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Bhatinov mask use on DSLR lens

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#1 kroberts26

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Posted 20 August 2019 - 07:56 AM

I'm just getting started in astrophotography using my DSLR and existing 35-105mm lens. Between my poor eyesight and a sticky focus ring on the lens my focusing has been just so-so. I bought a bhatinov mask that clips into my light pollution filter thinking that would give me better confidence that I was properly in focus. When I tried it this morning I was disappointed that focusing on a bright star and taking a picture resulted in a picture of the star and not the diffraction spikes I was expecting. Do bhatinov masks require a longer focal length to be effective or did I miss a step? There were intermittent high wispy clouds if that matters. 



#2 GamesForOne

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Posted 20 August 2019 - 08:14 AM

You need more exposure time to capture the dimmer diffraction spikes?

 

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#3 dpaigen

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Posted 20 August 2019 - 08:38 AM

I've used a Bhatinov mask on 18-140 and 70-200 lenses and it works just like it does on my 1000mm scope.  The diffraction pattern should be obvious, but (in a camera) you do have to zoom in (digitally & all the way) on a fairly bright star.  Perhaps you have it mounted wrong?  Can you post a picture of your setup?

 

Note that the diffraction pattern is not large.  I estimate around 5 times the number of pixels that the star itself takes up.  You won't see it without pixel peeping.

 

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#4 kathyastro

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Posted 20 August 2019 - 08:52 AM

You might need a longer exposure time, and you will for sure need to use the maximum magnification you can get on the LCD screen.  Otherwise, yes, the diffraction pattern will be too small.  Make sure that you are using a bright star.


Edited by kathyastro, 20 August 2019 - 08:53 AM.


#5 2ghouls

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Posted 20 August 2019 - 11:08 AM

Practically, I will just repeat what others have said: try longer exposure time (I often need 3-5 seconds with wide angle), and make sure it is a REALLY bright star.

 

But I will also commiserate. I have noticed using bahtinov masks with wide angle camera lenses is not nearly as easy as it is with longer lenses / telescopes where I can focus using live-view because the pattern shows up so well. I have found anything under 85mm f.l. to be a challenge, and requires the more time-consuming process of taking a few second long exposure, entering playback mode, zooming in all the way, changing the focus slightly, and taking another exposure, etc. I even bought the LonelySpeck SharpStar2 ($70) hoping it would work better than cheaper alternatives. IMO, it's not worth the money. Bahtinov mask focusing is just always harder at low f.l./ small aperture.



#6 kroberts26

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Posted 20 August 2019 - 11:20 AM

Thanks for the responses. The exposure was probably too short, and I definitely did not zoom in on the image to look for the spikes. Also, the star I shot was picked mostly because of convenience to my target (M31), not because it was super-bright. Good lessons for my next session.

 

David - I have no rig yet, just a Canon 5DM2 with lens on a tripod. I've ordered an Ioptron Skyguider Pro, should arrive later this week.



#7 kroberts26

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Posted 22 August 2019 - 06:54 AM

Update: Clear skies this morning (yea!). Shot a 10 sec exposure of Deneb with Bahtinov mask, then zoomed in 10x on that image. Did not see the "classic" diffraction spikes but did see some very small spikes which helped me get a better focus. Practice makes perfect, hoping for clear skies again tomorrow morning.



#8 dirac

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Posted 25 August 2019 - 10:02 PM

When you say that the bahtinov mask is where you put the light pollution filter, you mean a clip in filter? That is reallllly close to the sensor. If you want to see diffraction spikes you should put the mask on the lens, further from the sensor.



#9 kroberts26

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Posted 26 August 2019 - 03:37 AM

When you say that the bahtinov mask is where you put the light pollution filter, you mean a clip in filter? That is reallllly close to the sensor. If you want to see diffraction spikes you should put the mask on the lens, further from the sensor.

The bhatinov mask is attached to the light pollution filter that is screwed in to the front of the lens, so it is the first part of the optical train.




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