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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 ;-)



#32 Hello_There

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Posted 11 March 2021 - 06:46 PM

Ok, this is a stupid question but I don't quite understand the procedure of this, so you point the scope at the Sun, take out the eyepiece and look through the blocking filter? The Red glass in the blocking filter or something else? I have a Lunt 50mm so I don't see like the image demonstrated?


Edited by Hello_There, 11 March 2021 - 11:48 PM.

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#33 MAURITS

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Posted 12 March 2021 - 04:40 AM

Hey Bob, thanks for the briljand tip.

 

There is something I don't really understand.

 

First fine tune the Lunt singel LS60F front filter as you explained, than I place the second DS filter before the first filter and tune it the same manner.(with both filters on the scope).

 

Is this the way to go?

 

As I understand, after this you remove the double stack filter and tune it separate from the first one (but why ?), have I misunderstood this ...?



#34 BYoesle

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Posted 12 March 2021 - 01:19 PM

 

 I don't quite understand the procedure of this, so you point the scope at the Sun, take out the eyepiece and look through the blocking filter?

 

Yes, that's it exactly. You should center the Sun looking through the eyepiece first, then remove only the eyepiece, leaving the blocking filter in place.

 

 

 

 

The Red glass in the blocking filter or something else? I have a Lunt 50mm so I don't see like the image demonstrated?

 

You're looking at the etalon and blocking filter components, and the wavelength it is passing. You won's see the central obstruction shown with a Coronado etalon. I'm not sure if the method works the same with air pressure tuned etalons the way it does with tilt-tuned etalons - but it seems that it should. So you're on your own to discover if this will work for you.

 

 

 

First fine tune the Lunt singel LS60F front filter as you explained, than I place the second DS filter before the first filter and tune it the same manner.(with both filters on the scope).

 

Is this the way to go?

 

Hi Maurits,

 

Yes, that's what you can do for my double stacked etalons. The exception is that the second DS etalon might re-introduce reflections. So after doing the tuning, you may have to supply more tilt to remove these reflections. For me I just tune the primary etalon first, then the second with both in place at the same time.

 

As I said, your results may vary...


Edited by BYoesle, 12 March 2021 - 06:34 PM.

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#35 MAURITS

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Posted 12 March 2021 - 04:54 PM

Thanks Bob.



#36 PABresler

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Posted 14 March 2021 - 03:17 AM

I am just starting with a Daystar 60mm Scout. I was planning to use a ZWO 290M camera with Sharpcap. Can you tell me how to get the camera focused? How far is the sensor from the back of the filter?

 

Peter



#37 BYoesle

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Posted 14 March 2021 - 10:54 AM

Hi Peter,

 

Nope, sorry, never seen or used one. You might want to give DayStar a call. waytogo.gif


Edited by BYoesle, 14 March 2021 - 10:55 AM.


#38 hopskipson

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Posted 14 March 2021 - 05:06 PM

I am just starting with a Daystar 60mm Scout. I was planning to use a ZWO 290M camera with Sharpcap. Can you tell me how to get the camera focused? How far is the sensor from the back of the filter?

 

Peter

I'm not familiar with this scope.  Are you using the diagonal for imaging?  Have you tried either a Barlow or reducer to see if you can get it to focus?  When I use my Quark I find that using a 25mm eyepiece gets me close to focus with most ASI cameras.



#39 PABresler

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Posted 14 March 2021 - 05:15 PM

I do not plan to use the diagonal....just need the correct spacing between the filter and camera. I had a full view of the sun, but could not find the focus point.

 

Peter



#40 starzonesteve

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Posted 14 March 2021 - 07:51 PM

Hi Bob,

 

Nice to find this thread. It stimulated me to take out my Lunt DS100 today and the sun looked great. 6 or 8 nice proms and more solar activity than I have seen for a while.

 

On the simulated pictures in your first post, is that a central obstruction? I don't have one on my Lunt so I am guessing that I am just looking for uniformity across the sun's disc?

 

I was paying close attention today and note that some features on the sun are more visible in certain parts of the eyepiece's fov. Is this indicative of being off band, or just a reality of the way the etalons work?

