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Guiding with a Bahtinov Mask?

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#1 nxda

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Posted 24 April 2019 - 01:01 AM

So I was out the other night for the first time since last summer, just simply testing my gear to make sure everything still worked. And I'm happy to report it did! Always a good feelingsmile.gif

 

The skies that night were marginal at best and eventually there was an obvious thin layer of clouds overhead. You could still see stars through them just much dimmer. But as I watched the clouds and PHD, my guiding wasn't suffering? My CEM60-EC has always performed very well for me but lousy skies usually force me to shut down PHD and just let the encoder do the work alone. Even with the clouds my star profile was still really good and I just wasn't understanding why. Then I figured it out. I had not taking the Bahtinov Mask off my scope the whole time I was goofing around with the gear. 

 

This leads me to the question... For guiding purposes was the mask helping things? Was this real what I experienced or had I just nailed PA and the mount was just doing its job? It makes me wonder if I want to dump my OAG and go back to my old Borg guide scope and put a mask on it? If it results in better star profiles and more consistent guiding thru less atmospheric disruptions why not?

 

I'm very curious as to your thoughts on this!

 

Here is my guiding graph through the clouds... now this is pretty much the normal performance of the mount... but it was thru clouds!

 

Thanks,

Will

 

IMG_5955-C.jpg


Edited by nxda, 24 April 2019 - 01:09 AM.


#2 einarin

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Posted 24 April 2019 - 01:22 AM

PHD is pretty tolerant of finding centroid of stars - even if the shape is weird.

Bad things are doughnuts and saturated stars.



#3 johnsoda

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Posted 24 April 2019 - 11:22 AM

This is fascinating idea. Suppose PHD2 used the calculated intersection of the spikes as the guidestar position. I’m not saying it does now, but it could through what I would think would be easy changes, using spike detection like SharpCap has. I suspect this would be much less affected by seeing than the star itself. Of course, it would not work with an OAG for obvious reasons.

#4 nxda

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Posted 24 April 2019 - 01:50 PM

This really did blow me away when I realized what was happening. I think this was really the epitome of terrible seeing. Under normal circumstances I would have shut guiding down long before this and I certainly would not have been imaging at this point.

 

I am hoping folks with a more technical understanding of this can explain why this worked so well with PHD. My understanding is that seeing doesn't effect DSO imaging but it certainly effects guiding... which effects imaging...lol.gif

 

If this is perhaps a path to negate poor seeing conditions then that would be fantastic!

 

Continued thoughts are greatly appreciated.

 

Will


Edited by nxda, 24 April 2019 - 01:51 PM.


#5 StephenW

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Posted 24 April 2019 - 02:08 PM

That's nothing - you should try guiding and imaging with the mask still in place!  (yep, been there done that :))



#6 paolotto

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Posted 24 April 2019 - 02:37 PM

I clearly need another coffee... I’d started writing an email asking why the heck would this not work using an OAG.... sheesh...

But it makes intuitive sense that what might be a blob through wispy clouds might still have a center point with the mask on. I wonder if this might even work in good conditions if you’re having to use a saturated star, dispersing it’s light into a bahtinov pattern but still maintaining a calculable center spot. Fascinating.

#7 ahelms

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Posted 24 April 2019 - 05:57 PM

Looks like your image scale is around 2.5"/px. With that coarse of a resolution your data just doesn't have resolution to make any meaningful judgement.



#8 johnsoda

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Posted 24 April 2019 - 07:41 PM

Maybe there are possible diffraction patterns that might even be better.  For example, when you stop down a camera lens with a classic iris diaphragm, you get a spiked star.  Those multiple spikes might precisely position the star even better than a B mask.  Maybe there are even better ideas. I’ll let the diffraction experts think about it. 



#9 nxda

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Posted 24 April 2019 - 10:00 PM

Looks like your image scale is around 2.5"/px. With that coarse of a resolution your data just doesn't have resolution to make any meaningful judgement.

Can you explain? I am no guiding guru for sure. Are you saying my resolution is worse than general seeing of 2"/pp and that prevents meaningful judgement? If that is your position I completely get the logic.

 

But this experience for me shows there was significant improvement with the mask. I wish I had grabbed a shot of the graph after I removed the mask. The chasing of seeing became very significant. Any image produced would have been useless. I am definitely going to have to do some testing. I'm very intrigued.

 

Thanks

Will



#10 Stelios

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Posted 25 April 2019 - 02:30 AM

That's nothing - you should try guiding and imaging with the mask still in place!  (yep, been there done that smile.gif)

Any APOD's ?   step.giflol.gif roflmao.gif


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#11 ahelms

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Posted 25 April 2019 - 09:35 PM

Can you explain? I am no guiding guru for sure. Are you saying my resolution is worse than general seeing of 2"/pp and that prevents meaningful judgement? If that is your position I completely get the logic.

 

But this experience for me shows there was significant improvement with the mask. I wish I had grabbed a shot of the graph after I removed the mask. The chasing of seeing became very significant. Any image produced would have been useless. I am definitely going to have to do some testing. I'm very intrigued.

 

Thanks

Will

An analogy: you're tying to measure the thickness of a piece of paper with a ruler. Your instrument just lacks the precision required. You're trying to make a claim about better guiding but your guiding is already down in the noise of the centroid measurement. Check out your RMS RA and DEC values -- < 0.1 pixels.

 

Having a mask on the scope won't change seeing, I don't understand your claim of chasing seeing after removing the mask as seeing is determined by the atmosphere. If anything, the mask produces an image that works better for the centroid algorithm used by PHD (https://github.com/O...8/star.cpp#L138)



#12 gotak

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Posted 25 April 2019 - 09:43 PM

Looks like your image scale is around 2.5"/px. With that coarse of a resolution your data just doesn't have resolution to make any meaningful judgement.

I noticed that as well. 

 

In other bits of info I do believe Phd2 devs says don't go below 0.5 arc second per guide pixels as the centoid algorithms they have may not work well.


Edited by gotak, 25 April 2019 - 09:43 PM.


#13 nxda

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Posted 26 April 2019 - 08:27 AM

Ahelms.... I can't disagree with anything you've said. It makes complete sense. But I know how my setup and how it reacts when skies are bad. Something was very different here and the mask is a variable. This is why I pose the question. Hopefully by June the clouds will be gone and I can test... If I find the same response from the  mount I'll revisit and post the logs. I do appreciate you and others thinking about this. 

 

Will



#14 AhBok

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Posted 26 April 2019 - 04:00 PM

Many of us have tried guiding with a Bahtinov mask. I never tried stacking my images though! Who knows?!!
HaHaHa😜

#15 AhBok

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Posted 26 April 2019 - 04:01 PM

Many of us have tried guiding with a Bahtinov mask. I never tried stacking my images though! Who knows?!!
HaHaHa😜


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