Lunt 50 mod on a TV 102 iis?

[vineyard]

Hello,

I'm not sure whether this is the right category to put this in on SGL but it seems the closest.  Anyway, I think I have managed to make a mod that allows me to put the etalon of my Lunt 50THa on the back end of my 102mm refractor, and hey presto I think I have a 4" solar scope?

I'll break this post into two sections - the first is the mod itself, and the second is the journey that got me there (which was very interesting, and also entailed seeing first hand some pretty scary stuff!).  This way, if anyone is only interested in the mod, they don't need to bother reading the rest .

I should preface this by saying that I’m not an optics expert (so some of the reasoning below may be wrong, although it seems to have led to a place that works?) and of course the standard disclaimer: this is not a recommendation, I am not an expert, modify at your own peril!

THE MOD

Basically, if you unscrew the tube on a Lunt 50, you see the back bit (which has the ERF and behind that the etalon, the diagonal with the blocking filter etc).  The back bit has a male M68 thread.  So I cobbled together various adapters to create a combined adapter that threads on to that male M68 and then steps down to a 2" nose with a female thread.  Into that I put a UV-IR filter, and the nose can then be inserted into my 4" tube (like a normal 2” diagonal would).  And that's it.

So the final configuration has a 2" UV-IR filter, and then behind that the ERF that comes fitted on the Lunt 50 (and behind that the etalon, and then the blocking filter in the Lunt diagonal).  And the advantage is that I can use the better focuser on the 4” rather than the horrible helical on the Lunt.  Photos are attached.

I don't think it will work with every 4" tube - in particular I think this mod may need tubes that are designed for BV'ing (see the next bit for more on this).

THE JOURNEY

I got my hands on a barely used Lunt 50THa last year.  And after I managed to figure out how to see the Ha features ("burping" & greasing the tuner, putting a t-shirt over my head  ) and also how to find focus with an ASI camera (on which there is a separate thread elsewhere on SGL - @Rusted @Nigella Bryant @Pete Presland were particularly patient & helpful, thank you again!) I was so taken by what I saw that the 50mm aperture left me wanting more.

So the topic of PST mods came up, and @Peter Drew & @Merlin66 kindly picked up my open posted Q and were v helpful in sharing how those work, and the things to keep in mind (thank you both!).

While looking into that, and looking at the mounting cost involved with a donor scope, an ERF, a used PST etc, the Q came up in my head of whether my Lunt could be used instead of a PST.  Clearly I wasn’t the only one thinking of that b/c google then took me to this v useful thread on Solarchat.

Looking at that thread, and the f-ratios and the distances involved from the focal point to the sweet spot, started to get me cautiously excited.  Mostly b/c my 4” refractor is an old & now discontinued TV102iis.  This was designed for BV’ing and so the tube is actually shorter than a normal TV102 (to use it for monocular viewing, you have to attach an extension into which the diagonal is then inserted).  And the amount of travel on the focuser meant that I reckoned just using it as the stubby tube in BV configuration might allow the etalon unit to still find the sweet spot without having to physically change anything (since I am not going to take a hacksaw to my TV!).

So I knew I would have to make a “nose” that could thread on to the Lunt back-end (with its white tube removed) and then be inserted as if it was a diagonal into my refractor.  While I suspected that I would be able to find focus, the next big thing was energy rejection.

My ideal situation would be a front-mounted ERF.  But they are so expensive (of course its worth it when its your eyesight you’re talking about, so while I was prepared to go down that route, I kept looking to see if there were alternatives).  There was the possibility of internally mounted (ie smaller & cheaper) ERFs but I didn’t want to have to physically delve into the innards of my OTA every time I wanted to use it (I wouldn’t trust myself with the inside of my scope!).

And then I realised that the Lunt 50 has an ERF already in front of it.  However, I wasn’t prepared to just rely on that.  With 4x aperture area, and therefore at least 4x energy being brought to focus on the ERF (actually I think well in excess of that given the longer optical train and hence more convergent beam?), I wasn’t sure what the comfort margin in the Lunt ERFs energy density tolerance might be – and also I wasn’t sure whether that might degrade them faster.  This thread about the energy levels was also v helpful – and salutary about the need to be v careful (not only did opinion seem divided but the energy levels could also be quite high!).

And then I found this post which talks about adding a UV-IR filter in front for apertures above 80mm (obviously not to be used just by itself).

