Focusing Using a Tri-Bahtinov Mask - Forums

Topic starter

Frequent contributor

Apr 21, 2023

Hey all,

When I use a tri-bahtinov mask with my 4" or 6" refractor, it's hard to see the diffraction spikes unless 1) the sky is fully dark and 2) I am focused on a big, bright star. Unless these two criterion are met, I can's see the spikes.

Am I missing something here in terms of my technique using this focusing mask?

Thanks!
Jerry

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Robert Gillette

Apr 22, 2023 · Edited: Apr 22, 2023

Try increasing the exposure time and use 2x2 binning. Even with RGB filters, 10-15 seconds should be sufficient. A mag 3 star should do the trick.

CS, Bob

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Jerry Gerber

Topic starter

Frequent contributor

Apr 22, 2023 · Edited: Apr 22, 2023

Try increasing the exposure time and use 2x2 binning. Even with RGB filters, 10-15 seconds should be sufficient. A mag 3 star should do the trick.

CS, Bob

Thanks Bob, I've tried 10" exposures with an L-Pro filter, with 2:2 binning. It helps, but I still have to hunt for a bright star to see the spikes. If I zoom in I can see more stars with spikes but then the resolution suffers and it defeats the purpose of having sharp spikes to use for focusing..

Dave Rust

Apr 22, 2023

B-masks made of clear plastic usually display significantly brighter spikes than the black plastic masks, I've found.

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Jerry Gerber

Topic starter

Frequent contributor

Apr 22, 2023

Dave Rust:
B-masks made of clear plastic usually display significantly brighter spikes than the black plastic masks, I've found.

Thanks Dave, that's good to know. Might you know who sells clear plastic masks? I've been able to only find black ones.

Jerry

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Rob Foster

Apr 22, 2023

Hi Jerry, remind me what imaging capture software you use? In NINA , I slew to a brighter star in the quadrant of target, though usually before fully dark. I’ll zoom in on the star then loop at 2 sec exposures but at a significantly higher ISO or gain. I have found I don’t need the brightest star to achieve good diffraction spikes. Good suggestion on the 2:2 binning…. I’ll have to try that.

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Jerry Gerber

Topic starter

Frequent contributor

Apr 22, 2023

Rob Foster:
Hi Jerry, remind me what imaging capture software you use? In NINA , I slew to a brighter star in the quadrant of target, though usually before fully dark. I’ll zoom in on the star then loop at 2 sec exposures but at a significantly higher ISO or gain. I have found I don’t need the brightest star to achieve good diffraction spikes. Good suggestion on the 2:2 binning…. I’ll have to try that.

Hi Rob, hope all is well! I use ASIAIR Plus with an Android tablet. I've been trying 2:2 and even 3:3 binning with 5" or even 10" exposures, but the only way I can see the spikes is to zoom way in--but then the resolution begins to suffer and the spikes are not sharp. The spikes don't show up with the smaller, less bright stars. It seems the brighter the star the easier it is to see all the spikes and get to sharp focus.

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Nick Grundy

Apr 22, 2023

Jerry, i've got a simple adjustable one that's laser cut from some sort of stiff composite cardboardy crap. It works on the 130PHQ though. the trick is getting a decent star. I assume you've already polar aligned when you are focusing the main scope?

It looks like the weather is going to give you a few days to sort this out. If you are in a pinch you are welcome to borrow mine if you drive up this way. I never use it with the EAF installed.

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Dave Rust

Apr 22, 2023

Jerry
regarding clear plastic masks:

williams optics makes them, but they are for their specific diameter scopes (but one could fit yours).

I got mine on Etsy believe it or not. Go to Etsy.com and do a search. Several will show up.

Ebay has masks, but I didn’t see any clear ones today.

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Dave Rust

Apr 22, 2023

Oops, I’m wrong. William Optics makes masks for any scope now.

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Richard

Apr 22, 2023 · Edited: Apr 22, 2023

As noted by Dave, William Optics make bahtinov masks which are integrated into a dust cap but also sell adjustable masks which can fit a range of dew shield sizes. Here's some of the adjustable types:

https://www.firstlightoptics.com/search/for/Bahtinov/

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Jerry Gerber

Topic starter

Frequent contributor

Apr 22, 2023

Nick Grundy:
Jerry, i've got a simple adjustable one that's laser cut from some sort of stiff composite cardboardy crap. It works on the 130PHQ though. the trick is getting a decent star. I assume you've already polar aligned when you are focusing the main scope?

It looks like the weather is going to give you a few days to sort this out. If you are in a pinch you are welcome to borrow mine if you drive up this way. I never use it with the EAF installed.

Thanks Nick, I've got the tri-bahtinov mask for the 130PHQ but I think I need to learn how to use it better by increasing exposure. I usually focus before I polar align so that ASIAIR Plus plate solves successfully the first time, making PA faster.

I'm going to send you a PM about going to Stonyford...

Jerry Gerber

Topic starter

Frequent contributor

Apr 22, 2023 · Edited: Apr 22, 2023

Richard:
As noted by Dave, William Optics make bahtinov masks which are integrated into a dust cap but also sell adjustable masks which can fit a range of dew shield sizes. Here's some of the adjustable types:

https://www.firstlightoptics.com/search/for/Bahtinov/

Thanks Dave and Richard,

I'll check out what Williams Optics makes.

It looks like they make one that might fit on my 4" and 5" refractors, but nobody has them in stock..

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Jerry Gerber

Topic starter

Frequent contributor

Apr 22, 2023

Richard:
As noted by Dave, William Optics make bahtinov masks which are integrated into a dust cap but also sell adjustable masks which can fit a range of dew shield sizes. Here's some of the adjustable types:

https://www.firstlightoptics.com/search/for/Bahtinov/

I just purchased the Williams Optics focusing mask. It makes sense that being made of clear plastic it's going to transmit light and the diffraction spikes better than black masks. I think it will help.

Richard

Apr 23, 2023

Jerry Gerber:
Richard:
As noted by Dave, William Optics make bahtinov masks which are integrated into a dust cap but also sell adjustable masks which can fit a range of dew shield sizes. Here's some of the adjustable types:

https://www.firstlightoptics.com/search/for/Bahtinov/

I just purchased the Williams Optics focusing mask. It makes sense that being made of clear plastic it's going to transmit light and the diffraction spikes better than black masks. I think it will help.

Awesome. I have a Redcat 51 and FLT120 which have the integrated bahtinov mask in their dust cap covers. I've always used an auto focuser so I use the bahtinov mask just to get initial focus and let the auto focus routine run throughout the night. I would intentionally choose a brighter star near my imaging target for initial focus then slew to my target for imaging.

In a Bortle 5 zone with a OSC, I can usually usually take a 5s sub to get decent diffraction spikes. Using a 4.5nm narrowband filter with my monochrome camera or the l-extreme filter with OSC, I aim for 5-10s and bin 2x2.

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