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Recentering sweet spot possible?

Posted: Sun Feb 21, 2021 11:46 pm
by Gingerfish
I really never noticed that the sweetspot was off in my lunt 50. But after modding it, it became quite apparent that it was not in the middle! Is there any way to recenter it? I will be barlowing my mod to "zoom" in on the sweetspot, so having it centered would make life easier.

I found someone on CN who did this with a Lunt 60 by adjusted the focuser, but of course that is not really possible with the crappy helical focuser on the 50 (a FeatherTouch is next on the list of things to buy).

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Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 12:07 am
by justapictureposter
you can unscrew the ERF mount which also contains the negative collimator from the pressure tuner housing;

The etalon is protected in a sealed metal cylinder and slides right out like a piston . It is possible you can straighten it up a bit with some shims and rotate it around.

Dont have to touch the glass or anything, if it works cool, if not you gotta send it back for an internal spacer repair.


There is some wiggle room around the cylinder which holds the etalon. Just be careful not to drop anything!

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 1:49 am
by Gingerfish
Thanks!

Let me guess, it does not come out of the factory scratched like this? :?

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Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 3:02 am
by AndiesHandyHandies
Hi

I used a tilter in front of my PST Etalon and used auto-collimation from the front on the bare etalon on a refractor to set the etalon at 90 degrees to the optical axis.

I punched a hole near the edge of an old greetings card and drew a 1/4" black cross from it. Stand 20' infront of the telescope objective with the cross facing and centre the reflection from the front of the etalon looking through the hole. Move your head from side to side and up and down to centre the vignetting.

Improved the view.

Tried on my Maksutov but not as easy. Taken the tilter off.

Cheers. Andrew.

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 3:13 am
by justapictureposter
Gingerfish wrote:
Mon Feb 22, 2021 1:49 am
Thanks!

Let me guess, it does not come out of the factory scratched like this? :?

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absolutley not. Someone butchered it probably from attempting a mod as you are ; did you buy it from the "Gently used" portion of the lunt website or from a classified? A few vendors out there are notorious for selling used goods as well, something from neaf, something from a trade show. Something from an outreach event.


That is just awful to discover, and sadly something we all have dealt with buying telescope products.

The glue is certainly normal however, i remember it on mine.


from the appearance around the perimeter i believe I see it is slightly offset, and the etalon can be moved slightly to re-center and you should be okay. Again , be careful not to drop anything!

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 11:32 am
by Gingerfish
Thanks guys,

Yes, it was bought used. And yes, I know he tried to mod it, although just a "stage 1" mod according to himself :cry:

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 12:28 pm
by Gingerfish
Oh btw, if I push it too far, is there anyway to get it back out again? I see I can unscrew the black ring - if its not too damaged at the bottom thanks to the previous owner.

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 1:23 pm
by justapictureposter
I do not believe you will have any difficulties removing the etalon shell and putting it back in, but who knows what it looks like it side. It looks like the previous user, attached something to the front end with screws and thats where the digging marks came from.

The etalon itself should be sealed in its own cylinder behind that retainer ring, and it would resemble a beer can.

The cylinder should just slide right out without any friction like a bullet in a bolt action rifle.

It should easily slide back in the same way.

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 3:13 pm
by Gingerfish
Excellent! Will have a go when ever the rain stops again...

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 7:25 pm
by Gingerfish
I just noticed that Oliver at Beloptik has the LS50 under his repair service. I might ask him to do a service on it - just in case! I can see that the retainer ring has been unscrewed as well, so [no religion please] only knows what has been done inside!

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 8:08 pm
by marktownley
I don't think you'll achieve anything sending to Beloptik, there's nothing inside that can have anything done to it. I have the same my my LS50 with the sweet spot, the problem is the etalon cell is larger than the etalon, if I gently rock mine side to side I can hear the etalon moving in the cell. I did have the idea to use some small pieces of (electrical) tap around the periphery to take up the slop but have never got round to doing it

Re: Recentering sweet spot possible?

Posted: Mon Feb 22, 2021 8:32 pm
by Gingerfish
Ah ok ;) I'll have a look at it then, and see what can be done

Re: Recentering sweet spot possible?

