Lunt pressure tuned etalon for a 150 f/10 for imaging?

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Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by Rusted »

Sorry to be such a bore, but my PST sweet spot ring problems are becoming a serious nuisance and it may fail altogether.

Lunt are unwilling to discuss mods using their equipment. With perfectly sound reasoning regarding safety concerns.

Presumably the smaller and [therefore] cheaper Lunt etalons need to be moved towards the focuser from their original, Lunt, solar telescope positions.

I haven't found a separate Lunt 60mm, pressure tuned etalon for sale anywhere.
Does one exist? It might be almost affordable and usefully larger than the 50mm.

Is the 50mm worthwhile for a 150mm f/10 with a 1500mm focal length? Is it even doable?
Vignetting and size of sweet spot would seem to be potential issues with such a small etalon.

Thanks


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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by MalVeauX »

Hi,

So the Lunt 50mm is a small etalon (25mm?) so it will not have much bigger sweet spot than PST etalon.

The two interesting etalons from Lunt for this mod are the Lunt 60mm internal (the new modular one, its like they want us to mod this!) because its a 35mm aperture etalon (much bigger than 20mm and 25mm etalon with significantly bigger sweet spot for high res imaging use); and the Lunt 80mm double stack internal module, $1500ish, a big etalon. There's a few here on the boards (Brian!) that are using these mounted on the back of their scopes and have great results.

I have ordered one of the new Lunt 60mm modular scopes and will see how easy it is to mount on a larger F8 scope with DERF. It's already modular so no hack job to get it, just need to mount it and that's it. I uses F7 collimating lenses, so anything F8 will work, needs to be at -140mm for focus. And the modular etalon already has the back sorted with focuser and all. The only thing needed is an adapter flange to take the OTA tube to the inserting module itself and lock down as it normally would into its stock OTA tube and supply a DERF. I'll share everything as soon as I have it in my hands.

Ultimately again though I would suggest looking at the Lunt 60 modular scope and the Lunt 80 internal double stack module as potential filters for rear-mounting on any scope for this type mod. Pressure tuning. No tilt needed. Basic collimating lenses used. Still larger etalons than the typical tiny 20~25mm ones. Much bigger (35mm~40mm+).

Very best,


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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by Rusted »

Thank you very much Marty! :bow

Exactly the information I need to identify suitable etalons.
I'll start searching for the 60mm module from a dealer in Europe.

The obvious next question is suitability for my 150mm f/10 objective?
Is this likely to be beneficial or detrimental? Should I just use a focal reducer?


http://fullerscopes.blogspot.dk/

Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3, PST etalon, Lunt B1200S2, Assorted T-S GPCs, ZWO ASI174.
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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by AndiesHandyHandies »

Hi Marty,

Is the 40mm aperture etalon you quote for the Lunt 80mm scope. Does it use collimator lenses? And is it for a F7 telescope?

So a 80mm F7 OTA with that etalon would have all the Sun in the sweetspot like the PST?

Cheers. Andrew.


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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by MalVeauX »

Rusted wrote: Sun Jul 18, 2021 6:34 am Thank you very much Marty! :bow

Exactly the information I need to identify suitable etalons.
I'll start searching for the 60mm module from a dealer in Europe.

The obvious next question is suitability for my 150mm f/10 objective?
Is this likely to be beneficial or detrimental? Should I just use a focal reducer?
Both of them are mostly designed to work around F7 to F8 if you look at their respective configurations over the years and use collimating lenses and the etalon aperture can mask the scope aperture (such as the 70mm F6 scope that makes the 60mm F7 HA scope with its etalon in reality). In this case, any longer focal-ratio will work fine; you simply lose light, but nothing more than that. Faster than F7 is where things would be different, as you would likely mask the light cone with the etalon's aperture based on placement depending on the collimating lenses (ie, using a F7 collimating lens in a F5 beam would mask the aperture). So if you target F7 or longer focal-ratio, it should work without any special difference (other than light loss from focal-ratio speed).
AndiesHandyHandies wrote: Sun Jul 18, 2021 5:44 pm Hi Marty,

Is the 40mm aperture etalon you quote for the Lunt 80mm scope. Does it use collimator lenses? And is it for a F7 telescope?

So a 80mm F7 OTA with that etalon would have all the Sun in the sweetspot like the PST?

Cheers. Andrew.
The Lunt 60 uses a 35mm clear aperture etalon.