That got me thinking about adding a UV-IR filter to the nose I was already thinking about.  I chatted w Simon @ Widescreen Centre (since they sell Lunt ERFs and I wanted to see if I could find out specs of their transmission rates & energy tolerances).  When he realised why I was asking, he suggested I chat w Gerd Neumann at Astronomik.  So I called him up and described what I was thinking.  He was v helpful & approachable (this hobby is so great for that btw) and reckoned that that double combination of a UV-IR and the ERF ought to be sufficient (although it was of course on me what I did).  We also chatted about ideally what size image should be falling on the filter (since if it was too small, the intensity on the filter would be higher with all the Qs that raises, and if it was too broad, then the actual nose would start heating).  Interestingly the ideal size he independently recommended was about what the image would be if the focuser was completely in (ie the risk of the nose being heated was not there).

So I thought, ok I will try that and test it (worst case, I can use the filter for my night-time viewing!).

So once all the adapters & filters arrived, it was just a case of waiting for a sunny day and then – with a meat probe thermometer from the kitchen drawer also at hand – getting started.

First off, I just used the Lunt 50 as it is – took the diagonal out and used a sheet of paper placed at the focus of the image.  Measured the temperature of that image – a reasonably steady low 20s C.

Then I pointed the 4” at the sun (with all its back end taken out) and did the same thing.  Making v-e-r-y sure that I was not anywhere near looking through the tube!  The image on the paper was so intensely bright that it hurt even looking at it.  And the temperature readings were s-c-a-r-y.  Within 2-3 seconds, the temperature was in the high 70s C and still rising fast.  The thought of that happening to a retina just does not bear thinking about.  Those warnings about never looking at the sun through a scope are not idle!

Then I put the nose in with the UV-IR filter mounted.  And did the same thing w temperature reading.  It rose more slowly but was still getting to the low 50s C.  So those warnings about how these UV-IR filters should not be used by themselves to look at the sun are also not idle!

Then I threaded the nose on to the back bit of the Lunt and put that into the refractor.  With the diagonal out, I found that I could get an image to focus on the paper (yes!) and when I measured the temperature, it was stable and in the low 20s (yes!).

So the next thing was the moment of truth, of putting the diagonal on it, and looking through it (gingerly) myself.  It was lovely.  You could only see part of the disk (not the whole disk as with the Lunt 50 OTA).  I was running out of time before having to go to work, so I took some quick photos w/o any barlows etc (I also hadn't seen @MalVeauX handy tutorial video at that time, so I didn't capture all the right data on the prominences, and my processing is also clumsy - it was about 70 lights (not even a movie) stacked with AS!3 and then played around v-e-r-y clumsily w Registax, GIMP and Lightroom).

Anyway, sorry for what has turned into a v long post.  I will defer to the expertise of others on whether I’m doing something stupid or insane (or things to be careful about with regards to regular use).  There are some tweaks I plan on doing, but I’m feeling pretty comfortable about the setup – there’s a UV-IR filter followed by the Lunt ERF and then also the blocking filter in the diagonal (which I guess is an aperture stop rather than an energy rejector so I'm not relying on that), and the temperature readings were also reassuring.

I’d like to say a huge thank you to everyone who helped patiently with all my various questions on this rather unexpected journey.  What I like about where this has led to is that I can use the Lunt as a travel solar scope or for whole disk viewing, and then swap it in to the larger 4” for closeups (and more resolution?), while also being able to use the same 4” for night-time viewing.  A bit like the modular scopes that Lunt are now marketing for the price of a used car 😂

Cheers all,

Vin

 

 

Edited February 23, 2020 by vineyard
typo

[Nigella Bryant]

Brilliant Vin, many thanks for this write up. Sounds an interesting project,especially considering the cost of a 100mm lunt being astronomical, lol. I also wonder if you can double stack? 

Edited February 23, 2020 by Nigella Bryant

[Solar B]

Looks outstanding ☀ well  done , give me a day and I'll read it all and get back 😊

Brian 

[Rusted]

It is good to hear you were so careful and avoided testing it visually before confirming the temperatures involved in your mod.

The image looks very promising if you don't push it so hard in processing next time.  It is grossly over-sharpened.

 

Just as a gentle warning that you should never, ever take solar viewing for granted:

In my last 6" PST mod the emerging beam at the focuser was HOT after the 90mm internal D-ERF!
I had serious problems with my ASI 120MC camera using my old 6" Celestron PST mod.
It may even have permanently damaged my camera from tracking the sun for hours on end.
There is a persistent, translucent "mask" superimposed on the camera's video image.
The "mask" looks just like solar surface texture at the same scale I see on the monitor!

