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Tilt tuner or Pressure tuner ?

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#1 paulsky

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Posted 28 January 2020 - 02:53 PM

 Hello,

                                  
Perhaps it is a topic too treated, but does there really exist a difference between the visual image of a tilt tuner solar telescope and another that has a pressure tuner?
How does the use of one system or another affect the solar image that we appreciate through the eyepiece?
But if there really wasn't much difference why Lunt has opted for the pressure system?
How many of you would choose a solar telescope depending on whether it carries one system or another?

 

Best regards

Paul


Edited by paulsky, 28 January 2020 - 02:54 PM.


#2 MalVeauX

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Posted 28 January 2020 - 03:08 PM

Hi Paul,

 

I think it has to do with uniformity of being on band and shape of the sweet spot, favoring the pressure tuner.

 

Very best,


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#3 rigel123

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Posted 28 January 2020 - 03:22 PM

I honestly can't say as I have only viewed through my tilt tuned Lunt, which gives excellent views.  Lunt still has some tilt tuned 60mm scopes on sale at the moment I am assuming until they run out and then will only offer the pressure tuned.  According to most opinions the pressure tuned wins on uniformity as Marty mentions although I don't notice any variation in the image in my single stacked scope and the sweet spot is large enough that the full disk fits in nicely and zoomed in there is no sweet spot.

 

The only less than positive comments I have seen on the pressure tuned scopes is issues with the O rings that some people have had to replace due to the pressure leaking out but it does not seem to be a major problem.  


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#4 jwestervelt

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Posted 28 January 2020 - 05:33 PM

A tilt-tuned etalon that is mounted before the objective theoretically should have better performance in terms of a sweet spot since the sweet spot is a function of incidence angle.  With the etalon being the first optical element, and with the sun's distance, the incidence angle will likely be 0 degrees, +/- the tilt of the etalon itself. 

The downside of the tilt-tuned etalon is that the sweet spot is a linear gradient across the image, aligned with the tilt axis of the etalon.  A pressure-tuned etalon mounted before the objective would be superior in this regard, however most PT etalons are smaller than the objective and mounted somewhere behind the objective where angle of incidence becomes a function of focal ratio.  The sweet-spot in this case is circular, and can be large enough to adequately cover the entire disc of the sun, making it a non-issue.

PT etalons are sensitive to temperature and local atmospheric pressure.  Once tuned, atmospheric pressure changes wouldn't affect performance since they are sealed, but temperature changes will result in pressure changes due to the ideal gas law.  Luckily this will only require minor tuning during a session as the etalon comes up to temp.  PT etalons, as Warren stated, require a bit more maintenance with regard to lubricant and o-rings, but i haven't seen that many reports of o-ring failure, and replacement parts are cheap.


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#5 bigdob24

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Posted 28 January 2020 - 07:12 PM

I’ve observed thru both and have to say visually no difference.

PT is easier to use in my opinion and tilt is no big deal easy also.

As far as the o rings , if they are maintained with grease “Dow Corning High Vacuum Grease”

is what Lunt recommends, they should last a good long while . Doesn’t take much grease either, make sure they don’t dry out.

BD



#6 NC STARGAZER

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Posted 29 January 2020 - 12:29 AM

I bought my LS60 with a tilt tuner, and I really didn't like the so called "sweet spot." 
It's not sweet at all. It is more of a bright spot smack dab in the middle of the sun.

 

If you move the sun to the edge of the field, you can see that the surface closest to the center is brighter and the surface closest to the edge is darker.

Lunt advised that I purchase a pressure tuner and I did. It was a big improvement visually, but the bright spot still existed in my images,

which I can process out, but is a pain, so I just use my Lunt for visual use now.

John



#7 wrnchhead

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Posted 29 January 2020 - 08:24 AM

I only have experience with the PST but my replaced ITF removed the sweet spotting effect of the internal etalon for whatever reason. I get even views across the whole sun. In double stack mode the external etalon definitely gives a linear stripe which can be moved around by the action of tuning or tilting but it is definitely not a full view effect.

#8 rigel123

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Posted 29 January 2020 - 09:14 AM

I bought my LS60 with a tilt tuner, and I really didn't like the so called "sweet spot." 
It's not sweet at all. It is more of a bright spot smack dab in the middle of the sun.

