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Tiltable versus perssure tuned etalons

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#1 Helen P

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Posted 24 June 2022 - 09:39 AM

Hello,

 

Another question:

 

The older LUNT models have/had a tiltable internal etalon, the newer ones have a pressure tuned one. (eg the LS60THaDS60)

Do the pressure tuned etalons deliver better performance ?

Just wondering.

 

Helen


Edited by Helen P, 24 June 2022 - 09:41 AM.


#2 MalVeauX

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Posted 24 June 2022 - 10:37 AM

Hi,

 

It will boil down to this... tilting is mechanically easier and has some limits; pressure takes more work, but has better potential for high contrast uniformity. The issue with tilting is that if the CWL of the etalon too far red and you're tilting it to come on band, tilting blue shifts, and if you have to tilt a lot you end up with issues likely with banding and uniformity suffers. The ideal tilt will be minimal tilt, if any. But that's rarely the case unless the etalon was made for your elevation specifically. Elevation matters because of air pressure, so it will have limits in high elevation areas.

 

Overall pressure tuning is the way to go for uniformity and the implementation is pretty easy with how Lunt has it so it's not hard to tune. The biggest issue is you still have tilt tuned front mounted etalons with Lunt because they don't have a pressure version (yet). Currently all of Lunt's pressure tuning is with internal etalons.

 

Very best,


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#3 rimcrazy

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Posted 24 June 2022 - 11:07 AM

I had a Coronado 90 Max II which is tilt tuned and I now own a Lunt LS80MT which is pressure tuned.  From my experience I get far better contrast both visual and photographic on my Lunt then I ever got with the Coronado.  Other I see with photos from others that they seem to get great pics with their tilt systems but I can only speak from what I saw with mine.  


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#4 rigel123

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Posted 24 June 2022 - 11:33 AM

I have one of those older tilt tuned Lunts that I think I got spoiled by as I think it has great uniformity and I really haven't touched the tuning wheel on the internal etalon once I set it 10 years ago.  But I don't think that can be guaranteed for all of the Lunts of that age.  More people will have experiences like rimcrazy where the pressure tuned scopes give them much better results.


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#5 Helen P

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Posted 24 June 2022 - 01:49 PM

Hi,

 

It will boil down to this... tilting is mechanically easier and has some limits; pressure takes more work, but has better potential for high contrast uniformity. The issue with tilting is that if the CWL of the etalon too far red and you're tilting it to come on band, tilting blue shifts, and if you have to tilt a lot you end up with issues likely with banding and uniformity suffers. The ideal tilt will be minimal tilt, if any. But that's rarely the case unless the etalon was made for your elevation specifically. Elevation matters because of air pressure, so it will have limits in high elevation areas.

 

Overall pressure tuning is the way to go for uniformity and the implementation is pretty easy with how Lunt has it so it's not hard to tune. The biggest issue is you still have tilt tuned front mounted etalons with Lunt because they don't have a pressure version (yet). Currently all of Lunt's pressure tuning is with internal etalons.

 

Very best,

indeed, I have a front mounted tiltable dual stack on the Lunt 50. Visually it can be stunning, but photographically I'I'm struggling to get good results and never seem to be able to get the tuning quite right.  I wondered wether that also goes for the internal etalons.


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#6 MalVeauX

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Posted 24 June 2022 - 01:53 PM

indeed, I have a front mounted tiltable dual stack on the Lunt 50. Visually it can be stunning, but photographically I'I'm struggling to get good results and never seem to be able to get the tuning quite right.  I wondered wether that also goes for the internal etalons.

Tuning two etalons to both be on band with best uniformity is not easy. It takes practice to do it both visually or with a camera. The key is to get that first etalon (primary) on band with highest uniformity first. Then add the 2nd etalon and tune it. If you try to just tune both at the same time back and forth, it will be futile.

 

Check out this thread where Bob Y explains some handy approaches to tuning:

 

https://www.cloudyni...-youre-on-band/

 

Very best,


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#7 bigdob24

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Posted 24 June 2022 - 04:21 PM

indeed, I have a front mounted tiltable dual stack on the Lunt 50. Visually it can be stunning, but photographically I'I'm struggling to get good results and never seem to be able to get the tuning quite right.  I wondered wether that also goes for the internal etalons.

I’m guessing your primary etalon on the 50 is pressure tuned?

I thought the process is to tune the primary , add the double stack and adjust the tilt until the red glow reflection is out of the FOV and enjoy!!

Thats how I adjust mine and think I have a close to being on band view. 
I also add some power and look at the AR regions and look in them as fine tuning the PT for the best detail.

‘I’ve never messed with any cameras so don’t know if things need to be adjusted differently.

‘I’ll be watching this interesting thread



#8 Coconuts

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Posted 24 June 2022 - 05:47 PM

Marty: "The key is to get that first etalon (primary) on band with highest uniformity first. Then add the 2nd etalon and tune it. If you try to just tune both at the same time back and forth, it will be futile."

 

That's valuable advice!  I have a double stacked LS80THa, and my process to date has been to burp both to atmospheric pressure, and then screw in the first PT knob until limb features were most visible.  I would then screw in the second PT knob, hoping for an improved image.  I think that I usually find it, but the interaction is uncertain and a bit confusing.  I was impressed the other day, though, when I didn't burp, and just looked in after about four days.  Prominances were just as vivid as the day that the PTs were adjusted.  Those o-rings seem to really hold pressure well!