I had no plans to ruin another camera. So I have just built a new 150mm H-alpha OTA using the PST etalon and a new iStar f/10 objective.
Not only did I use a full aperture Baader D-ERF, this time, but I added a 2" KG3 filter from Beloptik and a 2" Baader CCD.
Both filters are placed before and intended to protect the PST etalon from UV/IR overheating with the 150mm clear aperture.
The sun is sulking behind the clouds so I have nothing to show for all my efforts and expense so far.

Take nothing for granted with solar telescopes!  Let's be very careful out there!!

[Stu]

17 minutes ago, Rusted said:

Take nothing for granted with solar telescopes!  Let's be very careful out there!!

Yes indeed, especially when modding them yourself! Nice mod Vin, interesting read and as Rusted says, glad you were cautious before looking into the eyepiece!

[Solar B]

Rusted .. sorry to hear of your over heating issues ... I must confess that I'm surprised 

your internal 90mm D-ERF did not cope with the heat even at 150mm with which I 

have experience of but glad you've opted for the safest option of a full aperture filter

given it's advantages of no internal OTA turbulence ect

Brian 

[Rusted]

I remember burning my hand when left in the red focused beam emerging from the empty focuser.
The D-ERF was obviously not blocking the heat as expected.
If that was true then the unprotected etalon was being heated by the beam.
I double checked the arrow on the edge of the D-ERF but there was no error.
Perhaps it was fortunate that I stopped observing visually and concentrated entirely on imaging.
I never felt any discomfort but the retina has no pain sensors.

[Solar B]

I'm still surprised as the D-ERF is still highly regarded , I hail from the days of 

the C-ERF but I know your not imagining your experiences so yes indeed 

Take nothing for granted with solar telescopes!  Let's be very careful out there!!

Wise words !

Brian 

 

[Macavity]

Nice work. It takes a while to work out all the threads involved. 😎
I took the Belt & Braces approach with a Front (too expensive! lol)
90mm Baader D-ERF for my ED80 / Lunt 50 Frankenscope thing...

I assume there are no doubts re. the *Baader* D-ERFs? But then 
the chip stays cool enough and I don't use it for visual anyway. 😏

Edited February 24, 2020 by Macavity

[Peter Drew]

As they say, YMMV, but I've never noticed any overheating problems using Baader D-ERF filters internally or externally with no other filter additions..  Working at F10 as PST mods do, there is not so much focused heat concentration as with faster F ratios.  Regarding the internal heat induced turbulence due to internal ERF's often cited as a big disadvantage, again I've not been aware of a problem and see this as no different between using a H Wedge in which the full solar heat enters the tube before any filtering.  Users report the best views using this method including elusive features.    😀  

[Solar B]

Using a full aperture ERF is more of an advantage for imagers as there would 

undoubtedly be some internal OTA turbulence during extended imaging sessions  

with just an internal one.

 

[Merlin66]

The issue of "burning lenses" has come up before.

I prepared a spreadsheet of the "energy balance" across a modded PST ( and any other telescope used for solar).

The "do not exceed" energy intensity, after much discussion, seems to be around 10 Kw/m^2.

The Baader D-ERF is good at what it does, just passing 27% of the total solar energy.......

 

Solar ignition.zip

[Rusted]

Whats this "solar energy" you're on about? We don't all live in perpetually sunny Oz!

Once we get any sunshine [at all] I shall be measuring temperatures furiously after both D-ERFs with the tools available to me.

A laser guided, handheld, thermal "gun" and an inside/outside thermometer on a lead.
I wondered if I shouldn't bond the "outside" sensor to a scrap of blackened copper foil.
I suppose it's only a matter of time before any thin piece of metal heats up in the focused beam.
The sensor is encapsulated in plastic so won't tolerate direct heating.

I can live with losing the 120MC to the eternal sunshine but losing the ASI174 would be too heavy a loss. 
Perhaps it is only the protective filter which is causing the mask effect on the ASI120? I'll have to look into that.

[Macavity]

Good stuff folks! "Standing on the shoulders of Giants" (Solar Forums!)
some pictures of my Lunt 50 / ED80 "Franken Scope" FRONT ERF cell. 😁

ONE DAY I might get my "Wood Technology prototype" (Hey it works!)
*machined properly* by local CNC firm... LIVE to see more Sunspots?! 🤔

The basic thing. "High precision" hardboard rings + "Tilted" Baader ERF!

