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Sweetspot - what's "normal" on a Lunt 60?

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#1 dswtan

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Posted 25 May 2019 - 02:51 PM

I have a Lunt LS60 PT purchased used here on CN a few years ago, and while it's basically fine, beautifully made, and seems well cared-for, I've always wondered if the sweetspot is optimal -- and if not, can it be improved, e.g. by Lunt repair.

 

I contacted Lunt a while back, but no response -- I assume they don't want to support used items unless it's maybe a repair request, which is fair enough. However, I'm still curious if the sweetspot performance I'm seeing is "normal". It seems narrow/small to me, but the only H-alpha experience I had before this was on a little PST. I do have a Quark now, but I'm still getting used to that and I'm not sure that's a valid comparison anyway. 

 

So here's a schematic of the "eye shape"/lenticular form of the sweetspot I'm experiencing -- which moves laterally (per arrows) when I adjust the pressure tuner. This shows the approx proportionate size of the sweetspot I'm seeing and the solar disk.

 

You'll note it's a lot smaller than the full disk. I see people post pictures of clear whole disks all the time, but I can only ever get part of the disk clear with my setup. Is this normal? Are the whole disk people just getting lucky with large sweetspots? Could mine be improved? Thanks for any insights!

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#2 descott12

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Posted 25 May 2019 - 04:11 PM

 

 

So here's a schematic of the "eye shape"/lenticular form of the sweetspot I'm experiencing -- which moves laterally (per arrows) when I adjust the pressure tuner. This shows the approx proportionate size of the sweetspot I'm seeing and the solar disk.

 

Steve,

Great picture. That is pretty much what I see with my new Lunt 50 but I couldn't articulate what I was seeing exactly. The picture is almost exactly what I see.  

 

Previous posts regarding this have been somewhat contradictory in that some will say that all scopes will have a sweet spot that doesn't cover the full FOV and others have said that sweet spot should cover most or all of the disk.


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#3 Eddgie

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Posted 25 May 2019 - 04:41 PM

One of the things that the early "Hype" for the pressure tuned Lunt 60 proclaimed was that it would be better about having an even band across the surface of the sun.  I read half a dozen reviews that proclaimed this.

 

My direct experience was that this was not true for every Lunt 60.  My 60 was purchased new.  I did call Lunt about it, and was told that this condition (not having the same tune across the field) was indeed normal and that the Etalons were optimized as well as possible to have the biggest possible in-tube part of the field.

 

Now Lunt offered to take it back to check it out, but said that if no actual defect was found, it would be returned.  I took this to say that "This is normal, and you should not expect a complete in-band view of the solar disk."

 

To date, the only scopes I have ever owned that showed a complete in-band sun was a PST (actually, 2 of the 3 PSTs showed an entire in-band disk).

 

My Meade 60 DS is pretty darn close to showing a full in band disk.

 

My Quark was the worst.. So bad in fact that I sent it back to see what is going on.  

 

Again, clearly a YMMV kind of thing I think, but based on my own ownership of the Lunt 60, this would appear to be a normal condition. 


Edited by Eddgie, 25 May 2019 - 04:45 PM.

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#4 TOMDEY

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Posted 25 May 2019 - 04:49 PM

Howdy! I've had three Lunts, all double-stacked. What you are showing there is pretty characteristic of mine. It takes about fifteen minutes on the sun before mine stabilize for wavelength and field... seems to me... And some days it seems better than others.

 

And, especially if you are doing double-stacked, the Doppler effect from the rotation of the sun and approaching/reseeding features... can masquerade as a faux ~sweet spot~ or at least further confuse things.

 

And, finally, the typical laptop display is angularly non-linear, so that the bottom of the screen exhibits lower contrast than the top, and the top can wrap-around, where blacks become white, etc. Lot of stuff going on there!

 

I believe that some of the very expensive DayStar filters are certified for large, precise field... effectively free of that sweet-spot mumbo jumbo. Those are their top of the line filters. I think they call them professional or research-grade or some such. You could easily spend $10k for one good etalon... just the etalon.

 

Here are a couple pics taken with mine (Lunts)... The sweet spot is sort of horizontal and centered. These are simple single exposures with very little processing. I avoid over-sharpening, as is generally favored among CN imaging folks... makes the sun look like a plate of spaghetti with ringing and lateral phase flip artifacts, that look like structure.     Tom

 

 

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#5 TOMDEY

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Posted 25 May 2019 - 04:49 PM

and this

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  • 124 solar H-alpha Lunt Tom snapshot.jpg

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#6 spacemunkee

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Posted 25 May 2019 - 04:49 PM

While mine is a tilt tune, I get the same, though only really pronounced when imaging and watching on screen as the brightness moves across disk when adjusting. But I just nail it as even as possible and still seems to get good detail across the whole disk.
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#7 TOMDEY

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Posted 25 May 2019 - 05:21 PM

And having had three Lunts (six tuners total) --- I have noticed that each one is different. PS: when you go double-stacked, rotating one tuner relative to the other can also help balance things out. Gets pretty complicated... takes quite a few sessions to begin to get an intuitive feel for getting the most out of the system. And sometimes the visual best seems to be different than the imaging best.

