Achieving sharp focus is an essential stage of any deep-sky astrophotography session. With so enough time and effort put into your polar alignment and auto-guiding precision, it will be a shame to spoil a photography because of poor focus. Over the years, I’ve experienced my good share of deep-sky astrophotography concentration mishaps. In many cases, I did not recognize how bad the concentration of my photograph was until I attempted to process the final image.
Avoid this unfortunate circumstance by simply using a simple tool that effectively confirms that your concentrate is really as sharp as could be, every time. Whichever type of video camera you use, a Bahtinov mask will help you attain a higher degree of reliability when concentrating your telescope. Unlike autofocus and superstar measuring software equipment, this ‘old school’ technique can be achieved without the utilization of an external pc, and takes only a minute.
In general, small the stars are in your image, the better. Among the reasons I really like apochromatic refractors including the William Optics FLT 132, is its ability to take colourful, pinpoint stars. However, this satisfying characteristic is merely present when your target is spot-on. Refractor telescope owners expect razor-sharp details within their photography, consequently mastering the art of focusing stars is crucial.
Several types of telescopes have better means of achieving a sharp focus than others. For instance, a Newtonian reflector creates its star diffraction spikes naturally of its design. These patterns could be dissected and tweaked to diagnose problems with collimation and find a sharper concentrate for astrophotography. In this post, even so, I’ll be describing ideas that are most readily useful to those capturing with a refractor telescope, that are using a manual focus routine.
Automatic and motorized focuser users have the ability to use software that tells them whenever a star is as sharp as likely.(Like the FWHM measurement in BackyardEOS or APT). If a capturing with a DSLR camera or committed astronomy camera and you simply want quickly confirm your focus can be on - utilizing a Bahtinov mask is among the easiest ways to do this. The latest type of this focusing device is the greatest I’ve ever used, because the improved design allows more light to feed the mask. The diffraction spikes Bahtinov mask from William Optics is usually a patented style - and it works exceptionally well.
You can use a Bahtinov mask right away, or simply pop one on the objective of your telescope to verify your best focusing attempts were indeed bang-on. I love the simplicity and visual aspect of diagnosing and correcting the superstar diffraction spike routine. Advanced imagers will most likely use sophisticated autofocus software to keep focus throughout the night, even adapting to changing temperatures.
I have installed a Pegasus Astro stepper electric motor kit on my Explore Scientific ED102 refractor. It nowadays has this ability to focus on its own, although I have certainly not deployed the autofocus features as of yet. The focused focuser program reads the celebrity size facts and communicates with the motor to make small changes to the focuser as wanted. A temperature probe reads the ambient temperature outside to decide if a focus tweak is required.
As for all of those other telescopes I take advantage of, the Bahtinov mask may make the focusing routine simple and straightforward.
If you haven’t heard about focusing using live view, it means you are quite not used to DSLR astrophotography, as a result I’ll briefly explain how it operates. Most DSLR cameras (including the Canon Rebel XS and beyond) have a feature referred to as ‘live view�,\’ in which a real-time photo is displayed on-screen.
Because it is so dark during the night, your live-view display screen may appear as pure black when looking through your telescope. This may be because:
- There is no need your DSLR’s ISO Sensitivity substantial enough
- You are so far out of focus that stars are not visible
- You aren’t on long exposure ‘light bulb mode’
- You aren’t pointed at a shiny enough star!
4 tips to ensure that a star appears on your live view screen:
- Purpose your telescope at a glowing star
First, make your daily life easier by pointing your telescope in shiny star such as Sirius, Betelgeuse, Vega, Deneb etc. These stars are all bright enough to seem on your own live-view screen.
- Use a higher ISO sensitivity
Next, make sure that your camera is defined to it’s highest ISO when focusing via live view. On the Canon T3i that I take advantage of, that occurs to be ISO 6400.
- Set camera to Manual Mode - Bulb Shutter speed
Ensure that your DSLR camera is defined to Bulb method - the longest possible publicity - earlier 30�. Slower shutter speeds will dim the superstar, and we wish it as glowing as possible for focusing.
- Adjust the target knob on your telescope
And lastly, focus your telescopes draw tube until you see a bright star appear on-screen. You may well be way out, hence make sure that you check end-to-end.(If you even now can’t reach concentration, you may need to acquire an extender tube)
A dual-speed Crayford design focuser really helps when adjusting emphasis as of this level of accuracy. If you are looking at investing in a refractor for astrophotography
The same settings apply for anyone who is by using a camera lens instead of a telescope. The just extra step you will have to take is to make certain that the zoom lens is defined to its most effective aperture.