The "outer" Cell:



I "sawed a bit off" my (FIRST EVER scope) *ST102* Dew Shield.
It fits the ED80 as well! (Thank you, Skywatcher "standards") 😛

This is the "Sandwiched" Cling Film cell, I ADD on to take the "Flats":



ED80 / Lunt 50 / Barlowed 2x (If I remember rightly) "Frankenscope".

If I am "spared", I might use the 90 mm D-ERF as an intermediate filter
in a BIGGER (than 80mm) Solar Frankenscope? Take care / HAVE FUN! 🥳

Edited February 25, 2020 by Macavity

[Nigella Bryant]

Brilliant Macavity, impressed

[vineyard]

Wow, fantastic - how big do you think the Earth would be compared to that feature?

[Macavity]

43 minutes ago, vineyard said:

Wow, fantastic - how big do you think the Earth would be compared to that feature?

It was pretty SMALL spot? This was the Full Disk: "12 May 2019"... according to File date!
Ages since I dragged out the Solar scope. Hang on in for the next Solar Max, Folks?!? 😛

Edited February 25, 2020 by Macavity

[daslolo]

On 23/02/2020 at 06:57, vineyard said:

 

I've seen this cellular structure before, it's an image processing artifact, what settings were you using?

Edited February 26, 2020 by daslolo

[vineyard]

That'd be intriguing.  I just panned round the disk taking shots of various bits, and the photos of this section of the disk were the only ones it appeared in (I processed the other sections too just to see if there was anything).  I do think I remember seeing the feature in the EP before I put the camera in, but maybe its a fake memory 😂

Re the setting, the camera (ASI178MC)  .txt file shows:

Bin = 1
Brightness = 10
Capture Area Size = 3096 * 2080
Colour Format = RAW8
Debayer Preview = ON
Exposure = 1926us
Flip = None
Gain = 134
Hardware Bin = OFF
High Speed Mode = OFF
Mono Bin = OFF
Output Format = *.PNG
Raw Format = OFF
StartX = 0
StartY = 0
 

The stacking was done with AS!3, and then I was testing out Registax, before playing with layers in GIMP and then Lightroom.  I'm afraid I can't remember the various curves & levels settings, b/c I was just going visually off the screen.

Looking fwd to finding out more (if you look along the bottom edge, there's also a slightly different textured dimple in the surface - although it is horribly oversharpened as Rusted says, and in less sharpened images its more pleasing to see - could that also be an artefact?)

Cheers,

[Peter Drew]

I think the real data is there but being masked by the processing regime.   😀

[vineyard]

1 hour ago, Peter Drew said:

I think the real data is there but being masked by the processing regime.   😀

That wouldn't surprise me Peter b/c I have no clue on the processing side  😂 ...slowly slowly trying-to-learn by doing (at the v least it passes the time & keeps me away from mischief 😄)

[Rusted]

Vin,

If you can bung a much less processed image up on here we can all have a play to see what can be done.
What does the image look like straight after AS!3? Before you "break it" in your later software.
Less is more. As people keep telling me!   If it's already overcooked after AS!3 then it may be beyond help. 

[vineyard]

Hi Rusted, I'm afraid I cleaned up my hard drive recently to create space, so the original image files of that particular one are somewhere on my external backup (all I have on my laptop of that image now is the jpg).  I'm not too sure how to extract only some files from a Time Machine as opposed to restoring the whole thing, and I don't really want to rewind my whole laptop back & lose all the cleaning up I did!

However, there was another batch of images I took that morning which I have not yet gotten around to doing anything with.  It's of pretty much the same quadrant of the sun, and when I randomly opened one image in LR and ramped up some balance levels, the same prominence does appear.  So I can offer up those originals in the dropbox linked below, in case that would be helpful?  Its only about 30 PNG files (vs the about 70 I think there were for the image earlier in this thread).  I've also included two sub-folders in that dropbox with some darks & biases I took that morning - unfortunately I didn't take any flats b/c I wasn't sure how to do that for an Ha scope 🤔

Cheers!

https://www.dropbox.com/sh/s5s0ibkhmdqfbnd/AAA6CP4--E6_BG0hMm7XxUAma?dl=0

[Merlin66]

Vin,

Just downloading your files....

PNG is not a good format to start with - TiFF or Fits would be better.

Normally you would use a fast frame camera and record in AVI or SER, then process the video through AS3! etc.

Keep you posted.

 

[Merlin66]

Vin,

OK I now see you've used a DSLR (?) for imaging.....

Unable to get a good stack from AS3! - it shows multiple artifacts.....

Edited February 26, 2020 by Merlin66