
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Below is my current version of the Orion Nebula, with data captured in December 2016, and January 2017.