 

If you move the sun to the edge of the field, you can see that the surface closest to the center is brighter and the surface closest to the edge is darker.

Lunt advised that I purchase a pressure tuner and I did. It was a big improvement visually, but the bright spot still existed in my images,

which I can process out, but is a pain, so I just use my Lunt for visual use now.

John

Almost sounds like the tuning isn't quite on.  In my LS60T when viewing with the Zoom eyepiece at 25mm, the area of the sweet spot can be put in the center of the FOV and it covers 3/4 of the entire area, plenty for the sun to be evenly illuminated across the disk when in the center of the FOV.  As I move the sun to the edges you can see the disk start to brighten and lose contrast on that limb going towards the edge of the FOV, then moving it across the opposite direction will start to brighten that limb going out of the FOV, but as long as the sun is centered it is well within the "sweet spot"  I made a drawing of what I see (crude at best) but it outlines how I find the "sweet spot" to look like in my set up.  Of course when zoomed in so that the sun is totally covering the FOV there is no actual sweet spot unless I adjust the tuner, which I haven't had to change in 7 years since first tuning it.

 

Telescope-view.jpg


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#9 drprovi57

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Posted 29 January 2020 - 11:23 AM

Almost sounds like the tuning isn't quite on.  In my LS60T when viewing with the Zoom eyepiece at 25mm, the area of the sweet spot can be put in the center of the FOV and it covers 3/4 of the entire area, plenty for the sun to be evenly illuminated across the disk when in the center of the FOV.  As I move the sun to the edges you can see the disk start to brighten and lose contrast on that limb going towards the edge of the FOV, then moving it across the opposite direction will start to brighten that limb going out of the FOV, but as long as the sun is centered it is well within the "sweet spot"  I made a drawing of what I see (crude at best) but it outlines how I find the "sweet spot" to look like in my set up.  Of course when zoomed in so that the sun is totally covering the FOV there is no actual sweet spot unless I adjust the tuner, which I haven't had to change in 7 years since first tuning it.

 

attachicon.gifTelescope-view.jpg

This post of how to tune the sweet spot is very valuable.  I do have a question, in your post you mention not having to adjust the "pressure tuner" for 7 years.  I tend to release the pressure from the tuner after each session and then have to re-adjust the pressure tuners again for a new session.  I have a Lunt DS LS80 scope and tuning both stacks can take 15-20 min.. I  mailing only do imaging.  QUESTION:  Is it ok to leave the Lunt pressure tuners in their "calibrated - sweet-spot" position of days, weeks, months, years?  I have read conflicting reports

 

THanks

Jason



#10 rigel123

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Posted 29 January 2020 - 09:04 PM

This post of how to tune the sweet spot is very valuable.  I do have a question, in your post you mention not having to adjust the "pressure tuner" for 7 years.  I tend to release the pressure from the tuner after each session and then have to re-adjust the pressure tuners again for a new session.  I have a Lunt DS LS80 scope and tuning both stacks can take 15-20 min.. I  mailing only do imaging.  QUESTION:  Is it ok to leave the Lunt pressure tuners in their "calibrated - sweet-spot" position of days, weeks, months, years?  I have read conflicting reports

 

THanks

Jason

Mine is a tilt tuned (sorry I didn’t make that clear) so I can’t comment on how long you can leave the PT.  My comment was in response to John, (NC Stargazer) comment about the sweet spot just being a bright spot in the middle which is not what I see in my tilt tuned scope that once set holds its tuning unless I accidentally rotate the tuning knob.


Edited by rigel123, 29 January 2020 - 09:06 PM.


#11 mikerepp

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Posted 30 January 2020 - 03:40 PM

Yes you can leave the pressure tuner set.  The next time you use the scope it should be very close so only a slight amount of adjustment is required.   Tuning is somewhat like focusing, to see various parts of a prom may require a tweak one way or another to bring out the best of the feature.  I leave mine pretty much set from one observation to the next.  Periodically you may have to unscrew it all the way and then screw it back in to "burp it".



#12 BYoesle

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Posted 31 January 2020 - 08:57 PM

A tilt-tuned etalon that is mounted before the objective theoretically should have better performance in terms of a sweet spot since the sweet spot is a function of incidence angle.  With the etalon being the first optical element, and with the sun's distance, the incidence angle will likely be 0 degrees, +/- the tilt of the etalon itself.

