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Live view focusing with Bahtinov mask

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#1 g450

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Posted 21 November 2019 - 10:47 AM

I am using a Sony A6500 attached to a Astro tech 72ED2 . I use the LCD screen for live view focusing  and bought a Bahtinov mask  sized for the scope to assist.  For some reason when I have it focused exactly according to the mask, my  developed stars are not as pinpoint as when I just try to get the roundest tiny star as possible minus the mask. .When I  am focused this way, I do see many sparkling pixels suddenly in view as well as the main star.  Has anyone else had this issue.  Is there any way to adjust the mask to make it work?


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#2 TOMDEY

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Posted 21 November 2019 - 11:12 AM

I can only think of just one thing that can cause a bias between the mask and actual best focus... that is systematically-imperfect wavefront. You might want to at least try rotating the mask to see if the best signature shifts with that rotation. The mask (of course) is based on the (necessary) assumption that your telescope is a perfect imager. To the extent that it is not, the mask will generally disagree with truly best focus.

 

[For the theoretically-inclined here... best focus is identically when the Zernike Power in the wavefront is driven to zero. It is consequent of the Zernikes being orthogonal on the unit circle.]   Tom

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#3 fmeschia

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Posted 21 November 2019 - 11:38 AM

Is it a traditional opaque mask, or one of those new clear acrylic masks?

Francesco


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#4 B 26354

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Posted 21 November 2019 - 11:41 AM

This probably won't be overly helpful, but....

 

I've tried using a Bahtinov mask with my Olympus OM-D E-M1 (mirrorless Micro-4/3rds) on my ES 120CF, but like you, I wasn't ever as successful with it as I am without it. The nice thing about the E-M1, is that its electronic viewfinder (EVF) has 2.3 times the resolution as its LCD screen. And both the LCD and the EVF image can be magnified by 3X, 5X, 7X, 10X and 14X... so by using the higher-resolution EVF at 14X, I'm able to achieve excellent focus.

 

Looking on the Sony website at the "Full Specifications" for the A6500... under "Focus Magnifier" in the "LCD Screen" section, it says that it's possible to magnify the viewed image by 5.9X and 11.7X.

 

But there is no "Focus Magnifier" listed in the "Viewfinder" section, so it appears that it's not possible to magnify the EVF's view... which is unfortunate, because your Sony's EVF has 2.5 times the resolution of the LCD screen.

 

I prefer not to use a laptop for camera/mount/imaging control... but perhaps there's a way for you to do so with your A6500, so that you'd be able to utilize a larger viewing-screen to achieve focus, either with the Bahtinov or without it?

 

Hopefully, those with more experience will chime in.  grin.gif


Edited by B 26354, 21 November 2019 - 11:42 AM.


#5 StarBurger

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Posted 21 November 2019 - 11:53 AM

I see exactly the same thing. Focus with Barty mask same as without, just going on smallest image. No difference.

I think I use the mask to justify having spent $10 on it....



#6 greenjuice

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Posted 22 November 2019 - 02:07 AM

I am using a Sony A6500 attached to a Astro tech 72ED2 . I use the LCD screen for live view focusing  and bought a Bahtinov mask  sized for the scope to assist.  For some reason when I have it focused exactly according to the mask, my  developed stars are not as pinpoint as when I just try to get the roundest tiny star as possible minus the mask. .When I  am focused this way, I do see many sparkling pixels suddenly in view as well as the main star.  Has anyone else had this issue.  Is there any way to adjust the mask to make it work?

Hi. I’m not sure from your description of your procedure, exactly what you are doing for setup but this is what I do; increase the zoom in live view, focus until you see uniform spikes from the mask then snap a picture. Don’t rely on just the visual focus, snap a picture, adjust, snap another, adjust, keep going ad- infinitum until you get a sharp uniform spike. Hope that helps.



#7 the Elf

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Posted 22 November 2019 - 02:30 AM

The mask must be perpendicular to the optical axis. If you have a dewshield it may have some tilt. It may be me but I'm troubled when the spikes are at an arbitrary angle so I always turn the mask until the center spike is straight. Exactly as Mel says, take a photo like 5 sec at ISO 12800. The spikes are much longer now than in the life view and easier to rate.

You do not state if you have a field flattener. If not the image is curved. If you set focus in the center it will be unsharp in the corners. I use an RC without field flattener. When I image small object that I'm going to crop I focus on a star in the center of the image. For large objects I move the focus star 1/4 of the image width to one side and focus there. This results in a bit loss of sharpness in the center but far more usable image almost up to the corners. FWHM maps may help to check this out.

For shorter focal lengths like 200mm the star size method may work better. When the star is small reduce the exposure time until it dims down (likely a 500th of a second!). Now changing focus changes the stars brightness. This leads to excellent results with my camera lenses. For the long focal lenght of the RC (1100mm with reducer) the Bahtinov mask is easier.

The "sparkling pixels" are probably the increasing noise from the sensor that gets hot in life view. A friend of mine owned a mirrorless Sony and we used it for a just for fun shot. The noise in life view was awful compared to my T3i. Alas I don't remember what model it was. In general smaller chips with many megapixels are noisy in life view unless the data is denoised by the image processing in the camera. Try to use the brightest star for focus, right now Vega and Capella are good candidates. You can possibly reduce the ISO value and the noise gets less. Hope this helps.



#8 g450

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Posted 22 November 2019 - 09:03 AM

In response to Francesco , the mask is opaque. In fact after I bought one I printed 4 of my own slightly different masks. They all give the same result. I also am using a Orion field flattener. Have also been taking some micrometer measurements of the scope when I am at apparent best focus .  I stack with DSS and develop with Star Tools. I appreciate everyone's input and the next clear night is in 72 hours so I will be doing some more experiments like rotating the dew shield. I have only been doing this for 6 months but am getting some decent results considering the amount of money I have spent and I don't live in Namibia 1000 miles from the nearest city lights. I am just like everyone else, trying to coax the best out of their equipment and software. I have attached a shot of Orion Nebula from last week with a Astronomik CLS filter in line. My stars seem a little bloated.

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  • Autosave131oriontry29nov.jpg

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#9 the Elf

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Posted 23 November 2019 - 03:57 AM

Your image is excellent. Can you post some data, number of subs, camera settings, etc? You stretched it quite far and the data are deep enough to do so. In that case the stars become bigger and bigger. Part of the bloating is longitudinal chromatic aberration. The blue is not in the same focus layer as the other colors. Only mirror telescopes are totally free from that. There are ways to deal with it in processing like split the image into color channels and do individual deconvolution or desaturate the teal color, in this case using a mask to protect the running man. If can remove the stars to stretch the nebula and combine this with a less stretched version of the stars. If you are doing this for 6 months only you are a genius. It took me far longer to get to anything decent at all.



#10 Sonya6500

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Posted 24 November 2019 - 08:36 AM

So this was 80 exposures of  60 seconds each, iso 1600. 25 flats, darks and bias frames. I do not do any guiding yet so I just try for a accurate polar alignment. I received the telescope in March of this year and built a solid pier for the drive motor.  I  would not make any claim of genius but just read a lot and watch You Tube videos on how others are doing it . Thanks for your input!! Tonight is very clear so I will be out there.




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