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Focus a DSLR and lens ?


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Hi all,

What method do you use to focus a DSLR with its lens - i.e. no telescope - on stars?  Unlike on the scope the stars seem very faint on capture liveview software such as APT.  I can zoom in and focus best I can and this seems reasonably OK but it seems a bit subjective.

I thought of getting or making a Baht mask and exposing the DLSR for 4-5s subs and homing in on focus that way.  The stars are so faint though that I was wondering whether the baht mask works in this scenario?

Appreciate a steer please guys?

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Have you turned on APT's autostretch for live view? Usually, I set a focus plan up to loop 5-10s exposures at Max ISO with Bahtinov mask on the lens and APT's image preview set to 1:1 instead of Fit (ensuring star remains in FoV of the preview display), Bahtinov Aid open in APT, then focus by hand following https://ideiki.com/astro/usersguide/bahtinov_aid.htm?ms=AAA%3D&st=MA%3D%3D&sct=MTAxNw%3D%3D&mw=MjQw

I do this on the brightest star I can find to conveniently slew to... not necessarily the target I'm imaging.

APT also has built in routines for autofocusing using Lens motors - see https://ideiki.com/astro/usersguide/eos_lens_control_and_auto_focus.htm?ms=AAA%3D&st=MA%3D%3D&sct=MTAxNw%3D%3D&mw=MjQw

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I've done this a few different ways. If you can find some distant streetlights, the AF can handle it. Otherwise, I just use trial-and-error with several exposures (live view is no good, as you say).

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I use FWHM mode in BackyardEOS. Just point to bright star in live view and start focussing. The smaller the value the better the focus. I am fairly new to this hobby but seems to have worked spot on so far. BackyardEOS has a free evaluation also so could give it a try for nothing. There is a Nikon version also.

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Obviously each lens is different but the last bit of advice on this I read was to practice manually focusing the camera during the day on infinity, which hopefully should be marked on the barrel, then to adjust a small way back and forth to get sharp focus.

I used this technique in Lapland over the winter and the results were sharp most of the time, then us the rule of 500 to prevent star trails.  Obviously turn off image stabilisation as (it does on my sigma anyways) cause havoc when on a tripod..

After that the slight adjustments are just trail and error until your happy..

HTH

Fozzie

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I use my 7 inch android tablet running dslr controller connected to my canon 1100d use it for manual vintage lenses and modern lenses. Turn the iso up the really faint stars show when in focus and wink off when not, once focused turn iso down.

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4 hours ago, happy-kat said:

I use my 7 inch android tablet running dslr controller connected to my canon 1100d use it for manual vintage lenses and modern lenses. Turn the iso up the really faint stars show when in focus and wink off when not, once focused turn iso down.

Same here.

You'll need a USB Mini B to USB Micro B lead/adapter to connect the DSLR to the mobile device. Larger the screen, the better as you can really fine tune the focus (x10 mag on the DSLR Controller App), although I prefer to use the ancient Moto-G due to it fitting in the bag nicely.

DSLR Controller is only available for Android.

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Good advice above, the one thing not to do is using a bright star on its own and think you have nailed the focus because its too subjective, it does get you close enough to start seeing the fainter stuff popping up. The next step is to zoom in on liveview and as Happy-Kat has mentioned look for a faint star that you can blink on/off with the slightest touch of the focusser.

Alan

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Many thanks.

Just playing around with wide field. It is much more immediate than mega long exposure CCD imaging, especially in UK skies, and it cna be done anywhere. Just want another string to my bow.

I have a Samyang 14mm F2.8 incoming....

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I haven't tried it but have read excellent DSLR imagers describe zooming in to pixel scale on faint stars and noting the stellar width in pixels. Also, they say, there comes a critical point at which very faint stars simply disappear unless in perfect focus.

Olly

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1 hour ago, ollypenrice said:

I haven't tried it but have read excellent DSLR imagers describe zooming in to pixel scale on faint stars and noting the stellar width in pixels. Also, they say, there comes a critical point at which very faint stars simply disappear unless in perfect focus.

Olly

This is a critical point, if the realy faint stars are in focus (ideally at the thirds intersection) then the jobs a good un. The quality of the lens will then determine how the bright stars appear but any bloating is easily fixed in post prosessing.

Alan

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Focus with a camera lens has always been my Nemesis, but thanks to Tooth_dr's generous offer of some mini Bahtinov masks for free recently, I managed this on Artcturus.  Not had a chance to take an image as yet, but was pretty pleased with this from the suburbs of London.

N.B. this was on a static tripod so a bit of trailing evident. 

This was with a mono CCD camera on the back of a nifty 50 Canon lens though, so perhaps not representative of a DSLR.  

I took notice of another tip I read somewhere to stick a lump of blue tack on the focusser ring once I have found focus to stop it slipping.  

Carole 

 

 

Arcturus with camera lens_6523.png

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"but the last bit of advice on this I read was to practice manually focusing the camera during the day on infinity, which hopefully should be marked on the barrel, then to adjust a small way back and forth to get sharp focus".

My 450D and my lenses dont seem to have an infinity marking anywhere on them. I use live view and then its trial and error.

Not that i do much,if any imaging. I have been known to put camera on photo tripod and aim at Andromeda.Took loads of short exposures and stacked with DSS. I was happy with result. Dont have image anymore.

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Yes mine doesn't have any infinity marking on it either, so that's what I did initially, focussed in the daytime and then put my own mark on the point of focus on a land object (with a bit of tippex).  This gets me in the ball park when I go outside at night.

Carole 

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In APT you can use HFD and FWHM (Tools->Focus Aid) in combination with Lens control to get quite good focus with LiveView x5 on bright star. If you have the full version give a try of the lens auto-focusing ;) This is one of the features I like a lot (not very modest from my side :glasses1:)

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I have a secondary 7" screen that connects to the HDMI port of my DSLR, and shows whatever is on the camera's rear screen. It has a focus assist feature that puts a red border around whatever is in focus. It also allows me to have the camera pointed at high objects and still see what's on the screen (my Nikon's rear screen is fixed), and see it larger than what the screen shows.

So, for faint objects, I first find a bright star that easily shows on the screen, and focus until the star turns red. It's pretty much an infinity focus; then I move to my intended subject and fire away.

I use this screen for multiple things; I have a cheap EP camera that fits on the end of an EP, and projects the image to the screen; works great for outreach with people who have difficulty seeing the objects in an EP (kids, old folks, etc.). It's not really much different from using a laptop's screen, except for the focus assist feature, and that it is very compact. It has better resolution than the camera's screen, too. This is the gizmo I'm using:  https://www.bhphotovideo.com/c/product/1398766-REG/elvid_7_4k_on_camera_monitor.html

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