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Histogram of H-Alpha; help needed

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#1 contefrederic

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Posted 05 August 2018 - 06:50 PM

I just setup a new Lunt 80mm THa, single stack, scope.  I reset the etalon and am trying to tune the image to 656nm, but am having trouble with my old eyes making out the granulations, etc.  I put a CCD on it and created a histogram but do not know what to expect when I hit the sweet spot.  I went through the pressure turns but the histogram only narrowed as I turned the black tuning knob in.  I read somewhere that the histogram should flatten out, but I do not know what that looks like.  

I have Sharpcap and CCDOps for histogram generation and a Truis SX-964 and a ZWO 174MM for CCDs.

Any suggestions or, better, yet, examples of what the histogram should look like on a single stack Lunt when tuned?

Thank you



#2 Joe47

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Posted 05 August 2018 - 07:15 PM

If you are referring to granulation they are seen in White Light not Ha.
Joe

#3 MalVeauX

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Posted 05 August 2018 - 08:17 PM

You would have seen it very clearly, as a pencil drawing. My guess is you were not close to focus and so it stayed too blurry to know if you were on-band or not.

 

Point it at the edge of the solar disc, focus until it's sharp-ish. Then tune, slowly, until you see surface features start to develop in the form of spicule fingers. Note the position/value. Fine tune from there.

 

And by the way you absolutely can see photospheric convection cells in HA with most single stacks that are not super tight sub-angstrom as they leak through.

 

Very best,


Edited by MalVeauX, 05 August 2018 - 08:18 PM.


#4 contefrederic

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Posted 05 August 2018 - 11:54 PM

Thank you, MalVeaux, for the advice on focusing.  Will be out tomorrow to try, again.  I will magnify the image centered on the limb.  I can see many prominences but not sure if that means I am close or not.

Wish me luck.

Cheers,



#5 contefrederic

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Posted 06 August 2018 - 07:46 AM

Joe,

Yes, you are correct.  Some granulations can be seen in white light, but, are better seen in H-Alpha.  

 

Here is a good reference for you:

http://www.astronomy...how.com/sun.htm

 

Fred



#6 FlankerOneTwo

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Posted 06 August 2018 - 05:55 PM

Hi,

 

If you are referring to tuning the etalon, you are usually trying to find the point of maximum contrast. This is represented by the width of the histogram (difference between darkest and lightest pixels) as shown in the image below, and also to a degree by the "flatness" of the histogram, meaning that pixels are widely distributed in intensity, not all clumped together.

high_low_contrast-histogram.jpg

 

In practice I believe this should be where the image is darkest as you have then made the dark pixels as dark as possible - you can then increase the exposure or gain as needed to stretch the brightest pixels to the right. You simply watch the peak of the histogram as you advance the tuning piston. You will see the top of the histogram move leftward (darken) and at some point begin to move rightward (brighten) again. The maximum contrast point should be at the darkest image. If the resulting image is too dark, then you adjust the exposure or gain to brighten it so that the top end is where ever you like it to be (I usually run it fairly high, 80+% or so). At the moment I usually don't use gamma as there is sufficient bit depth after stacking to do it all in post processing unless you do extreme histogram manipulations (which can be very dramatic), but there's no doubt that some folks like to image with gamma on and get great results. It can make it easier to judge what the final result will be. I might change my approach after more experimentation.

 

I usually crank the gain up high first so I can focus on the spicules on the limb, then reduce gain and tweak tuning for contrast. I do find I need to burp the piston from time to time, especially if the weather has changed significantly or I haven't used the Lunt in a while.

 

I'll let the true experts chime in if they have different techniques - always learning something new here!



#7 BYoesle

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Posted 07 August 2018 - 07:33 PM

Hi Fred,

 

Forget the histogram. For that matter, forget imaging until you have mastered visual observation. For a beginner I would recommend going to the GONG H-alpha Network:

 

http://halpha.nso.edu/

 

If there are any filaments on these images, try to observe them and tune to get them to be as dark as possible - then you are on band, and any prominences that are on the limb should then be visible as well. Seeing prominences is good, and you are near to on-band if you can currently see them. Be patient - solar minimum is a difficult time to cut your H alpha teeth...

 

 

Some granulations can be seen in white light, but, are better seen in H-Alpha.

 

What you are referring to as "granulations" are not the underlying convection cells of the photosphere (which would be very subtle at best - e.g. "invisible" - and would require excellent seeing with a hi-res single stack filter system so you can see them from the parasitic continuum leakage coming through the chromosphere lying above: http://www.astrosurf...-contrast-2.htm ). They are indeed more easily observed in white-light continuum (especially near the limb), if not narrow band non-H alpha wavelengths such as the G-band, CaK, etc:

 

http://www.astrosurf...il_hr_2017.html

 

The website you cite is completely erroneous in this instance for nomenclature. Instead, you are seeing what is referred to as the "chromospheric network" of spicules, mottles, fibrils, plage, and other features of the chromosphere:

 

https://www.prairiea...sun-in-h-alpha/

 

https://www.astrolea...ing_Program.pdf


Edited by BYoesle, 08 August 2018 - 06:34 AM.

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#8 BYoesle

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Posted 14 August 2018 - 01:55 AM

For imaging here's what Patrick's diagrams of the change of the histogram look like for focusing in FireCapture:

 

Histogram focusing.jpg


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