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Tuning trick for knowing you're on-band.

beginner observing solar
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#26 BYoesle

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Posted 01 February 2021 - 02:07 PM

 

I did notice after 10 to 20 minutes my scope would start drifting off band and I would have to readjust the ethlons.  I have a Lunt DS 100mm pressure tuned scope and am wondering if I need to replace the O rings in the ethlons (the scope is new to me, but is ~4 1/2 years old) or if the band drift is due to something else I've not considered?

 

Hi Rick,

 

The blocking filter temperature change will not affect the etalon CWL, but may make the image dimmer as the BF cools significantly and the BF CWL shifts blueward and the transmission of the etalon is dimmed. This could be what you're seeing. Warm the BF up with a dew heater or other, and see what happens.

 

I you get actual etalon "drift" off-band, you either have a pressure leak O-ring issue, or the etalon temperature and spacer thicknesses are changing, and you have to re-tune the etalon to account for this etalon gap thickness change. Air temperature and barometric pressure changes can also affect the etalon CWL, and it might ultimately be difficult to differentiate what is going on. But in the end, etalons do sometimes need slight tuning adjustments.



#27 jackk

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Posted 01 February 2021 - 02:48 PM

Interesting.

I usually do imaging. What I use to determine if I’m on band or not with my single pressure tuned etalon is to check the histogram level. The image gets dimmer and more contrasty when approaching on-band, so the histogram moves to the left. If I tune past the on-band position, the histogram will start to move to the right again. Thus, on-band is achieved with the lowest histogram level for a correctly exposed image.

I wonder, how the approach I described above correlates with your visual approach? Not disagreeing at all (I don’t consider myself an expert in any sense anyway), just would like to understand the whole picture.

Are you aware of any videos that show this process live? Seems very useful. 



#28 Stickman

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Posted 01 February 2021 - 03:04 PM

Good chance it was the colder temp affecting your blocking filter.  I have found heating the BF helps in cold temps.  Lunt has a BF heater.  Some people use dew heaters.may 

        Thanks to you and Bob for both responding to my post, I appreciate your suggestions and insight.  I eventually would like to start imaging, so I'm inclined to initially purchase a solution for heating the BF.  If that doesn't resolve the issue, the O rings are pocket change. Thanks again!

 

         Rick  



#29 BYoesle

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Posted 02 February 2021 - 04:12 AM

See this post, which may help to explain what Gustavo mentions about histogram "darkening" with etalon tuning, and later also reveals a similar effect on the histogram for focusing. Both apparently due to improvements in contrast:

 

Histogram focusing.jpg

 

For both focusing and tuning - whether for visual observation or imaging, I feel it's best to learn the use the Mark I eyeball, properly trained via some experience. Imaging ROI focusing by observing how fine the fleeting details become by variances in seeing is better IMO for critical focusing versus using the histogram - and the same goes for tuning. However, as with most things, YMMV depending on equipment, technique, and experience.


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#30 alvacouch

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Posted 10 February 2021 - 01:53 PM

What should I be seeing with a Lunt 50 pressure tuned single-stack when I try this method? I understand how it works with a tilt tuner but not what to look for with a pressure tuner. Thanks! 


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#31 BYoesle

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Posted 10 February 2021 - 08:17 PM

Hi Alva,

 

I believe it looks the same, but its been awhile since I've tried it with one. Just give it a try and let us know what you see and if it seems to work ;-)




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