Posted: Thu Feb 25, 2021 12:09 pm
by Gingerfish
I just can't seem to get a break! Went out to do some proms imaging, Focused, started imaging and went to get some coffee. Came back 5 minutes later to a off band sun. So there is definitely a leak somewhere, and I I don't think it's from the PT, as I have two new o rings in there. I guess it has to go back to Lunt themselves now, as there is no service in the UK I presume? :roll:

Lesson learned I guess, don't buy a used solar scope! :cry:

Re: Recentering sweet spot possible?

Posted: Thu Feb 25, 2021 9:17 pm
by marktownley
Theres not many places it can be leaking from - have you checked the allen stud on the end of the PT knob?

Re: Recentering sweet spot possible?

Posted: Thu Feb 25, 2021 10:49 pm
by Gingerfish
Yes, I went over everything. Even regreased the o rings again. Same problem. Lunt wants $150 to fix it, which isn't shocking. But it's still going to hurt when you add in shipping and duty :cry:

Still, cheaper than a new scope I guess...

Re: Recentering sweet spot possible?

Posted: Fri Feb 26, 2021 5:53 am
by marktownley
It can only be on either the o-rings, seals around front and back collimating lenses or the stud.

Sounds extreme, but you might want to pressurize the module up and briefly immerse it in a bucket of water - like you would with a bicycle inner tube to look for a puncture. It won't be in there for long - maybe 10 seconds while you look for the bubbles. It's a pressure vessel so it has in effect some waterproofing by definition. Just don't leave it in! Like I say, i'm talking seconds here to find the bubble train...

Re: Recentering sweet spot possible?

Posted: Fri Feb 26, 2021 12:32 pm
by Gingerfish
Smart! Bubbles are coming from the front end :(

Re: Recentering sweet spot possible?

Posted: Fri Feb 26, 2021 6:45 pm
by marktownley
PTFE tape? Re-seat the front threaded section?

Re: Recentering sweet spot possible?

Posted: Fri Feb 26, 2021 10:48 pm
by Gingerfish
Right, I will have to take it apart then :o Surprisingly, Lunt doesn't even mind! They just told me if I damage it, it will cost me more to fix.
I was told there was silicone inside, but does anyone know what kind? I presume it will be gasket sealant or something similar that can take pressure, and doesn't shrink or harden?

Re: Recentering sweet spot possible?

Posted: Fri Feb 26, 2021 11:11 pm
by justapictureposter
If i remember correctly its the same stuff you see on the front on the retaining ring. Glad to see you have decided to take it apart now despite the nightmare you heard about.

Remember, the only thing you must fear is dropping something. DO NOT DROP ANYTHING.

Lay a soft cushion down on top of whatever you open it up on, and a paper towel or napkin on top of that. You cannot mess anything up unless you scratch or drop it.

As far as re-sealing it up with the glue; dont go crazy with it. You just need to dab it around the perimeter with your finger; its design is intended to block air and thats it. Clean up any excess with an alcohol wipe.

Re: Recentering sweet spot possible?

Posted: Sat Feb 27, 2021 1:41 am
by Gingerfish
Cheers! Well I am more relaxed about the whole thing now that Lunt gave the OK. I just need to order a pin wrench, the one I had was too thick. Looks like I need a 2mm.

Btw, this first lens, is that one of the collimation lenses I see in the drawings? Anything I should worry about here?

I have one of these, would that do for sealing it back up?

https://www.amazon.co.uk/gp/product/B00 ... UTF8&psc=1

Re: Recentering sweet spot possible?

Posted: Sat Feb 27, 2021 1:57 am
by justapictureposter
i cannot recommend a glue other than it must be completely vapor free

A good pair of tweezers works to remove the retainer ring if you widen them out a bit.

I see nothing wrong with the loctite, but it must vapor free. So you may want to contact oliver smie over at beloptik to link you to a good glue.

It must be universally rated for hot and cold use and be vapor free.

Re: Recentering sweet spot possible?

Posted: Sat Feb 27, 2021 2:54 am
by Gingerfish
I don't know if it is vapor free or not. I will send Oliver an email

Re: Recentering sweet spot possible?

Posted: Sat Feb 27, 2021 2:33 pm
by Gingerfish
In the meantime, I took of the retainer ring. And yeah, the seal has most definitely been broken! But WHY would you be in here?! To remove dust?

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