 

I've always wanted to have the top performance DayStar, but the cost is pretty prohibitive, even for us obsessive amateurs. I guess, as in most everything --- you get what you pay for!    Tom


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#8 Eddgie

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Posted 25 May 2019 - 06:25 PM

Yeah, on my Meade Solarmax II, there is indeed some improvment DS vs SS.  If I am careful, I can get the entire disk in band but not the disk and the prominences.  To see the prominences, I have to slightly pan the scope,  That is fine though because normally I put the Proms dead center when I want to examine them.   

 

Anyway, it appears that getting a full disk sun that is all in exactly the same tune is not common with a single stack, but maybe with the DS, since you are going narrower, the difference in band may be harder to see?  Don't know, I just know that with care, I can tune the DS for a pretty much full in-band disk. 


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#9 rigel123

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Posted 25 May 2019 - 10:07 PM

My tilt tune Lunt 60 has a Sweet Spot that covers 2/3 of the FOV and I can get the entire disk inside that area. I just got the DS module this year, but I have always been able to fit the entire disk inside the Sweet Spot with my zoom eyepiece at 25mm.
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#10 KarlL

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Posted 25 May 2019 - 10:33 PM

Call Faye at Lunt.


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#11 dswtan

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Posted 30 November 2020 - 03:23 PM

Epilogue from OP, and kudos to Lunt -- though original problem remains on my used LS60T: 

 

A year on, the not-so-sweet spot continued to bother me and the sun was starting to get active again. So I re-contacted Lunt for them to take a look, and address any other matters. They seem to have a clearer statement about servicing older scopes on their site now.

 

I got a response, and after a while it turned out that there was a technical issue in our email communications before -- I have an older ISP (Earthlink, no laughing please) email address that was bouncing their mails. We moved to Yahoo, and bingo, we could communicate! (Phone was not convenient.)

 

To summarize, they replaced the blue filter (a standard low-cost item on these scopes I understand), and all well on return. Sweet spot is still basically the same, but I am enjoying the scope again especially with new excitements like AR2786 happening now.

Great interactions with Faye and Benjamin at Lunt! Really nice experience. I look forward to upgrading to a nice LS80 one day when funds allow. Thanks Lunt, and the folks in this thread. waytogo.gif


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#12 james7ca

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Posted 01 December 2020 - 02:45 AM

I'm glad you updated this "old" thread as I was having uniformity issues with my Lunt LS50 that I purchased a few years ago (circa 2017). I actually spoke to a Lunt representative at a local astronomy fest shortly after I purchased my LS50 and he told me that the LS50 wasn't designed for photography. But, I've actually gotten some passable images although the full-disk performance seems poor (my "sweet" spot may be one third of less than the full disk when using a Skyris 445M camera, sensor size 6.26mm x 5.01mm at 1280x960 pixels).

 

Today I experimented with rotating the scope to see if I could take two sequences and use the best half of each to produce a better image. The combined image ended up having fairly sharp looking and uniform edges/limbs (top, bottom, left, right) but the center of the sun turned out very soft and somewhat ugly looking, so I may have to try both repositioning, retuning, and scope rotation to produce a uniform full disk image (by combining several different sequences).

 

I also do DSO imaging and what I've found is that very few samples of any scope really do that well over an "extended" field (even when they are marketed as flat field or when used with a corrector/flattener). Thus, those who actually end up with a good performing scope are indeed very lucky. My rate of finding a "passable" scope seems to be less than 70% and I don't think I have yet gotten a really good sample, a few that were okay or somewhat better, a few more with real issues but which were still usable -- meaning "passable", and then a few that were outright rejects (two of those were returned, but I still have one scope that was unusable for astrophotography and for which the dealer told me "I wasn't buying a Takahashi" -- so that scope is now more useful as a door stop than for anything like imaging). frown.gif


Edited by james7ca, 01 December 2020 - 02:48 AM.

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#13 lakeorion

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Posted 01 December 2020 - 07:50 AM

Me too.  I had dreams of saving for a small Ha scope and taking pictures.  Looks like those dreams will be put on hold until there is a paradigm shift / new tech available.




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