 

Per your (and Bob's, in the on band linked post) process, I would not have the second etalon in the light path while adjusting the first, and only install it after the first was on band. To do that, I will need a dummy mechanical etalon to let me attach the focuser and eyepiece with just the first etalon in place.  It would be easy to measure its thickness, and Precise Parts is there to make one.  I'll give it a try.

 

All the best,

 

Kevin


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#9 KMH

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Posted 24 June 2022 - 06:23 PM

Marty: "The key is to get that first etalon (primary) on band with highest uniformity first. Then add the 2nd etalon and tune it. If you try to just tune both at the same time back and forth, it will be futile."

 

That's valuable advice!  I have a double stacked LS80THa, and my process to date has been to burp both to atmospheric pressure, and then screw in the first PT knob until limb features were most visible.  I would then screw in the second PT knob, hoping for an improved image.  I think that I usually find it, but the interaction is uncertain and a bit confusing.  I was impressed the other day, though, when I didn't burp, and just looked in after about four days.  Prominances were just as vivid as the day that the PTs were adjusted.  Those o-rings seem to really hold pressure well!

 

Per your (and Bob's, in the on band linked post) process, I would not have the second etalon in the light path while adjusting the first, and only install it after the first was on band. To do that, I will need a dummy mechanical etalon to let me attach the focuser and eyepiece with just the first etalon in place.  It would be easy to measure its thickness, and Precise Parts is there to make one.  I'll give it a try.

 

All the best,

 

Kevin

Kevin,

 

Maybe I'm misunderstanding your "dummy etalon" comment, but wouldn't you just attach the focuser directly to the primary etalon  in the manner of a conventional single stack?  It seems like adding a spacer between the primary etalon and the focuser would prevent you from reaching focus??

 

Kevin



#10 Coconuts

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Posted 24 June 2022 - 06:29 PM

Kevin:  My comment was based on the assumption that the focal plane is a constant distance behind the first etalon, irrespective of whether a second etalon is present or not.  But I don't know this to be the case.

 

All the best,

 

Kevin



#11 KMH

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Posted 24 June 2022 - 07:29 PM

Kevin:  My comment was based on the assumption that the focal plane is a constant distance behind the first etalon, irrespective of whether a second etalon is present or not.  But I don't know this to be the case.

 

All the best,

 

Kevin

I don't know for sure either, but I thought the double stack module would also need to have its own set of collimating/refocusing lenses to create a parallel beam through the etalon, which would push the focal plane back.

How do you like your DSII?  I'm considering one myself.

 

Thanks,

Kevin



#12 Coconuts

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Posted 24 June 2022 - 08:06 PM

Kevin:  I like it a lot. 

 

I bought it used about six years ago, and unfortunately, during a very distracted period after Hurricane Irma roughed up our homes in the VI, I forgot to bring it indoors while under a Telegizmos 365 cover for an extended period.  Afterwards, the scope had changed a lot, and was no longer functional.  The first mediation was to replace a blue glass filter within the blocking filter.  That had become very frosted, which was a recognized aging issue, but replacing it didn't resolve the problem.  So I sent it back to the factory, where they determined that the blocking filter had to be replaced, and some repair was performed on the collimating lens.  The repair charges were modest.  When I got it back, it was working as good as new.

 

I am approaching completion on a much more convenient permanent configuration that will keep the solar scope in a safe place, and yet make solar (and night sky) observation and imaging extremely easy and comfortable.  I will be presenting on this project on Friday, July 29th at Stellafane.

 

All the best,

 

Kevin



#13 hopskipson

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Posted 25 June 2022 - 08:50 AM

Hi Kevin,

 

I got a second-hand Lunt 80 double-stack as well.  It came with a spacer with the single-stack scope serial number.  I use this to get the primary etalon on-band and then remove it to add the double-stack etalon and tune that.  I would think you are missing the Lunt spacer.

 

On a side note, when I sent my double-stack etalon to Lunt for repair, they were kind enough to send a set of thumb screws to replace the Allen key set screws.  This made swapping and tuning much more effortless.



#14 Coconuts

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Posted 25 June 2022 - 12:37 PM

James:  Thanks; I will reach out to Lunt and purchase their spacer.  That makes more sense than getting PP to make one.

 

All the best,

 

Kevin



#15 Coconuts

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Posted 26 June 2022 - 03:41 PM

"James:  Thanks; I will reach out to Lunt and purchase their spacer.  That makes more sense than getting PP to make one."

 

Dawn finally broke on Marblehead...  Ya know what makes even more sense?  Doing nothing.  I was neglecting the fact that the Lunt DSII unit is mostly collimated light, and so while ~ 54 mm thick, it is only 23 mm thick optically.  So all I have to do is refocus!  The Feathertouch focuser on the LS80THa has 65 mm of travel, so as long as I don't fully insert the 2" barrel of the B1200 blocking filter assembly, I can easily reach focus with and without the DSII.  I will explore how much easier tuning is starting with the stock etalon, and only adding the DSII once the primary is on band.

 

All the best,

 

Kevin


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