 

The downside of the tilt-tuned etalon is that the sweet spot is a linear gradient across the image, aligned with the tilt axis of the etalon.  A pressure-tuned etalon mounted before the objective would be superior in this regard, however most PT etalons are smaller than the objective and mounted somewhere behind the objective where angle of incidence becomes a function of focal ratio.  The sweet-spot in this case is circular, and can be large enough to adequately cover the entire disc of the sun, making it a non-issue.

A bit of clarification. An talon is only exactly "on-band" when light rays pass through normal to the etalon plates. But the Sun - being an extended object 0.5 degree in diameter - can never have all its rays pass through an etalon normal to it. When centered on-axis, the central ray will, but the rays from the edge will subtend a 0.25 degree angle, and an etalon placed before the objective will have zero instrument angles and no increase of the field angles. Unfortunately there is no optical system that can change this.

 

Image1.jpg

 

Every etalon has an "acceptance angle" which is by definition one-half the diameter of the "sweet spot," more technically referred to as the "Jacquinot spot." The Jacquinot spot is defined as the field in which essentially monochromatic viewing/imaging occurs, and outside of which the image begins to shift "off band." For an air-spaced etalon of about 0.7 FWHM, the Jacquinot spot is about 1 degree in diameter. Since the limb of the Sun subtends 0.25 degrees off-axis, its "field angle" is 0.25 degrees compared to the center of the on-axis center of the Sun. When such an etalon is placed ahead of the objective, there are no instrument angles or field angle magnification, and the entire disc of the Sun fits well within the Jacquinot spot, and the large size of the spot allows for tilt tuning to be accomplished, and if the etalon is suitably close to being on-band without much tilting, no banding or other untoward effects will be noted.

 

With the smaller pressure tuned etalons mounted behind the objective in a collimator and refocusing lens system, the collimator lens renders on axis rays parallel (i.e. as if at infinity and no instrument angle) and therefore the focal ratio of the instrument is irrelevant. What becomes relevant is the field angle magnification, which just like for an eyepiece, increases the angle of the off-axis rays. It can be seen that if the focal length of the collimator lens is one-half the objective focal length, the field angle magnification becomes 2x, and the field angle for the Sun will subtend 0.25 x 2 = 0.5 degree. Therefore when centered on axis, the Sun's limb will lie 0.5 degrees off axis, and the limb of the Sun will lie essentially on the circumference of the "sweet spot." If you shift the image of the Sun off-axis, part of it will fall outside the Jacquinot spot, and appear as Warren has depicted in his post above. If the etalon is tilt-tuned, tilting may introduce some degree of banding due to the increased tilt of the field angles perpendicular to the tilt axis. Theoretically, pressure tuning would not introduce any banding.

 

It is when the etalon is placed near the focus (usually using a Barlow or a telecentric based lens system), such as with the DayStar and Solar Spectrum filters, the focal ratio is increased, reducing the convergence angles. If the on-axis light cone of the converging rays exceed the acceptance angle, the filters bandpass will widen beyond the on-band acceptance angle. If the on-axis convergence angles are too large, the filter will not be anywhere near its theoretical FWHM - tyically you need to be at a focal ratio of 45 or greater to be close to the specified FWHM. Additionally, if the lens system is not truly telecentric, the off-axis rays may also be shifted in incident angle beyond the acceptance angle, and the field angle angle of the Sun's limb will lie 0.25 degrees beyond the already tilted central rays. Thus the importance of utilizing a true telecentric lens system of long enough focal ratio so as not to exceed the acceptance angle of the etalon for good full disc contrast, uniformity, and specified bandpass.

 

Image2.jpg


Edited by BYoesle, 01 February 2020 - 10:26 AM.

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#13 paulsky

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Posted 01 February 2020 - 09:52 AM

                                 Wow!
Thank you very much, I keep learning !!!

Paul



#14 dscarpa

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Posted 08 February 2020 - 12:34 PM

 My had since 2009 single tilt etalon Lunt 60's performace is  good I've never been tempted to get a pressure tuner for it. The sweet spot is about 80% of the FOV and centered. Tuning is easy and is a set it and forget it proposition. David 


Edited by dscarpa, 08 February 2020 - 12:36 PM.

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