The Lunt 80mm uses a 50mm clear aperture etalon. This is probably one of the biggest clear aperture etalons you'll find that is pressure tuned, air gap, and for the cost ($1500!). That also directly means you can potentially get a bigger sweet spot out of this.

Both scopes use collimator lenses, yes, all etalon systems that are air gap seem to use collimator lenses that I'm aware of from all the line ups. They happen to use F7~F8 collimator lenses, but you could potentially swap these out for any other two matched pairs, for F10 or whatever you want. But their current configurations are F7~F8 from Lunt depending on the year/model I think (I don't have both so I cannot confirm directly).

So an 80mm aperture scope with a 50mm aperture etalon will have a large sweet spot. As long a the resulting sweet spot is 0.5 degrees or so, you get the full disc in the sweet spot. It's all about the field angle & acceptance angle.

The Lunt 80mm double stack module is on its own (no collimation lenses, etc) and has a big 50mm etalon in there. $1500ish USD. You'll need to mount this with collimating lenses of your choice.

The Lunt 60mm modular scope comes with a full scope of course, and a modular etalon system where it comes out with a focuser and everything and has collimating lenses already for F7 roughly, it's a 35mm clear aperture etalon. You get a complete scope in this one, and probably can just use a flange adapter to use it on another scope around F8 no problem with -140mm into the OTA for focus with no further modification (my theory) to the actual etalon assembly (just the donor OTA).

So I opted to use the 60mm despite it having a smaller etalon for the simpler approach to the mod. For now at least.

======================================

Mounting Lunt etalons on rear of scope is not new though; several have done it successfully. It's just more pricey, in theory, than a PST mod. So usually people try to mod a PST mod or Lunt 35/50mm, cheaper. But for the price, I think the Lunt 80mm internal DS module is one of the best options for cost potentially for this kind of install.

Very best,


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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by AndiesHandyHandies »

Hi

Thanks for the reply.

The 50mm bare etalon looks good for a 100mm 'PST' mod. 100 x F10 = 1000mm so need 50mm 500mm fl collimators.

Thanks. Andrew.


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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by Rusted »

Thanks again Marty. :bow

All good stuff to add to our options.


http://fullerscopes.blogspot.dk/

Baader 160mm D-ERF, iStar 150/10 H-alpha objective, 2" Baader 35nm H-a, 2" Beloptik KG3, PST etalon, Lunt B1200S2, Assorted T-S GPCs, ZWO ASI174.
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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by AndiesHandyHandies »

Hi Rusted,

Had a discussion with Rodastro about collimators and tele-centrics and we reckon manufacturers choose which to use based on hiding faults in the etalon design.

For Quarks:
If you look on CVs site an old school 4x long barlow performs just as well as a telecentric. The problem is finding one.

A long cheap Revelation 3x Barlow, baffled before and after the lens for a dark background, gives me sharp images in my 90mm Triplet but with a sweet spot. OK for my visual reports of prominences. Mark Townley heads up there.

For air spaced, PST:
I did a bodge on my Meade 127mm Triplet sliding a 2x ED barlow nosepiece up the focuser and putting my PST mod in. At F28 the view was as good as Pedros images. CV refers despite the theory to a PST etalon working at F28, which is either a co-incidence or a reference to my experience. Because it was about 200mm infront of focus rather than 100mm it gave 4x. Rodastro did a star drift test to confirm that.


If you go over my 100mm PST suggestion for full disk I think you are better with a Quark as its in the focused beam so it is not aware of how big the objective is. If you are using a large long focus telescope you are in mosaic territory for full disk images anyway. Daytime seeing is a limit anyway.

Apollo Lasky, Skyburner, said you can use barlows to extend the FNo for a F10 PST mod, and Rodastro uses home made reducers to shorten his 6" F15 to get the right FNo for his Quark.

So you can experiment with telecentric for a air spaced and collimator for a mica spaced depending on what you want to achieve. Or long barlow with either.

Start with a etalon and then choose a OTA or OTAs, with barlow or reducer, to suit your needs. Full disk or partial.


Cheers. Andrew.


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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by AndiesHandyHandies »

Hi

The proposed 'PST' has a sweet spot the size of the Sun.

Cheers. Andrew.


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Re: Lunt pressure tuned etalon for a 150 f/10 for imaging?

Post by GreatAttractor »

F/7 is not strictly necessary, one can go slower (at the cost of the the sweetspot covering a smaller angular size). A fellow astrophotographer does well with Lunt 50THa etalon + 100 mm f/11 achro: [link](https://astropolis.pl/topic/49688-moja- ... 4ce-w-h-a/).


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