A lens place to an aperture of F/4 or faster allows a good amount of star light to reach the sensor.
BackyardEOS was built to help astro-imagers enhance their acquisition process found in the field. There is a fantastic function within BackyardEOS which has presented me with an increased level of focus accuracy than ever before: FWHM.
###How exactly to use FWHM (Total Width Half Maximum)
FWHM is a accuracy focusing aid feature incorporated with BackyardEOS. It is discovered within the ‘body and target’ tab at the top kept of the display. This function associates a benefit on the celebrity you have decided on in a target screen. Using the live look at method within BackyardEOS will highlight a real-time photograph of a bright superstar in your discipline of view.
“Full Width Half Optimum may be the width of a stars image at one half its peak. Focus is achieved when you get the lowest benefit for the same superstar over time, indicating a tighter superstar. BackyardEOS implements FWHM by calculating the standard deviation (the square base of the variance) of most pixel values in an exceedingly small selected area.”
Remember to use the same camera configurations as discussed earlier even while performing this. Manual Mode, Light bulb Shutter Speed, Superior ISO, and Pre-Focus.
First, make sure the Live view switch is pressed in the low right-palm side of the BackyardEOS interface. If your camera settings happen to be correct, and your concentration is usually close, you should visit a number of stars on screen.
I prefer to pick a medium-sized celebrity within the framework as my target star. Double click the target home window around your chosen star. This will present a zoomed-in preview screen at the very top right of the user interface.
You will now notice lots below this zoomed star image, which may be the value we will monitor to achieve a higher accuracy of focus. The Zoom field centre can zoom in on the superstar if desired, but I prefer to use the default 3X zoom.
Adjust the okay focus knob on your own telescope watching as the number connected with that star changes. The goal is to receive the star no more than possible, with the cheapest number. There is absolutely no set number to attain, as superstars vary in proportions. On a mid-sized star, I usually reach a number only 3.6, give or have.
After you have reached the cheapest number easy for that star, go ahead and lock your target into position working with the lock screw on your own telescope.
Do not work with a Bahtinov mask if you work with the FWHM function of BackyardEOS. The image above is definitely misleading because you is only going to be adjusting target and discussing the FWHM quantity when the mask is certainly off. The Zoom container is employed for an close up consider the star diffraction structure only. A real reading of the superstar size can only be attained when the mask can be off. With regards to concentrating with a Bahtinov mask, or the FWHM function, it’s one or the other.
I recently used a Bahtinov mask for the very first time with my Explore Scientific ED 102, to find if I could get an even sharper focus on my astrophotography photos. The model I used was created by Kendrick Astro Instruments and was built for telescopes 90mm - 105mm.
Update: In February 2018 I had an opportunity to try out the brand new Diffraction Spikes Bahtinov mask from William Optics. The new mask creates a superstar diffraction pike pattern that’s 3 times brighter than a traditional mask (like the one used in this content). I noticed an enormous difference when focusing my DSLR through the FLT 132 refractor.
While aiming a bright star, you just place the Bahtinov focusing mask on your own telescopes objective zoom lens and secure it into place using the provided rubber tabs. The openings in the mask produce a set of diffraction spikes on the superstar that will aid in the precision of our focus.
The same process as previously defined using Live take on your DSLR, or within BackyardEOS is applied when focusing with the mask installed. The difference is certainly, you will no longer use the FWHM worth to adapt focus. This period, you need to pay attention to the star structure created.
For my own small refractor, the live view image was not bright enough to have a useful star diffraction routine. Instead, I used short preview exposures (the Snap Image button) and made changes back and forth.
After about 3 slight focus adjustments, I could produce the ideal star diffraction pattern to point that my own focus was as sharp as possible.
The nice thing about the Bahtinov focus mask is that the star diffraction pattern gives you a useful visual aid. Slight alterations in focus are evident in the celebrity pattern right away.
If you watched my training video, you’ll understand that I am not convinced a Bahtinov mask is needed to reach perfect give attention to a small refractor. Of course, that is assuming you possess the BackyardEOS program and utilize the FWHM method.
I believe that larger, more quickly telescopes like a Newtonian Reflector would benefit more from a good Bahtinov mask. These telescopes collect more light and thus give a brighter image when working with live view. This would enable you to make concentrating adjustments to the celebrity diffraction pattern instantly.
If you ask me, the FWHM procedure (with out a Bahtinov mask) manufactured results as sharp as with the Bahtinov mask. I might skip the Bahtinov concentrating mask in potential imaging periods, and continue to count on the accurate browsing provided using FWHM in BackyardEOS.
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