 

Also, does it make sense to completely depressurize my 2nd etalon, use your technique to get on band with the first one, then try a second iteration of your technique to get on band with both etalons pressurized? Or is it necessary to take the scope apart, remove the 2nd etalon altogether, then tune the first one? Or, should I just attempt to create the most uniform image with both etalons in place?

 

Sorry so many questions. I love my Lunt DS100. I would like to think I'm getting everything from it that it has to offer. Thanks for your insight and experience.

 

Steve



#41 BYoesle

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Posted 14 March 2021 - 08:52 PM

Hi Steve,

 

When using an eyepiece, you're looking to get the darkest filaments and the brightest plage in active regions:

 

P Zetner SHS H alpha crop SM.jpg

Click for larger.

 

The technique described in this thread is to take the eyepiece out (leaving the blocking filter in place) and observe for the brightest and most uniform appearance of the etalon(s) - not the "sun's disc." This is what I do with my double stacked tilt-tuned Coronado filters - and yes they therefore show the central obstruction that you wouldn't see with the unobstructed Lunt etalons. It's my estimation that you're basically illuminating the etalon(s) with collimated H alpha light, and observing when you are centered on the H alpha line, and how uniform the etalon gap is as well. YMMV. 

 

I was paying close attention today and note that some features on the sun are more visible in certain parts of the eyepiece's fov. Is this indicative of being off band, or just a reality of the way the etalons work?

I'm assuming you have the double stacked internal etalons, and yes it sounds like the "off-band" area is what you're likely seeing in known as the Jacquinot ("sweet") spot, which is fairly common with collimator-based internal etalon telescopes, and made worse with double stacking internal etalons. See here and here

 

It also can indicate your etalons might be not thermally stabilized, or have another issue altogether that's where this etalon uniformity test comes in. Narrowband solar filter systems are a world unto themselves, you'll need to get experience in solar observing to know what is good and not so good.

 

... does it make sense to completely depressurize my 2nd etalon, use your technique to get on band with the first one, then try a second iteration of your technique to get on band with both etalons pressurized? Or is it necessary to take the scope apart, remove the 2nd etalon altogether, then tune the first one? Or, should I just attempt to create the most uniform image with both etalons in place?

With my tilt-tuned front (external) DS I leave both in place and do the primary etalon first, then adjust the second.

 

With your pressure tuned internal DS, I would as a beginner opt for removing the secondary DS etalon and try the single stack primary etalon first, then add the DS etalon and see how that behaves. After a while leaving both in place and tuning will become easier if not intuitive. At some point you may even want to opt for ditching the internal DS etalon and go with a front mounted 100 mm etalon for double stacking, which will give you better contrast uniformity with the internal primary etalon.

 

Let us know how it works out. waytogo.gif

 

Addendum: I personally highly value contrast uniformity extended enough to include at least the entire solar disc - at least for a 100 mm and smaller aperture telescope. When moving the Sun's disc off-center you might see some detail change off-band, but there should be a region fairly close to the center where the entire disc of the Sun is on-band. If this is not the case, you might want to give Lunt a call.


Edited by BYoesle, 14 March 2021 - 08:59 PM.


#42 starzonesteve

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Posted 14 March 2021 - 11:29 PM

Thanks, Bob. There is a lot to mull over here.

 

One technical question. When you look for etalon uniformity without the eyepiece in, do you place your eye directly over the blocking filter as you would if there was an eyepiece in place, or do you pull your eye back a bit? When I was attempting this it appeared to be necessary to have the sun centered and when I moved my eye around there was a variety of ghost images, which made the observation less than straightforward for me.



#43 BYoesle

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Posted 15 March 2021 - 01:00 PM

Hi Steve,

 

I would keep my eye centered - us a peephole collimating eyepiece or a small de-lensed old eyepiece. Due to the close proximity of the refocusing lens, it might be necessary to view from farther away to get a clearer view of the etalon itself.

 

Again, you will have to see how well this works for you and your particular instrument and its filter arrangement.


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